WO1998030504A1

WO1998030504A1 - Aerated removal of nitrogen pollutants from biologically degradable wastewaters

Info

Publication number: WO1998030504A1
Application number: PCT/AU1998/000011
Authority: WO
Inventors: Mervyn Charles Goronszy
Original assignee: BISASCO PTY Ltd
Current assignee: BISASCO PTY Ltd
Priority date: 1997-01-09
Filing date: 1998-01-09
Publication date: 1998-07-16
Anticipated expiration: 1999-07-09
Also published as: HRP980009A2; HRP980009B8; HRP980009B1; SI20157A; AU5468698A; AUPO453897A0

Abstract

A cyclic activated sludge method and apparatus uses multi-reactor facilities (typically 2, 3, 4, or multiples of 2, e.g., 6, 8, 12, 16, etc.), without separate secondary clarifiers, to accommodate a continuous hydraulic profile. Each reactor is configured with an admixture volume as an initial unaerated mixing zone followed by repetitive mixing gradients to nucleate and promote large activated sluge floc sizes followed by a second intermittently aerated hydraulically connected reaction zone using partial baffle walls. The second reaction zone in each reactor is collectively, but out of phase, operated with successive air-on and air-off sequences. The air-off sequence enables solids-liquid separation and a depletion of biomass oxidation reduction potential to take place after an air-on sequence, with subsequent withdrawal of a fraction of the reactor upper level liquid contents using a variable rate moving box weir decanter. Sludge containing heterotrophic and autotrophic micro-organisms is directed from the second zone to the initial volume for admixture with biodegradable wastewaters containing more than 3:1 parts of BOD:TKN. The period of each air-on sequence depends upon the relative oxygen demanding mass load input. There is provision for influent flow during an extended air-off sequence after removal of the supernatant fraction. The air-on sequence typically operates with filling of influent into the reactor. Air-on sequencing in the second reactor volume is used to mix and elevate the oxidation reduction potential of the settled separated microbial culture from negative to mixed positive values. Air-off sequencing in the second reactor volume provides separated liquid-solids phases with a dynamic depletion to negative oxidation reduction potential (about -150 mV reference hydrogen electrode).

Description

TITLE: AERATED REMOVAL OF NITROGEN POLLUTANTS FROM BIOL- -OGICALLY DEGRADABLE WASTEWATERS

TECI-INICAL FIELD The present invention relates to a method of removal nitrogenous and other nutrient pollutants and carbonaceous oxygen demand from industrial and domestic wastewaters using cyclically aerated activated sludge processing. BACKGROUND ART

The practice of removing nitrogen and other nutrient pollutants has. using conventional activated sludge technology, been complicated and hence expensive. These conventional processes include the Bardenpho process, the UCT process, the modified UCT process, the VIP process, to name a few. which all use secondary settling basins and major sidestream and recycle stream flows to accomplish their objective.

The procedure we have invented and called the 2A20 DN process is much simpler, more cost effective and occupies a much smaller footprint. In large multi basin facilities treating the wastewater from up to about a million population equivalents, cost savings of at least 30% over conventional technologies can be gained. For typical domestic wastewater containing less than 2 mg/L of non degradable nitrogen, effluent total nitrogen of less than 5 mg/L is achieved through the 2A20DN cyclic activated sludge technology described here. This technology, by its method, focuses on minimum energy use and optimum use of simple to construct water retaining structures, using any convenient geometry. These can be rectangular, square, circular, orthogonal, with upright or sloping walls. The method of operation that we have described here specifically interferes with the growth of nitrate nitrogen forming bacteria and hence avoids its formation. This results in a reduction of organic carbon required for dentrification and a net saving in treatment energy. The process described herein provides the practitioner with a choice. The concurrent nitrification - dentrification process we have described also minimises the reduction in alkalinity that necessarily accompanies conventional prior art processes. DISCLOSURE OF THE INVENTION

It is an object of the present invention to overcome or at least ameliorate some of the problems associated with the prior art.

According to one aspect the present invention provides a method for the acclimated combined growth of heterotrophic and autotrophic micro-organisms for the removal of nitrogen, organics and other nutrient pollutants in a wastewater using an admixture reactor, comprising the steps of: providing an initial admixture of influent wastewater with a flow of acclimated culture, from a second sequentially aerated reactor volume, at least during an unmixed unaerated time period followed by an aerated - mixed time period, causing the oxidation reduction potential of the mixture to progressively decrease in the admixture reactor to less than about -100 mV to about -200 mV (when compared to a hydrogen reference electrode). passing the mixture to the second reactor volume which time selectively receives a controlled flow of oxygen containing gas during at least a part of the wastewater inflow time, during controlled time sequences which limit the mixed liquid phase dissolved oxygen concentration to predetermined values causing a preferential growth of heterotrophic and chemolithoautotrophic microbial species, whereby the end of aeration sequence mean substrate removal rate in a cycle is at least four times the minimum endogenous substrate removal rate: interrupting the introduction of wastewater and the introduction of oxygen containing gas to provide the formation of a settled sludge layer in which the oxidation reduction potential rapidly decreases to less than about -100 mV to about -200 mV (compared to hydrogen reference electrode) and before the next aeration and wastewater inflow sequence, varying the volume of liquid with the admixture reactor.

