WO2015062613A1

WO2015062613A1 - Control system for a wastewater treatment facility

Info

Publication number: WO2015062613A1
Application number: PCT/DK2014/050359
Authority: WO
Inventors: Mikkel Holmen Andersen; Peter Andreasen
Original assignee: Unisense Environment A/S
Priority date: 2013-11-04
Filing date: 2014-11-03
Publication date: 2015-05-07

Abstract

The present invention relates to a wastewater control system wherein at least part of the system adjusts parameters of the nitrification periods and denitrification periods based on measurements of N₂0. The invention furthermore relates to processes and computer systems for controlling wastewater treatment facilities.

Description

Control system for a wastewater treatment facility

Technical field of the invention

The present invention relates to a control system of a wastewater treatment facility comprising nitrification and denitrification periods. In particular, the present invention relates to a control system for reducing the overall emission of N2O from the facility.

Background of the invention

Biological water treatment is commonly used to reduce the nitrogen pollution content of water. These biological processes may include alternating nitrification and denitrification periods, which can be implemented continuously or

sequentially. Such a process consists of the introduction of a water to be treated into a biological reactor within which aerated and anoxic phases are implemented.

N2O is produced naturally during the microbial processes of nitrification (N) and denitrification (DN). It has been observed that nitrification-denitrification processes in wastewater treatment facilities results in the discharge into the atmosphere of nitrogen protoxide (N2O) also called nitrous oxide. Unlike oxygen (O2) nitrous oxide (N2O) is well soluble in pure water and other aquatic liquids including wastewater and significant amounts of N2O can be accumulated in the wastewater liquid during both the microbial processes of nitrification and denitrification. Due to the lower solubility of oxygen, a high aeration rate is needed to maintain the oxygen demand needed for a successful aerobic oxidation of ammonia (NH₄ ⁺) to nitrate (NO3 ) via nitrite (NO2-). However, the content of N2O in air is low (~320 ppb) and during the aeration N2O is degassed from the wastewater liquid in to the aeration air leaving the wastewater liquid.

N2O is a gas with a powerful greenhouse effect. It is especially 300 times more powerful than carbon dioxide. Beyond its contribution to the heating of the atmosphere, N2O also takes part in the destruction of the ozone layer. The discharge into the atmosphere of nitrogen protoxide exerts a negative impact on the environment. CA0281638 and FR2954306A describe systems for controlling N2O emission from wastewater treatment facilities.

Hence, improved control of the nitrification/denitrification process in a wastewater treatment facility would be advantageous, and in particular a more efficient and/or reliable process resulting in overall lower emission of N2O would be advantageous.

Summary of the invention

The present invention relates to a control system for wastewater treatment facilities preferably facilities employing alternating nitrification and denitrification periods.

The present inventors have identified by careful monitoring of the N2O generated during nitrification and shortly after switching to denitrification (by terminating oxygen supply) that the level of N2O and/or the rate of change of N2O can be used to optimize the N and DN cycles to minimize the overall oxygen consumption and minimize the generation of N2O (which is a strong green house gas). Thus, an object of the present invention relates to the provision of a system for controlling an alternating wastewater treatment facility.

In particular, it is an object of the present invention to provide a system for controlling an alternating wastewater treatment facility that solves the above- mentioned problems of the prior art with N2O emission.

Thus, one aspect of the invention relates to a system arranged for controlling aeration means (3) in an associated wastewater treatment facility (1), the system being adapted to

- controlling the aeration means (3), for aerating the wastewater (2);

- receive signal input from one or more N2O sensors (4) which can real time measure the amount of N2O in the wastewater (2); and

- optionally, being adapted to control means (5) for adding a carbon source to the wastewater; wherein the system further being adapted to a) prolong a denitrification (DN) period tn+i-DN relative to a previous

denitrification (DN) period tn-DN if

i. the level of N2O measured by the one or more N2O sensors (4) in an nitrification (N) period tn-N between tn-DN and tn+i-

DN is above a predefined first threshold level SI; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in an nitrification period tn-nitrification between tn-DN and tn+i-DN is above a predefined first threshold level SI'; and/or b) reduce the denitrification period tn+i-DN relative to the previous

denitrification period tn-DN if

i. the level of N2O measured by the N2O sensors (4) in the

nitrification period tn-nitrification between tn-DN and tn+i-DN) is below or equal to a second threshold level S2; and/or ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN)is below or equal to a second threshold level S2'; and/or c) maintain the length of the denitrification period tn+i-DN relative to the

length of the previous denitrification period tn-DN if

i. the level of N2O measured by the N2O sensors (4) in the

nitrification period tn-N between tn-DN and tn+i-DN is in the range equal to or below the threshold level SI to above the threshold level S2; and/or

ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN is in the range equal to or below the threshold level SI' to above the threshold level S2'; and/or d) increase the amount of addition of one or more carbon sources to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors (4) in a nitrification (N) period tn-N between tn-DN and tn+i-

DN is above a predefined third threshold level S3; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in an nitrification period tn-nitrification between tn-DN and tn+i-DN is above a predefined third threshold level S3'; and/or e) decrease the amount of addition of one or more carbon sources to a

denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the N2O sensors (4) in the

nitrification period tn-nitrification between tn-DN and tn+i-DN is below or equal to a fourth threshold level S4; and/or ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN is below or equal to a fourth threshold level S4'; and/or f) maintain the amount of addition of one or more carbon sources to a

nitrification period tn-N between tn-DN and tn+i-DN is in the range equal to or below the threshold level S3 to above the threshold level S4; and/or

ii. the rate of change of N2O measured by the N2O sensors (4) in the intervening nitrification period tn-N between tn-DN and tn+i-DN is in the range equal to or below the threshold level S3' to or above the threshold level S4'; wherein a denitrification period tn-DN is ended by increasing the oxygen level in the wastewater (2), by increasing the aeration through the aeration means (3), to an oxygen level which promotes nitrification.

Another aspect of the present invention relates to a system arranged for controlling aeration means (3) in an associated wastewater treatment facility (1), the system being adapted to

- controlling the aeration means (3), for aerating the wastewater (2);

- optionally being adapted to control means (5) for adding a carbon source to the wastewater; wherein the system further being adapted to a) prolong a denitrification (DN) period tn+i-DN relative to a previous

denitrification (DN) period tn-DN if

i. the level of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a predefined fifth threshold level S5; and/or

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a predefined fifth threshold level S5'; and/or b) reduce the denitrification period tn+i-DN relative to the previous

denitrification period tn-DN if

i. the level of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is below or equal to a predefined sixth threshold level S6; and/or

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is below or equal to a predefined sixth threshold level S6'; and/or c) maintain the length of the denitrification period tn+i-DN relative to the

length of the previous denitrification period tn-DN if

i. the level of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S5 to above the threshold level S6; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S5' to above the threshold level S6'; d/or d) increase the amount of addition of a carbon source during a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors

(4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a seventh threshold level S7; and/or

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a seventh threshold level S7'; and/or e) decrease the amount of addition of a carbon source to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is below or equal to an eight threshold level S8; and/or

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is below or equal to an eight threshold level S8'; and/or f) maintain the amount of addition of a carbon source to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors

(4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S7 to above the threshold level S8; and/or

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S7' to above the threshold level S8'; wherein a denitrification period tn-DN is ended by increasing the oxygen level in the wastewater (2), by increasing the aeration through the aeration means (3), to an oxygen level which promotes nitrification.

Another aspect of the present invention relates to a process for controlling aeration means (3) in a wastewater treatment facility according to the invention. Yet another aspect of the present invention relates to a computer program enabling a processor to carry out the process according to the invention.

Brief description of the figures

Figure 1 shows 0.5 hour data from an alternating activated sludge laboratory system with a volume of 600 ml_. Data from dissolved oxygen (O2), nitrous oxide (N2O), nitrate/nitrite (NO3 /NO2 ) and nitrite (NO2 ) sensors are all collected by a PC logging at IHz. The figure shows a nitrification phase and the relation between oxygen and N2O formation.

Figure 2 shows 0.5 hour data from an alternating activated sludge laboratory system with a volume of 600 ml_. Data from dissolved oxygen (O2), nitrous oxide (N2O), nitrate/nitrite (NO3 /NO2 ) and nitrite (NO2 ) sensors are all collected by a PC logging at IHz. The figure shows a nitrification phase and the relation between oxygen level, nitrite addition and N2O formation.

Figure 3 shows 0.5 hour data from an alternating activated sludge laboratory system with a volume of 600 ml_. Data from dissolved oxygen (O2), nitrous oxide (N2O), nitrate/nitrite (NO3 /NO2 ) and nitrite (NO2 ) sensors are all collected by a PC logging at IHz. The figure shows a nitrification phase and the relation between oxygen level, high nitrite addition and N2O formation.

