IES20000480A2 - Waste water treatment plant - Google Patents

Abstract

A waste-water treatment plant for the biological treatment of organic material in waste-water using micro-organisms is provided. The plant comprises (a) a reactor (3) adapted to receive liquor and retain it in contact with a bacterial population to digest organic material; (b) an aeration means (4) adapted to provide oxygen levels within the reactor sufficient to maintain bacterial population growth; (c) a denitrification tank (5) adapted to receive liquor from the reactor and retain it in contact with denitrifying micro-organisms; and (d) a clarification tank (6) adapted to allow sludge to settle. The plant is adapted to return to the reactor (3) at least a portion of the settled sludge. A process for the biological treatment of organic material in waste-water using micro-organisms is also described. <Figure 1>

Description

The present invention relates to a process for the biological treatment of organic material in waste water using microorganisms and to a plant for carrying out such a process. The process and plant according to the invention are suitable for the treatment of all types of waste water streams that contain organic waste.

Background Art In most societies there is a requirement that waste water is treated before discharge into the environment, to a defined standard measured by Biochemical Oxygen Demand (BOD) and the Suspended Solids content. For example a typical standard for domestic sewage discharge into a river or water course may have a specification of 20:30 BOD:SS (parts per million) or less. Waste water with a high BOD discharged into natural waters such as rivers encourages the growth and proliferation of microorganisms. These micro-organisms consume any dissolved oxygen in the rivers which results in an environment which cannot support higher forms of life such as plants or fish.

In many countries there are also regulations controlling the standards for the level of nutrients, particularly nitrates and phosphates, which are allowed to be discharged into inland waters. Inorganic ions such as phosphates and nitrates are produced as a result of the biological breakdown of the organic waste in the water. If the effluent containing these inorganic ions is left untreated it may encourage the growth of extensive algal blooms which subsequently may cause damage to the surrounding environment. It is therefore a requirement of a waste water treatment process that it can incorporate a treatment process for the removal of nitrate and phosphate to within the specified levels.

Most forms of domestic, agricultural and industrial waste water contain both organic and inorganic materials either in solution or in the form of suspended solids or colloidal suspensions. The treatment offtTe^JHin^pi^allv typically prrfth.nhti OPEN TO PUBLIC INSPECTION UNDER SECTION 28 AND RULE 23 JNL No.. ΙΕ ο 0 ο 4 8 ο quantities of organic sludge which must be either disposed of or subjected to further treatment.

There is a variety of waste water treatment processes known in the art, all of which operate in essentially the same manner. Most of these systems produce an effluent that is well below the BOD and suspended solid levels allowed by the regulatory authorities in many countries. A typical treatment for a waste water stream involves an initial screening step for the removal of gross inorganic solids and grit and sand. Following the initial screening step the waste water contains suspended and colloidal organic material and dissolved organics and inorganics. In the first biological treatment stage (primary treatment) the liquor flows continuously into a tank. The residence time in the tank is sufficient to allow the precipitation of suspended organic matter (primary sludge). The primary sludge deposited at the bottom of the tank is removed for storage and later treatment. The supernatant liquor containing colloidal and dissolved matter then passes continuously to the second stage tank (secondary treatment) where it is treated biologically in the presence of air (aerobic treatment). The biomass treats the organic material as food and converts some of it to carbon dioxide and water. The rest of the organics are converted into biomass. The overflow from the tank passes to a third vessel which is designed to separate the biomass from the water in which it is suspended. Biomass sinks to the bottom of the tank as a secondary sludge. The resulting overflow now contains very low levels of organics and suspended solids. It is discharged to an appropriate water course, for example, natural receiving waters such as a river, lake or sea. Alternatively, the discharge from smaller plants may also include soak-aways to ground water. The primary and secondary sludge are combined and are usually treated further to reduce the volume and converted to a form suitable for use, for example as fertiliser. The disposal of waste sludge is a major problem for most operators of waste water treatment plants. In particular the treatment and disposal costs of the waste sludge are a significant proportion of the overall water treatment process.