Preferably, the time cycles, and equipment interaction of each reactor are automatically controlled. For preference, sensors for the measurement of oxidation - reduction potential, dissolved oxygen concentration, mixed liquor suspended solids concentration are in contact with a velocity stream of said mixture at least during an aeration period. j -

For further preference, the mixture suspended solids concentration in the second reactor volume at bottom water level is about 5000 to 6000 mg/L with an operating sludge age that provides the proliferation of the autotrophic micro-organisms for a designated range of operating temperature and effluent nitrogen concentrations. Preferably, a stoichiometric mass of process oxygen in a cycle is transferred during preselected sequences of time and set point dissolved oxygen concentrations.

Typically, the acclimated culture contains general higher life forms, such as Vorticella. Ciliates. Protozoa. Rotifer. Nematodes and the like.

In a preferred form, the step of causing mixing of wastewater and acclimated culture followed by nucleation and activated sludge floe growth in an admixture volume of said reactor through the placement of liquid flow paths respectively near wall surfaces and air - liquid surfaces of said reactor. For preference, the step of controlling the decrease in the oxidation reduction potential to below about -200 mV (compared to reference hydrogen electrode) in the admixture volume is done by means of entrainment of selectively placed oxygen containing gas bubbles. Preferably, the near surface liquid removal rate that takes place in an air off sequence is process compatible and does not cause the entrainment of solids from within the settled solids layer.

In another preferred form, a plurality of reactors are used, whereby the net flow in and out of the plurality of reactors is continuous whereby flow into and out of each reactor is interrupted for at least a part of the time.

Preferably, the varying volume is produced by a moving box weir within the reactor connected by a plurality of downcomers to a central rotating drum shaft for moving the weir to a set bottom water level position and then returning it to an out of liquid rest position. According to a further aspect the present invention provides apparatus for the removal of nitrogen, organics and other pollutants in a wastewater using an admixture reactor by the acclimated combined growth of heterotrophic and autotrophic microorganisms, comprising:: means for continuously providing an initial admixture of influent wastewater with a flow of acclimated culture, from a second sequentially aerated reactor volume. means for, at least during an unmixed unaerated time period followed by an aerated - mixed time period, causing the oxidation reduction potential of the mixture to progressively decrease in the admixture reactor to less than about - 100 mV to about - 200 mV (when compared to a hydrogen reference electrode), means for passing the mixture to the second reactor volume which time selectively receives a controlled flow of oxygen containing gas during at least a part of the wastewater inflow time, during controlled time sequences which limit the mixed liquid phase dissolved oxygen concentration to predetermined values causing a preferential growth of heterotrophic and chemolithoautotrophic microbial species, whereby the end of aeration sequence mean substrate removal rate in a cycle is at least four times the minimum endogenous substrate removal rate; means for interrupting the introduction of wastewater and the introduction of oxygen containing gas to provide the formation of a settled sludge layer in which the oxidation reduction potential rapidly decreases to less than about- 100 mV to about-200 mV (compared to hydrogen reference electrode) and means for varying the volume of liquid with the admixture reactor before the next aeration and wastewater inflow sequence.

In one preferred form, the invention incorporates the use of cyclically aerated activated sludge processing for the removal of nitrogenous pollutants and carbonaceous oxygen demand from industrial and domestic wastewaters. The invention relates to the design and operation of a wastewater - activated sludge admixing reactor which is configured to efficiently function for mixing and particle nucleation using high liquid velocities through orifice jetting at a stationary wall position and much lower coagulating - flocculating liquid velocities at surface air interfaces which, combined. promote to floe nucleation and growth, in a multi-cell arrangement. The admixing volume is connected, by pipe or other hydraulic means, to a second reactor volume which is subject to cycled aeration sequences. Cycling of aeration sequences and operating loading conditions insures a repetitive cycling of bulk phase oxidation reduction potential from about - 100 mV to -about 200 mV (hydrogen reference) throughout the treatment process which also assists with the control of filamentous activated sludge bulking. This form of the invention combines an activated sludge reactor configuration operated for repetitive sequenced positive to negative oxidation reduction potential cycling and treated effluent removal using a lowering box weir decanter connected to a rotating drum shaft by a plurality of downcomers for the removal of BOD and nitrogen through simultaneous aerated nitrification - denitrification.