Figure 4 shows a schematic drawing of data during an alternating activated sludge process. Data from a dissolved nitrous oxide (N2O) sensor were used to calculate the N2O rate of change (dINhO/dt) indicated by straight lines. The aerated nitrification phases are denoted N, and the anoxic denitrification phases denoted DN. The lines represent the N2O rate of change at different time points during nitrification and at the beginning of denitrification. Figure 5 shows two hours data from an alternating activated sludge laboratory system with a volume of 600 ml_. Data from a dissolved nitrous oxide (N2O) sensor were used to calculate the N2O rate of change (dINhO/dt, 30 second period), all collected by a PC logging at IHz. The nitrification phase (N) is marked with gray areas and the denitrification phase (DN) is marked with white areas. The black-lined box marks the preferred range in the current system from triggering a control action according to the invention.

Figure 6 shows two hours data from an alternating activated sludge laboratory system with a volume of 600 mL. Data from dissolved oxygen (O2), nitrous oxide (N2O), nitrate/nitrite (NO3 /NO2 ) and nitrite (NO2 ) sensors are all collected by a PC logging at lHz. The nitrification phase (N) is marked with grey areas and a section number for reference and the denitrification phase (DN) is marked with white areas.

Figure 7 shows eight hours data from an alternating activated sludge system with a volume of 4.000 m³. Data from a dissolved nitrous oxide (N2O) sensor were used to calculate the N2O rate of change

in this case calculated over 8 minutes), all collected by the central SCADA system. The nitrification (N) is marked with grey areas and the denitrification phase (DN) is marked with white areas. The black-lined box marks the preferred range in the current system from triggering a control action according to the invention.

Figure 8 shows eight hours data from an alternating activated sludge system with a volume of 4.000 m³. Data from dissolved nitrous oxide (N2O), ammonium

(NH₄ ⁺) and nitrate (NO3 ) sensors are all collected by the central SCADA system. The nitrification (N) is marked with grey areas and the denitrification phase (DN) is marked with white areas. Figure 9 shows the same eight hours data from an alternating activated sludge system as in figure 7, but data from a dissolved nitrous oxide (N2O) sensor were used to calculate the N2O rate of change (dN20/dt) without applying an averaging over several minutes. Figure 10 shows eight hours data from an alternating activated sludge system with a volume of 4.000 m³. Data from a dissolved nitrous oxide (N2O) sensor were used to calculate the N2O rate of change (dN20/dt, in this calculated case over 8 minutes), all collected by the central SCADA system. The nitrification (N) is marked with grey areas and the denitrification phase (DN) is marked with white areas. The black box marks the preferred range in the current system from triggering a control action according to the invention.

Figure 11 shows eight hours data from an alternating activated sludge system with a volume of 4.000 m³. Same time-period as in figure 10. Data from dissolved nitrous oxide (N2O), ammonium (NH₄ ⁺) and nitrate (NO3 ) sensors are all collected during a relatively low ammonium inflow by the central SCADA system. The nitrification (N) is marked with grey areas and the denitrification phase (DN) is marked with white areas.

Figure 12 shows eight hours data from an alternating activated sludge system with a volume of 4.000 m³. Data from a dissolved nitrous oxide (N2O) sensor were used to calculate the N2O rate of change

in this case calculated over 8 minutes), all collected by the central SCADA system. The nitrification (N) is marked with grey areas and the denitrification phase (DN) is marked with white areas. The black box marks the preferred range in the current system from triggering a control action according to the invention.

Figure 13 shows eight hours data from an alternating activated sludge system with a volume of 4.000 m³. Same time-period as in figure 12. Data from dissolved nitrous oxide (N2O), ammonium (NH₄ ⁺) and nitrate (NO3 ) sensors are all collected during a period of relatively low ammonium inflow changing into a high

ammonium inflow with nitrate build-up and a failing aeration valve. The

nitrification (N) is marked with grey areas and the denitrification phase (DN) is marked with white areas.

Figure 14 shows examples of system responses based on N2O measurements during a nitrification period. Figure 15 shows examples of system responses based on N2O measurements in the beginning of a denitrification period.

Figure 16 shows examples of system responses based on N2O measurements during a nitrification period determining to maintain nitrification. After one round of response the cycle can be repeated. Figure 17 shows examples of system responses based on N2O measurements during a nitrification period determining to switch to a denitrification period. Figure 18 shows examples of system responses based on NOx, nitrite, and/or nitrate measurements during a denitrification period determining to switch to a nitrification period (top) or maintain a denitrification period (below). After one round of response to maintain denitrification the cycle can be repeated. Figure 19 shows: Top: a schematic view of a possible nitrification/denitrification zone on a wastewater treatment facility.1) wastewater treatment facility; 2) wastewater; 3) aeration means; 4) N2O sensors; 5) means for adding carbon source; 6) wastewater inlet; and 7) wastewater outlet. Below: A more detailed schematic view of a wastewater treatment facility.

The present invention will now be described in more detail in the following. Detailed description of the invention

Control length of denitrification periods based on N2O measurements in the nitrification period

As described above it has been realized that by monitoring the N2O levels and/or rate of change of N2O during nitrification, it is possible to minimize the overall emission of N2O from the wastewater treatment facility. In an aspect the invention relates to a system arranged for controlling aeration means (3) in an associated wastewater treatment facility (1), the system being adapted to

- controlling the aeration means (3), for aerating the wastewater (2);

receive signal input from one or more N2O sensors (4) which can real time measure the amount of N2O in the wastewater (2); and

- optionally, being adapted to control means (5) for adding a carbon source to the wastewater; wherein the system further being adapted to a) prolong a denitrification (DN) period tn+i-DN relative to a previous denitrification (DN) period tn-DN if

i. the level of N2O measured by the one or more N2O sensors (4) in an nitrification (N) period tn-N between tn-DN and tn+i- DN is above a predefined first threshold level SI; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in an nitrification period tn-nitrification between tn-DN and tn+i-DN is above a predefined first threshold level SI'; and/or b) reduce the denitrification period tn+i-DN relative to the previous

denitrification period tn-DN if

i. the level of N2O measured by the N2O sensors (4) in the

nitrification period tn-nitrification between tn-DN and tn+i-DN is below (or equal to) a second threshold level S2; and/or ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN is below (or equal to) a second threshold level S2'; and/or c) maintain the length of the denitrification period tn+i-DN relative to the

length of the previous denitrification period tn-DN if

i. the level of N2O measured by the N2O sensors (4) in the

nitrification period tn-N between tn-DN and tn+i-DN is in the range equal to or below the threshold level SI or above the threshold level S2; and/or

ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN is in the range equal to or below the threshold level SI' to above the threshold level S2'; wherein said denitrification period tn-DN is ended by increasing the oxygen level in the wastewater (2), by increasing the aeration through the aeration means (3), to an oxygen level which promotes nitrification.

Denitrification Denitrification is a microbiologically facilitated process of nitrate reduction that may ultimately produce molecular nitrogen (N₂) through a series of intermediate gaseous nitrogen oxide products.

This respiratory process reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter. The preferred nitrogen electron acceptors in order of most to least thermodynamically favorable include nitrate (NO3 ), nitrite (N0₂ ^~), nitric oxide (NO), nitrous oxide (N₂0) finally resulting in the production of dinitrogen (N₂) completing the nitrogen cycle. In general, denitrification occurs where oxygen, a more energetically favorable electron acceptor, is depleted, and bacteria respire nitrate and/or the other oxygenated nitrogen forms as a substitute terminal electron acceptor. Due to the high concentration of oxygen in our atmosphere, denitrification only takes place in anoxic environments where oxygen consumption exceeds the oxygen supply and where sufficient quantities of nitrate are present. Denitrification is commonly exploited to remove nitrogen from sewage, industrial, and municipal wastewater.

The process is performed primarily by heterotrophic bacteria (such as Paracoccus denitrificans and various pseudomonads), although autotrophic denitrifiers have also been identified (e.g., Thiobacillus denitrificans).

Denitrification is the multi-stepped, anoxic reduction of nitrate (NO3 ) to dinitrogen gas (N₂) thorough and generally proceeds through some combination of the following intermediate forms under oxidation of carbon:

NOB^"→ N0₂-→ NO→ N₂0→ N₂ (g) or more precisely: 2INO3- + 12H⁺ + 10e-→ 5 N₂ (g) + 6 H2O

Carbon in organic matter (here presented as glucose C6H1206) is oxidised : C5H12O6 + 6 H2O→ 6CO2 + 24H⁺ + 24e^"

During denitrification in a wastewater treatment facility appropriate amount of methanol, ethanol, acetate, glycerine, and/or proprietary products, COD or BOD may be added to the wastewater to provide a carbon source for the denitrification bacteria. Carbon addition during denitrification increases the rate of change of NO3-, NO2- and N2O.

