WO2015052279A1

WO2015052279A1 - A method and a system for wastewater nitrogen removal

Info

Publication number: WO2015052279A1
Application number: PCT/EP2014/071636
Authority: WO
Inventors: Julián CARRERA MUYO; Eduardo ISANTA MONCLÚS; Julio PÉREZ CAÑESTRO
Original assignee: Universitat Autonoma De Barcelona
Priority date: 2013-10-10
Filing date: 2014-10-09
Publication date: 2015-04-16

Abstract

The method comprises performing a partial nitritation of wastewater in a biofilm reactor, by means of performing a closed loop control for regulating the ammonium concentration within the biofilm reactor based on the ammonium concentration at the outlet of the biofilm reactor, and calculating and varying the value of an ammonium concentration setpoint of said closed control loop based on the nitrogen concentration at the outlet of or inside the biofilm reactor. The system is adapted to implement the method of the invention.

Description

A method and a system for wastewater nitrogen removal

Field of the Invention

The present invention generally relates, in a first aspect, to a method for wastewater nitrogen removal, which comprises performing a partial nitritation of wastewater in a biofilm reactor by means of an ammonium concentration closed loop control, and more particularly to a method comprising varying the ammonium concentration setpoint for the closed loop control based on nitrogen concentration.

A second aspect of the invention relates to a system adapted to implement the method of the first aspect.

Background of the Invention

The most sustainable (energy-neutral) wastewater treatment plants that are just starting to be developed are using Anammox for nitrogen removal (Figure 1 ).

With the configuration in Figure 1 , the main challenge for the success of the treatment is the difficulty of maintaining stable the nitritation process, since nitrite- oxidizing bacteria (NOB) may develop in such a granular sludge, producing nitrate, competing with anammox for nitrite and reducing the efficiency of the treatment considerably.

Since the publication in 2010 of the idea of applying anammox for sewage treatment (Kartal, Kuenen and van Loosdrecht 2010 Science 328:702-3), several research groups started to focus on the development of such a treatment. As identified by Hu et al. (2013a), the main challenge for applying anammox in the mainline of municipal wastewater treatment is to achieve a high-rate process with good biomass retention and a low effluent nitrogen concentration at low water temperatures (8-15 °C). Recently, a breakthrough was made in the application of the anammox process in this temperature range (Hu et al. 2013b). A laboratory scale one-stage system running at 25 °C could be adapted very rapidly (10 days) to operate at 12 °C. The system was operated for over 300 days without nitrite accumulation or detectable NOB activity and was able to remove over 90 %of the supplied nitrogen at 12 °C (Hu et al. 2013b). This latest study reported the proof of principle for the application of one-stage systems at low temperatures; nevertheless, further studies are necessary to be able to determine the feasibility of the application of such a process at pilot and eventually full scale. In particular that study utilized a sequencing batch reactor (SBR), which probably would not be the best alternative for the application of the technology at full scale in municipal treatment plants. More complex approaches have been followed by Wett and coworkers, based on the previously developed DEMON process, which requires cyclones to separate anammox enriched granules whereas the nitritation is mainly linked to biomass in suspension (Wett 2007). Last results showed good performance in an Austrian WWTP (Strass), as presented in recent publications (Wett et al., 2013).

In parallel, the group Verstraete and coworkers, have presented some progress in the application of anammox at low temperatures with biofilm reactors, although results still are preliminary and with no good conversions (De Clippeleir et al., 2013).

On the other hand, ES2334321 B1 and Bartroli et al., 2010 disclose a partial nitritation method of wastewater in a biofilm reactor containing ammonia-oxidizing bacteria, comprising performing a closed loop control for regulating the ammonium concentration within the biofilm reactor based on the ammonium concentration at the outlet of the biofilm reactor.

The method disclosed in ES2334321 B1 is applied to wastewater with a high ammonium concentration, the goal of said method is to obtain a full nitritation, and therefore the ammonium set point is set to a fixed low value, independently of inflow characteristics, which makes the obtained effluent unsuitable for feeding, among others, an anammox reactor. References:

Kartal, Kuenen and van Loosdrecht 2010 Sewage treatment with anammox. Science 328:702-3.

Hu Z, Lotti T, van Loosdrecht M, Kartal B. 2013. Nitrogen removal with the anaerobic ammonium oxidation process Biotechnol Lett (2013) 35:1 145-1 154.

Hu Z, Lotti T, de Kreuk M, Kleerebezem R, van Loosdrecht M, Kruit J, Jetten MSM, Kartal B. 2013. Nitrogen Removal by a Nitritation-Anammox Bioreactor at Low Temperature. Applied and Environmental Microbiology 79(8): 2807-2812.

Wett B (2007) Development and implementation of a robust deammonification process.

Water Sci. Technol. 56:81-88.

Wett B, Omari A, Podmirseg SM, Han M, Akintayo O, Gomez Brandon M, Murthy S, Bott C, Hell M, Takacs I, Nyhuis G, O'Shaughnessy M, 2013. Going for mainstream deammonification from bench to full scale for maximized resource efficiency. Water Science & Technology 68 (2), 283-289.

De Clippeleir H, Vlaeminck SE, De Wilde F, Daeninck K, Mosquera M, Boeckx P, Verstraete W, Boon N, 2013. One-stage partial nitritation/anammox at 15 °C on pretreated sewage: feasibility demonstration at lab-scale. Appl. Microbiol. Biotechnol. DOI 10.1007/s00253-013-4744-x.