Admixing of biodegradable wastewater with an activated sludge microbial catalyst under defined conditions of relative flow proportions and mixing energy determine the net floe size (mean diameter) of the activated sludge and its liquid phase oxidation reduction potential. True anaerobic conditions, with sulfide generation. require an oxidation - reduction potential (reference hydrogen electrode) of about -400 mV. At the other end of the scale, highly aerated mixtures of wastewater and activated sludge exhibit an oxidation reduction potential of about 300 mV. It is known that numerous biological reactions can be facilitated through exposure to reaction conditions delineated by oxidation reduction potential. In this context, the terms oxic. anoxic and anaerobic, which have conventional (historical) definition, became meaningless as we have found it possible to generate biological reactions under aerated conditions (oxic by former definition) which have been known to require anoxic conditions (by former definition).

The benefit of dissolved oxygen in microbial metabolism is not removed by simple definitions. Rather the vagaries of what is meant by the former definitions are focused and simplified. Whilst the introduction of an oxygen containing gas into a mixture of activated sludge and wastewater would under normal convention imply an aerobic reaction condition, a somewhat different picture emerges from an analysis of mixing energy (liquid flow short-circuiting) dissolved oxygen concentration and microbial metabolic activity. We have found that predominantly negative oxidation reduction reaction environments are relatively easy to manipulate in full-scale plant using conventional fine bubble diffused aeration systems with certain ratios of activated sludge to wastewater and aeration intensity.

In the absence of other oil and grease removal technology, the admixture volume can also be designed to operate as a surface grease and oil collector which then requires its own removal management considerations. BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a graph of SQR/AOR variation with basin dissolved oxygen concentration achieved with an embodiment of the invention;

Figure 2 shows nitrification equations in accordance with an embodiment of the invention;

Figure 3 shows a schematic diagram of the processing system according to one embodiment of the invention; Figure 4 shows a schematic diagram of one module of the system of Figure 3:

Figures 5-9 show various shapes of the basins which may be used in embodiments of the invention;

Figures 10 and 1 1 show two forms of cross sectional shape of the basins which may be used in embodiments of the invention; and Figure 12 shows a schematic cross sectional view of a typical admixture structure according to one embodiment of the invention. MODES FOR CARRYING OUT THE INVENTION

The admixture volume can be of any geometry. For effective nitrogen removal from typically medium strength domestic wastewaters requires an admixture volume of around 40 minutes mean retention time at average dry weather flow conditions. Sparge pipes, or other gas injection means, are strategically located in upward flowing segments of the admixture volume to effect, when required, short term (less than 21 minute) preprogrammed operation for oxygen reduction potential (ORP) modification.

Upflow velocities in the admixture volume are designed to be non-settling for most expected particle sizes. Special raw wastewater circumstances, such as high dissolved oxygen (in excess of 2 mg/L). low readily available soluble substrate (less than 30 mg/L). low BOD/TKN ratio (less than 3.0/1) require a special design of admixture reactor which includes an ability to provide hydrolysis of influent readily degradable particulate substrate, among other related factors. Elements of the design and mode of operation are very important to the functions of biological nitrification and denitrification as designed and intended for a works; and also to the net use of readily degradable soluble substrate and to the hydrolysis of readily degradable particulate substrate to soluble readily degradable substrate. 1 mg of influent degradable VSS generates about 1.4 mg of equivalent readily degradable soluble COD.

The preferred admixture volume embodiment is essentially designed to operate as a plug-flow unit through a multi-cellular arrangement that will insure a high degree of dispersion. It should be configured and fitted to provide selectable reaction environments within a cell or in successive cells. By choice the admixture volume can function as totally aerobic, initially anoxic and then anaerobic then anoxic to anaerobic, initially aerobic then anoxic to anaerobic and then anoxic to aerobic etc. etc. The specific environment - reaction time can be manipulated simply through operating selected valves: open, closed or partially open and intermittently used.

The preferred admixture embodiment is designed to operate within a range of substrate to microorganism ratios without flow adjustment. The primary design and function of the admixture volume is to maximize enzymatic transfer of the readily degradable (soluble) fraction of the influent organics which is then followed by depletion of the liquid phase oxidation reduction potential. Through design, receptive microorganisms are provided with a saturated substrate environment to enhance initial rates of substrate removal and maximize the mass conversion to the collectively called intracellular - storage compounds. In so doing advantage is taken of substrate affinity which is influenced by both micro-organism growth rate and the ratio(s) of substrates available. The ability of heterotrophic bacteria to increase their uptake affinity for limiting nutrients is well documented, particularly for sugars and for phosphate with depression of catabolic enzymes as the most commonly described regulation mechanism. Alternative pathways can be forced when an environmental condition such as oxygen tension can be used to limit metabolic access to the substrate carbon source. The oscillating feed starve mechanism of operation of the two reactors in combination enhances general storage product formation and oxidation reduction potential of the reaction environments..