It is to be understood that when the term "switch to denitrification" and/or "switch to nitrification" it refers to a change in the oxygen supply leading to the switch.

Nitrification Nitrification is the biological oxidation of ammonia/ammonium with oxygen, then into nitrite followed by the oxidation of these nitrites into nitrates. Degradation of ammonia to nitrite is usually the rate-limiting step of nitrification.

The oxidation of ammonia into nitrite is performed by at least two groups of organisms, ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). AOB can be found among the β-proteobacteria and gammaproteobacteria. In soils the most studied AOB belong to the genera Nitrosomonas and

Nitrosococcus. Although in soils ammonia oxidation occurs by both AOB and AOA, AOA dominate in both soils and marine environments, suggesting that

Thaumarchaeota may be greater contributors to ammonia oxidation in these environments.

Nitrification also plays an important role in the removal of nitrogen from municipal wastewater. Most nitrogen in the liquid of pre-clarified municipal wastewater, settled in for example a wastewater clarifier, is on the reduced form as in

NH3/NH₄ ⁺. Furthermore, when the solid organic matter fraction from the settler tank is degraded NH3/NH₄ ⁺ is released. The conventional removal of the nitrogen is nitrification, followed by denitrification whereby the nitrogen is released as the gas N2. As also explained above, the cost of this process resides mainly in aeration (bringing oxygen in the reactor) and the addition of an external carbon source (e.g., methanol) for the denitrification.

Nitrification is a process of nitrogen compound oxidation (effectively, loss of electrons from the nitrogen atom to the oxygen atoms) :

2 NH3 + 3 O2→ 2 NO2- + 2 H2O + 2 H⁺ (Nitrosomonas)

2 NO2^" + O2→ 2 NO3- (Nitrobacter, Nitrospina)

N is oxidised :

NH3 +2H2O→ NO2- + 6H⁺ + 6e

NO2- + H2O→ NO3- + 2H⁺ + 2e^"

Oxygen is reduced :

O2 + 4e- + 4H⁺→ 2H₂0

Nitrifier denitrification process

An important biological N2O production process pathway co-exists with the nitrification pathway, namely the nitrifier-denitrification pathway. The nitrifier- denitrification pathway is accomplished by several bacteria species, mainly ammonia -oxidizing bacteria (AOB). The nitrifier-denitrification pathway plays a key role in N2O production by AOB, especially under anoxic to suboxic conditions, e.g. in the bulk wastewater or within bacterial flocks. During low-oxygen conditions in activated sludge processes, a significant amount of N2O can be produced by the reduction of NO2^" to NO, and to N2O by autotrophic AOBs.

Activated sludge is a process for treating sewage and industrial wastewaters using air and a biological floe composed of bacteria and protozoa. The anammox reaction

Anammox, an abbreviation for ANaerobic AMMonium Oxidation, is a globally important microbial process of the nitrogen cycle.

In another embodiment, aerobic and anaerobic ammonium oxidation are active simultaneously in a one-stage biofilm reactor with flock or granule biomass. This is also named the deammonification process. The presence of heterotrophic bacteria (DB) and nitrite-oxidizing bacteria (NOB) in the biofilm or flock cannot be avoided, especially if there is biodegradable carbon source available and high oxygen concentration supplied.

Dissolved oxygen (DO) is a significant parameter influencing the overall nitrogen removal rate and activity of different microorganisms in the system including production of N2O. DO concentrations should stay at a certain level to allow AOB to produce a sufficient amount of nitrite (NO2 ) for the anammox reaction but low enough to avoid causing anammox inhibition by a high level of nitrite (NO2 ) to effect or supporting increasing nitrite oxidizing bacteria (NOB) growth.

By controlling the reactions phase lengths, that is the nitrification phase and the anammox (incl. any concurrent denitrification) phase length, by the introduction of alternating aerated and non-aerated phases inside the same system, and by carefully monitoring N2O levels and/or N2O rate of change during nitrification and/or in the beginning of a anammox/denitrification period, it is possible to minimizing the overall N2O emission by adjusting the length of an aerated or non- aerated period.

BOD and COD:

BOD (Biochemical Oxygen Demand) is a measurement of the amount of organic pollution that can be oxidized biologically in a sample of water. COD (Chemical Oxygen Demand) is the total measurement of all chemicals in the water that can be oxidized.

Chemical names used:

Rate of change

In the present context "rate of change" (e.g. of N2O) relates to the change (increase or decrease) of measured levels of the compound over a given period. Rate of change may be given as (dN₂0/dt) for N2O.

Periods

In the present context, the term "period" in relation to nitrification (N) and denitrification (DN) relates to the length of the individual N and DN periods. A nitrification period is given as period tn-N and a denitrification period is given as period tn-DN. Thus, in an alternating system a sequence of periods will be tn-DN -> tn-N-> tn+l-DN^ tn + l-N-> tn+2"DN^ tn + 2"N^ tn + 3"DN^ tn+3"N and SO forth.

Preferably, the length of a nitrification period is in the range 10-60 minutes, such as 10-30 minutes. Preferably, the length of a denitrification period is in the range 10-60 minutes, such as 10-30 minutes. As described for the present invention, the length of the periods can be adjusted according to measured values in the wastewater to be treated or the wastewater under treatment.

Sequential Batch Reactors (SBR)

In another embodiment, sequencing or sequential batch reactors (SBR) are the processing tanks for the treatment of the wastewater. SBR reactors treat wastewater in repeated or sequential batches where measurements from the former batch can provide input to the following batch. The operation of an SBR is based on a fill- and -draw principle, which consists of four steps— fill/mix, react, settle, and decant. These steps can be altered for different operational

applications.

During the fill and mixing phase, the basin receives influent wastewater. The influent is mixed during filling with the activated sludge in the reaction, creating an environment for biochemical reactions to take place. Mixing and aeration can be varied during the fill phase to create a denitrification (DN) or nitrification (N) environment. Often the majority of denitrification takes place in the mixed-fill phase when the aeration is off. During the following "react phase" no wastewater enters the basin and the mechanical mixing and aeration units are on. Nitrification occurs by allowing the mixing and aeration to continue. Finally, aeration and mixing are stopped and the activated sludge is allowed to settle before the effluent is decanted and the cycle is repeated. Wastewater

In the present context "wastewater" refers to industrial or municipal effluents such as anaerobic digester supernatants, effluents from the treatment of sludges by wet oxidation, gas treatment condensates, condensates from treatment of wastewater sludge, discharge leach, slaughterhouse and dairy effluents, liquid pig manure or any other type of effluent charged with nitrogen in nitrate, nitrite or ammonium form. As described above N2O measurements can be used in the system either as actually measured levels and/or as rate of change over time. In a preferred embodiment rate of change is used. In an even more preferred embodiment rate of change in the beginning of denitrification period is used, since these

measurements are not disturbed by the addition of oxygen. With addition of oxygen in gas form (for example as air), the content of N2O (g) in that oxygen containing gas will be low, e.g. in air ~320 ppb. During the aeration N2O (aq) will therefore be degassed as N2O (g) from the wastewater liquid in to the aeration air leaving the wastewater liquid. This removal of N2O (aq) stops immediately with the termination of gas addition.

Control carbon supply in the denitrification period based on N2O measurements in the nitrification period

In another aspect the invention relates to a system arranged for controlling aeration means (3) in an associated wastewater treatment facility (1), the system being adapted to

- controlling the aeration means (3), for aerating the wastewater (2);

being adapted to control means (5) for adding a carbon source to the wastewater; wherein the system further being adapted to a) increase the amount of addition of one or more carbon sources to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors (4) in an nitrification (N) period tn-N between tn-DN and tn+i-

DN is above a predefined third threshold level S3; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in an nitrification period tn-nitrification between tn-DN and tn+i-DN is above a predefined third threshold level S3'; and/or b) decrease the amount of addition of one or more carbon sources to a

nitrification period tn-nitrification between tn-DN and tn+i-DN is below (or equal to) a fourth threshold level S4; and/or ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN is below (or equal to) a fourth threshold level S4'; and/or c) maintain the amount of addition of one or more carbon sources to a

ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN is in the range equal to or below the threshold level S3' to above the threshold level S4'; wherein said denitrification period tn-DN is ended by increasing the oxygen level in the wastewater (2), by increasing the aeration through the aeration means (3), to an oxygen level which promotes nitrification. In this system N2O measurements are used to control the addition of carbon to the denitrification period.

