NITRIFICATION/DENITRIFICATION OF WASTE MATERIAL

Abstract: A method for treating a waste material in which the material is subjected to periods of aeration with air or other oxygen-containing gas, to bring about biological oxidation and nitrification, alternating with periods during which aeration is discontinued to allow development in the material of anaerobic conditions leading to biological denitrification of the material. Aeration is discontinued when the rate of change of dissolved oxygen concentration in the material reaches or exceeds a predetermined value, for example 0.1 to 4 mg/l hr, and resumed after the nitrate concentration in the material is reduced to a desired level, for example after 4 hours. The method preferably takes place in a single vessel (1). Waste enters via inlet (13) and exits via outlet (14); waste entry and exit are preferably continuous. Flow of air to diffuser (12) can be cut-off by means in conduit (11), such means being controlled by a signal processor which monitors the ratio of change of the dissolved oxygen levels sensed by sensor (9). Chemicals for the removal of phosphorous nutrients may be added to the waste material prior to or during the treatment.

Description: NITRIFICATION/DENITRIFICATION OF WASTE MATERIAL The present invention relates to a method and apparatus for the treatment of waste materials, particularly raw sewage, to convert them to more environmentally acceptable materials. The invention is particularly concerned with methods utilizing intermittent aeration. The present invention will be described with particular reference to treating raw sewage. It should be understood, however, that the method and apparatus of the present invention may be applicable to the treatment of other waste materials. Generally, there are three levels of treatment for municipal sewage; a primary treatment in which relatively large sized solids are separated from the liquid phase; a secondary treatment in which material requiring oxygen for its degradation is removed, such material being known as biochemical oxygen demand (BOD) ; and a tertiary treatment in which nitrogen and phosphorus nutrients are removed. The most commonly used secondary treatment processes are the activated sludge process and modifications thereof, in which waste material, such as for example raw sewage, is mixed with an activated sludge comprising biologically active material and the mixture is continuously aerated with oxygen or air to promote degradation of the waste material. This process is known as the completely mixed activated sludge process (CMAS) and results in the biological oxidation of the organic carbon of the waste material to carbon dioxide and the production of additional biomass (sludge) which must be removed for disposal. Nitrogen present in the waste in the form of ammonia or organic nitrogen compounds is simultaneously oxidised by biological action to nitrate in a reaction known as nitrification and the nitrate is discharged in the liquid effluent from the treatment plant. Typically, aeration is controlled so as to maintain a constant concentration of dissolved oxygen (DO) in the liquid phase of the waste material under treatment, for example, as shown in Figure 1 of the accompanying drawings. While the process can be very efficient in reducing the BOD of the waste, the continuous aeration required is often associated with high aeration energy costs, especially in extended aeration plants. Also, disposal of the sludge can incur high costs. A further disadvantage of the CMAS system is that the nitrogen nutrients present in the liquid effluent can stimulate undesirable algal and plant growth in the body of water into which the effluent is discharged, and if the receiving waters are destined for potable supplies the nitrate thereby introduced may endanger the health of young infants. The most common tertiary treatment method for the removal of nitrogen nutrients from waste materials such as sewage is the biological nitrification and denitrification process. With the activated sludge system, nitrification of the incoming waste material is achieved by controlling the average cell retention time (sludge age) to allow nitrifying bacteria to accumulate in the reaction vessel. The nitrifying bacteria convert the ammonia and organic nitrogen in the waste into nitrate. Denitrification, a biological process in which nitrates are converted to nitrogen gas, can only occur under anaerobic conditions. In activated sludge plants denitrification has been encouraged in two ways, the first of which involves separating the oxidative nitrifying stage from the anaerobic denitrifying stage by carrying out the latter in a separate denitrification vessel, usually with the addition of a reductant such as methanol. The second method involves the use of a single vessel provided with alternating aerobic and anaerobic zones or with continuous recycling of the mixed liquor from the