The principle behind the design is to provide for the capture of a soluble carbon source under maximum controlled conditions and to direct that carbon source so that it is available in a controlled environment that permits the removal of ammonia and provides an electron balance velocity whereby the ammonia oxidation product is essentially removed as a nitrogen containing gas. A large overall aerobic reactor fraction, enhances the physical storage transfer and subsequent use therein.

Not easily recognized is the equivalent pulse type of loading condition that is imposed upon the biomass through the design of the admixture volume. This mode of feeding maximizes enzymatic storage.

Three functions are served by the admixture volume, all of which involve the generation and use of storage polymers. In all cases soluble COD is accumulated in the cells as storage polymer. The energy for this process is derived from the oxidation of a minor fraction of the COD with oxygen, nitrite or nitrate, or in the case of an anaerobic condition from the hydrolysis of polyphosphate.

Feeding substrate in an oxygen limited environment also results in maximum polymer formation. Hence there is a decided benefit to transform under at least two reaction conditions, anoxic and anaerobic, depending upon the fraction of available soluble substrate relative to the mass of P and TKN in the raw wastewater.

The metabolic pathway for organic carbon use is relatively well known. Firstly under aerated conditions, heterotrophic removal utilizes organic carbon as its carbon and energy sources for oxidation and phosphorus polymerization. In this case the organic matter degradation follows the glycolysis/pyruvic acid/Krebs cycle metabolic pathways. This scheme is highly efficient in terms of energy production, yielding 38 ATP (Adenosine Triposphate) molecules per mole of glucose oxidized. This is the principle reason that this mechanism is the preferred scheme over other possible pathways, in the presence of free dissolved oxygen.

During denitrification. under zero dissolved oxygen concentration, organic matter is also used for bacterial catabolism and anabalism. The metabolic pathways are the same as for oxidation with dissolved oxygen, but less energy is generated. As a result of a shorter pyruvic acid decarboxylation cycle, and because the resulting product of the former does not enter the Krebs cycle at the beginning, only 26 ATP's are produced per mole of glucose utilized. Under anaerobic conditions, in the fermentative process. organic matter is used for energy and syntheses with a resulting energy gain of only 2 ATP's per mole of glucose employed. The ATP molecules formed come from the glycolysis pathway, with no energy generated during the fermentative pathway. This is a very inefficient process and will only occur when the environmental conditions are not suitable for the previous one to occur.

Another very important process can also take place, the formation and consumption of storage products being defined as "those cellular compounds which undergo rapid synthesis in the presence of soluble exogenous substrate and rapid degradation upon exhaustion of the external feed supply'^". The two most common carbon and energy storage compounds are glycogen, and poly -2-hydroxybutyrate

(PHB). Relatively little is known regarding the metabolic pathways utilized during the synthesis and degradation of these compounds. Glycogen is a glucose polymer which provides cells with a reserve of carbohydrate. Its degradation is accomplished through the glycolysis pathway. End products will depend upon the prevailing environmental conditions. The glycolysis pathway is also used for its synthesis, in the reverse way.

Carbon dioxide and intermediary products of the Krebs cycle (e.g. succinate and malate) are used as substrates for the synthesis of glycogen, when there is a surplus of exogenous organic material and ATP is not needed by the cell.

In nitrification, carbon incorporation is achieved through the Calvin cycle, using carbon dioxide as the sole source of carbon.

Nitrification is traditionally concluded to be a two stage reaction (simplified).

( 1 ) NH₄" + 1.5 0₂ Nitromsomonas NO ^~ ₂ + 2H⁺ + H₂0 (240-342 k J/ml)

(2) NO ₂ + 0.5 0₂ Nitrobacter NO ₃ + (63 - 99 k J/ml.)

The first stage pathway is believed to occur in three steps, from an oxidation state of -3 to +3. It is believed that energy derives from the oxidation of hydroxylamine (NH₂0H) and that ATP is produced by oxidative phosphorylation when the electrons pass through the electron transport chain. NH₄ ^{+(" )} + 0.5 0₂ → NH₂ ^{H )}0H + H⁺

? + 1 → NO ₂ (+3)

The nitrifiers are strictly aerobic when growing on their respective substrate: anoxic or anaerobic conditions are not lethal to them. Under these conditions it has been shown that Nitrobacter can reduce nitrate and that Nitrosomonas europa can reduce nitrite in the presence of hydroxylamine. The overall equation for nitrifier synthesis and nitrification is written as

NH₄ ^{^} + 1.830, + 1.98 HC0 → 0.021 C₅H₇N0₂ + 0.98 NO 7+ 1.041 H₂0 + 1.88 H₂C0₃

For nitrification then 4.3 g0₂/gN0₃.N formed. The earlier equations suggest a mass balance of 4.57 g0₂/g NH₄.N removed or oxidized. Nitrifier cell yield is 0.17 g cells/g NH₄.N removed. Alkalinity reduction is 8.63 g HC0₃ /g NH₄_N removed or 7.14 CaC0₃/gNH₄ N removed.