Bartroli, A., Perez, J., Carrera, J., 2010. Applying ratio control in a continuous granular reactor to achieve full nitritation under stable operating conditions. Environ. Sci. Technol. 44, 8930-8935.

Alex J, Benedetti L, Copp J, Gernaey KV, Jeppsson U, Nopens I, Pons MN, Rosen C, Steyer JP, Vanrolleghem P. 2008. Benchmark Simulation Model no. 1 (BSM1 ). IWA.

Wett B, Alex J, 2003. Impacts of separate rejection water treatment on the overall plant performance. Water Science and Technology 48(4): 139-146.

Abma WR, Driessen W, Haarhuis R, van Loosdrecht MCM, 2010. Upgrading of sewage treatment plant by sustainable and cost-effective separate treatment of industrial wastewater. Water Science & Technology 61 (7):1715-1722.

Wanner, O., Reichert, P., 1996. Mathematical modeling of mixed-cultures biofilms.

Biotechnol. Bioeng. 49, 172-184.

Reichert, P., 1998. AQUASIM 2.0-Computer program for the Identification and

Simulation of Aquatic Systems, Version 2.0, EAWAG, CH-8600 Dubendorf,

Switzerland.

Jubany, I., Carrera, J., Lafuente, J., Baeza, J.A., 2008. Start-up of a nitrification system with automatic control to treat highly concentrated ammonium wastewater.

Experimental results and modelling. Chem. Eng. J. 144, 407-419.

Perez, J., Costa, E., Kreft, J.U., 2009. Conditions for partial nitrification in biofilm reactors and a kinetic explanation. Biotechnol. Bioeng. 103 (2), 282-295. Description of the Invention

It is an object of the present invention to offer an alternative to the state of the art, with the purpose of providing a method for wastewater nitrogen removal including a partial nitritation process which overcomes the drawbacks of the known partial nitritation processes, offering a dynamic and more evolved control of the ammonium concentration adaptable to the current circumstances of the process, in order to obtain an effluent suitable for, among others, feeding an anammox reactor.

To that end, the method for wastewater nitrogen removal of the first aspect of the invention comprises performing a partial nitritation of wastewater in a biofilm reactor, wherein said biofilm reactor contains ammonia-oxidizing bacteria, and the method comprises performing a closed loop control for regulating the ammonium concentration within said biofilm reactor based on the ammonium concentration at the outlet of said biofilm reactor.

Contrary to the known methods, particularly contrary to the partial nitritation method disclosed in ES2334321 B1 where no proposal for calculating and varying the ammonium concentration setpoint is done, the method of the first aspect of the invention comprises calculating and varying the value of an ammonium concentration setpoint of said closed control loop based at least on the nitrogen concentration at the outlet of or inside said biofilm reactor.

Said nitrogen concentration generally refers to the sum of nitrite and nitrate concentrations at the outlet or inside said biofilm reactor.

Generally, ammonium enters the biofilm reactor through a main stream, in which case, for a preferred embodiment, the method comprises performing said regulation of ammonium concentration within the biofilm reactor by regulating the flow-rate of a side stream also entering said biofilm reactor.

Said ammonium concentration at the outlet of said biofilm reactor and/or said nitrogen concentration at the outlet of or inside the biofilm reactor is/are online measured (preferably both of them), whether directly at said points of the biofilm reactor (at the outlet thereof, for the ammonium concentration, and at the outlet or inside the biofilm reactor, for the nitrogen concentration), near said points, or in another point or points of the system (within or external to the biofilm reactor) which allow a calculation/estimation of the ammonium concentration at the outlet of the biofilm reactor and/or of said nitrogen concentration at the outlet of or inside the biofilm reactor from the measurements made at said another point or points of the system.

The method of the first aspect of the invention comprises calculating and varying the value of the ammonium concentration setpoint of the closed control loop also based on a desired ratio of ammonium and nitrite concentrations at the outlet of said biofilm reactor, hence the method can be used for feeding a further stage with an effluent having the specific ratio of ammonium and nitrite concentration required for that further stage.

Said calculation of the value of the ammonium concentration setpoint is performed, according to a preferred embodiment, according to the following expression: o + l

where TAN refers to the total ammonium concentration, SP to setpoint, NO^~ is the sum of nitrite and nitrate, and b to said desired ratio of ammonium and nitrite concentrations. For a preferred embodiment, said desired ratio of ammonium and nitrite concentration is between 0.5 and 2, preferably between 1.1 and 1 .3.

The method of the first aspect of the invention further comprises, for a preferred embodiment, feeding an anammox reactor with the effluent of the biofilm reactor, and using said anammox reactor for performing an anammox reaction with the ammonium and nitrite contained in said biofilm reactor effluent in order to achieve a further nitrogen removal. Said preferred ratio of ammonium and nitrite concentration of between 1 .1 and 1 .3 is optimal for the feeding of said annamox reactor, and thus is used therefor.

By providing the partial nitritation to be achieved in a separate reactor, the problem previously described in the Background of the Invention section related to the NOB developing is circumvented, as maintaining partial nitritation without nitrate production (i.e. repressing NOB activity) is achieved by the method of the first aspect of the invention, particularly by means of the control strategy described above, thus enhancing the good efficiency of the nitrogen removal for main stream.

Preferably, the ranges of operating conditions for the partial nitritation are: temperature inside the biofilm reactor is between 8 and 35 °C, the ammonium concentration in said main stream is between 30 and 100 g N/m³ and the COD (Chemical Oxygen Demand) concentration is between 1 and 125 g/m³. These ranges of conditions differ considerably from the ones of the partial nitritation of ES2334321 B1 .