aerobic zone to the anaerobic zone. Various configurations have been employed to establish an anaerobic zone by, for example, turning off banks of aerators either at the pojLnt where sedimented sewage enters the reactor or at some point along the reactor, or both. In these anaerobic zones sewage, returned sludge or mixed liquor suspended solids (MLSS) are introduced in order to reduce the nitrate which has been formed in the aerated part of the reactor or has returned to the reactor with the recycled sludge. Although this second method avoids the cost of added chemicals, it is subject to operational problems such as incomplete nitrification, the presence of ammonia in the effluent which is undesirable ecologically, and variable effluent quality. Overall, biological nitrification and denitrification plants can be complex and often involve high capital costs and/or high operating costs. A well established tertiary treatment process for the removal of phosphorus nutrients is the chemical dosing method which uses alum, iron salts or lime before or after aeration. In some plants chemicals are added both before and after aeration. The process is usually performed in a small vessel having a low hydraulic residence time which results in the added chemicals being in contact with the waste water for a relatively short time. To achieve good removal of phosphorus it is therefore often necessary to add well in excess of the theoretical amounts of these chemicals. The present invention, which we will refer to as the "AAA-CMAS process" , is essentially an improvement of the completely mixed activated sludge process in which air is introduced into the aerator intermittently rather than continuously so as to create alternating aerobic and anaerobic conditions for treatment of the waste. This process is capable of achieving substantial reductions in energy consumption, sludge production and in the amount of nitrogen nutrients remaining in the treated waste. Additionally, chemical dosing for the removal of phosphorus nutrients may be carried out concurrently with the AAA-CMAS process to achieve substantially complete phosphorus removal with the use of close to the stoichiometric amounts of the chemicals required. The AAA-CMAS process differs from similar processes such as the intermittent-cycle, extended-aeration system in that the aerobic and anaerobic conditions in the process are controlled by monitoring the rate of change in dissolved oxygen concentration during the aerobic stage rather than by a timer. Additionally, in the preferred form of the AAA-CMAS process the flows of influent and effluent are continuous and it is a completely mixed process not a plug flow process of the Pasveer, Carousel, Orbal or other oxidation ditch type. According to one aspect of the present invention there is provided a method for treating a waste material in which the material is subjected to periods of aeration with air or other oxygen-containing gas, to bring about biological oxidation and nitrification, alternating with periods during which aeration is discontinued to allow development in the material of anaerobic conditions leading to biological denitrification of the material and wherein aeration is discontinued when the rate of change of dissolved oxygen concentration in the material reaches or exceeds a predetermined value; and aeration is resumed after a period sufficient to allow the nitrate concentration in the material to be reduced to a desired level. Preferably, the anaerobic period, that is the period of discontinuance of aeration, is sufficiently long for substantially complete denitrification to take place. Preferably, the method is carried out in a completely-mixed continuous-flow system, wherein influent material to be treated is continuously added to the system and treated effluent is continuously discharged from the system. It is also preferred to carry out the method in a single vessel. Thus, the invention also provides apparatus for carrying out the above-described method which comprises a reaction vessel, inlet means for continuously supplying influent waste material to the vessel, outlet means for continuously withdrawing treated effluent from the vessel, mixing means for continuously and completely mixing the contents of the vessel, aeration means for intermittently supplying air or other oxygen-containing gas to the vessel, said reaction vessel being provided with a sensing means for sensing the rate of change of dissolved oxygen concentration in the material in the reaction vessel and means to interrupt the supply of gas to the vessel for a period when the said rate reaches or exceeds a predetermined value. Typically, the present invention allows an activated sludge process to be operated under oxygen limiting conditions instead of substrate limiting conditions. By controlling the time for which air is supplied and the time for which the supply of air is stopped, there is a