There are numerous aerobic processes which have been developed over the years for the biological treatment of waste water to yield an environmentally acceptable effluent. The process described above is referred to as an activated sludge process. It ΙΕ ο 0 0 4 8 Ο involves the production of an activated mass of micro-organisms which is capable of stabilising waste aerobically. In this process the secondary tank has an aeration system fitted to ensure that the dissolved oxygen levels are sufficient for the biomass to respire and grow. A particular feature of such a process is that a proportion of the separated secondary sludge is returned to the aeration tank. This assists in maintaining the biomass population. The balance of settled sludge not returned to the process is stored for further treatment and eventual disposal.

In an activated sludge plant the biomass within the aeration tank consists of about 95% bacteria. The bacteria self-select to flourish on the organic waste presented to them. A low population level of bacteria when presented with a rich source of food and oxygen will develop quickly in an almost explosive growth pattern known as the exponential phase. The population quickly outgrows the food supply. At this point the population has to fall back on its own cellular resources (known as endogenous respiration). The ! bacteria begin to die off and the cells lyse releasing nutrients and assisting the maintenance of the population. There are also more complex organisms within the biomass such as protozoa and rotifers which prey on the bacteria. During endogenous J ·( respiration the bacteria are in a weakened state and the predator population flourishes. In most activated sludge plants a balance is quickly achieved with a constant inflow of organics and a microorganism population selecting to treat the waste.

There are a number of activated sludge processes in use which operate in essentially the same manner in that they all produce an excess of sludge which must be disposed of after further treatment. Sludge is always produced during the biological treatment process and generally is disposed of to agriculture. It may be disposed of as the raw untreated sludge in solid or liquid form or as a dried or composted material with Ί appropriate substances such as domestic waste or lime based products such as cement kiln dust. It is known in the art that excess sludge within the sewage treatment works may be reduced in volume by an aerobic or anaerobic digestion process. The anaerobic process has the advantage of producing methane which is used as a source of fuel for combined heat and power (CHP) plant often supplying enough electrical energy to run the works. However, the digestion only reduces the volume of sludge by around 40-50% and the overall process is not particularly efficient. The cost of treating sludge produced by the conventional waste water treatment plants can also be quite high.

Attempts have been made in the prior art to reduce the quantity of sludge produced by extending the aeration periods. Such attempts were partially successful but the rate of oxidation was low and relied upon the natural lifetime of the biomass. In general the plant sizes were larger and the costs of the treatments higher than conventional processes.

In the activated sludge process the ammonia produced from the destruction of the influent organics is converted to nitrite and nitrate. The process known as nitrification is a common feature of the bacterial treatment of organic waste. The nitrate produced is commonly discharged in the effluent stream. As mentioned previously the discharge of effluent containing inorganics such as nitrates can be detrimental to the surrounding environment. This has resulted in recent regulatory trends to place a limit on the levels of nitrates which may be discharged, particularly to inland waters.

Waste water treatment apparatus for denitrification employing a biological denitrification method is known in the art. For example, Japanese Patent Laid Open No. 42850/1979 discloses a waste water treatment apparatus comprising inter alia a denitrification tank. However, the system requires a large capacity treatment tank.

United States Patent No. 5,766,454 discloses a waste water treatment system comprising an aerobic tank and an anaerobic tank. Waste water enters the aerobic tank where it is aerobically treated. The effluent then flows into the anaerobic tank where it is I anaerobically treated to remove nitrogen in the waste water. The system described in this patent is suitable for home use only and thus is not suitable for catering for large volumes of influent. The system also produces sludge which has to be pumped out from the aeration means upon servicing the system.

U.K. patent application number 2 099 807 describes a waste water treatment apparatus for denitrification where nitrification and denitrification are performed IE 0 0 ο 4 8 ο simultaneously in a reaction tank. The nitrification/denitrification process is described using a variety of scenarios all of which rely upon a high mixed liquor concentration, that is, above 10,000ppm for efficiency of the process. The process disclosed therein requires a “deep shaft” type reactor in which the oxygen levels are varied to suit the nitrification/denitrification stage. The patent describes a series of aerated and unaerated tanks to convert remaining ammonia and remove residual nitrate. The process relies on reaction within the reactor to provide denitrification. The activated sludge is returned to the re-aeration tank from the reaction tank. In this apparatus the BOD must be more than 30ppm in the reaction tank.