Control length of a denitrification period based on N2O measurements in the beginning of the same denitrification period

- controlling the aeration means (3), for aerating the wastewater (2);

denitrification (DN) period tn-DN if

denitrification period tn-DN if i. the level of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is below (or equal to) a predefined sixth threshold level S6; and/or

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is below (or equal to) a predefined sixth threshold level S6'; and/or c) maintain the length of the denitrification period tn+i-DN relative to the

length of the previous denitrification period tn-DN if

i. the level of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S5 to above the threshold level S6; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S5' to above the threshold level S6'; wherein said period of a denitrification period tn-DN is ended by increasing the oxygen level in the wastewater (2), by increasing the aeration through the aeration means (3), to an oxygen level which promotes nitrification. Thus, in this aspect the length of a denitrification period is based on input on N2O in the beginning of said denitrification period. Control carbon supply in a denitrification period based on N2O measurements in the beginning of the denitrification period

In yet an aspect the invention relates to a system arranged for controlling aeration means (3) in an associated wastewater treatment facility (1), the system being adapted to

- controlling the aeration means (3), for aerating the wastewater (2); receive signal input from one or more N2O sensors (4) which can real time measure the amount of N2O in the wastewater (2); and

- optionally being adapted to control means (5) for adding a carbon source to the wastewater; wherein the system further being adapted to a) increase the amount of addition of a carbon source during a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a seventh threshold level S7'; and/or b) decrease the amount of addition of a carbon source to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors

(4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is below (or equal to) an eight threshold level S8; and/or

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is below (or equal to) an eight threshold level S8'; and/or c) maintain the amount of addition of a carbon source to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S7 to above the threshold level S8; and/or

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S7' to above the threshold level S8'; wherein said period of a denitrification period tn-DN is ended by increasing the oxygen level in the wastewater (2), by increasing the aeration through the aeration means (3), to an oxygen level which promotes nitrification. Thus, in this aspect the addition of one or more carbon sources to the denitrification period is based on input on N2O in the beginning of said denitrification period. The measured N2O levels and/or rate of change in the beginning of a

denitrification period may be measured within different time intervals from switching to DN. Thus, in an embodiment these measurement are performed within 8 minutes from switching to DN, such as within 5 minutes, such as within 4 minutes or such as within 3 minutes.

To make proper measurements of levels of N2O and rate of change of N2O it is important to be able to constantly monitor the facility. Thus, in an embodiment, the N2O level is measured at least one time per minute during nitrification and/or denitrification, such as at least two times, such as at least three times per minute.

The different threshold levels may be adjusted by the skilled person depending e.g. on the input load to the system and/or legal demands on emission of N2O, and/or fluctuation of the year and other parameters important to keep the facility running. In an embodiment, the threshold levels SI and/or S3 and/or S9 are in the range 0.1-15 N-mg/L, such as 0.2-10 N-mg/L, such as 0.2-5 N-mg/L or such as 0.2-1.5 N-mg/L. In another embodiment the threshold levels SI' and/or S3' and/or S10' are in the range +/- 0-30 N-mg/L/h, such as +/- 0-20 N-mg/L/h, such as +/- 0-10 N- mg/L/h, such as +/- 0.5-8 N-mg/L/h or such as +/- 0.5-4 N-mg/L/h.

In yet an embodiment the threshold levels S2 and/or S4 are in the range 0-15 N- mg/L, such as 0.01-5 N-mg/L, such as 0.01-1 N-mg/L, or preferably such as 0.01- 0.5 N-mg/L.

In yet another embodiment the threshold level S2' and/or S4' are in the range +/- 0-30 N-mg/L/h, such as +/- 0-20 N-mg/L/h, such as +/- 0-10 N-mg/L/h, such as +/- 0.1-2 N-mg/L/h or such as +/- 0.2-0.5 N-mg/L/h.

In an embodiment, the threshold levels S5 and/or S7 are in the range 0.1-15 N- mg/L, such as 0.2-10 N-mg/L, such as 0.2-4 N-mg/L. In another embodiment, the threshold levels S5' and/or S7' are in the range +/- 0-30 N-mg/L/h, such as +/- 0-20 N-mg/L/h, such as +/- 0-10 N-mg/L/h, or such as +/- 0.5-8 N-mg/L/h.

In yet an embodiment, the threshold levels S6 and/or S8 are in the range 0-15 N- mg/L, such as 0.01-5 N-mg/L, such as 0.01-1 N-mg/L, or preferably such as 0.01- 0.5 N-mg/L.

In yet another embodiment, the threshold level S6' and/or S8' are in the range +/- 0-30 N-mg/L/h, such as +/- 0-20 N-mg/L/h, such as +/- 0-10 N-mg/L/h, such as +/- 0.1-2 N-mg/L/h or such as +/- 0.2-0.5 N-mg/L/h.

In yet an embodiment, the threshold level Sll is in the range 0-15 N-mg/L, such as 0.01-5 N-mg/L, or such as 0.01-2 N-mg/L. The table below summarizes threshold ranges, which may find use according to the present invention. The skilled person may adjust values depending on the specific requirement for a wastewater facility.

As shown in figure 7, in the table above and in the corresponding embodiments, the thresholds levels for the rate of change may have both positive and negative values. Preferably, the positive values are used for thresholds. The system according to the invention may also control other means for controlling the reactions taking place in the facility. Thus, in an embodiment, the system is further adapted to control means for adding microorganisms promoting the nitrification phase, and/or control means for adding microorganisms promoting the denitrification phase, and/or control means for transferring part of the wastewater to a storage tank.

Some of the threshold levels may be identical, since the system may be set to either prolong a denitrification based on N2O measurements as defined according to the invention and/or be set to supply one or more carbon sources to the wastewater. Thus, in an embodiment the threshold level SI is equal to the threshold level S3 and/or the threshold level SI' is equal to the threshold level S3'.

In another embodiment the threshold level S2 is equal to the threshold level S4 and/or the threshold level S2' is equal to the threshold level S4'. In a further embodiment the threshold level S5 is equal to the threshold level S7 and/or the threshold level S5' is equal to the threshold level S7'.

Some sensors may not be able to make the measurements in the liquid phase but can only make measurements in the gas phase above the wastewater, the latter being less desirable. Thus, in yet an embodiment the one or more N2O sensors (4) is arranged for measuring real time the amount of N2O in liquid wastewater (2). In a further embodiment, the one or more N2O sensors (4) are electrochemical N2O sensors. Such sensors are e.g. provided by Unisense A/S.

As described above the system according to the invention may adjust the amount of carbon added during the denitrification phase to promote the reactions.

Depletion of a carbon source during the denitrification phase results in a slower denitrification rate and a carbon source can be added to increase the nitrite turnover to N2. Thus, in yet another embodiment the one or more carbon sources is selected from the group consisting methanol, ethanol, acetate, glycerine, and/or proprietary products, COD or BOD. The skilled person may use other carbon sources depending on which material is available. This could also be carbon rich (and easily convertible) material from e.g. breweries, dairies, fermentation plants and alike.

The carbon source may also be recycled sludge from the treatment facility.

Maintaining nitrification period Besides being able to control the length of a denitrification period and/or the carbon supply during a denitrification period based on N2O measurements it may also be able to control the nitrification periods. Thus, in an aspect the invention relates to a system adapted to maintain a nitrification process, when an increase in the aeration level through the aeration means (3), in response to a measured amount of N2O at time tl above an upper ninth threshold level S9 results in a lower amount of measured N2O at time t2; and/or - an increase in the aeration level through the aeration means (3), in

response to a measured rate of change of N2O at time tl above an upper ninth threshold level S9', results in a lower amount of measured rate of change of N2O at time t2; and/or a reduction in the aeration level through the aeration means (3), in response to a measured amount of N2O at time tl below (or equal to) a lower tenth threshold level S10, results in higher amounts of measured N2O at time t2 below S9; and/or a reduction in the aeration level through the aeration means (3), in response to a measured rate of change of N2O at time tl below (or equal to) a lower threshold level S10', results in higher amounts of measured rate of change of N2O at time t2 below S9'; and/or

- a steady aeration level through the aeration means (3), in response to a measured amount of N2O at time tl in the liquid wastewater is equal to or below the upper ninth threshold level S9 and above the lower tenth threshold level S10, results in a steady amount of measured N2O at time t2; and/or

- a steady aeration level through the aeration means (3), in response to a measured rate of change of N2O at time tl in the liquid wastewater is equal to or below the upper ninth threshold level S9' and above the lower tenth threshold level S10', results in a steady amount of measured rate of change of N2O at time t2; wherein the time t2 is later in time than the time tl. It is to be understood that the nitrification period is maintained by supplying an amount of oxygen, which promotes a nitrification process.

Controlling switch from nitrification to denitrification

Similar, a system capable of controlling when to switch from nitrification to denitrification based on N2O measurements may also be advantageous. Thus, yet an aspect of the invention relates to a system adapted to switch from a

nitrification period to a denitrification period, when an increase in the aeration level in response to a measured amount of N2O at time tl above the upper ninth threshold level S9, results in an increased amount of measured N2O at time t2; and/or an increase in the aeration level in response to a measured rate of change of N2O at time tl above the upper ninth threshold level S9', results in an increased amount of measured rate if change of N2O at time t2; wherein said switch to a denitrification process is conducted by reducing the oxygen level in the wastewater (2), by reducing aeration through the aeration means (3), to an oxygen level which promotes denitrification; wherein the time t2 is later in time than the time tl. In a preferred embodiment rate of change is used to determine switch from N to DN. It is also to be understood that the measurements tl and t2 of N2O is performed during a nitrification period.