substantial period of time of the process during which the requirement for free dissolved oxygen is not satisfied and accordingly, the oxygen contained within the nitrate groups present in the reaction vessel contents is utilized. Thus, the process of the present invention typically comprises three periods, one in which the concentration of dissolved oxygen is increasing, a second in which the concentration of dissolved oxygen decreases to zero, and a third period in which the concentration of dissolved oxygen is maintained at zero. The present invention also allows the activated sludge process to be operated under dynamic conditions instead of steady state conditions since the environment is changing periodically, such as for example in the amount of oxygen present which is available to react with the organic materials. The basic equation for the oxygen transfer rate is: where dc/dt = rate of change in dissolved oxygen concentration K τ Lia = overall oxyen transfer coefficient C = saturation concentration of oxygen in the liquid C = sensed or actual concentration of oxygen in the liquid R = microbial oxygen uptake rate. Under dynamic conditions, the value of dc/dt is not zero but variable. This fact is relied upon in the practice of this invention. Control means may be provided to stop aeration when the rate of change of the dissolved oxygen concentration reaches or exceeds a predetermined value. The present invention will now be described by way of example with particular reference to the accompanying drawings in which: FIGURE 1 is a schematic graph of dissolved oxygen concentration as a function of time for a conventional activated sludge process in which there is continuous aeration. FIGURE 2 is a graph similar to Figure 1 for an activated sludge process under "off-peak loading", "average loading" and "peak loading" conditions in which the aeration is regularly periodically interrupted for a fixed length of time, e.g., by timer means. It illustrates the wastage of added oxygen (high DO level) that occurs during off-peak and average loadings and the low and insufficient DO level attained under peak loading conditions. FIGURE 3 is a graph similar to Figure 2 showing the DO levels achieved in an activated sludge process operated in accordance with the method of the present invention; FIGURE 4 is a schematic diagram of one form of an experimental system for evaluating the method of the present invention; and FIGURE 5 is a comparison of biomass production rate between the process of the present invention operated with various air-on/air-off periods and the conventional completely mixed and continuously aerated activated sludge (CMAS) system; FIGURE 6 is a comparison of mixed liquor suspended solids (MLSS) which is indicative of sludge production, effluent nitrogen concentration, effluent BOD, sludge volume index (a measure of sludge settling characteristics) , pH, and effluent suspended solids concentration, between the process of the present invention and the conventional CMAS system under normal operating conditions in a 150 person-equivalent extended aeration plant. The potential theoretical advantages of the invention can be listed as follows: Energy saving: A saving in aeration energy can be obtained from two sources; the oxygen made available from nitrate during the air-off period and the high oxygen transfer rate when the system is switched from anaerobic to aerobic conditions. It is expected that there will be 2.86 mg of oxygen available for each mg of nitrate denitrified according to the half reactions of electron acceptors and oxygen and nitrate: 1/2 H 2 0 - 1/4 0 2 + H + + e~ 1/10 N 2 + 3/5 H 2 0→l/5 NO-, " + 6/5 H + + e~ When the aeration period starts, there will be present an amount of unmetabolized substrate which has been accumulated during the air-off period. This accumulated substrate will keep the dissolved oxygen concentration, C, low since the oxygen present will be consumed in degrading the organics of the waste material, resulting in a high oxygen transfer rate and microbial oxygen uptake rate, R, as shown: where R = microbial oxygen uptake rate (mg/L.hr) C = oxygen concentration in the wastewater at saturation (mg/L) C = oxygen concentration in the wastewater (mg/L) _a = overall oxygen transfer coefficient (hr ) dc/dt = change of oxygen concentration in wastewater per unit time (mg/L.hr) When the dissolved oxygen concentration is low, the driving force C -C will be high for the oxygen transfer. Nitrogen nutrient removal: The method of the present invention makes it possible to create within the one vessel clearly defined aerobic periods for nitrification, sandwiched between clearly defined anaerobic periods for denitrification. Typically, the duration of the anaerobic period is related to the duration of the aerobic period. Nitrification requires the continuous presence of oxygen while denitrification can only occur in the absence of oxygen. With the