During assimilatory nitrate reduction, the enzyme assimilatory nitrate reduction catalyzes the transformation of nitrate to ammonia by following the inverse mctabloic rate of nitrification, the resulting ammonia being used for synthesis. This mechanism takes place in the presence of dissolved oxygen. Denitrification is written as

N0₃ ^" ^• NO ₂ - ^■ N0_gas →N₂0_gas ► N 2 gas +5 +3 +2 + 1 0.

Reduction of nitrate to nitrite occurs by enzyme dissimilatory nitrate reduction, utilizing electrons from cytochrome b of the electron transport chain.

Both PHB and glycogen can provide the organic carbon for denitrifiction. This factor is maximized in the use of the preferred admixture volume embodiment.

For nitrite removal (using CH₃0H as the C source for simple explanation): NO ₂ + 0.67 CH₃OH + H⁺ → 0.04 C₅H₇N0₂ + 0.48N₂ + 0.47 C0₂₊ + 1.7 H₂0

In the past speculation has been given to the desirability of stopping the oxidation process at the nitrite stage and to denitrify from there. From the point of view of nitrification less electron acceptor is required. The admixture volume is designed to maximize total nitrogen removal while at the same time minimizing oxygen input and carbon requirements for denitrification in an activated sludge process. An endogenous respiration reduction can also take place.

C₅H₇ N0₂ + 4.6 NO J→ 5C0₂ + 2.8 N, + 4.60H^" + 1.2 H₂0

It is possible for denitrification to take place in the presence of dissolved oxygen, provided that an anoxic period has occurred beforehand in the reaction stream, during which time the appropriate enzymes are formed. A maximum dissolved oxygen (DO) concentration of 0.2 mg/L is suggested to describe such a low dissolved oxygen condition.

While the actual mechanisms involved may be somewhat conjectural, ammonia oxidation can be controlled as to the formation of nitrite and nitrate nitrogen. Operation under reaction conditions with an excess of ammonia and a limit on the mass supply of process oxygen promotes a removal of ammonia nitrogen with an increase in oxidized nitrogen as the limitation on oxygen supply is removed. Under these circumstances DO is also limiting being generally less than 0.1 mg/L.

By increasing the mass supply of oxygen a point is reached whereby both limiting nitrate and ammonia nitrogen exist, also with limiting dissolved oxygen. As the mass rate of flow of process oxygen is increased, ammonia becomes limiting and nitrate is shown to increase. With approximate stoichiometric relationships, less nitrogen incorporation for new cellular growth can be shown. There exists a range of concentration where stored carbon is not limiting and dissolved oxygen is limiting whereby limiting concentrations of both nitrate and ammonia nitrogen are maintained with an optimal net removal of nitrogen. Maximizing stored carbon mechanisms (e.g. PHB, glycogen etc.) and minimizing net residual dissolved oxygen concentration while maintaining an equilibrium between net oxygen demand and net oxygen supply has been shown to be instrumental to the generation of net nitrogen removal during aeration. We believe that an initial carbon storage mechanism which takes place under negative oxidation reduction potential followed by operation under process oxygen limitation whereby the liquid phase dissolved oxygen concentration is generally less than 0.2 mg/L during reaction, is in combination responsible for a shunt mechanism which causes ammonia nitrogen to transform to nitrate nitrogen followed by a stored carbon interaction to assist with the removal of the relatively unstable nitrite nitrogen so formed by enzymatic reduction.

Additionally there is a Nitrosomonas species that rapidly assimilates nitrite or nitrate nitrogen under aerated positive dissolved oxygen reaction conditions.

With cyclic activated sludge treatment the multi-cell admixture volume is sized to maximize the rate of depletion liquid phase of oxidation reduction potential, i.e. to make sure participating microorganisms approach an initial saturated storage state. Recognizing that the introduction sludge is at a reduced metabolic activity as measured by an SOUR of around 7 - 10 mg0₂/gVSS/hr. In the initial admixture volume (measured under artificial dissolved oxygen concentration enhancement) this rate escalates by a factor of 3+ representing the advanced level of enzymatic transfer of the available substrate. Measurements taken in full-scale reactors have always shown the level of concentration dynamics as described above. Using biomass that exhibits endogenous reactivity (OUR) only it is not possible to maintain the same level of aerated nitrogen removal.

