BIOLOGICAL WASTEWATER TREATING SYSTEM

Abstract: A modified activated sludge system is provided wherein wastewater containing BOD and phosphate values is admixed with biomass-containing recycled sludge under conditions effecting sorption of BOD in the biomass (A), then said wastewater is subected to aeration in an oxidation zone (B) under conditions effecting oxidation of BOD, including a dissolved oxygen level in said zone of less than one part per million. By such operation, considerable savings in power requirements for oxygen transfer to the mixed liquid are had, while developing and maintaining a dense non-bulking biomass having the properties of good removal of phos­ phate from the waste-water influent. By operating the system at high influent throughput rate (F/M), external oxygen requirement for BOD removal is minimized.

Description: BIOLOGICAL WASTEWATER TREATING SYSTEM TECHNICAL FIELD This invention relates generally to improvements in the treatment of municipal sewage and/or industrial wastewater by the activated sludge process. It is particularly concerned with the control of operating conditions to enhance selective production and maintenance in the system of active biomass essentially free of filamentous growth, such that the attained sludge has favorable settling characteristics with the capability of substantial removal of phosphate values from the incoming wastewater. Under the selected operating conditions of the system according to the invention, the stated desired objectives are attained at lower consumption of oxygen per unit of BOD removal than heretofore required. BACKGROUND OF THE PRIOR ART The prior art pertinent to the instant invention is discussed at length in U.S. Patent 4,056,465 issued to one of the present inventors. The present invention is directed to improvements over the system disclosed in said patent. The selective production of a biomass species capable of removing phosphate values and producing a non-bulking sludge of rapidly settling characteristics is attained in accordance with said prior patent by strictly maintaining anaerobic conditions during an initial operating stage where incoming wastewater and recycled sludge from secondary clarification are mixed. Under these conditions the proliferation of undesired high surface area microorganisms is avoided while substantial quantities of BOD are sorbed from the influent wastewater by organisms having the capability of doing so under anaerobic conditions. The initial anaerobic zone, as described in the patent, may then be followed by an oxygenated aerobic zone where the food initially sorbed in the anaerobic zone is oxidized and any remaining BOD is sorbed and oxidized. During this aerobic stage the energy previously lost by hydrolysis of polyphosphates is recouped and polyphosphates are reformed and stored within the aerated biomass, thus removing phosphate from the mixed liquor. If denitrification of the wastewater is also desired, the patent indicates that an anoxic zone may be interposed between the anaerobic and the oxygenated aerobic zone. The term "anaerobic" is defined in the aforesaid patent "as the state existing within a sewage treating zone which is substantially free of NO, (i.e. less than 0.3 ppm and preferably less than 0.2 ppm expressed as elemental nitrogen) wherein conditions are maintained such that the dissolved oxygen concentration (DO) is less than 0.7 ppm and preferably less than 0.4 ppm". The term "anoxic" is defined in the aforesaid patent "as the condition existing within a sewage treating zone wherein BOD is metabolized by nitrates and/or nitrites in initial total concentrations higher than about 0.5 ppm expressed as nitrogen, and dissolved oxygen is less than 0.7 ppm, preferably at less than 0.4 ppm". As further described in the patent, in order to assure adequate oxygen presence in the aerobic oxygen ated zone to effect desired metabolism of BOD and the desired phosphate uptake, the dissolved oxygen content (DQ) of that zone should be maintained above 1 ppm and preferably above 2 ppm. In the several operating examples of the patent the average DO employed in the total aerobic zone is close to or above 6 ppm. Systems of the type disclosed in Figure 1 of U.S. Patent 4,056,465 having an initial anaerobic zone followed by an oxidation zone are sometimes denominated systems. ,stems. Systems of the type illustrated in Figure 2 of said patent, having an anoxic zone intermediate the anaerobic and oxidation zones are referred to as "A/A/O" or "A2/O"TM systems. BRIEF SUM!