For an embodiment, said side stream carries reject water coming from an anaerobic digester, the method comprising performing said regulation of the flow-rate of said side stream from a discharging buffer tank interspersed between said anaerobic digester and the biofilm reactor.

According to an embodiment, the main stream comes from a main output of a very-high-load activated sludge and settler producing biomass and organic particles and feeding with the same said anaerobic digester through, the anaerobic digester generating biogas and said reject water from the received biomass and organic particles.

Optionally, the method of the first aspect of the invention further comprises controlling the dissolved oxygen concentration inside the biofilm reactor based on the nitrogen concentration in the main stream.

A second aspect of the present invention concerns to a system for wastewater nitrogen removal, comprising:

- a biofilm reactor containing ammonia-oxidizing bacteria, and

- a closed loop control configured and arranged for regulating the ammonium concentration within said biofilm reactor based on the ammonium concentration at the outlet of said biofilm reactor, said closed loop control comprising an ammonium concentration setpoint.

Contrary to the known systems, particularly contrary to the system of ES2334321 B1 , the system of the second aspect of the invention comprises processing means configured and arranged for calculating and varying the value of the ammonium concentration setpoint of said closed control loop based at least on the nitrogen concentration at the outlet of or inside the biofilm reactor.

For an embodiment, the system of the second aspect of the invention comprises first and second measuring means for, respectively, measuring the ammonium concentration at the outlet of the biofilm reactor and the nitrogen concentration (particularly the sum of nitrite and nitrate concentrations) at the outlet of or inside the biofilm reactor, said first and second measuring means being arranged to provide said processing means with the measured values, the processing means being configured for performing said calculation of the value of the ammonium setpoint based on said measured values and on a desired ratio of ammonium and nitrite concentrations at the outlet of said biofilm reactor.

For an embodiment, the system of the second aspect of the invention further comprises:

- a very-high-load activated sludge and settler having an input for receiving sewage, a main output connected to a main stream input of the biofilm reactor for providing the latter with ammonium, being configured for producing biomass and organic particles therefrom, and having a secondary output for delivering said biomass and organic particles;

- an anaerobic digester having an input connected to said secondary output of the very-high-load activated sludge and settler to receive said biomass and organic particles, and being configured to generate biogas and reject water there from, through respective first and second outputs;

- a discharging buffer tank arranged for receiving said reject water from said second output of said anaerobic digester and for feeding the biofilm reactor, through a side stream input thereof, with said reject water according to a flow-rate regulated under the control of said control means in order to perform said regulation of the ammonium concentration within the biofilm reactor; and

- an anammox reactor with an input connected to an output of the biofilm reactor to receive the effluent coming there through, and configured for performing an anammox reaction with the ammonium an nitrite contained in said biofilm reactor effluent in order to achieve a further nitrogen removal. For other embodiments, the biofilm reactor (with its associated ammonium loop control) of the system of the second aspect of the invention can be connected to stages other than the ones of the above cited very-high-load activated sludge and settler, digester and anammox reactor, or in combination with some of said cited stages (generally with the anammox stage).

The system of the second aspect of the invention is adapted to implement the method of the first aspect.

Brief Description of the Drawings

The previous and other advantages and features will be better understood from the following detailed description of embodiments, with reference to the attached drawings, which must be considered in an illustrative and non-limiting manner, in which:

Figure 1 schematically shows a conventional sewage treatment with Anammox, basic configuration (based on Kartal, Kuenen and van Loosdrecht 2010 Science 328:702-3). 1 : Very-high-load activated sludge + settler; 2: Anaerobic digester; 3:

Granular sludge nitritation - Anammox reactor.

Figure 2 shows an embodiment of the system of the second aspect of the invention for main stream treatment, with the next block units: 1 : Very-high-load activated sludge + settler; 2: Anaerobic digester; 3: Granular sludge partial nitritation reactor or biofilm reactor (also called in the present specification as cold ANFIBIO); 4:

Granular sludge Anammox reactor. The closed loop control of the second aspect of the invention is applied to biofilm reactor 3.

Figure 3 is a block diagram of the control strategy of the method and system of the invention, to obtain partial nitritation in the aerobic granular sludge reactor (unit 3 in Figure 2) operating in continuous mode. TAN: total ammonia nitrogen. SP: setpoint.

[TAN]sp station: calculates the desired ammonium setpoint to keep the adequate concentrations ratio between of ammonium and nitrite.

Figure 4 show, by means of two graphs, an experimental demonstration of performance of partial nitritation obtained with the biofilm reactor of the second aspect of the invention, at lab scale.

Figure 5 shows a basic layout of the system of the second aspect of the invention, simulated with a model described in the next section, where: 1 : Very-high-load activated sludge + settler. Only considered to determine the buffer capacity (2500 m³) with regard to dynamics of ammonium concentration; 2: Anaerobic digester (not described with the model); 3: Granular sludge partial nitritation reactor (cold ANFIBIO), i.e. the biofilm reactor; volume used in simulations 250 m³; 4: Granular sludge Anammox reactor, volume used in simulations 2000 m³. 5: Buffer tank used to regulate the reject water inflow for the biofilm reactor 3.