U.K. patent number 2 298 195B describes a process in which the production of sludge is reduced or even eliminated by the deactivation of a proportion of the bacteria in the biomass. In this way the total number of bacteria in the reaction zone can be maintained constant while maintaining a high rate of bacterial growth and aerobic digestion. It is a feature of activated sludge processes that they depend upon the facility of the bacteria to clump together naturally (flocculation). This ensures good and quick settlement in the separation phase and little carry over in the final effluent. The patent i relates to a system in which the settled biomass is returned in full to the reactor and a high shear is applied to the floe, breaking them up and thereby allowing easier predation by rotifer and protozoan microorganisms in the digestion process.

The process described in the above-mentioned U.K. patent produces low volumes of sludge. However, as a consequence of digesting the sludge in situ, the system produces effluent containing high levels of inorganic nutrients such as nitrates. There is therefore a need for an improved process which produces zero or very low sludge volumes and which produces an effluent which is within the acceptable limits for discharge into inland waters.

Summary of Invention The present invention provides a waste water treatment plant for the biological treatment of organic material in waste water using microorganisms, comprising (a) a ΙΕ ο 3Π4 8 ο reactor adapted to receive liquor and retain it in contact with a bacterial population to digest organic material; (b) an aeration means adapted to provide oxygen levels within the reactor sufficient to maintain bacterial population growth; (c) a denitrification tank adapted to receive liquor from the reactor and retain it in contact with denitrifying microorganisms; and (d) a clarification tank adapted to allow sludge to settle, the plant being adapted to return to the reactor at least a portion of the settled sludge. Preferably a proportion of the sludge from the clarification tank is returned to the denitrification tank.

The invention also provides a transportable waste water treatment plant for the biological treatment of organic material in waste water using microorganisms, sized to fit into a standard transport container (freight container) attachable to a means of transport, preferably a container lorry. The plant may be suitable for treatment of waste water for populations of up to 1000 persons, preferably 500. Multiple units of the plant may be J'l ! used for populations of greater than 1000.

Preferably the invention provides a waste water treatment plant for the biological treatment of organic material in waste water using microorganisms, (a) a reactor adapted to receive liquor and retain it in contact with a bacterial population to digest organic material; (b) an aeration means adapted to provide oxygen levels within the reactor sufficient to maintain bacterial population growth; (c) a denitrification tank adapted to receive liquor from the reactor and retain it in contact with denitrifying microorganisms; and (d) a clarification tank adapted to allow sludge to settle, the plant being adapted to π return to the reactor at least a portion of the settled sludge; wherein the plant is transportable and sized such that it can fit into standard transport containers.

The plant according to the invention may be sized such that it can fit into standard transport containers. One such arrangement comprises the packaging of the plant to fit a standard transport container attachable to a means of transport. The term standard container herein means standard sizes of transport container such as 20 foot and 40 foot containers. Thus the plant is suitable for populations of between 200 and 1,000 people, preferably about 500 people. A treatment plant fitting inside a standard 40 foot container would be capable of handling the waste water from a community of up to 500 lEfl 0Π4 8 o persons. Multiple units such as those according to the invention could treat larger populations on a modular basis. The miniaturised apparatus therefore allows ease of shipment and temporary or permanent installation.

Preferably the plant has the capacity to treat waste water for populations of up to 500 persons and further preferably up to 5000 persons.

Preferably, the plant has a pre-contact tank adapted to allow intimate mixing of screened waste water with sludge returned from the clarification tank. The intimate mixing aids the removal of nutrients such as nitrates from the system.

The reactor is preferably capable of retaining incoming liquor for 1-24 hours.

It is a feature of the present invention that it requires an intensive aeration system. Preferably the aeration means comprises a jet aerator. The aerator allows for vigorous aeration and intimate mixing between the liquor and the microorganisms within the reactor. This allows the biomass to have excellent contact with the oxygen from the . ,1 t air and allows rapid respiration and bacterial growth.

Preferably, the waste water treatment plant according to the invention further comprises a clarification system which is a duplex clarification system suitably adapted to handle the heavy solid loadings from the reactor and denitrification system. It is a feature of the invention that for miniaturisation purposes the clarification system may be combined with the denitrification tank. Additionally, the floor of the denitrification tank may comprise an inclined plate so as to maximise the settling of the sludge. The denitrification tank may also comprise an agitator.

In a conventional waste water treatment plant, a typical plant would comprise screening and degritting, primary settlement, pre-contact tank, aeration, nutrient removal and final clarification. In addition there would be a requirement for sludge storage, dewatering and disposal thereby requiring a large plant size.