The skilled person would know which oxygen levels are required to promote either nitrification or denitrification. In the present invention, these levels are controlled by the oxygen supply means placed in the facility. Oxygen may be supplied by addition of atmospheric air or by addition of more concentrated forms of oxygen. Thus, in an embodiment the level of O2 in the wastewater is reduced to or kept in the range 0-0.1 mg/L, to maintain a denitrification period or switch to a

denitrification period from a nitrification period, such as below 0.1 mg/L, or such as in the range 0.01-0.1 mg/L. In another embodiment the level of O2 in the wastewater is adjusted to or kept at a level in the range above 0.1 to 10 mg/L to maintain a nitrification period or to switch to a nitrification period from a denitrification period, such as in the range 0.5-10 mg/L, such as in the range 0.5- 5 mg/L or such as 0.5-2 mg/L.

In an embodiment, the measured aeration level is based on an average of one or more O2 sensors.

The systems according to the invention may also be set to control the switch from denitrification to nitrification based on further input. Thus, in an embodiment the system is further adapted to

switch from a denitrification period to a nitrification period, when the measured amount of NOx, nitrite, and/or nitrate is below (or equal to) an eleventh threshold level Sll; wherein said switch to a nitrification period is conducted by increasing the aeration level through the aeration means (3) to a level which promotes nitrification.

To be able to make more precise measurements, and to avoid "one point of failure", more than one N2O sensor is preferably present in the facility. Thus, in an embodiment, the measured N2O level is based on an average of the one or more N2O sensors.

It is to be understood that the facility may also comprise one or more inlets (6) for a stream of unprocessed wastewater and one or more outlets (7) for a stream of wastewater processed by the treatment facility. The facility may also comprise a storage tank for unprocessed and/or processed wastewater.

It is also to be understood that known regulations such as regulating inlet of wastewater or other inlets or outlets based on e.g. NH₄ measurements and other standard regulation processes can be combined with the present invention.

Wastewater treatment facility

The system(s) according to the invention may be implemented in a wastewater treatment facility. Thus, an aspect of the invention relates to a wastewater treatment facility (1) comprising a system according to any of the preceding claims. In an embodiment, the facility is for purifying wastewater through alternating nitrification and denitrification periods.

Process for controlling aeration means in an associated wastewater treatment facility

In a further aspect, the invention relates to a process for controlling aeration means (3) in an associated wastewater treatment facility according to the invention. Thus, the invention also relates to a process for controlling aeration means (3) in a wastewater treatment facility (1), the process comprising

- controlling the aeration means (3), for aerating the wastewater (2);

receive signal input from one or more N2O sensors (4) which can real time measure the amount of N2O in the wastewater (2); and - optionally, controlling the means (5) for adding a carbon source to the wastewater; wherein the process further comprising a) prolonging a denitrification (DN) period tn+i-DN relative to a previous

denitrification (DN) period tn-DN if

i. the level of N2O measured by the one or more N2O sensors (4) in an nitrification (N) period tn-N between tn-DN and tn+i- DN is above a predefined first threshold level SI; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in an nitrification period tn-nitrification between tn-DN and tn+i-DN is above a predefined first threshold level SI'; and/or b) reducing the denitrification period tn+i-DN relative to the previous

denitrification period tn-DN if

i. the level of N2O measured by the N2O sensors (4) in the

nitrification period tn-nitrification between tn-DN and tn+i-DN) is below or equal to a second threshold level S2; and/or ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN)is below or equal to a second threshold level S2'; and/or c) maintaining the length of the denitrification period tn+i-DN relative to the length of the previous denitrification period tn-DN if

i. the level of N2O measured by the N2O sensors (4) in the

ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN is in the range equal to or below the threshold level SI' to above the threshold level S2'; and/or d) increasing the amount of addition of one or more carbon sources to a

denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors

(4) in a nitrification (N) period tn-N between tn-DN and tn+i- DN is above a predefined third threshold level S3; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in an nitrification period tn-nitrification between tn-DN and tn+i-DN is above a predefined third threshold level S3'; and/or e) decrease the amount of addition of one or more carbon sources to a

nitrification period tn-nitrification between tn-DN and tn+i-DN is below or equal to a fourth threshold level S4; and/or ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN is below or equal to a fourth threshold level S4'; and/or f) maintaining the amount of addition of one or more carbon sources to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the N2O sensors (4) in the

In a similar aspect the invention relates to a process for controlling aeration means (3) in a wastewater treatment facility (1), the process comprising

- controlling the aeration means (3), for aerating the wastewater (2);

- optionally controlling the means (5) for adding a carbon source to the

wastewater; wherein the process further comprising a) prolonging a denitrification (DN) period tn+i-DN relative to a previous

denitrification (DN) period tn-DN if

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a predefined fifth threshold level S5'; and/or b) reducing the denitrification period tn+i-DN relative to the previous denitrification period tn-DN if

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is below or equal to a predefined sixth threshold level S6'; and/or c) maintaining the length of the denitrification period tn+i-DN relative to the length of the previous denitrification period tn-DN if

i. the level of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S5 to above the threshold level S6; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S5' to above the threshold level S6'; and/or d) increasing the amount of addition of a carbon source during a

denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a seventh threshold level S7; and/or

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a seventh

threshold level S7'; and/or e) decreasing the amount of addition of a carbon source to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification

(DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors

(4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is below or equal to an eight threshold level S8; and/or

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is below or equal to an eight threshold level S8'; and/or f) maintain the amount of addition of a carbon source to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S7 to above the threshold level S8; and/or

Computer system

In an aspect the invention relates to a computer program enabling a processor to carry out the process(es) according to the invention.

A computer program may be stored and/or distributed on a suitable medium, such as a computer program product, such as a computer readable medium, such as an optical storage medium or a solid-state medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. It may be understood that the processor may be operatively connected with appropriate elements, such as an actuator for controlling the aeration means for aerating wastewater, and/or

- a data connection operatively connected to the one or more N2O sensors so as to enable receiving signal input from the one or more N2O sensors, and/or

- optionally, an actuator for controlling the aeration means for adding a

carbon source to the wastewater.

It should be noted that embodiments and features described in the context of one of the aspects of the present invention also apply to the other aspects of the invention.

All patent and non-patent references cited in the present application, are hereby incorporated by reference in their entirety. The invention will now be described in further details in the following non-limiting examples. Examples

Example 1

Control of an alternating activated sludge process Materials and Methods:

A 700 ml bioreactor was filled with approximately 600 ml activated sludge from an alternating activated sludge process of a local municipal wastewater treatment plant. The bioreactor was stirred magnetically and the temperature was regulated with a thermostat to 20°C. Furthermore, aeration was supplied through an air stone from an adjustable air-pump. The reactor was equipped with sensors for oxygen and N20 (aq) and with two biosensors for NOx and N02-.

All sensors were logged real-time every second using a MultiMeter and

SensorTrace Basic software. All equipment were from Unisense A/S. During the experiments, the activated sludge was feed stock solutions of KNO3, KNO2, NH₄CI, and complex carbon sources in the form of Tryptic Soy Broth medium, all of which were prepared in water.

Results:

The following results are with reference to figure 1. The timeline starts with a denitrification tn-DN (no oxygen) period followed by a nitrification period tn-N

(oxygen present) and ends with yet a denitrification period tn+i-DN(no oxygen) The oxygen content was measured and is apparent in the figure.

The activated sludge from an alternating activated sludge process contained low level of both ammonium and COD. Initially the bioreactor was maintained at low ammonium and COD load. Hereafter the bioreactor was feed by injections of

NH₄CI and COD from stock solutions (Figure 1). During the first period the oxygen level was keep at a relatively high level around 60 μΜ or 2 mg/L and a small increase in the amount of N₂0(aq) was detected at an apparent rate of 2.5 μΜ/hour. At time 0.65 hours the oxygen supply was decreased to below 0.5 mg/L as detected by be oxygen sensor signal. This resulted in an increase of produced N2O (aq) at an apparent rate of 17 μΜ/hour in the liquid as detected directly with the dissolved N2O sensor. From time 0.7 hours the air supply was increased again and the apparent N2O (aq) formation rate dropped quickly to zero resulting in a steady N2O. The concentration of NO3^" (NO_x-N0₂) increased steadily during the nitrification period as ammonium nitrogen (NH₄ ⁺) were oxidised. At time 0.74 hours the reactor was spiked with ammonium to 100 mg/L final concentration and the N2O formation started to increase. An increase in oxygen addition (time 0.75 hours) was therefore needed to maintain the apparent N2O formation rate as low as possible. Following the initial N2O formation test during the aerated nitrification process, at time 0.94 hours the air supply was shut off to allow for the denitrification process to proceed. Characteristically, the

concentration of NOx immediately started to decrease indicating that the denitrification process had started. Organic carbon and ammonia was added to the process at this point (0.94 hours). In this first case no significant change in N2O level was detected immediately after the oxygen level was shut of, but soon hereafter (time 0.95 hours) the N2O level decreased at a rate of -150 uM/hour due to N2O reduction in the denitrification process. In conclusion, the control of a sufficient oxygen level can be guided by the realtime N2O formation rate and/or level and can be used to minimize the N2O emission by reduction of its formation.