#ARY OF THE INVENTION It has now been found that substantial savings in operating costs can be achieved in accordance with the system and operating mode of the present invention, with effective BOD removal while obtaining dense sludge of good settling characteristics and desired high to adequate removal of phosphate from the wastewater influent. These cost savings result chiefly from lower oxygen consumption and lowered power requirements for oxygen mass transfer than in either (1) the conventional wholly aerobic activated sludge systems of the prior art using air or more concentrated oxygen gas, or (2) other disclosed systems employing one or more anaerobic stages in conjunction with one or more aerobic stages, as in u.s. Patent 4,056,465. The operation of the present system differs from those previously known, in that the instant system is not bound by a minimum dissolved oxygen concentration in the oxidation zone, whereas the prior art required a dissolved oxygen concentration or NOX equivalent of at least 1 ppm and preferably substantially greater than 2 ppm. In operation of the system in accordance with the present invention, the influent wastewater and recycled sludge are initially mixed in a BOD sorption zone to effect transfer of soluble BOD from the aqueous phase of the mixed liquor to the solid sludge. Following sorption, the mixed liquor is passed to a BOD oxidation zone where the food sorbed in the initial zone is oxidized and additional BOD may be sorbed and oxidized. In the initial BOD sorption zone conditions are so maintained that at least 25% and preferably at least 50% of the soluble BOD5 content of the influent wastewater is transferred from the aqueous phase of the mixed liquor to the solid sludge. In the initial sorption zone conditions are controlled to avoid excessive oxidation of BOD in that zone particularly during the period from startup until substantially steady state operation is achieved. Once the system has achieved "steady state a higher percentage of oxidation can be permitted to occur in the sorption zone without suffering rapid washout of the desired species of biomass generated. The biological stress which results in the selection of preferred biomass occurs in the BOD sorption zone. In the BOD oxidation zone which follows, energy is generated by the metabolization of BOD as a result of oxidation, which energy is utilized in the growth of biomass and removal of phosphate values from the bulk liquor to the interior of the biomass. The preferred species of biomass selectively generated and propogated under the initial conditions maintained in the BOD sorption zone according to the present invention, display a characteristic which is not ordinarily found in most conventional activated sludge systems. It has been observed that the rates of oxygen uptake are relatively slow in the BOD oxidation zone of the present process as compared to conventional activated sludge systems. Because of the slow oxygen uptake, the system of the present invention can be operated as a high rate system with minimum consumption per unit of BOD removed. BRIEF DESCRIPTION OF THE DRAWINGS The single figure of the accompanying drawing is a schematic and diagrammatic side view of a simplified system for practice of the invention. DETAILED DESCRIPTION OF THE INVENTION Referring to the accompanying drawing, a modified activated sludge treating facility is represented, in many respects similar to that depicted in Figure 1 of U.S. Patent 4,056,465. The wastewater to be treated, generally but not necessarily as clarified wastewater from a primary sedimentation tank or clarifier (not shown), initially enters the BOD sorption zone A through the inlet 11. In the sorption zone A the influent wastewater is admixed with recycled sludge settled in sedimentation tank or secondary clarifier 12 and recycled to zone A by line 13. A minor portion of the settled sludge is removed by line 14. The purified supernatant liquid is sent via line 15 to receiving streams or reservoirs with or without further treatment as need be. As shown, zone A is preferably partitioned to provide two or more liquid treating sections in order to afford plug flow of the liquid through the BOD sorption zone A. It has been found that by provision of physically partitioned sections or the hydraulic equivalent thereof, there is better assurance of achieving the desired freedom from filamentous growth and thereby attaining good sludge characteristics even under adverse conditions. Such adverse conditions, for example, include operation with low concentrations of BOD wherein high surface area biomass would have an advantage in competing for sorption of BOD at low concentration. Bypassing of untreated BOD through the BOD sorption zone is minimized. In the particular embodiment illustrated, zone A is shown as partitioned into two sections or chambers 16 and 17, each equipped with stirring means 19. The liquid passes in proximate plug flow through the several sections of zone A and is discharged into BOD oxidation zone B. While zone A is shown as having two partitioned sections 16 and 17, it will be understood that three or more such sections may be employed. Zone A