Preferably the waste water treatment plant allows for reduction in plant area (footprint). Typically such reduction would be up to half the size of a plant with sludge storage or up to one third the size of plants with separate sludge treatment. The efficiency of the process disclosed herein reduces the overall plant size and removes the need for sludge storage or treatment.

In the process according to the present invention a primary settlement stage is not required. The untreatable gross solids such as plastic, textiles and grit are removed. The control of oxygen levels within the reactor, to suit the nitrification and denitrification processes, such as required in GB 2 099 807 is not required by this process. Preferably in the process according to the invention the level of oxygen within the reactor does not have to be varied to suit nitrification and denitrification.

Preferably the invention provides a process for the biological treatment of organic Ί ' ' material in waste water using microorganisms wherein the nitrification step in the microbiological treatment of waste water is carried out separately from the denitrification step whereby the level of dissolved oxygen in the reactor is greater than 1 ppm.

Preferably, in the process according to the invention the mixed liquor suspended solids concentration in the reactor is less than 10,000 ppm and is preferably in the range 6000-8000ppm.

Organic material, both dissolved and suspended are bacterially treated in the h waste water treatment plant. The various bacterial species digest the organic waste, converting the material for their own growth and degrading the contained nitrogen not required to ammonia. This ammonia is used by other bacteria and oxidized under the strongly aerobic conditions within the reactor to nitrite and ultimately to nitrate ion. The bacteria are sensitive to a variety of conditions including pH, temperature and oxygen levels. A level of dissolved oxygen of greater than lppm is essential for efficient conversion to ammonia and subsequent conversion to nitrite and nitrate to occur.

Preferably the invention provides a process wherein the nitrogenous material found in organic waste in the waste water and from the decomposition and digestion of biomass is rapidly converted to ammonia.

Preferably the process allows a rapid reduction of BOD in the reactor with a dissolved oxygen (DO) level of at least 3ppm at all times. This allows rapid uptake of organics and quick conversion of ammonia to nitrate.

Preferably, the conversion capabilities of the process are such that less than lppm of free ammonia is present. The bacterial species that convert ammonia to nitrate and nitrite are sensitive to changes in temperature, pH and substrate conditions. In particular they are sensitive to dissolved oxygen levels. In the process according to the invention the dissolved oxygen levels are preferably maintained at 3 ppm or more at all times using an efficient aeration system such as the jet aerator. This assists and maintains the growth of nitrifying organisms. As a consequence ammonia levels are therefore kept low compared to some conventional systems which may have residual levels of ammonia of 3 or 4 ppm. ; ' 1 5 Denitrification requires different bacterial species and the level of dissolved oxygen is critical to the process of denitrification. It is a feature of the process according to the present invention that it does not rely on reaction within the reactor to provide any denitrification, only the conversion of ammonia to nitrate. The mixed liquor in the reaction tank is allowed to overflow into a separate tank where the dissolved oxygen is kept low to allow the denitrifying bacteria to flourish. A source of organic carbon is I required and the process provides two sources for this. The reactor is run to allow sufficient carbonaceous material to remain untreated. This is one source of organic carbon. Thus the denitrifying bacteria are used to convert both the nitrate to nitrogen and use the remaining organic carbon. The second source arises from a proportion of the sludge which is returned from the clarifier to the denitrifying tank.

I Preferably in the process according to the invention a proportion of the sludge returned from the clarifier is passed to the denitrification tank wherein the oxygen levels in said denitrification tank are less than lppm.

Preferably 5 -15% of the sludge returned from the clarifier is passed to the denitrification tank. The returned sludge contains a proportion of viable denitrifying organisms thereby increasing the population of denitrifying organisms in the denitrification tank. This improves the efficiency of the denitrification process and adds carbonaceous material essential to a proper denitrification reaction. The recycling of a small proportion of the settled sludge from the clarifier assists in the efficiency of the denitrification process.

In normal sewage treatment plants a proportion of the sludge produced is wasted. In the process according to the invention the sludge contains adsorbed nitrate and also cellular proteinaceous and other nitrogenous materials which are converted to nitrate in the reactor. Because the sludge is re-circulated in the present invention the process has to process levels of nutrient which are higher than in conventional plants.