present invention, it is possible to control the air-on and air-off times to achieve the required degree of nitrification and denitrification by sensing the rate of change in dissolved oxygen concentration in the waste material and comparing it to a pre-selected value. Sludge reduction: When the organic substrate is metabolized to produce ATP (Adenosine-5-triphosphate) , the energy carrier of the cell, electrons are transported through a carrier chain to oxygen. ' This process is termed the oxidative phosphorylation process. It is thought the transfer of a pair of electrons from an organic substrate to oxygen can give rise to two molecules of ATP if the initial dehydrogenation is effected by a flavoprotein, and to three molecules of ATP if the dehydrogenation is mediated by a pyridine nucleotide-linked dehydrogenase. At these sites, phosphorylation is 'coupled' to electron transfer. However, phosphorylation can be uncoupled by chemicals such as dinitrophenol or by applying oxygen stress to the microorganisms. This uncoupling of phosphorylation causes a more rapid electron transfer without producing the usual amount of ATP for biomass production. This means that when the activated sludge biomass is subjected to oxygen stress, a transition from anaerobic to aerobic conditions will result in the uncoupling of oxidative phosphorylation. The nature of this uncoupling of the electron transfer chain differs from that which occurs when microorganisms are continuously exposed to anaerobic conditions and its effect upon biomass production is far greater. It is thought that more energy is lost as heat and less energy is trapped to form the ATP needed for cell synthesis with the result that under conditions of oxygen stress the microorganisms change their metabolic pathways to achieve minimal cell growth i.e., low sludge production. Phosphorus removal: When sludge production is reduced by applying oxygen stress to the activated sludge system in accordance with the present invention, it is possible to achieve substantially complete removal of phosphorus by simultaneously adding chemicals such as for example alum, iron salts or lime to the reaction vessel containing the waste under treatment. Because the chemicals remain in contact with the waste for a relatively long period of time, the efficiency of phosphorus removal is markedly increased and it becomes possible to use near to the stoichiometric amounts of the added chemicals to effect total phosphorus removal. The saving in chemical dosage can be of considerable economic benefit in operation of the process. Additionally, the capital and energy costs for the plant can be reduced because a separate reaction vessel and associated mixing means are not required for phosphorus removal. Generally, the process of the present invention after it has been in operation for at least one complete cycle may be understood by consideration of the following points with reference to Figure 3 "Average Loading": (1) At the beginning of the reaeration period, denoted by A in Figure 3, C=0 and the overall oxygen transfer rate ( [K J_ja (C5-C) ] is high compared to the microbial oxygen uptake rate R. Values of dc/dt will increase very rapidly initially such as is illustrated in Figure 3 in the period from t ~ to t, . (2) After the initial reaction period, the rate of change in dissolved oxygen (DO) concentration in the water, dc/dt, slows down due to the microbial oxygen uptake in the presence of accumulated substrate and ammonia and organic nitrogen. As R increases, dc/dt will increase at a much slower rate than that of the initial aeration period, as illustrated in the period from t, to t_ of Figure 3. During this period, the uptake of oxygen by the microorganisms leads to both the oxidation of the carbonaceous substrate and the oxidation of ammonia and organic nitrogen to nitrate nitrogen. The oxidation of the carbonaceous substrate is generally rapid and most of the oxidation that occurs during the period t.. - t_ is due to nitrification i.e., the conversion of ammonia and organic nitrogen to nitrate. (3) At the end of the previously described period in (2) above when all the ammonia and organic nitrogen has been converted to nitrate through the biological nitrification process, the microbial oxygen uptake rate, R, will decrease while the rate of change in DO concentration, dc/dt will increase as indicated in Figure 3. When the oxygen sensor and associated control means detects that dc/dt is equal to or greater than the pre-determined value 0, as shown by the period t„ - t.. in Figure 3, the air-on and air-off controller terminates the air supply as indicated at point B in Figure 3. Typically, the value of 0 is in the range of 0.1 to 4 mg oxygen/l.hr and preferably in the range 1 to 3 mg oxygen/l.hr. During the air-off period, the concentration of DO in the waste decreases rapidly to zero due to the continued respiration of the microorganisms. Thereafter, in