Figure 6 shows in a graph, for the Scenario A described in the next section, the flow-rates of main stream (imposed to test diurnal variability) and side stream (regulated by the control loop of the system of the second aspect of the invention, to keep the desired ammonium concentration in the biofilm reactor 3). Integrated average of side stream flow-rate yields 2.5 % of the main stream, meaning 35% of the total N treated in the WWTP (Waste Water Treatment Plant).

Figure 7 shows, also for the Scenario A, the variability assumed for ammonium concentration in the main stream. Integral ammonium concentration average yields 37.7 glM/rm³. The variability is the same for Scenario B, also described in the next section.

Figure 8 also associated to Scenario A, shows the effluent of the partial nitritation reactor (cold ANFIBIO, unit 3 in Figures 2 and 5). Note how the control strategy produces an effluent with the adequate ratio between ammonium and nitrite concentrations, as to feed the subsequent anammox reactor (unit 4 in Figures 2 and 5).

Figure 9, also regarding Scenario A, shows the effluent from the anammox reactor (unit 4 in Figures 2 and 5). Further polishing may include removal of nitrate by heterotrophic denitrification.

Figure 10 shows, also for Scenario A, the Ammonium concentration in the cold ANFIBIO reactor (unit 3 in Figures 2 and 5) and ammonium setpoint. Note how in large fraction of the time measurement is very close to setpoint. Flow-rate of main stream has been plotted for direct comparison of the effects on ammonium concentration in the reactor.

Figure 1 1 , also regarding Scenario A, shows the Ammonium concentration in the cold ANFIBIO reactor (unit 3 in Figures 2 and 5) and ammonium setpoint. Flow-rate of side stream and inflow ammonium concentration in the main stream have been plotted for direct comparison of the effects on ammonium concentration in the biofilm reactor 3.

Figure 12 shows in a graph, for the Scenario B described in the next section, the flow-rates of main stream (imposed in the scenario to test diurnal variability) and side stream (regulated by the control loop of the system of the second aspect of the invention, to keep the desired ammonium concentration in the reactor). Integrated average of side stream flow-rate yields 1 .0 % of the main stream, meaning 22% of the total N treated in the WWTP.

Figure 13 depicts, for Scenario B, the effluent of the partial nitritation reactor (cold ANFIBIO, unit 3 in Figures 2 and 5). Note how the control strategy produces an effluent with the adequate ratio between ammonium and nitrite concentrations, as to feed a subsequent anammox reactor (unit 4 in Figures 2 and 5).

Figure 14, also for Scenario B, shows the effluent from the anammox reactor (unit 4 in Figures 2 and 5). Further polishing may include removal of nitrate by heterotrophic denitrification.

Figure 15 shows, for Scenario B, the Ammonium concentration in the cold ANFIBIO reactor (unit 4 in Figures 2 and 5) and ammonium setpoint. Note how in large fraction of the time measurement is very close to setpoint. Flow-rate of main stream has been plotted for direct comparison of the effects on ammonium concentration in the reactor.

Figure 16, also for Scenario B, shows the Ammonium concentration in the cold ANFIBIO reactor (unit 4 in Figures 2 and 5) and ammonium setpoint. Flow-rate of side stream and inflow ammonium concentration in the main stream have been also plotted for direct comparison of the effects on ammonium concentration in the reactor.

Detailed Description of Several Embodiments

The system of the second aspect of the present invention is schematically depicted in Figures 2 and 5, and it includes at least the biofilm reactor 3 (where partial nitritation is performed) integrated in a system including the rest of illustrated units (1 , 2, 4 and 5), said rest of illustrated units belonging or not to the system of the invention, depending on the embodiment, i.e. for an embodiment the system of the invention only comprises the biofilm reactor 3 (and advantageously also unit 5) and is to be integrated in a system comprising the rest of illustrated units, while for another embodiment all the illustrated units are comprised by the system of the second aspect if the invention.

The control strategy included in the system and performed by the method of the present invention is illustrated in the block diagram of Figure 3, for an embodiment, were the ammonium closed loop is the one included in the square area indicated as "TAN CONTROL LOOP" and includes a "TAN probe" for measuring the ammonium concentration at the outlet of the reactor 3 (see Figures 2 and 5) to be compared with the ammonium setpoint [TAN]_Sp, and a "controller" and "pump" which, based on the ammonium setpoint and measurement comparison, acts on the side stream, i.e. on the reject water, entering the reactor 3 (see Figures 2 and 5) in order to control the ammonium concentration in the bulk liquid therein to repress NOB activity and provide an adequate ratio to feed a subsequent anammox reactor.

The loop indicated as "[TAN]_SP MANAGEMENT" includes the block indicated as

" NO^~ analyzer" which on-line measures the sum of nitrite and nitrate concentration in reactor 3, and a "[TAN]_Sp station" in charge of calculating an varying the [TAN]_Sp, from inputs received from the "TAN probe" block regarding the ammonium concentration measurements, and from the " NO^~ analyzer" regarding the sum of nitrite and nitrate concentration measurements.

Although in Figure 3 two blocks are depicted regarding reactor 3, it has been done only for clarity sake, as both refer to the same reactor: biofilm reactor 3 (Figures 2 and 5).

By the control strategy depicted in Figure 3, an effluent with suitable conditions for the subsequent anammox reactor 4 (see Figures 2 and 5) is obtained. Instead of using only the ammonium concentration measurement (as it was the case of the ammonium control loop of ES2334321 B1 ), additional on-line measurements are performed to continuously provide the adequate ratio between ammonium and nitrite to feed the subsequent anammox reactor 4.