The invention thus provides a process comprising an efficient nitrogen reduction regime.

More preferably, the invention provides a process wherein the treatment of the waste sludge generated both from the treatment of the waste water and the denitrification process is integral in the treatment of the waste water. This results in a highly efficient and cost effective process. It is estimated that the treatment costs per cubic metre of waste water inclusive of the integral sludge treatment are equivalent to conventional activated sludge plants.

Preferably the process allows clarification of the final liquor and nutrient removal so that the resulting effluent is within the regulatory limits for discharge. The required limits are set by the regulatory authorities and are subject to change. At present typical 8® 0048 0 values are at least 20ppm BOD (Biochemical Oxygen Demand), 30ppm SS (Suspended Solids) and less than 5ppm Nitrate.

Preferably the invention provides a process wherein all of the sludge from the clarification tank is returned continuously to the pre-contact tank and eventually to the reactor prior to digestion. The return of all sludge to the pre-contact tank and eventually to the reactor, in combination with the rapid bacterial respiration and shear deactivation and removal of bacteria by predators ensures continuous and complete or almost complete sludge digestion and organics removal within one reactor system.

There is therefore no need for separate sludge treatment systems. Conventional systems require sludge storage tanks and separate plant and equipment to treat the generated sludge to a level suitable for the disposal route. Preferably, the process according to the invention produces zero or very low levels of waste sludge. The process allows a reduction to zero of the volume of organic waste sludge produced by the process of the invention.

Preferably the invention provides a process wherein the effluent from the clarifier meets the required regulatory limits for biochemical oxygen demand, suspended solids and nitrate levels. Following the final clarification step the liquor may be discharged to internal waterways. The liquor may be passed on to a further stage for phosphate removal prior to discharge or recycling for secondary applications.

Preferably the invention provides a process wherein the final effluent has a BOD in the range of 10-20 mg/l.

Preferably the invention provides a process wherein the final effluent has a COD in the range of 38-60 mg/l. The COD (chemical oxygen demand) provides a quick method of determining organic content of a waste water by using chemical oxidation of dichromate ion. The test takes three hours to perform as opposed to the five days of the BOD test. It is possible to correlate with reasonable accuracy the BOD figure from the COD result. COD is always greater than BOD because there are always compounds that are resistant to biological degradation or are toxic to biological species (not usually present in domestic sewage but may be present in industrial waste) but which can be oxidised by chemical means. Once the correlation is achieved COD can be used to control plant operating parameters.

Still further preferably, the invention provides a process wherein the level of suspended solids in the final effluent is in the range of 14-30 mg/1.

Preferably, the level of nitrates in the final effluent is less than 5 mg/1.

Preferably the invention provides a process wherein the level of TUN (total unoxidised nitrogen) as N is in the range of 2.0 - 3.2 mg/1., the level of NH4 as N is less than lmg/1, and the level of NO3 as N is 5 mg/1 or less.

Brief Description of the Figures The invention will be further described by way of example only, and with reference to the accompanying drawing in which: I Fig. 1 is a schematic representation of the waste water treatment plant according to the invention.

Detailed Description of Invention 1.

In the process according to the invention the influent is screened using conventional commercial techniques. The screened waste water is passed continually direct to the pre-contact tank (2) without any primary settlement. In the pre-contact tank (2) it is intimately mixed with the returned sludge from the clarification tank (6). The intimate mixing aids the removal of nutrients such as nitrates from the system. The liquor from the pre-contact tank (2) overflows directly into the reactor (3) whose volume is determined by the nature of the waste water. The volume typically ranges from a 6-24 hour retention. Once in the reactor (3) the liquor comes into contact with the bacterial ΙΕ ο ο ο 4 8 ο organisms which feed on the dissolved, colloidal and suspended organic matter therein.

The aerator (4) within the reactor (3) causes intimate mixing which allows the biomass to have excellent contact with the oxygen from the air and allows rapid respiration and bacterial growth. The growth rate is dependant upon nutrient levels, oxygen availability, temperature and pH. The latter three criteria can be controlled by plant operation and it is therefore the nutrient availability that will control the nature of the microbial biomass.