It is only with a biomass that exhibits a sufficiently initial higher OUR (elevated level of stored carbon) that selective aerated nitrogen removal can be maintained. The admix reactor design maximizes soluble substrate uptake caused by elevated substrate to biomass loading, reduces (markedly) the liquid phase oxidation reduction potential, assists with the hydrolysis of particulate BOD to soluble compounds that are immediately enzymatically removed, causes release of polyphosphate, thus generating a highly reactive stored carbon source. Operation at a dissolved oxygen limitation of 0, 1 , 2.5 mg/L. results in a beneficial AOR/SOR ratio relative to energy use (air flow rate) and ι: dissolved oxygen differential as shown in Figure 1 for Alpha of 0.65, Beta.98. 100 metres elevation).

Stopping the reaction at the nitrite stage represents a used 1.5 moles of oxygen in place of two moles used for the conventional nitrate formation. This may convert to a 25 percent oxygen saving alone. On the other hand, only 0.5 moles of oxygen may be required for the nitrate formation reaction.

The availability of oxygen and inorganic carbon nutrients are deterministic on the ammonia nitrogen oxidation process.

According to present understanding the availability of an organic carbon source and the unavailability of dissolved oxygen is a requisite for denitritification and denitrification.

Ammonia nitrogen is removed by assimilation into micro-organisms. The nitrogen contained therein can be. through digestion processes, made available for either nitrite or nitrate formation.

According to our findings, operation under cycles of reduced net dissolved oxygen concentration, does not. according to theory, grossly affect the net removal of ammonia, within the concentrations, cycle times etc. that are used in practice. Net oxidation rates, under proposed conditions of operation, while variable, are around 3 -3.5 mg/mgVSS/hr. (around 20°C).

For an intermittently aerated activated sludge process (2A20™) aeration input is designed to operate under a stepped - time dissolved oxygen profile. The first step is at or near zero dissolved oxygen; the second step is less than 1.0 mg/L; the third step is greater than 1.0 mg/L but less than 2.5 mg/L. The upper maximum operating dissolved oxygen is itself functional on the end of aeration sequence dissolved oxygen concentration. The duration of each step depends upon the operating cycle. Discounting the non-aerated fill sequence and by way of example a four hour cycle, the first step is likelv to be for 60-80 minutes, the second step 20-40 minutes and the third step 20-40 minutes. The remainder of the time is allocated to solids liquid separation and to effluent withdrawal. Other time cycles can be used with shortened stepped times. This mode of operation is an effective shunt to the growth of Nitrobacter and hence the formation of nitrite with the caveat that process oxygen is made available at a rate that meets oxygen demand.

The admixture preferred embodiment is designed to function principally between two modes, positive to negative redox potential. (The status of the soluble compounds in the wastewater during commissioning will determine the exact method of operation). For phosphorus removal we need a generation of starved PHB or PHV. These stored carbon compounds are effective carbon sources for denitritification and denitrification. Alternatively the same nitrogen removal processes can be obtained through the glycogen pathway. The preferred reactor embodiment has as a minimum provision for hydrolysis of readily degradable influent particulate substrate.

From the nitrification equations shown in Figure 2 it can be shown that 3.23 mgL^"1 of dissolved oxygen are required to oxidise 1 mgL^" of ammonia nitrogen to nitrite nitrogen and 1.1 1 mgL^" of dissolved oxygen for complete oxidation to nitrate. The total is therefore 4.33 mgL^" . Oxidising to the nitrite states saves 26 percent of the total nitrate stoichiometric oxygen. Oxygen saving by denitritification is then 2/3 of the traditional 2.86 g0₂/gN0₂-N which has a BOD and COD equivalence of around 1.32 - 2.93 mg/mgN0₂-N reduced. Programmed aeration operation as per step changes in a variable volume reactor results in a lower energy use for the same transfer efficiency. This is an additive saving in energy which accompanies the mass saving in oxygen.

Those experienced in the art will recognise that the wastewater to be treated in this process may have passed through any number of combinations of pre-treatment unit operations which may include screening, grit removal, oil and grease removal, primary settling, pH correction, alkalinity addition, nutrient addition.