Example 2

Control of an alternating activated sludge process

The following results are with reference to figure 2, which is a continued timeline from figure 1. The timeline starts with a denitrification period followed by a nitrification period and ends with yet a denitrification period as observed by the measured oxygen level.

From time 1.07 hours the air supply was increased (to switch from denitrification to nitrification) to a measured oxygen level of about 1 mg/L and maintained at same rate to have a controlled apparent N2O formation rate, in this case N2O below 25 μΜ/hour (figure 2). The initial NO2^" concentration were relatively high (should be avoided) which almost prevented the oxidation of NH3 to NO3^" during this nitrification instance (as seen by a very slow increase in NOx), and instead a relatively high rate of N2O formation as end product from NH3 oxidation was observed. Adding a nitrite (NO2 ) spike to the process at time 1.2 hours to a final level of approximately 160 μΜ clearly increased the apparent rate of formation of N2O from 23 μΜ/hour to 55 μΜ/hour further indicating that the apparent N2O formation rate can indicate the concentration level of NO2^". In other words a higher formation rate of N2O (aq) measured directly and real-time in the nitrification process can be used both 1) to shift to denitrification and 2) to estimate the amount of nitrites formed during the nitrification process and thereby the length of the following denitrification process needed to reduce all the NO2^" to N2(g) It is also apparent that in this period (1.20 min to 1.25 min) there was almost no NO2^" and NO3^" produced, i.e. a high fraction of the NH3 being oxidised ended up in the N2O (aq) pool.

Furthermore, the higher level of NO2^" resulted in a significant increase in the apparent N2O formation rate when the oxygen decreased. This is clearly detected at time 1.25 hour where the apparent N2O formation rate increased from 55 μΜ/hour to 150 μΜ/hour.

Without being bound by theory, it is believed that this increase in N2O formation after shutting off aeration shows the correct production rate of N2O during nitrification :

During nitrification, there may be a formation of N2O but the aeration will degas some of the produced N2O (the more aeration, the more degassing). The measured rate of formation is therefore the real rate of formation where the degassing due to aeration is subtracted. Right after the aeration has been shut off, there will still be some oxygen present and the nitrification can proceed for a short time. The measured rate of formation is therefore in this period the real rate of formation (no degassing due to aeration). A high rate in this situation indicate a high level of NO2^" and a long denitrification period is needed to reduce all the NO2- to N₂ (g).

Example 3

Control of an alternating activated sludge process. The following results are with reference to figure 3, which is a continued timeline from figure 2. The timeline starts with a denitrification period followed by a nitrification period and ends with yet a denitrification period as observed by the measured oxygen level.

5

The aeration was turned on at times 1.39 hours and increased to 2 mg/L oxygen (figure 3). Compared to the situation in figure 2 (aeration beginning at time 1.07 hours), the oxygen level was here almost double whereas the NO2^" and NOx level was comparable to the situation at time 1.07 hours. The higher oxygen level 10 ensured that the NO2^" level remained the same indicating that the amount of

oxygen was sufficient to convert NO2^" to NO3^" and all N oxidation proceeded to NO3^" which insured a low steady state level of N2O with an apparent formation rate of zero uM/hour. The increase in NO3^" can be observed by the increase in the NOx sensor signal.

15

Having established a steady-state nitrification process in which a steady oxidation of NH₄ ⁺ to NO3^" was ongoing and no increase in N2O or NO2^" was seen, the process was then spiked with NO2^" to a final level of approximately 190 μΜ at time 1.53 hours. The apparent rate of formation of N2O clearly increased from 0 μΜ/hour to

20 40 μΜ/hour showing that an insufficient amount of oxygen or microbe capacity was available to convert the additional NO2^" to NO3^" to finalize the nitrification despite the higher aeration. The change of apparent N2O formation rate could again be used as indicator for a too high level of NO2^", and the oxidation of NH3 was thus converted to N2O. The nitrification process continued and the increase in

25 NO3^" could be observed by the increase in the NOx sensor signal, without

significant increase in NO2- .

As in the former example 2, the higher level of NO2^" resulted in a significant increase in the apparent N2O formation rate when the aeration was shut off. This was clearly detect at time 1.72 hour, where the apparent N2O formation rate 30 increased from 6.55 μΜ/hour to 200 μΜ/hour just before it began to decrease at a rate of -71 μΜ/hour due to the initiation of the denitrification.

Example 4

Data from municipal wastewater treatment plant

Methods A local municipal wastewater treatment plant with a capacity of 200.000 PE (Population Equivalents) was fully equipped with standard sensors for ammonium, nitrate, and oxygen and also equipped with N2O sensors in one of four 4.000 m³ activated sludge tank running an alternating activated sludge process. The plant operated a multi-stage process where the primary settled and grid removed sludge was treated in a digester, and the remaining wastewater was lead to the activated sludge process. Aeration means (2) in the alternating activated sludge tank was provided for by a turbo pump through diffusers at the bottom of the tank and the air supply was regulated by an adjustable valve operated during the change for aerated to anoxic phases. Wastewater was lead in to the tank during the beginning of the anoxic phase and outlet from the tank was during the beginning of the aerated phase. The data in a SCADA system were collected at a process temperature of approximately 19°C and data from a dissolved nitrous oxide (N2O) sensor were used to calculate the N2O rate of change (dINhO/dt). The aerated nitrification phases (N), and the anoxic denitrification phases (DN) were controlled via the SCADA system and the real-time N2O level and rate of change were calculated from 10s data point(s). The wastewater treatment plant had a clear diurnal cycle with a high loading during the late morning. Figures show 7-13 data from Medium, Low and High loading.

Shown in figures 7 and 8 are data from the full-scale wastewater treatment plant during a period of relative high ammonium concentration of 10 mg N-NH₄+/L As depicted in figure 8 the N2O (aq) concentration increased during the same period as well as the nitrate concentration, the latter due to a clearly incomplete denitrification process. Furthermore, it was seen that the N2O (aq) concentration increased rapidly as soon as the nitrification period was ended by turning the aeration off. The rapidly N2O (aq) concentration increase can also be identified by looking at the rate of change,

as depicted in figure 7. A significant increase of dINhO (aq)/dt values is identifiable after each nitrification period (black- lined box marks the preferred range), in this case calculated over an 8 minute window using linear regression on the N2O (aq) concentration data. The data in figure 9 is as in figure 7 but in this case calculated over a 1 minute window using linear regression on the N20(aq) concentration data, showing that the rate of change,

often needs to be calculated over several minutes to provide data for the preferred range in the current system for triggering a control action according to the invention.

In conclusion, the fixed nitrification and denitrification periods provide no means to pursue a completion of the denitrification process and thus minimize the N2O formation in the following nitrification period.

Shown in figures 10 and 11 are data from the full-scale wastewater treatment plant during a period of relative high ammonium concentration of 10 mg N- NH₄+/L Unlike data in figures 7 and 8, neither the N₂0(aq) concentration nor the nitrite concentration increased during the period as in this case the denitrification period is long enough to process the nitrates and nitrites formed during the previous nitrification period. Likewise, the N2O (aq) concentration did not increase rapidly as soon as the nitrification period was ended by turning the aeration off, which also resulted in a minor change in the rate of change, dN₂0(aq)/dt, as depicted in figure 10. In conclusion, in some cases the fixed nitrification and denitrification periods can be sufficient or overly long for a completion the denitrification process and thus minimization of the N2O formation. Shown in figures 12 and 13 are data from the full scale wastewater treatment plant during a period of relative high and increasing ammonium concentration from 10 to 20 mg N-NH₄ ⁺/L As depicted in figure 13 the N2O (aq) concentration increased significantly during the period as well as the nitrate concentration, the latter due to a clearly incomplete denitrification process. Furthermore, it can be seen that the N2O (aq) concentration increased rapidly as soon as the nitrification period ended in the second half of the figure where the nitrate concentration was high. The rapidly N2O (aq) concentration increase can also be identified by looking at the rate of change, dN₂0(aq)/dt), as depicted in figure 12. A significant increase of dN₂0(aq)/dt values is identifiable in the second half of the figure. Clearly, in this case, the fixed nitrification period and incomplete denitrification during the fixed denitrification period resulted in an increasing and accumulating N2O concentration in the wastewater. Particularly in this case, addition of a carbon source would intervene the process by increasing the denitrification rate. Also noteworthy in figure 13 is the failing air valve closure at time 212 hours, resulting in the complete nitrification of ammonium to nitrate. In the following denitrification period the N2O (aq) is also almost fully processed. Due to the now much lower ammonium load no further significant accumulation of nitrate or N2O (aq) is realized.