and B may be separate interconnected vessels provided with suitable means for effecting substantial uni-directional flow of liquid from zone A to zone B with minimal back mixing. Aeration of the liquid is effected in zone B in known manner; thus, as shown, compressed air may be admitted into the bottom of the oxidation zone by spargers 20. If desired, instead of or in addition to spargers, the oxygenated zone may be provided with mechanical aerators. Also, instead of air, oxygen of any desired purity may be admitted to zone B, in which event suitable means for covering all or part of the zone may be required. In practice, some oxidation, preferably as up to no more than about 1% of the total BOD5 in the influent may occur in zone A, but normally substantially all the oxidation occurs in zone B. As illustrated in the drawing, zone B is partitioned into two liquid treating sections 26 and 27, although, as will be understood, a larger number of such sections may be employed if so desired. One of the reasons for staging in zone B is because phosphate uptake is observed to be of first order relation with respect to soluble phosphate concentration; thus the low value of phosphate in the effluent is best obtained with plug flow configuration. Zone A is herein designated a BOD sorption zone of a wastewater treatment plant. The term "BOD sorption zone" with respect to the described system of the present invention has reference to and is defined as that zone of a wastewater treatment plant in which the influent wastewater and recycled sludge are initially mixed and in which at least 25% and preferably at least 50% of the soluble BOD5 content of the influent wastewater is transferred from the aqueous phase of the mixed liquor to the solid sludge. The term "soluble BOD5" refers to biological oxygen demand which passes through a 1.25 micron glassfiber filter, exclusive of oxygen needed for oxidation of nitrogen values. To obtain the stated extent of transfer of the soluble BOD from the aqueous phase to the solid sludge, it is important that the following conditions be had. 1. The F/M ratio in the sorption zone, as hereinafter described, is maintained at less than 10 and preferably at less than 5. F is the weight of the total BOD5 introduced by the influent wastewater per day and > is the weight of biomass measured as mixed liquor volatile suspended solids, i.e. MLVSS in the BOD sorption zone. 2. During the initial operation from startup and until substantially steady state operation is achieved, it is essential that less than 2% and preferably less than 1% of the influent total BOD5 be oxidized, whether by oxygen or other oxidizing agents present in the BOD sorption zone. 3. After steady state operation is attained, good performance can be had with a somewhat higher extent of oxidation effected in the BOD sorption zone. Even in such event it is best that less than 5% and preferably less than 3% of the influent total BOD, be oxidized with oxygen and/or. other oxidizing agents in the sorption zone. As above set out, the principal oxidation of BOD present in the influent wastewater takes pla'ce in oxidation zone B. The term "oxidation zone" as employed with reference to the present system is defined as that zone of a wastewater treatment plant in which means for oxygen mass transfer are employed and the mixed liquor from the BOD sorption zone is contacted with oxygen and/or oxidizing agents under conditions and for a time sufficient to oxidize at least 30 percent of the total BOD5 that was present in the initial wastewater influent. As has already been indicated above, it is important that oxidation be limited in the BOD sorption zone even during steady state operation, and during the startup phase of the present process care be taken and conditions maintained so that no more than 2% and preferably less than 1% of the total BOD5 be satisfied by reaction with either oxygen or other oxidizing agents (such as nitrite and/or nitrate--NOx--) in the BOD sorption zone. To assure that the extent of oxidation in the BOD sorpticn zone is below or not greater than the indicated maxima, one or more of the following steps may be taken: A. The vessel (or vessels) constituting zone A may be provided with a blanket of nitrogen or other inert gas at the liquid surface to avoid access thereto of atmospheric air; or a loose fitting cover my be provided at or above the liquid surface, or a rigid cover may be provided above the liquid surface with or without an inert gas blanket. Instead of or in addition to these indicated ways of limiting the extent of oxidation, if any, that could take place in the BOD sorption zone, nitrogen purge gas may be admitted into zone A. B. Means for gaseous mass transfer are excluded from the BOD sorption zone. The zone is equipped with stirrers, as illustrated, for example, by reference 19 in the drawing, as opposed to spargers, surface aerators or other gas-liquid mass transfer devices. 