The above mentioned control of the side stream entering biofilm reactor 3 is performed, for the embodiment of Figure 5, by means of a discharging buffer tank 5 arranged for receiving the reject water produced in anaerobic digester 2 and dose it (i.e. to regulate its flow-rate) to biofilm reactor 3. Said dosing is done on demand to keep a desired ammonium concentration (setpoint). This procedure has been named by the present inventors as DOSIS (DOsing Side Stream).

Next, evidences which support the performance of the present invention are given, in the form of both: experimental results at laboratory scale and a theoretical model based study.

Experiments at laboratory scale: With a laboratory scale reactor it has been obtained a partial nitritation (ca. 50% oxidation of inlet ammonia to nitrite, without nitrate production) of a low strength synthetic wastewater (70 mgN L^"1) at 12.5 °C for several weeks (50 days) and 120 days at a temperature equal or lower than 15°C. The volume of the reactor is 2.5 L. Results are presented in Figure 4. Model based study:

To show the suitability of the control strategy based on the regulation of the side stream (DOSIS), a mathematical model has been used. For the implementation of the control system, the manipulated variable will be the flow-rate of the side stream (i.e. reject water, as depicted in Figures 2 and 5). The anaerobic digester 2 will have a discharging buffer tank 5 from which the flow-rate of reject water can be regulated and used for control purposes. The model based study shows the performance of the invention in case of diurnal variability in terms of flow-rate and ammonium concentration of the wastewater.

Wastewater treatment configuration and reactor description for simulations:

The average flow rate used in the simulations is ca. 2- 10⁴ m³ d^"1, with an average biodegradable COD in the influent of 300 g m^"3 (ca.1 - 10⁵ p.e). Side stream has been assumed to have a constant ammonium concentration of 1^■ 10³ g/m³. The temperature used for the simulations in all reactors was set to 15°C and a pH of 7.5 was assumed.

Side stream requirements for steady state operation have been calculated with the model as presented in table 1 . Note how for steady state the amount of side water required is low and due to variability larger amounts are required for control purposes. Table 1 . Side stream requirements for steady state as predicted by the model.

The influent dynamics may produce a higher demand of side stream for control purposes. Diurnal variations and seasonality may be challenging for the control strategy. To test the effect of a rather strong diurnal variability, the pattern proposed for the Benchmark Simulation Model no. 1 (BSM1 , (Alex et al., 2008), for dry weather has been used. The pattern imposed is rather extreme, and many WWTP's may have rather less variability, i.e. a lower amount of side stream will be required for control purposes. This diurnal variability in terms of ammonium concentration and flow-rate is used as an example, seasonality or storm events could also be similarly handled by the control strategy. If required, the DO concentration in biofilm reactor 3 (Figures 2 and 5), could be also manipulated to decrease eventually the conversion of the reactor 3 during low nitrogen-load events. Inflow variations in ammonium concentration will be buffered in the biological reactor 1 removing COD. To take into account this buffering capacity, a volume 2500 m³ has been considered. The basic layout described with the model as well as the volumes of the reactors considered in the simulations are presented in Figure 5. More details regarding the operating conditions of the reactors are found below, at the end of the present section. Two different scenarios have been considered:

Scenario A. The side water stream has been assumed to be produced with a flow-rate of 2.5 % of the main stream, meaning ca. 35% of the total nitrogen. This range is likely to occur when a very-high-load activated sludge + settler system to remove COD and biogas production is used as in Figure 1 . Results are presented in Figures 6- 1 1 .

Scenario B. Existing WWTP's conventionally produce a lower amount of N in the side stream. A potential application of the technology would be the retrofitting of two- stage biological systems (A B plants) (see Wett and Alex, 2003 for a description of an A/B plant). For instance, the unit 1 in Figure 4 could be a high loaded A-stage with intermediate clarification and a separate sludge cycle. And B stage could be mainly devoted to nitrogen removal through the proposed system (units 3 and 4 as shown in Figure 4). Therefore, in this scenario a side stream with a flow-rate of 1 .0 % of the main stream has been considered, meaning 22% of the total nitrogen. Results are presented in Figures 12-17.

It is interesting to mention at this point that the anaerobic digester 2 could be easily fed with an external wastewater containing COD and N (see for instance Abma et al., 2010). In this case an extra amount of side water would be produced, and the results will be equivalent to those presented for the first scenario (scenario A).

Model description:

Biofilm model, kinetics and parameters:

A one-dimensional biofilm model was developed to simulate the nitrifying biofilm airlift reactor performance based on Wanner and Reichert (1996) and implemented in the software package AQUASIM (Reichert, 1998), v.2.1 d.

The biomass species described as particulate compounds in the biofilm matrix were four in the partial nitritation reactor 3: ammonia-oxidizing bacteria (AOB), nitrite- oxidizing bacteria (NOB), heterotrophic bacteria and inert biomass. Biofilm area was described as a function of the granule radius, to correctly simulate the biofilm geometry (for further details see below Eq. 5). And two in the anammox reactor 4: anammox bacteria (AMX) and inert biomass. Total biofilm area was defined as a function of granule size and number of granules. A detachment rate was used to keep a constant biofilm thickness in steady state at a predefined value. Detached biomass from the biofilm was considered as active following the same kinetics defined for the biomass in the biofilm. Attachment of biomass onto the biofilm surface has been neglected. For the sake of simplicity external mass transfer has been neglected. The porosity of the biofilm was fixed as 80% and kept constant during all the simulations. Initial fractions of particulate compounds were 10% AOB, 8% NOB, 2% heterotrophic biomass in the partial nitritation reactor , whereas 20% AMX was assumed in the anammox reactor 4. The microbial kinetics and the stoichiometry used are detailed in Tables A1 -A3. Growth of AOB and NOB included inhibition by free ammonia (FA) and free nitrous acid (FNA) as proposed by Jubany et al. (2008). Other parameters related to the biofilm and used in the model are detailed in Table A4.