Example 5

Control of an alternating activated sludge process by adjustment of nitrification (N) and denitrification (DN) periods.

Methods

A 700 ml bioreactor was filled with approximately 600 ml activated sludge from an alternating activated sludge process of a local municipal wastewater treatment plant. The bioreactor was stirred magnetically and the temperature was regulated with a thermostat to 20°C. Furthermore, aeration was supplied through an air stone from an adjustable air-pump. The reactor was equipped with sensors for oxygen and N20 (aq) microsensors and with two biosensors for NOx and NO2. All sensors were logged real-time every second using a MultiMeter and SensorTrace Basic software. All equipment was from Unisense A/S. During the experiments the activated sludge was fed stock solutions of KNO3, KNO2, NH₄CI, and complex carbon sources in the form of Tryptic Soy Broth medium, all of which were prepared in water.

Results:

The bioreactor with fresh activated sludge from an alternating activated sludge process was loaded with 20 mg N-NH₄/L NH₄CI and COD from stock solutions (figure 6). During the nitrification period (figure 6, section 1) from time 9.4 hours to 9.8 the oxygen level was kept at a relatively high level around 60 μΜ or 2 mg/L and a steady increase in the amount of N2O was detected at an apparent rate of 10 μΜ/hour (mean dINhO/dt over 30s, Figure 5.) At time 9.83 hours the oxygen supply was stopped and the oxygen level drops to approximately 0 mg/L as detected by the oxygen sensor signal. As soon as the oxygen supply was decreased a significant increase of N20(aq) formation at an apparent rate of 66 μΜ/hour was detected directly with the N20 (aq) sensor. The apparent N2O (aq) formation rate after turning of the oxygen was clearly indicative for nitrification process under high load and oxygen limiting condition in the activated sludge. Following the first aeration phase the denitrification process was allowed to proceed and the N2O (aq) level dropped rapidly due to N2O reduction in the denitrification process. The rate of denitrification was further enhanced by adding COD to the bioreactor at time 9.95 hours resulting in a small increase in the N₂0(aq) level. The N2O (aq) level remains low and constantan until it with N02- reaches zero. At the end of the first denitrification phase the bioreactor was loaded with another 20 mg N-NH₄/L NH₄CI but also NO2^" to approximately 2.3 mg N-NO2 /L, both from stock solutions. During the next nitrification period (figure 6, section 2) from time 10.0 hours to 10.14 the oxygen level was kept at a relatively high level around 60 μΜ or 2 mg/L, but due to the higher ammonium and nitrite concentrations a very fast increase in the amount of N2O was detected at an apparent rate of 30 μΜ/hour. Due to the fast increase in N2O (aq) level during the nitrification phase, the oxygen supply was stopped at time 10.14 hours and the oxygen level dropped to approximately 0 mg/L as detected by be oxygen sensor signal. As soon as the oxygen supply decreased a very significant increase of N2O (aq) formation at an apparent rate of 120 μΜ/hour was detected with the N20 (aq) sensor. Again the apparent rate N2O (aq) formation rate after turning of the oxygen was clearly indicative for a nitrification process under high concentration of ammonium and nitrite.

Due to the fast increase in N₂0(aq) level during the nitrification phase and the high rate of formation after stopping the oxygen supply, the denitrification phase was prolonged and the COD was added to promote the denitrification process, which was allowed to proceed to the end as detected by the decrease of the N₂0(aq) and NO2- level.

Following the above denitrification process, a lower load of ammonium was applied to the bioreactor by adding 10 mg N-NH₄/L NH₄CI from stock solution. During the next nitrification period (figure 6, section 3) from time 10.44 hours to 10.65 the oxygen level was kept at a relatively high level around 60 μΜ or 2 mg/L, and a steady increase in the amount of N2O was detected at an apparent rate of 12 μΜ/hour. Due to the short and slower increase in N2O (aq) level the nitrification phase was continued for a further period of time. The oxygen supply was stopped at time 10.65 hours and the oxygen level droped approximately 0 mg/L as detected by the oxygen sensor signal. As soon as the oxygen supply decreased a smaller increase of N2O (aq) formation peaking at an apparent rate of 55 μΜ/hour as detected with the N2O (aq) sensor indicative for nitrification process under lower load as compared to the previous cycle. Therefore the following denitrification process time was shortened before a lower load of ammonium cycle was repeated (figure 6, section 4) from time 10.8 to 11.13 hours in which the nitrification phase was prolonged with a similar rate of N2O formation as in the previous nitrification phase and was ended without any significant N2O (aq) increase after the oxygen supply was stopped.

Conclusion

By carefully monitoring N2O levels and/or N2O rate of change (during nitrification and/or in the beginning of a denitrification period) it is possible to adjust the length of a DN period or the amount of carbon supply to the DN period or a coming DN period, thereby minimizing the overall N2O emission.

Claims

1. A system arranged for controlling aeration means (3) in an associated wastewater treatment facility (1), the system being adapted to

- controlling the aeration means (3), for aerating the wastewater (2);

denitrification (DN) period tn-DN if

denitrification period tn-DN if

i. the level of N2O measured by the N2O sensors (4) in the

nitrification period tn-nitrification between tn-DN and tn+i-DN) is below or equal to a second threshold level S2; and/or ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN)is below or equal to a second threshold level S2'; and/or c) maintain the length of the denitrification period tn+i-DN relative to the length of the previous denitrification period tn-DN if

i. the level of N2O measured by the N2O sensors (4) in the

ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN is in the range equal to or below the threshold level SI' to above the threshold level S2'; and/or d) increase the amount of addition of one or more carbon sources to a

nitrification period tn-nitrification between tn-DN and tn+i-DN is below or equal to a fourth threshold level S4; and/or ii. the rate of change of N2O measured by the N2O sensors (4) in the nitrification period tn-N between tn-DN and tn+i-DN is below or equal to a fourth threshold level S4'; and/or maintain the amount of addition of one or more carbon sources to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the N2O sensors (4) in the

2. A system arranged for controlling aeration means (3) in an associated wastewater treatment facility (1), the system being adapted to

- controlling the aeration means (3), for aerating the wastewater (2);

denitrification (DN) period tn-DN if

i. the level of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a predefined fifth threshold level S5; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a predefined fifth threshold level S5'; and/or b) reduce the denitrification period tn+i-DN relative to the previous

denitrification period tn-DN if

length of the previous denitrification period tn-DN if

i. the level of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S5 to above the threshold level S6; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is in the range equal to or below the threshold level S5' to above the threshold level S6'; and/or d) increase the amount of addition of a carbon source during a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a seventh threshold level S7'; and/or e) decrease the amount of addition of a carbon source to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is below or equal to an eight threshold level S8'; d/or f) maintain the amount of addition of a carbon source to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+ i-DN is in the range equal to or below the threshold level S7' to above the threshold level S8'; wherein a denitrification period tn-DN is ended by increasing the oxygen level in the wastewater (2), by increasing the aeration through the aeration means (3), to an oxygen level which promotes nitrification.

3. The system according to any of the preceding claims, wherein the one or more N2O sensors (4) is arranged for the amount of N2O in the liquid wastewater (2).

4. The system according to any of the preceding claims, wherein the system is adapted to maintain a nitrification process, when an increase in the aeration level through the aeration means (3), in response to a measured amount of N2O at time tl above an upper ninth threshold level S9 results in a lower amount of measured N2O at time t2; and/or an increase in the aeration level through the aeration means (3), in response to a measured rate of change of N2O at time tl above an upper ninth threshold level S9', results in a lower amount of measured rate of change of N2O at time t2; and/or a reduction in the aeration level through the aeration means (3), in response to a measured amount of N2O at time tl below or equal to a lower tenth threshold level S10, results in higher amounts of measured N2O at time t2 below S9; and/or a reduction in the aeration level through the aeration means (3), in response to a measured rate of change of N2O at time tl below or equal to a lower threshold level S10', results in higher amounts of measured rate of change of N2O at time t2 below S9'; and/or

- a steady aeration level through the aeration means (3), in response to a measured rate of change of N2O at time tl in the liquid wastewater is equal to or below the upper ninth threshold level S9' and above the lower tenth threshold level S10', results in a steady amount of measured rate of change of N2O at time t2; wherein the time t2 is later in time than the time tl.