the absence of free dissolved oxygen, the microorganisms commence to utilize the oxygen of the nitrate for respiration, using the incoming organic carbon of the sewage as the electron donor for denitrification, i.e., the reduction of nitrate to nitrogen. The length of the denitrification period will depend on the amount of nitrate that is present at the start of the anaerobic period, and this in turn will depend on the length of the aerobic period during which nitrification has occurred. Thus, there is a specific relationship between the length of the aerobic period available for nitrification, and that of the following anaerobic period required for denitrification. Once this relationship is known, or once a proportionality constant between these two periods has been established, the air-on and air-off controller can be arranged to determine the anaerobic period automatically from the aerobic period in accordance with the relationship (t_, - t. = k (t n - t ? ) where k is a proportionality constant. This is illustrated in Figure 3 which also shows that t. corresponds to the value of t„ for the subsequent aeration cycle. If the control of the operation of the aeration means in accordance with the present invention were not provided, the plot of dc/dt against time would continue similarly to that shown by the profile of Figure 2 and would result in excess oxygen being supplied as indicated by the shaded areas in Figure 2. Further operation of the apparatus of the present invention is now described. During peak loading as shown in Figure 3 or when shock loading situations occur, the air-on time, t n ~t_., will increase because of the accumulated substrate and organic or ammonia nitrogen available for oxidation. The air-off time t,-t. will also increase because of the increased quantity of nitrate that must be denitrified. Should the air-on time t„-t 3 exceed a pre-determined value, t, , the controller is arranged to call in a second or even a third air blower to ensure that the air-on time t_-t_ will not be unduly prolonged, i.e., greater than a selected value. During off-peak periods, the air-on time t n -t_ and the air-off time t--t. would normally be short. Under these conditions, the controller is arranged to ensure that the minimum air-on time will always be equal to or greater than a preselected value, t before the blower is switched off. This is desirable to prevent "hunting" of the system and possible damage to the motors driving the blowers through too frequent on-off switching. Typically, t is of the order of 30 minutes. Generally, the operation of the apparatus in accordance with the present invention is as follows: 1. The air-on period is controlled by comparing the rate of change of DO concentration, dc/dt, which is constantly monitored by the controller, with a pre-selected value 0. Air is on when dc/dt < 0 or when < tm; When t_-t_. = t, , the controller calls in a second blower; Air is switched off when dc/dt > 0 and when t n ~t.. > m Generally, any suitable oxygen sensor means may be employed to detect and monitor the dissolved oxygen concentration in the reactor vessel contents, together with any suitable electronic circuitry to determine the rate of change of the oxygen concentration and to effect control of the aeration of the reactor contents in accordance with the principles of the invention. 2. The air-off period t_-t, is first determined experimentally from the time required under air-off conditions for the concentration of nitrate in the previously aerated waste to fall to zero. Once the proportionality constant, k, between the air-on period from t n -t-, and the air-off period from t-.-t. has been established, the subsequent air-off periods can be readily derived from the equation t 3" t 4 = k( Q .-t 3 ) . 3. The present invention allows the activated sludge system to be operated at variable air-on and air-off times according to the flow rate and concentration of the influent. Its minimum air-on and air-off times are tm and ktm resp c ectively. Its maximum air-on and air-off times are governed by the time factor t, which is the time the controller calls in an additional air blower or blowers should the influent flow rate and concentration be such that the existing air blower is unable to raise the rate of change in DO concentration to 0 at time t, . The present invention will now be illustrated by the following non-limiting examples. EXAMPLE I The reactor vessel used to assess the AAA-CMAS system of the present invention for energy and sludge reduction and nitrogen removal was as shown in Figure 4. The reactor (1) was constructed from 155 mm diameter PVC tubing and had a volume of 18 1. A square perspex settling arm (2) with a volume of 3.2 1 was attached at an angle which permitted the settled solids to return to the reactor. A stirrer (3) with two blades (4,5) was used to ensure complete mixing during air-off periods. The lower blade (5) was