In a preferred embodiment shown in Figure 3 the treatment system may have four reactor modules 1 into which influent wastewater is directed according to a set and predetermined flow sequence. Each module, as shown in Figure 4. contains a constructed reaction volume for cyclical aeration and non-aeration 2; a constructed volume for the addition of raw influent with a flow of biomass from the constructed reaction volume at least during an aeration sequence 3, whereby mixing takes place naturally and with out mechanical equipment, a means 4 for stopping the flow of influent wastewater (motorised valve or weir gate), a pump means 5 for directing reaction volume contents to the admixture volume . a pump means 6 for the removal of waste biomass from either the reactor volume or the admixture volume 2, an oxygen transfer means 7 for the generation of air bubbles during the pumping of air thereto 8. a moving weir liquid conveyance mechanism 9 driven by motor means 10 to cause layers of near surface liquid to be collected in a horizontally configured box via a plurality of pipes 1 1 connecting the horizontal box to a central rotating drum shaft which transports the collected liquid out of the reactor volume by gravity. The horizontally configured box is positioned such that the leading edge, which is fitted with an adjustable weir, accepts the flowing supernatant liquid from a near surface position with the use of an attached self moving floating screen guard which positively excludes surface solids. The trailing edge of the moving box is constructed to always be at a higher elevation than the leading edge, to effectively prevent the collection and transport of surface floating solids 12. End plates complete the surface isolation which prevents the flow of floating surface solids into the collection box. Air locking within the drum shaft is prevented through the attachment of strategically located air - flow pipes. Typically each module will operate to transport up to 2.5 metres of liquid depth of near surface waters and to contain up to a bout 3.5 metres of liquid depth after the near surface liquid removal operation. Each module houses sensing instrumentation 13, 14, 15 and 16 for the automatic monitoring of dissolved oxygen concentration 13, oxidation reduction potential 14, mixed liquor suspended solids concentration 15, and liquid depth 16. The sensors 13, 14. 15 and 16 are used, in conjunction with a computer control programme, associated software, and a number of control algorithms, to operate each of the four modules 1 in a manner that optimises energy use for a specified treatment objective, and which enables the repetitive cyclic function of the four modules to take place with automatic adjustment of the aeration cycling via the single sensor measurement of the rate of change (increase or decrease) of dissolved oxygen concentration. It will be understood by those experienced in the art that a four modular operation offers a simple means for common blower station use whereby each of two reactor modules are serviced by a single blower station. This arrangement furnishes aeration processing in each module pair of up to 12 hours per day using either 12. 8, 6, 4 hour cycles per module per day. The setting of the duration for oxygenation in each aeration cycle is important to the processing technique which regulates the various enzyme, catalyse and intracellular storage mechanisms. Through proper operation of the initial enzyme transfer and subsequent intracellular storage mechanisms which are caused to take place at greater than 100 mV (hydrogen scale reference), less than 0 mV and at a pH that is not less than 6.0 units it is possible to select a biological culture and biological reaction circumstance which does not determinate at the nitrate form. By adhering to these principles of operating it becomes possible to cause the removal of ammonia nitrogen, presumably to terminal completion to nitrogen or nitrogen containing gases without the major formation and accumulation of nitrate nitrogen. This has a major benefit, especially in low alkalinity wastewaters of allowing the process to operate without marked fluctuation in alkalinity (measured as equivalent CaC0₃). a circumstance which further suggests the non-use of the nitrate mechanism in obtaining the net removal of nitrogenous forms.

In one preferred embodiment of the invention there are no less than four volumes in the admixture reactor. Successive cycle times are not necessarily constant. The duration of each air-on time sequence is a process variable from cycle to cycle as is the aeration intensity within a cycle. A portion of the sludge is pumped for admixture with the incoming wastewater at least during an aeration sequence. A portion of the sludge in the second reactor is removed at least during an air-off sequence. While multiple basins present the optimum embodiment, for the process of the invention it may be practised in a single treating basin with the reactor volumes as described. It will be clear to those experienced in the art that an individual module can have a number of alternative cross sections, and arrangements to make best use of common wall construction. The enclosing structure can be of either upright construction or sloping wall construction. Regular, rectangular, square, circular and octagonal quadrant. These various shapes and configurations are illustrated in Figures 5 to 9. Referring to Figure 12, there is shown a cross sectional view of the admixture reactor according to one embodiment. The inflow enters through port 20 and flows as indicated through a number of baffles 21 to the second zone. The baffles and mixture flows are arranged to provide the desired operating conditions. Sludge is fed back from the second zone through pipe 22.

It is highly desirable in the practice of the invention to cause the admixture of wastewater and activated sludge solids containing heterotrophic and autotrophic microorganisms (with others), under non-aerated reaction conditions whereby the demand specific oxygen utilisation rate of the mixture is at least three times the rate of the activated sludge culture before admixture, which is acclimated to growing on the admixed wastewater, whereby the admixture reaction volume is connected by liquid means to a second reactor volume which operates at the same or a different liquid surface, such that the combined retention of the contents of the second reactor volume relative to the admixture reactor volume is less that 24 hours, such that the oxidation reduction potential in the admixture reactor rapidly depletes to negative values (circa - 150 mV, hydrogen electrode reference) of oxidation reduction potential. In those facilities where the wastewater undergoes a primary settling unit operation, it is possible to supplement the resulting carbon content of the wastewater entering the admixture volume with a primary settled solids or with the liquid product of a primary solids fermentation process. Similarly in situations of insufficient organic and inorganic carbon nutrients, these may be introduced through the admixture volume. Other chemical additives may be introduced directly into the second reactor volume.