Thus where there is a nutrient rich stream feeding a low biomass population in highly aerated conditions bacterial exponential growth will occur. As the bacterial population exceeds the level of nutrient capable of sustaining it, it ceases to grow and the levels of protozoa and rotifers increase to prey on the bacteria. The bacteria therefore enter the zone of endogenous respiration. The bacteria die resulting in cell lysis and release of nutrients which encourage further growth.

I -1 There is an intensive aeration system (4) within the plant. The bacteria are in :· I < intimate contact with the oxygen ensuring that substrate uptake is enhanced and that the process quickly becomes substrate limiting. Thus food availability comes only from endogenous respiration and cell lysis. It is at this point that if a proportion of the bacterial population is deactivated the population pressures on the remaining bacteria will be removed and the growth phase will resume.

The bacterial biomass within the system typically forms clumps or floe of ? ί bacteria. These clumps form the bulk of respiring bodies and remove the organic ' I ' substrate. In the reactor a shear force is exercised on the floe of bacteria which quickly Γ· breaks them up into their constituent bacteria, severely reducing their activity. The use of a high shear mixing system in conjunction with the aeration device (4) ensures that a proportion of the floe are regularly deactivated.

In the process according to the invention the nitrogenous material found in the organic waste and from the decomposition and digestion of biomass is rapidly converted to ammonia. The ammonia is quickly converted by nitrifying bacteria within the reactor to nitrate. In the process according to the invention the high oxygen levels and long sludge age assist greatly in this conversion process.

« U U 4 ο (I The liquor, following an appropriate residence time in the reactor, (by appropriate is meant the time deemed to be sufficient to allow the required level of reduction of BOD) flows into a pre-clarifier and denitrification tank (5). The denitrification tank (5) is an agitated non-aerated vessel designed to ensure good contact between the mixed liquor and the denitrifying bacteria held in the tank. The denitrifying filter acts as both a nitrogen removal system and a biological polishing filter.

The process described herein operates at a higher level of suspended solids than those found in conventional activated sludge plants. The apparatus therefore provides a duplex clarification system which is purpose built to handle the heavy solids loadings from the reactor and denitrification system.

Following the steps of pre-clarification and de-nitrification, the liquor flows into the final clarifier (6). Any remaining sludge settles and is returned initially to the precontact tank (2) and ultimately to the reactor (3). A proportion of the sludge returned from the clarifer (6) is passed to the denitrification tank (5). The resulting final effluent is then of an acceptable standard that meets the regulatory requirements and is ready for discharge. The effluent may be discharged into inland waterways or passed to a further I stage for phosphate removal prior to discharge or recycling for secondary applications.

The efficiency of the process according to the invention will be further illustrated by the following example.

Example The plant and process according to the invention were used to biologically treat organic material in waste water using microorganisms. The plant and process were operated as hereinbefore described.

The efficiency of the process was examined by carrying out analysis over 8 sampling periods. The analytical results for the influent (untreated effluent), final effluent and the activated sludge are presented below.