5. The system according to any of the preceding claims, wherein the system is adapted to switch from a nitrification period to a denitrification period, when an increase in the aeration level in response to a measured amount of N2O at time tl above the upper ninth threshold level S9, results in an increased amount of measured N2O at time t2; and/or an increase in the aeration level in response to a measured rate of change of N2O at time tl above the upper ninth threshold level S9', results in an increased amount of measured rate if change of N2O at time t2; wherein said switch to a denitrification process is conducted by reducing the oxygen level in the wastewater (2), by reducing aeration through the aeration means (3), to an oxygen level which promotes denitrification; wherein the time t2 is later in time than the time tl.

6. The system according to any of the preceding claims, wherein the system is further adapted to

switch from a denitrification period to a nitrification period, when the measured amount of NOx, nitrite, and/or nitrate is below an eleventh threshold level Sll; wherein said switch to a nitrification process is conducted by increasing the aeration level through the aeration means (3).

7. The system according to any of the preceding claims, wherein the N2O level is measured at least one time per minute during nitrification and/or denitrification, such as at least two times, such as at least three times per minute.

8. The system according to any of the preceding claims, wherein the threshold levels SI and/or S3 and/or S9 are in the range 0.1-15 N-mg/L, such as 0.2-10 N- mg/L, such as 0.2-5 N-mg/L or such as 0.2-1.5 N-mg/L.

9. The system according to any of the preceding claims, wherein the threshold levels SI' and/or S3' and/or S10' are in the range +/- 0-30 N-mg/L/h, such as +/- 0-20 N-mg/L/h, such as +/- 0-10 N-mg/L/h, such as +/- 0.5-8 N-mg/L/h or such as +/- 0.5-4 N-mg/L/h.

10. The system according to any of the preceding claims, wherein the threshold levels S2 and/or S4 are in the range 0-15 N-mg/L, such as 0.01-5 N-mg/L, such as 0.01-1 N-mg/L, or preferably such as 0.01-0.5 N-mg/L.

11. The system according to any of the preceding claims, wherein the threshold level S2' and/or S4' are in the range +/- 0-30 N-mg/L/h, such as +/- 0-20 N- mg/L/h, such as +/- 0-10 N-mg/L/h, such as +/- 0.1-2 N-mg/L/h or such as +/- 0.2-0.5 N-mg/L/h.

12. The system according to any of the preceding claims, wherein the threshold levels S5 and/or S7 are in the range 0.1-15 N-mg/L, such as 0.2-10 N-mg/L, such as 0.2-4 N-mg/L.

13. The system according to any of the preceding claims, wherein the threshold levels S5' and/or S7' are in the range +/- 0-30 N-mg/L/h, such as +/- 0-20 N- mg/L/h, such as +/- 0-10 N-mg/L/h, or such as +/- 0.5-8 N-mg/L/h.

14. The system according to any of the preceding claims, wherein the threshold levels S6 and/or S8 are in the range 0-15 N-mg/L, such as 0.01-5 N-mg/L, such as 0.01-1 N-mg/L, or preferably such as 0.01-0.5 N-mg/L.

15. The system according to any of the preceding claims, wherein the threshold level S6' and/or S8' are in the range +/- 0-30 N-mg/L/h, such as +/- 0-20 N- mg/L/h, such as +/- 0-10 N-mg/L/h, such as +/- 0.1-2 N-mg/L/h or such as +/- 0.2-0.5 N-mg/L/h.

16. The system according to any of the preceding claims, wherein the threshold level Sll is in the range 0-15 N-mg/L, such as 0.01-5 N-mg/L, or such as 0.01-2 N-mg/L.

17. The system according to any of the preceding claims, wherein the system is further adapted to control means for adding microorganisms promoting the nitrification phase, and/or control means for adding microorganisms promoting the denitrification phase, and/or control means for transferring part of the wastewater to a storage tank.

18. The system according to any of the preceding claims, wherein the threshold level SI is equal to the threshold level S3 and/or the threshold level SI' is equal to the threshold level S3'.

19. The system according to any of the preceding claims, wherein threshold level S2 is equal to the threshold level S4 and/or the threshold level S2' is equal to the threshold level S4'.

20. The system according to any of the preceding claims, wherein the threshold 5 level S5 is equal to the threshold level S7 and/or the threshold level S5' is equal to the threshold level S7'.

21. The system according to any of the preceding claims, wherein the one or more N2O sensors (4) is arranged for measuring real time the amount of N2O in liquid wastewater (2).

10 22. The system according to any of the preceding claims, wherein the one or more N2O sensors (4) are electrochemical N2O sensors.

23. The system according to any of the preceding claims, wherein the one or more carbon sources is selected from the group consisting methanol, ethanol, acetate, glycerine, and/or proprietary products, such as COD or BOD.

15 24. The system according to any of the preceding claims, wherein the level of O2 in the wastewater is reduced to or kept in the range 0-0.1 mg/L, to maintain a denitrification period or switch to a denitrification period from a nitrification period, such as below 0.1 mg/L, or such as in the range 0.01-0.1 mg/L.

25. The system according to any of the preceding claims, wherein the level of O2 20 in the wastewater is adjusted to or kept at a level in the range above 0.1 to 10 mg/L to maintain a nitrification period or to switch to a nitrification period from a denitrification period, such as in the range 0.5-10 mg/L, such as in the range 0.5- 5 mg/L or such as 0.5-2 mg/L.

26. The system according to any of the preceding claims, wherein the measured 25 aeration level is based on an average of one or more O2 sensors.

27. The system according to any of the preceding claims, wherein the system is further adapted to switch from a denitrification period to a nitrification period, when the measured amount of NOx, nitrite, and/or nitrate is below or equal to an eleventh threshold level Sll; wherein said switch to a nitrification period is conducted by increasing the aeration level through the aeration means (3) to a level which promotes nitrification.

28. The system according to any of the preceding claims, wherein the measured N2O level is based on an average of the one or more N2O sensors.

29. A wastewater treatment facility (1) comprising a system according to any of the preceding claims.

30. The wastewater treatment facility (1) according to claim 29, wherein the facility is for purifying wastewater through alternating nitrification and

denitrification periods.

31. A process for controlling aeration means (3) in a wastewater treatment facility (1), the process comprising

- controlling the aeration means (3), for aerating the wastewater (2);

- optionally, controlling the means (5) for adding a carbon source to the wastewater; wherein the process further comprising a) prolonging a denitrification (DN) period tn+i-DN relative to a previous

denitrification (DN) period tn-DN if

i. the level of N2O measured by the one or more N2O sensors (4) in an nitrification (N) period tn-N between tn-DN and tn+i- DN is above a predefined first threshold level SI; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in an nitrification period tn-nitrification between tn-DN and tn+i-DN is above a predefined first threshold level SI'; and/or b) reducing the denitrification period tn+i-DN relative to the previous denitrification period tn-DN if

i. the level of N2O measured by the N2O sensors (4) in the

denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors (4) in a nitrification (N) period tn-N between tn-DN and tn+i-

DN is above a predefined third threshold level S3; and/or ii. the rate of change of N2O measured by the one or more N2O sensors (4) in an nitrification period tn-nitrification between tn-DN and tn+i-DN is above a predefined third threshold level S3'; and/or e) decrease the amount of addition of one or more carbon sources to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the N2O sensors (4) in the

ii. the rate of change of N2O measured by the N2O sensors (4) in the intervening nitrification period tn-N between tn-DN and tn+i-DN is in the range equal to or below the threshold level

S3' to or above the threshold level S4'; wherein a denitrification period tn-DN is ended by increasing the oxygen level in the wastewater (2), by increasing the aeration through the aeration means (3), to an oxygen level which promotes nitrification.

32. A process for controlling aeration means (3) in a wastewater treatment facility (1), the process comprising controlling the aeration means (3), for aerating the wastewater (2);

receive signal input from one or more N2O sensors (4) which can real time measure the amount of N2O in the wastewater (2); and optionally controlling the means (5) for adding a carbon source to the wastewater; wherein the process further comprising a) prolonging a denitrification (DN) period tn+i-DN relative to a previous

denitrification (DN) period tn-DN if

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a predefined fifth threshold level S5'; and/or b) reducing the denitrification period tn+i-DN relative to the previous

denitrification period tn-DN if

ii. the rate of change of N2O measured by the one or more N2O sensors (4) in the DN period tn+i-DN within 10 minutes from switching to the DN period tn+i-DN is above a seventh threshold level S7'; d/or e) decreasing the amount of addition of a carbon source to a denitrification (DN) period tn+i-DN relative to the addition to the previous denitrification (DN) period tn-DN if i. the level of N2O measured by the one or more N2O sensors

33. A computer program enabling a processor to carry out the process according to any one of claim 31 or claim 32.