enclosed by a baffle (6) to minimize turbulence in the settling arm. The temperature within the reactor was maintained at 20°C by a thermostatically controlled immersion heater (7) . The reactor was fitted with a pH probe (8) and a dissolved oxygen sensor (9) immersed in the waste. An air-inlet line (11) connected a compressed air supply (not shown) to a diffusing inlet (12) at the base of the reactor (1) . A solenoid valve (not shown) was installed in the air-inlet line. The solenoid was actuated by a controller (not shown) which processed the signal from the oxygen sensor (9) to determine the rate of change in DO concentration and closed the solenoid valve when a pre-selected rate of change was reached or exceeded. An overriding timer control (not shown) to actuate the solenoid valve was also fitted to allow experimental operation of the system for pre-selected aerobic and anaerobic periods in order to obtain kinetic data on the operation of the process under various air-on/air-off regimes. Raw sewage was collected and allowed to settle for 30 minutes and then the supernatant was pumped to a stirred, refrigerated (3 to 5°C) storage tank (not shown) where yeast extract was added to increase the total Kjeldahl nitrogen (T N) concentration to 60 to 90 mg/1. Fresh feed was collected in this way every two days. This mixture served as the influent and was pumped continuously to the reactor .(1) from the storage tank by a variable speed peristaltic pump (13) . The reactor contents were initially seeded with solids from a local sewage treatment plant. Sludge was discharged continuously from an outlet (14) on the low side of the reactor by a constant speed peristaltic pump (15) , the rate of discharge being controlled to give the desired sludge age. A steady state corresponding to a given sludge age was readily achieved. Each operating condition was maintained for a period of four weeks before daily samples were taken for analysis. Three air-on and air-off conditions; i.e. 2 hr air-on/4 hr air-off; 3 hr air-on/3hr air-off; 4 hr air-on/2 hr air-off; and continuous air-on were examined for three different sludge ages; i.e., 8, 12 and 22 days. The results of laboratory-scale experiments are given in the Table. They show that it is possible to treat sewage without continuous aeration, and that the system of the present invention removed twice to four and one half times the amount of nitrogen than the amount removed when the reactor was operated as a conventional continuously aerated CMAS system. Effect of the aeration regime on nitrogen removal and MLVSS in an experimental activated sludge unit Air Sludge TOC TKN Oxidized N N MLVSS Hydraulic on/off Age In Out In Out In Out removed retention (h) time (days) (mg/1) (mg/1) (mg/1) (mg/1) (mg/1) (days) 2 on/ 8 122 26 75 56 .03 .4 19 357 1.18 4 off 12 124 35 67 54 .02 .4 13 549 1.18 22 142 41 87 70 .02 .9 16 957 1.18 U3 I 3 on/ 8 107 20 82 49 .02 8.2 26 235 1.18 3 off 12 115 19 81 18 .02 37.4 26 340 1.18 22 102 11 94 20 .02 30.0 44 400 1.18 4 on/ 8 86 17 56 22 .02 22 13 229 1.20 2 off 12 96 16 66 16 .02 31 19 350 1.20 22 96 16 62 16 .02 30 16 475 1.20 CMAS 8 123 23 77 32 .02 40 4 320 1.35 continuous aeration 12 98 14 72 26 .02 35 11 395 1.20 22 117 16 62 23 .02 33 6 560 1.35 The difference in biomass production was examined by plotting the specific biomass growth rate (as measured in terms of units of mixed liquor volatile suspended solids (MLVSS) produced per day per unit of MLVSS present in the reactor) versus the specific substrate utilization rate (as measured in terms of units of total organic carbon (TOC) utilized per day per unit of MLVSS present in the reactor. The results are shown in Figure 5 where the intercepts on the horizontal axis represent the amount of substrate used for maintenance energy production and the intercepts on the vertical axis represent the amount of biomass production foregone in favour of the production of maintenance energy. Figure 5 clearly shows that the AAA-CMAS system, when operated under 3 hr air-on/3 hr air-off conditions, used more substrate for maintenance energy production than either the conventional CMAS system or the AAA-CMAS system when operated with different air-on/air-off periods. There was very little difference in the amount of substrate used for maintenance energy production between the AAA-CMAS_ system operated under 4 hr air-on/2 hr air-off conditions and the conventional CMAS system. This was due to the fact that both systems were not subject to oxygen stress. The low utilization of substrate for maintenance energy production when the AAA-CMAS system was operated under 2 hr air-on/4 hr air-off conditions indicates that the process was being operated under oxygen-limiting conditions. Not all of the organic substrate was oxidized, so that the high MLVSS values represent not only the activated sludge biomass, but also the unmetabolized organic suspended solids. EXAMPLE 2 A full scale trial