Claims

CLAIMS:

1. A method for the acclimated combined growth of heterotrophic and autotrophic micro-organisms for the removal of nitrogen, organics and other nutrient pollutants in a wastewater using an admixture reactor, comprising the steps of: providing an initial admixture of influent wastewater with a flow of acclimated culture, from a second sequentially aerated reactor volume, at least during an unmixed unaerated time period followed by an aerated - mixed time period, causing the oxidation reduction potential of the mixture to progressively decrease in the admixture reactor to less than about - 100 mV to about -200 mV (when compared to a hydrogen reference electrode), passing the mixture to the second reactor volume which time selectively receives a controlled flow of oxygen containing gas during at least a part of the wastewater inflow time, during controlled time sequences which limit the mixed liquid phase dissolved oxygen concentration to predetermined values causing a preferential growth of heterotrophic and chemolithoautotrophic microbial species, whereby the end of aeration sequence mean substrate removal rate in a cycle is at least four times the minimum endogenous substrate removal rate; interrupting the introduction of wastewater and the introduction of oxygen containing gas to provide the formation of a settled sludge layer in which the oxidation reduction potential rapidly decreases to less than about -100 mV to about -200 mV (compared to hydrogen reference electrode) and before the next aeration and wastewater inflow sequence, varying the volume of liquid with the admixture reactor.

2. A method according to claim 1 wherein the time cycles, and equipment interaction of each reactor are automatically controlled.

3. A method according to claim 2 wherein sensors for the measurement of oxidation - reduction potential, dissolved oxygen concentration, mixed liquor suspended solids concentration are in contact with a velocity stream of said mixture at least during an aeration period.

4. A method according to claim 3 where the mixture suspended solids concentration in the second reactor volume at bottom water level is about 5000 to 6000 mg/L with an operating sludge age that provides for the proliferation of the autotrophic microorganisms for a designated range of operating temperature and effluent nitrogen concentrations.

5. A method according to claim 4 where a stoichiometric mass of process oxygen in a cycle is transferred during pre-selected sequences of time and set point dissolved oxygen concentrations.

6. A method according to claim 1 wherein the acclimated culture contains general higher life forms, such as Vorticella, Ciliates. Protozoa. Rotifer. Nematodes.

7. A method according to claim 1 including the step of causing mixing of wastewater and acclimated culture followed by nucleation and activated sludge floe growth in an admixture volume of said reactor through the placement of liquid flow paths respectively near wall surfaces and air - liquid surfaces of said reactor.

8. Λ method according to claim 1 wherein the step of controlling the decrease of the oxidation reduction potential to below about -200 mV (compared to reference hydrogen electrode) in the admixture volume is done by means of entrainment of selectively placed oxygen containing gas bubbles.

9. A method according to claim 1 wherein a near surface liquid removal rate that takes place in an air off sequence is process compatible and does not cause the entrainment of solids from within the settled solids layer.

10. Λ method according to claim 1 wherein a plurality of reactors are used, whereby the net flow in and out of the plurality of reactors is continuous whereby flow into and out of each reactor is interrupted for at least a part of the time.

1 1. A method according to claim 10 whereby the net flow into the reactors is continuous whereby flow into each reactor basin is interrupted for at least a part of the time.

12. A method according to claim 1 wherein said varying volume is produced in said second volume by a moving box weir within said second volume connected by a plurality of downcomers to a central rotating drum shaft for moving said weir to a set bottom water level position and then returning it to an out of liquid rest position.

13. Apparatus for the removal of nitrogen, organics and other nutrient pollutants in a wastewater using an admixture reactor by the acclimated combined growth of heterotrophic and autotrophic micro-organisms, comprising:: means for providing an initial admixture of influent wastewater with a flow of acclimated culture, from a second sequentially aerated reactor volume, means for. at least during an unmixed unaerated time period followed by an aerated - mixed time period, causing the oxidation reduction potential of the mixture to progressively decrease in the admixture reactor to less than about - 100 m V to about - 200 mV (when compared to a hydrogen reference electrode), means for passing the mixture to the second reactor volume which time selectively receives a controlled flow of oxygen containing gas during at least a part of the wastewater inflow time, during controlled time sequences which limit the mixed liquid phase dissolved oxygen concentration to predetermined values causing a preferential growth of heterotrophic and chemolithoautotrophic microbial species, whereby the end of aeration sequence mean substrate removal rate in a cycle is at least four times the minimum endogenous substrate removal rate; means for interrupting the introduction of wastewater and the introduction of oxygen containing gas to provide the formation of a settled sludge layer in which the oxidation reduction potential rapidly decreases to less than about- 100 mV to about-200 mV (compared to hydrogen reference electrode) and means for varying the volume of liquid within the admixture reactor before the next aeration and wastewater inflow sequence.

14. Apparatus according to claim 13 wherein said means for varying includes means for varying the second volume comprising a moving box weir within said second volume connected by a plurality of downcomers to a central rotating drum shaft for moving said weir to a set bottom water level position and then returning it to an out of liquid rest position.