QG n 4 8 0 Table 1 (Influent) Parameter pH BOD mg/1 COD mg/1 *SS mg/1 * TUN as N mg/1 *TP as P mg/1 Sample Period 1 7.6 150 250 110 32 5.7 Sample Period 2 7.9 215 325 185 34 7.6 Sample Period 3 7.6 42 115 30 17 2.6 Sample Period 4 7.6 82 180 80 20 4.0 Sample Period 5 7.6 130 280 210 35 5.2 Sample Period 6 8.1 70 160 6324 -i 3.2 Sample Period 7 7.6 170 300 180 41 6.6 Sample Period 8 7.5 135 390 158 34 7.8 Maximum 8.1 215 390 210 41 7.8 Minimum 7.5 42 115 30 17 2.6 Average - 124 250 115 30 5.3 *SS = Suspended Solids; TUN= Total Unoxidised Nitrogen; TP = Total Phosphorous O0Q480 Table 2 (Final Effluent) Parameter PH BOD mg/l COD mg/l *SS mg/l *TUN as mg/l NFUas N mg/l NO3 as N mg/l *TP as P mg/l Sample Period 1 7.8 11 50 16 2.7 < 1 12 3.9 Sample Period 2 8.1 14 55 25 3.2 < 1 11 4.1 Sample Period 3 7.6 19 60 20 2.7 <1 10 2.7 Sample Period 4 7.8 13 60 14 2.0 < 1 9.2 2.9 Sample Period 5 8.0 12 38 14 3.0 < 1 9.7 2.9 Sample Period 6 8.1 10 48 20 2.5 < 1 12 2.8 Sample Period 7 7.8 18 45 30 3.2 < 1 18 3.2 Sample Period 8 7.8 20 55 27 3.2 < 1 14 4.2 Maximum 8.1 20 60 30 3.2 < 1 18 4.2 Minimum 7.6 10 38 14 2.0 < 1 9.2 2.8 Average - 15 51 21 2.8 <1 11 3.3 *SS = Suspended Solids; TUN= Total Unoxidised Nitrogen; TP = Total Phosphorous 080480 Table 3 (Activated Sludge) Parameter *SS mg/l *VSS mg/l VSS: SS Settled Solids ml/1/30 min *S.V.I. Sample Period 1 4250 3400 0.80 660 156 Sample Period 2 4190 3385 0.81 690 165 Sample Period 3 3900 3110 0,80 600 154 Sample Period 4 4100 3300 0.80 680 166 Sample Period 5 4020 3260 0.81 700 174 Sample Period 6 4200 3415 0.81 750 ;;· 179 Sample Period 7 3530 2890 0.82 775 219 Sample Period 8 6300 5120 0.81 800 127 Maximum 6300 5120 0.82 800 219 Minimum 3530 2890 0.80 600 127 Average 4311 3485 0.81 707 168 *SS= Suspended Solids; VSS= Volatile suspended solids; S.V.I. = Sludge volume index IE0 0 0 48 0 Table 4 (Plant Loading) Parameter BOD (in) kg/d P.E.(1) MLSS kg F/M(2) ratio (kg/kg/d) Sample Period 1 7.2 120 54 0.13 Sample Period 2 10.3 172 54 0.19 Sample Period 3 2.0 33 50 0.04 Sample Period 4 3.9 65 53 0.07 Sample Period 5 6.2 103 52 0.12 Sample Period 6 3.4 57 54 0.06 Sample Period 7 8.2 137 45 0.18 Sample Period 8 6.5 108 81 0.08 Maximum 10.3 172 81 0.19 Minimum 2.0 33 45 0.04 Average 6.0 100 55 0.11 (1) P.E.= Population Equivalent, 0.06kg BOD per person per day, (Reference, Urban Waste Water Treatment Regulations 1994). (2) F/M= Food / Microorganism ratio The performance of the plant over the sampling period was illustrated by the % BOD removal and the total nitrogen removal as % N.

Table 5 (Plant performance) Total BOD removal % Total Nitrogen removal as N (%) Sample Period 1 93 54 Sample Period 2 94 58 Sample Period 3 55 25 Sample Period 4 84 44 Sample Period 5 91 64 Sample Period 6 86 40 Sample Period 7 89 48 Sample Period 8 85 49 Maximum 94 64 Minimum 55 25 Average(1) 88 i $1 (1) The average refers to the average value over the test period. The average was 5 calculated from the average concentration in versus the average concentration out.

Claims (5)

1. A waste water treatment plant for the biological treatment of organic material in 5 waste water using microorganisms, said plant comprising (a) a reactor adapted to receive liquor and retain it in contact with a bacterial population to digest organic material; (b) an aeration means adapted to provide oxygen levels within the reactor sufficient to maintain bacterial population growth; (c) a denitrification tank adapted to receive liquor from the reactor and retain it in contact with denitrifying 10 microorganisms; and (d) a clarification tank adapted to allow sludge to settle, the plant being adapted to return to the reactor at least a portion of the settled sludge.

2. A waste water treatment plant according to claim 1, adapted to allow a proportion of the sludge from the clarification tank to be returned to the denitrification tank. 15

3. A transportable waste water treatment plant for the biological treatment of organic material in waste water using microorganisms, wherein said plant is sized to fit into a standard transport container attachable to a means of transport, preferably a container lorry.

4. A process for the biological treatment of organic material in waste water using 20 microorganisms wherein the nitrification step in the microbiological treatment of waste water is carried out separately from the denitrification step whereby the level of dissolved oxygen in the reactor is greater than lppm.

5. A waste water treatment plant according to any one of claims 1 to 3, and a process according to claim 4, substantially as hereinbefore described with reference 25 to the accompanying drawing.