6. Care must be taken to avoid introduction of excessive quantities of any oxidizing agent, such as nitrate and/or nitrite (NOx) into the zxn sorption zone. The latter involves control of not only NO that might be present in the wastewater influent but also NOx that might be recycled to that zone from a downstream source in the system. Wastewater normally contains little or no NOx in the influent due to reduction of any nitrates and,'or nitrites by BOD in the presence of microorganisms in the sewer lines feeding the treatment plant. A potential source of NOx is from recycled mixed liquor from the BOD oxidation zone of nitrifying biological systems, i.e. those which are designed to effect oxidation of ammoniacal BOD to NOx. In such systems, as illustrated x in Figure 2 of U.S. Patent 4,056,465, wherein an anoxic zone is interposed between the initial anaerobic treating zone and the oxygenated aerobic zone, a portion of the mixed liquid from the aerobic zone is recycled to the intermediate anoxic zone to effect reduction of NOx therein. The present invention can be applied to systems including an anoxic zone positioned between the BOD sorption zone (A) and the oxidation zone (B). For the purpose of calculating extent of BOD oxidation in this invention, oxidation of BOD effected in the anoxic zone, shall be counted as though it had occurred in the oxidation zone. In such nitrifying systems, however, the quantity of NOX admitted to the BOD sorption zone needs to be controlled by avoiding recycle of mixed liquor from the aerobic zone of such system to the BOD sorption zone and by also controlling the NO; content in the recycled sludge from the clarifier underflow. The NO concentration in the sludge recycle can be x controlled by providing sufficient residence time in the clarifier and the sludge recycle line to permit adequate removal of NOx by reduction to elemental nitrogen through reactive contact with the biomass present in the sludge recycle liquor. Conventional biological wastewater treating systems have not been able to produce dense, active filamentousfree sludge and also to remove substantial phosphorous values without the addition of chemicals. This indicates that the biology of the process of the present invention does not occur naturally but is the result of the stress applied in order to produce the steady state biology of the desired characteristics indicated. Startup of the system of the present invention requires that a stringent biological stress be applied in order that the preferred organisms can outgrow conventional ones. This is accomplished by operation of the BOD sorption zone with minimal presence of oxidizing agents in order to maximize the applied stress. In this case, maximum stress is effected by maximum exclusion of oxygen and other oxidizing agents from the initial BOD sorption zone. Thus, those organisms which can utilize a nonoxidative source of energy, i.e. hydrolysis of polyphosphates, have an advantage in that only they have the energy required to effect active transport of BOD from the bulk liquor across the microorganism cell wall to the interior of the organism. To the extent that the energy for BOD transport is provided in this fashion, polyphosphate containing microorganisms have an advantage in sorting BOD, (or food supply) which permits these organisms to dominate the population upon successive cycles through the system. This is not true in conventional activated sludge systems. During initial operation in application of the process of the present invention, from startup and until steady state operation is attained, as is indicated above, care must be taken to minimize the extent of oxidation of BOD values in the BOD sorption zone. In practice the maximum permitted content of oxygen and/or otter oxidizing agents, such as nitrate and/or nitrite, expressed as oxygen equivalent, should be such that less than 2% and preferably less than 1% of the tota BOD5 contained in influent wastewater is oxidized in that zone. When these precautions are observed during startup and initial operation of the system, both good sludge properties and increasing phosphate removal are evidenced within from two to six weeks. Attainment of steady state operation with these desired properties can be accelerated by addition of "seed sludge" contain ing polyphosphate values. The operability of the system of the invention is dependent upon the initial presence of an effective BOD sorption zone. However, the biology of the present system is functional even in the event of excessive oxidation in the sorption zone until the desired active sludge species becomes displaced by conventional sludge. Such displacement has been observed to take place relatively slowly, as in a period of about one to two months. Once the