Modeling the TAN control loop: One of the key aspects of the development of the mathematical model was to provide a powerful approach able to simulate the control strategy, as described above with reference to Figure 3. The control strategy has two different closed-loops: (i) one to maintain the TAN concentration in the bulk liquid (i.e., the reactor effluent, considering a well-mixed liquid phase in the reactor 3) and, for the embodiment here described, (ii) a second one to control the DO concentration in the bulk liquid.

For the mathematical description of the DO control loop, aeration was introduced as a dynamic process only active in the bulk liquid phase. A high value for the volumetric gas-liquid oxygen transfer coefficient (k_La= 10⁴ d^"1) was selected. The oxygen solubility used was equal to the DO setpoint (Perez et al., 2009):

d[DO]

k_La([DO]_SP - [DO]) (1 ) dt

Where [DO] is the dissolved oxygen concentration in the bulk liquid, and [DO]_Sp is the DO concentration setpoint. The setpoint will be kept constant in the range 1 -4 mg/L.

For the total ammonia nitrogen (TAN) control loop, an ad hoc expression was developed, because the control loop has the side water flow-rate (Q_Si_de) as manipulated variable:

Where Q_Si_de,o is known as the bias of the control action, i.e. the default value of flow-rate. The controller will always act either increasing or decreasing Q_s\de around Qsi_de.o- [TAN] is the total TAN concentration in the bulk liquid phase. [TAN]_SP is the TAN concentration setpoint. a is the proportional gain of the controller, easily tuned in each simulation depending on the particular operating conditions. The principle of the performance of the expression is similar to that applied in a conventional proportional control law. The action will be stronger when the measured value of TAN concentration is far from the setpoint, whereas when TAN concentration is approaching the setpoint, the action of the controller will be weaker. [TAN]_Sp management:

The TAN concentration setpoint will be varied on demand depending on the concentration of total nitrogen in the reactor. A ratio of [TAN]/[TNN] between 1.1-1.3 is required to feed the subsequent anammox reactor. An additional measurement of NO^~ will be used to estimate the total nitrogen and calculate the adequate TAN concentration setpoint in the so called [TAN]_SP station (see Figure 3).

As already explained above, the [TAN]_SP station calculates on-line the required ammonium concentration depending on the measured NO^~ concentration in the reactor

3 bulk liquid phase:

[NO_X ]+ [TAN]

[TAN] \SP ^~ (3) b + \ Where b is the desired ration between ammonium and nitrite concentration as to feed the subsequent anammox reactor 4, i.e. [TAN]/[TNN] ratio in Figure 3.

Biological Processes:

Nitrification was defined as a two-step process with a first oxidation of ammonium to nitrite by ammonia-oxidizing bacteria (AOB) and a subsequent oxidation of nitrite to nitrate by nitrite-oxidizing bacteria (NOB).

Stoichiometric and kinetic parameter values together with their corresponding rate expressions are presented in Table A1 , Table A2 and Table A3. All the kinetic parameters were taken from literature. A temperature correction was applied through Eq. (S1 ) to the kinetic parameters corresponding to heterotrophic biomass (i.e.,

and b_H) as proposed by Henze et al. (2000).

0.07(r-20°C)

k(T) = k(20°C)e (S1 ) where k is μ,τκιχ,Η or b_H and T is the temperature (°C).

The values corresponding to the maximum specific growth rates of autotrophic bacteria ^_max,AOB and ,τκιχ,ΝθΒ) were calculated with the equations proposed by Jubany et al. (2008):

^[ '

6.69 - 10V^{5295 /(273+r)}

A » (PH, T) = _{ι + (2 05 0} -9 _{/ l 0} -_{pH ) + (l 0}-_PH _{/ Ι Μ 0}-_{Ί )} (S3)

On the other hand, the decay rate expression for both AOB and NOB were calculated as in Volcke et al. (2010):

^AOB 0-05 · -_msoiAOB (S4) Ο_Β = 0Λ5· _{μαιΆκΝΟΒ} (S5)

The terms TAN and TNN were used instead of ammonium and nitrite because they are the true compounds analyzed in the chemical analyses. Eqs. (S6) and (S7), derived from acid-base equilibriums, were used for the calculation of the free ammonia (FA or NH₃) and the free nitrous acid (FNA or HN0₂) concentrations in equilibrium with TAN and TNN, respectively.

TNN 47

FNA = (S7)

K_a - 10^ + 1 14

The ratio between the ionization constant of the ammonia equilibrium (K_b) and the ionization constant of water (K_w) is related to the temperature as shown in Eq. (S8) and the temperature effect on the ionization constant of the nitrous acid equilibrium (K_a) is shown in Eq. (S9) (Anthonisen et al., 1976) 6344

exp (S8)

273 + T )

- 2300

K exp (S9)

273 + T

The kinetics for each of the processes considered i.e. growth and decay of each kind of bacteria are shown in Table A2. In addition, the oxygen limitations, substrate and non-competitive inhibitions for AOB and NOB growth processes were also considered. AOB inhibition by TAN and NOB inhibition by TNN were described with a Haldane model while AOB inhibition by TNN and NOB inhibition by TAN were described with a non-competitive model.

