Classifications

• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F3/00—Biological treatment of water, waste water, or sewage
  • C02F3/006—Regulation methods for biological treatment
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F3/00—Biological treatment of water, waste water, or sewage
  • C02F3/02—Aerobic processes
  • C02F3/12—Activated sludge processes
  • C02F3/1205—Particular type of activated sludge processes
  • C02F3/1215—Combinations of activated sludge treatment with precipitation, flocculation, coagulation and separation of phosphates
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F3/00—Biological treatment of water, waste water, or sewage
  • C02F3/30—Aerobic and anaerobic processes
  • C02F3/301—Aerobic and anaerobic treatment in the same reactor
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F3/00—Biological treatment of water, waste water, or sewage
  • C02F3/30—Aerobic and anaerobic processes
  • C02F3/308—Biological phosphorus removal
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2209/00—Controlling or monitoring parameters in water treatment
  • C02F2209/02—Temperature
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2209/00—Controlling or monitoring parameters in water treatment
  • C02F2209/06—Controlling or monitoring parameters in water treatment pH
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2209/00—Controlling or monitoring parameters in water treatment
  • C02F2209/22—O2
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2209/00—Controlling or monitoring parameters in water treatment
  • C02F2209/44—Time
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
  • Y02W10/00—Technologies for wastewater treatment
  • Y02W10/10—Biological treatment of water, waste water, or sewage

Definitions

• 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.
• 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)
• BOD biochemical oxygen demand
• 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.
• CMAS completely mixed activated sludge process
• 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.
• 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.
• DO dissolved oxygen
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the anaerobic period that is the period of discontinuance of aeration, is sufficiently long for substantially complete denitrification to take place.
• 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.
• 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.
• the present invention allows an activated sludge process to be operated under oxygen limiting conditions instead of substrate limiting conditions.
• oxygen 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.
• 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.
• R microbial oxygen uptake rate
• Control means may be provided to stop aeration when the rate of change of the dissolved oxygen concentration reaches or exceeds a predetermined value.
• 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.
• 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.
• MMS mixed liquor suspended solids
• 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:
• 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)
• Nitrogen nutrient removal 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.
• 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.
• 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.
• chemicals such as for example alum, iron salts or lime
• 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.
• 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.
• 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 trez - t.. in Figure 3, the air-on and air-off controller terminates the air supply as indicated at point B in Figure 3.
• 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.
• the concentration of DO in the waste decreases rapidly to zero due to the continued respiration of the microorganisms.
• 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.
• 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.
• 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.
• 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.
• t is of the order of 30 minutes.
• 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;
• 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.
• 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, .
• t 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 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.
• 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.
• 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
• 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.
• AAA-CMAS process 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.

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• Life Sciences & Earth Sciences (AREA)
• Chemical & Material Sciences (AREA)
• Biodiversity & Conservation Biology (AREA)
• Microbiology (AREA)
• Hydrology & Water Resources (AREA)
• Engineering & Computer Science (AREA)
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• Analytical Chemistry (AREA)
• Purification Treatments By Anaerobic Or Anaerobic And Aerobic Bacteria Or Animals (AREA)
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• 1985
  • 1985-12-20 EP EP86900031A patent/EP0205496A1/de not_active Withdrawn
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  • 1985-12-20 AU AU52373/86A patent/AU595177B2/en not_active Ceased

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