system has achieved "steady state", a somewhat higher percentage of oxidation may be permitted to occur in the BOD sorption zone without suffering rapid washout of polyphosphate-containing sludge. Washout is detected by the deterioration in the settling properties of the sludge and reduced ability to remove phosphate. If an upset does occur, normal operation can be restored in time by strictly limiting the permitted extent of oxidation in the BOD sorption zone in the same manner as during an initial startup. While, as explained above, the biological stress which results in the selection of a preferred biomass occurs in the BOD sorption zone, the function of the BOD oxidation zone (B) is to generate energy by oxidation of BOD This energy is used for growth of biomass and transfer of phosphate values from the bulk liquor to the interior of the biomass. Removal of phosphate and storage in the biomass as polyphosphates is estimated to;require from about 1 to 5% of the energy generated by oxidation of the BOD. It is therefore believed that the preferred biomass of the present process does not occur in most conventional activated sludge systems, since the species which do not have the energy requirement for phosphate sorption and storage are able to use more energy for growth and will therefore predominate, unless the BOD is apportioned in a sorption zone as described in accordance with the present invention. It has also been observed that the rates of oxygen uptake are relatively slow in the BOD oxidation zone of the present process as compared to conventional activated sludge systems. For instance, an oxygen uptake rate of 30 milligrams of oxygen per gram of VSS per hour at 200C is rarely exceeded in the practice of the present invention, whereas in conventional aeration, the oxygen uptake rate can be over twice as great, as described, for example, in U.S. Patent 3,864,246. This indicates that the BOD sorbed in the sorption zone is stored in a form which is only slowly available for oxidation. The result is that the initial oxygen uptake rate is slow and further decreases slowly with time. Because of the foregoing observations, the present system when operated as a high rate system, requires minimal oxygen per unit of BOD removed Unoxidized BOD, of course, is wasted with the sludge. However, sufficient oxidation must occur to oxidize at least 30% of the total BOD5 fed to the system. If BOD is not oxidized to a sufficient extent, the sorption of fresh BOD upon recycle of the clarifier underflow to the initial sorption zone, is inhibited. This has an adverse effect, in that upon repeated cycles, progress ively lesser amounts of influent BOD is sorbed in the initial zone and progressively larger amounts of unsorbed BOD is transferred to the oxidation zone where it is sorbed and metabolized by conventional microorganisms. Under such conditions, the polyphosphate accumulating miçroorganisms eventually wash out. It has been found that the minimum oxidation required for the system is 30% of the total BOD5 in the influent. In a high rate system, oxygen consumption varies from 30 to 100% of the BOD5 in the influent. A high rate system is defined in terms of overall F/M, as one having an F/M overall ratio greater than or equal to about 0.3. F is the weight of total BOD5 introduced by the wastewater influent per day and M is the weight of MLVSS contained in all zones of the present system, including BOD sorption zone, BOD oxidation zone and anoxic zone, if present. with an F/M overall of less than about 0.3 and down to about 0.08, oxygen consumption is from about 80 to 150% of the total BOD5 content of the influent. Oxygen requirements in excess of 100% are attributed to the fact that BOD5 expresses only about two-thirds of BOD (infinite). It has been observed that when operating high rate systems, i.e. at an overall F/M in excess of 0.3, the present system utilizes substantially less oxygen than that of conventional activated sludge systems. Moreover, even at these high throughput rates, the present system continues to remove BOD and substantial quantities of phosphates while maintaining a desirable non-filamentous biomass species producing sludge having excellent settling properties. The obtained sludge has an SVI generally less than about 100 and a clarifier underflow having a solids concentration of greater than about 1%. To maintain these desired characteristics of the system, conditions must be controlled such that at least about 30 to 40% of the total BOD5 is oxidized in the system. Failure to do so results in progressive worsening of sludge properties and lower phosphate removal. An important consequence of causing the biological selection to occur in the BOD sorption zone is that in doing so there is no longer a requirement for a minimal dissolved oxygen concentration (DO) to be maintained in the oxidation