A person skilled in the art could introduce changes and modifications in the embodiments described without departing from the scope of the invention as it is defined in the attached claims.

Table A1. Stoichiometric Matrix

j Process 'TAN ■>TNN ■>NO_Q X A, OB X NOB XH X A, MX X,

1 Growth of XAOB -(3.43-Y_AOB)YAOB -1/YAOB YAOB

2 Decay of X_AOB

3 Growth of X_NOB -(1.14-Y_NOB) /YAOB -1/Y. NOB 1/Y, NOB

4 Decay of X_NOB -1

5 Aerobic growth of X_H -(1-YH)YH -1/YH

6 Decay of X_H -1

7 Growth of X_AMX -1/YAMX -1 YAMX-1/1.14 1/1.14

8 Decay of X, AMX -1

Units g 0₂ m^"a g N m^"a g N m^"a g N m^"a g COD m^"a g COD m^"a g COD m^"a g COD m^"a g COD m^"a g COD m^"a

Table A2. Kinetic rate expressions Process Process rate (cT¹) Reference

^■X,

Growth ^Ι,ΤΝΝ,ΑΟΒ ⁺ ^TNN ^,fJubany et

^Ξ,ΤΑΝ,ΑΟΒ ⁺ TAN ^{+ '}

of X, AOB K Ι,ΤΑΝ,ΑΟΒ al. (2008)

K I, TAN, NOB

Growth ^" ^KI,T. Jubany et of XNOB

al. (2008)

Decay Volcke et

"NOB „ ς, ^■X NOB

of XNOB ^ ^NΟ, ,Ν ^MΟΒ» ' ^ ^URO, al. (2010)

Growth ^o₂ S_s Henze et of X_H al. (2000)

Decay , Henze et

6 b Ή„- X_v

of X_H al. (2000)

Growth Volcke et maxAMX „ „ „ ^■X ^ A,MX

Of XAMX tr ' ^n al. (2010)

Decay , ^o₂ Volcke et

AMX ^' „ „ AMX

of XAMX ^Ο, ,ΑΜΧ + ^O, al. (2010) Table A3. Kinetic parameters (15 °C and pH 7.5)

Symbol Definition Value Unit References

Ammonia oxidizing bacteria

(AOB)

M-max.AOB Maximum specific growth rate 0.47 d-¹ Jubany et al. (2008) bAOB Decay rate 0.02 d-¹ Volcke et al. (2010)

YAOB Growth yield 0.18 g COD g^"1 N Jubany et al. (2008) o2,AOB Affinity constant for oxygen 0.2 mg 0₂ L^"1 Wett et al. (2013) s.TAN Affinity constant for TAN 1 .1 mg TAN L^"1 Volcke et al. (2010) |,TAN,AOB Inhibition coefficient for TAN 9004 mg TAN L^"1 Jubany et al. (2009) |,TNN,AOB Inhibition coefficient for TNN 1762 mg TNN L^"1 Jubany et al. (2009)

Nitrite-oxidizing bacteria (NOB)

M-max.NOB Maximum specific growth rate 0.55 d-¹ Jubany et al. (2008) bNOB Decay rate 0.03 d-¹ Volcke et al. (2010)

YNOB Growth yield 0.08 g COD g^"1 N Jubany et al. (2008) o2,NOB Affinity constant for oxygen 0.35 mg 0₂ L^"1 Wett et al. (2013) s.TNN Affinity constant for TNN 0.5 mg TNN L^"1 Volcke et al. (2010) |,TNN,NOB Inhibition coefficient for TNN 192 mg TNN L^"1 Jubany et al. (2008) |,TAN,NOB Inhibition coefficient for TAN 145 mg TAN L^"1 Brockmann and

Morgenroth (2010)

Heterotrophic bacteria (H) |J-max,H Maximum specific growth rate 4.3 d-¹ Henze et al. (2000) b_H Decay rate 0.29 d-¹ Henze et al. (2000)

YH Growth yield 0.67 g COD g^"1 N Henze et al. (2000) o2,H Affinity constant for oxygen 0.2 mg 0₂ L^"1 Henze et al. (2000) s,s Affinity constant for substrate 4 mg COD L^"1 Henze et al. (2000)

Anammox bacteria (AMX)

|J-max,AMX Maximum specific growth 0.01 d-¹ Volcke et al. (2010) rate bAMX Decay rate 0.0005 d-' Volcke et al. (2010)

YAMX Growth yield 0.17 g COD g^"1 N Volcke et al. (2010) θ,ΑΜΧ Inhibiting constant for oxygen 0.01 mg 02 L^"1 Volcke et al. (2010) s.TAN Affinity constant for TAN 0.03 mg N L^"1 Volcke et al. (2010) s.TNN Affinity constant for TNN 0.05 mg N L^"1 Volcke et al. (2010)

Table A4. Diffusivity coefficients

Parameter Symbol Value Unit References

Diffusivity of 0₂ in water D^ 2.2 - 10^"4 m² d^"1 Picioreanu et al.

(1997)

Diffusivity of NH₄ ⁺ in water D_TAN 1 -9 - 10^"4 m² d^"1 Picioreanu et al.

(1997)

Diffusivity of N0₂ ^" in water D_TNN 1 -7 - 10^"4 m² d^"1 Picioreanu et al.

(1997)

Diffusivity of N0₃ ^" in water D_N03 1 -7 - 10^"4 m² d^"1 Picioreanu et al.