of the AAA-CMAS process was carried out between July 1983 and May 1984 at the Helen Close Neighbourhood Sewage Purification Plant at Yarra Glen, Victoria, Australia. The plant was operated for 7 months in the conventional continuously aerated CMAS mode and then for 4 months in the AAA-CMAS mode in which the air-on and air-off periods were determined from the rate of change in the dissolved oxygen concentration as hereinbefore described. The power consumption of the plant when operated in the CMAS mode was 3400 KWH per quarter (3 months) . This was reduced to 2200 KWH per quarter when the plant was operated in the AAA-CMAS mode in which for average loading the air-on and air-off periods were 2 hr and 4 hr respectively. The following observations can be made from the results shown in Figure 6. 1. Increase in the oxygen stress in the system brought about by the change from CMAS to AAA-CMAS operation caused a decrease in the mixed liquor suspended solids (sludge production) from an average concentration of about 7500mg/l to about 4000 mg/1. 2. The nitrogen concentration in the effluent was reduced from an average of 20 mg/1 to about 5 mg/1. 3. There was no deterioration in the effluent BOD and in the suspended solids discharged during the trial period of operation under AAA-CMAS conditions. Desirable improvements were evident in the reduced sludge volume index and the higher operating pH close to 7. EXAMPLE 3 Investigations carried out at the Yarra Glen plant operating in the AAA-CMAS mode showed that phosphorus nutrients, present in the incoming sewage at a level of 9 mg/1, could be substantially removed by simultaneous alum addition to the aeration tank. The ratio of aluminium to phosphorus used was 1.03, i.e., a slight excess over the stoichiometric requirement for phosphorus removal The effluent from the plant was found to have a phosphorus content of less than 1 mg/1.

Claims: CLAIMS ! . A method for treating a waste material in which the material is subjected to periods of aeration with air or other oxygen-containing gas, to bring about biological oxidation and nitrification, alternating with periods during which aeration is discontinued to allow development in the material of anaerobic conditions leading to biological denitrification of the material; characterized in that aeration is discontinued when the rate of change of dissolved oxygen concentration in the material reaches or exceeds a predetermined value; and aeration is resumed after a period sufficient to allow the nitrate concentration in the material to be reduced to a desired level. 2. A method as claimed in Claim 1, characterised in that the period of discontinuance of aeration is sufficiently long. for substantially complete denitrification to take place. 3. A method as claimed in Claim 1, characterised in that the period of discontinuance of aeration is directly related to the length of the aeration period. 4. A method as claimed in any one of Claims 1 to 3, characterised in that the method is carried out in a completely-mixed, continuous-flow system, wherein influent material to be treated is continuously added to the system and treated effluent is continuously discharged from the system. 5. A method as claimed in any one of Claims 1 to 4, characterised in that all steps of the method are carried out in a single vessel. 6. A method as claimed in any one of Claims 1 to 5, characterised in that chemicals for the removal of phosphorus nutrients are added to the waste material prior to or during treatment. 7. A method as claimed in any one of the preceding claims, characterised in that said predetermined value of the rate of change of oxygen concentration is from about 0.1 to about 4 mg oxygen/l.hr. 8. A method as claimed in Claim 7, characterised in that said predetermined value is from about 1 to about 3 mg oxygen/l.hr. 9. A method as claimed in any one of the preceding claims, characterised in that the waste material is sewage. 10. Apparatus for treating waste material which comprises a reaction vessel, inlet means for continuously supplying influent waste material to the vessel, outlet means for continuously withdrawing treated effluent from the vessel, mixing means for continuously and completely mixing the contents of the vessel, aeration means for intermittently supplying air or other oxygen-containing gas to the vessel, characterised in that said reaction vessel is provided with sensing means for sensing the rate of change of dissolved oxygen concentration in the material in the reaction vessel and means to interrupt the supply of gas to the vessel for a period when the said rate reaches or exceeds a predetermined value.

Inventors: BRIDGER JOHN STEPHEN, IP SZE-YUEN, MILLIS NANCY FANNIE, BRIDGER, JOHN, STEPHEN, IP, SZE-YUEN, MILLIS, NANCY, FANNIE
Applicants: COMMW SCIENT IND RES ORG, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR
Application year: 1985
Main classification: C02F3
IPC codes: C02F3