zone This is in contrast to prior art teachings (as found for example in U.S. Patents 3,864,246 and 4,162,153) wherein minimal DO levels of at least 1 ppm and generally more than 2 ppm of dissolved oxygen are required. The maintenance of the high dissolved oxygen levels is manifested in higher power requirements. For instance, with a system employing atmospheric air oxidation, by reduction of the DO level from 2 ppm to a level of less than 1 ppm, the energy requirement for a given quantity of oxygen transfer from the gas to the liquid phase of the system is reduced from about 14 to 25%. In the present system, the dissolved oxygen level is not the controlling factor, but rather the percent of total BOD oxidized in the respective zones. The startup period of the present system is defined as the time required for the system to develop biomass of the desired type as evidenced by the steady state BOD in phosphate removal and sludge settling properties. It can also be defined as the time required for the system to recover from operational upsets. The startup period can be substantially reduced by seeding a system with sludge previously conditioned in an operational system of the present type. The extent of time reduction to attain steady state will be appreciated by comparison of the operations of Example 1A and lB, the results of which are respectively tabulated in Tables 1 and 2. Example 1A A laboratory unit having 5 equal stages of 1.6 literseach in the BOD sorption zone (A) and also in the downstream oxidation zone (B), was operated with wastewater from the Allentown, Pennsylvania sewage plant primary clarifier effluent. Data on the perfor mapce of the unit from the fourth week after startup is reported in Table 1, as weekly averages. The DO in the BOD sorption zone was maintained from the initial startup at less than 1 ppm and that in the oxidation zone at about 7 ppm. Substantial removal of phosphate was evident toward the end of the two-month startup period. Net removal of phosphate was 5 ppm with 59% of the soluble BOD5 sorption occuring in the sorption zone. Sludge properties showed steady improvement throughout the operating period. In week 49 the configuration of the unit was modified to provide 5 equal stages in the BOD sorption zone of 1.2 liters each and 5 equal stages in the oxidation zone of 1.6 liters each. Table 1 Startup Without Seeding week week week week week week Period 4 5 6 7 8 9 IDT (hr) 2.0 2.1 4.1 4.2 4.4 3.9 MLVSS, ppm 2139 2853 3751 3711 4291 4393 Zone A DO, ppm 0.7 0.7 0.5 0.3 0.1 0.2 Zone B DO, ppm 7.0 6.8 7.8 8.0 7.2 7.4 F/M Overall .87 .61 .21 .20 .14 .18 BODs/P5 16.8 28.9 17.3 13.0 11.0 14.6 BOD , ppm Infl. 98 111 107 84 88 112 ¯Effl. 10 7.1 4.6 4.7 9.9 3.6 TSS, ppm Infl. 99 81 63 98 76 64 Effl. 51 48 50 37 50 53 BOD ppm Infl. 148 152 131 131 105 129 Effl. 34 29 19 15 20 18 P, ppm s Infl. 5.8 3.8 6.2 6.5 8.0 7.7 Effl. 4.3 3.6 4.3 4.7 4.8 2.7 Net removed 1.5 0.2 1.9 1.8 3.2 5.0 SVI, ml/g VSS 85 73 53 70 65 55 SVI = Sludge Volume Index (Mohlmann) BODT= Total BOD5 BOD = Soluble BOD5 P5 = Soluble phosphate expressed as elemental phosphorous, P T$S = Total Suspended Solids Example 1B A pilot plant having a BOD sorption zone, A, consisting of three 58 gallon stages and a BOD oxidation zone, B, consisting of four 147 gallon stages, was initially seeded with 7.5 gallons of liquor containing about 1% of a phosphate-removing sludge, obtained from steady state operation of a laboratory A/O unit. Three weeks after addition of the seed sludge, stable operation for phosphate removal was attained. Incremental soluble phosphate removal was 4.4 ppm and BOD, sorption in zone A was 39% of the influent BOD5. A summary of the operation is reported in Table 2. TABLE 2 Conditioning by Seeding PERIOD WEEK 1 WEEK 2 WEEK 3 IDT, (hr) 2.12 2.12 2.12 MLVSS Yig/L 2815 2796 2708 DO, ppm A zone 0.2 0.2 0.2 B zone 4.0 4.0 4.0 F/M Overall 0.48 0.48 0.66 A 2.11 2.08 2.90 BOD , ppm % sorption in A 22 33 39 TSS, ppm Infl. 138 123 103 Effl. 32 24 21 BOSS, ppm Infl. 125 117 156 Effl. 25 24 25 Pst ppm Infl. 5.4 5.9 5.7 Effl. 4.8 2.3 1.3 Net Removed 0.6 3.6 4.4 SVI, ml/g VSS 35 42 29 Results obtained in the runs made during operation of the system as described in Example 2, showed that DO alone is not paramount in effecting removal of phosphate from wastewater as long as a sufficient BOD is oxidized in the oxidation zone (B). Accordingly, it follows that aeration power can be reduced by operating the oxidation zone at DO levels below 1 ppm. Example 2 The pilot plant unit was the same as that operated in Example 1B. It was operated for approximately two weeks at a high DO level in the oxidation zone (10 ppm). At another time it was operated for one week ct low DO level (0.27 ppm). During the low DO operating period all of the stages in the oxidation zone (B) were maintained at a level below 1 ppm. In both periods