(1997) Diffusivity of organic . ₂ Picioreanu et al.

Ds 1 .0 - 10^" m d^"

substrate in water (1997)

Assumed, in the range

Diffusivity coefficient

Ediff 0.5 dimensionless proposed by Bishop et inside biofilm

al. (1995)

Claims

\ - A method for wastewater nitrogen removal, comprising performing a partial nitritation of wastewater in a biofilm reactor, wherein said biofilm reactor contains ammonia-oxidizing bacteria, and the method comprises performing a closed loop control for regulating the ammonium concentration within said biofilm reactor based on the ammonium concentration at the outlet of said biofilm reactor, wherein the method is characterized in that it comprises calculating and varying the value of an ammonium concentration setpoint of said closed control loop based at least on the nitrogen concentration at the outlet of or inside said biofilm reactor.

2.- The method of claim 1 , wherein ammonium enters the biofilm reactor through a main stream, the method comprising performing said regulation of ammonium concentration within the biofilm reactor by regulating the flow-rate of a side stream also entering said biofilm reactor.

3. - The method of any of the previous claims, wherein said ammonium concentration at the outlet of said biofilm reactor and/or said nitrogen concentration at the outlet of or inside the biofilm reactor is/are online measured.

4. - The method of any of the previous claims, wherein said nitrogen concentration refers to the sum of nitrite and nitrate concentrations at the outlet or inside said biofilm reactor.

5.- The method of any of the previous claims, comprising calculating and varying said value of said ammonium concentration setpoint of said closed control loop also based on a desired ratio of ammonium and nitrite concentrations at the outlet of said biofilm reactor.

6. - The method of claim 5, wherein said calculation of the value of the ammonium concentration setpoint is performed according to the following expression:

_[TANlp [TAN]

L^J b + l

where TAN refers to the total ammonium concentration, SP to setpoint, NO^~ is the sum of nitrite and nitrate, and b to said desired ratio of ammonium and nitrite concentrations.

7. - The method of claim 5 or 6, wherein said desired ratio of ammonium and nitrite concentration is between 0.5 and 2, preferably between 1.1 and 1 .3.

8. - The method of claim 7, further comprising feeding an anammox reactor with the effluent of said biofilm reactor, and using said anammox reactor for performing an anammox reaction with the ammonium and nitrite contained in said biofilm reactor effluent in order to achieve a further nitrogen removal.

9. - The method of claim 2 or of any of the previous claims when depending on claim 2, wherein said side stream carries reject water coming from an anaerobic digester, the method comprising performing said regulation of the flow-rate of said side stream from a discharging buffer tank interspersed between said anaerobic digester and the biofilm reactor.

10. - The method of claim 2, wherein said main stream comes from a main output of a very-high-load activated sludge and settler producing biomass and organic particles and feeding with the same said anaerobic digester through, the anaerobic digester generating biogas and said reject water from the received biomass and organic particles.

1 1 . - The method of any of the previous claims, wherein the temperature inside said biofilm reactor is between 8 and 35 °C, the ammonium concentration in said main stream is between 30 and 100 g N/m³ and the COD concentration is between 1 and 125 g/m³.

12.- The method of any of the previous claims, further comprising controlling the dissolved oxygen concentration inside the biofilm reactor based on the nitrogen concentration in the main stream.

13. - A system for wastewater nitrogen removal, comprising:

- a biofilm reactor (3) containing ammonia-oxidizing bacteria, and

- a closed loop control configured and arranged for regulating the ammonium concentration within said biofilm reactor (3) based on the ammonium concentration at the outlet of said biofilm reactor (3), said closed loop control comprising an ammonium concentration setpoint;

wherein the system is characterized in that it comprises processing means configured and arranged for calculating and varying the value of said ammonium concentration setpoint of said closed control loop based at least on the nitrogen concentration at the outlet of or inside said biofilm reactor (3).

14. - The system of claim 13, comprising first and second measuring means for, respectively, measuring the ammonium concentration at the outlet of said biofilm reactor (3) and the nitrogen concentration, particularly the sum of nitrite and nitrate concentrations, at the outlet of or inside the biofilm reactor (3), said first and second measuring means being arranged to provide said processing means with the measured values, the processing means being configured for performing said calculation of the value of the ammonium setpoint based on said measured values and on a desired ratio of ammonium and nitrite concentrations at the outlet of said biofilm reactor (3).

15. - The system of claim 13 or 14, further comprising: - a very-high-load activated sludge and settler (1 ) having an input for receiving sewage, a main output connected to a main stream input of the biofilm reactor for providing the latter with ammonium, being configured for producing biomass and organic particles therefrom, and having a secondary output for delivering said biomass and organic particles;

- an anaerobic digester (2) having an input connected to said secondary output of the very-high-load activated sludge and settler to receive said biomass and organic particles, and being configured to generate biogas and reject water there from, through respective first and second outputs;

- a discharging buffer tank (5) arranged for receiving said reject water from said second output of said anaerobic digester (2) and for feeding the biofilm reactor (3), through a side stream input thereof, with said reject water according to a flow-rate regulated under the control of said control means in order to perform said regulation of the ammonium concentration within the biofilm reactor (3); and

- an anammox reactor (4) with an input connected to an output of the biofilm reactor (3) to receive the effluent coming there through, and configured for performing an anammox reaction with the ammonium an nitrite contained in said biofilm reactor effluent in order to achieve a further nitrogen removal.