no attempt was made to control available oxygen, and oxygen was abundantly available in the oxidation zone as indicated by the relatively high oxygen consumption in excess of 1.5 pounds oxygen used per pound of BOD removed (unfiltered influent minus filtered effluent), i.e. lb 02/lb BODR(U-F). The high DO operation showed a better phosphate removal than that at low DO, because of the higher overall F/M and higher BODIs/P ratio. The data recorded in operation of Example 2 is reported in Table 3. TABLE 3 IDT (hr) 2.10 1.59 T,OC 24 22.5 Avg. MLVSS, ppm 2600 2683 Avg. DO, ppm A zone 0.2 0.2 B zone 10 0.27 F/M overall 0.74 0.63 A zone 3 26 2.77 BOD5/P5 14 11.2 BOD , ppm Infl. 76 55 s Infl. 76 55 Effl. 2.5 2 TABLE 3 (Cont'd.) % eorbed in A 31 57 TSS, ppm Infl. 124 104 Effl. 22 21 BODT,ppm Infl. 153 112 Effl. 9.2 9.5 P , ppm Infl. 5.8 4.9 Effl 1.0 1.9 Net removed 4.8 3.0 SVI 19 23 It is apparent from Example 2 operation that a substantial power for oxygen transfer in the oxidation zone can be saved. Thus, it is calculated that a saving of 23% in power requirements could be realized by operating the oxidation zone (B) at a DO level of 0.5 ppm, for example, as compared to operation at 10 ppm. This calculation is based on the use of relatively pure oxygen to the system as determined by the solubility of oxygen in water at atmospheric pressure. It is further evident from the Example 2 results that phosphate removal in a system according to the invention is not greatly affected by whether high or low DO exists in the oxidation zone. It therefore appears that satisfactory performance of an A/O system can be had during a period of stable operation when the DO in the oxidizing zone is at a level below 1 ppm, provided that no more than 2% of the influent total BOD5 is oxidized in the BOD sorption zone and that no less than 30% of the influent total BOD5 is oxidized in the overall system. The following Example 3 shows that even though the desired species of phosphate-removing microorganisms are added to a conventional activated sludge system employing oxygenation in the initial treating stage, these initial predominant microorganisms become washed out and the system loses its capacity for phosphate removal. These results indicate the importance of maintaining the required biological stress in the BOD sorption zone of an A/O system. In this case, the BOD sorption zone was absent. Example 3 A system comprising 5 equal oxidation stages of 1.2 volumes was fully seeded with an activated phosphateremoving sludge obtained from an operating A/O unit. Performance for the subsequent four weeks after seeding is reported in Table 4. At the end of the fourth week the net phosphate removal deteriorated from an initial 3.8 ppm to 2.3 ppm. The sludge properties also deteriorated as indicated by an increase in SVI from 54 to 80. The configuration of the system was modified so that during the third and fourth week of operation there was an initial oxidation stage of 0.4 volumes followed by four equal oxidation stages of 1.4 volumes each. TABLE 4 Waskout in the absence of a BOD sorption zone Week Week Week Week 1 2 3 4 IDT (hr) 2 2 2 2 MLVSS, ppm 5566 4114 3520 2640 DO, ppm 6 6 5 6 F/M, overall .37 .66 .86 .61 BOD5/P5 36.7 37.8 21.2 15.1 BOD5 Pp1nf1.ppm 147 198 101 nfl, 189 147 198 101 Effl. 1.7 1.3 2.1 1.6 TSS, ppm Infl. 32.6 64 61 29 Effl. 18 22 16 13 BOD, ppm lnfl. 174 178 239 125 Effl. 7 7 6 4 P ppm Infl. 5.2 3.9 9.4 6.7 Effl. 1.4 0.3 3.4 4.4 Net removed 3.8 3.6 6.0 2.3 % removed 73 92 64 34 SVI 54 73 98 80 lb. d2/lb. BOD 1.2 1.1 0.83 1.1 (U-F) As has been indicated above even though a stable operation has been achieved and biomass of the desired phosphate removing properties developed, it is important that the BOD sorption zone be maintained under conditions limiting oxidation in that zone. Thus includes provision of adequate, time in the secondary clarifier and the recycle sludge transfer line to avoid the presence of substantial quantities of NO in the sludge recycled to x the BOD sorption zone. Example 4 A series of runs were carried out during a period of about six months to determine the effect of F/M overall on oxygen consumption and on net removal of phosphorus values from municipal wastewater in an A/O unit. The stable A/O unit was fed with Rochester, NY, wastewater during the period having the indicated configurations shown in Table 5. As can be seen by plotting the data reported in Table 5 the consumption of oxygen was reduced with increase in F/M overall while net removal of phosphorus values was highest in the F/M overall range of about 0.3 to about 0.8 In all of the above, the percent of influent BOD oxidized in the sorption zone was less than one percent. TABLE 5 lb O, Net P5 lb BODR BOD, Removed Week F/M Overall (U-F) P5 ppm Configuration 1 .61 0.59 8.22 1.8 2 1.32 0.79 15.1 1.5 3 0.81 0.84 9.3 1.6 4 0.40 1.38 10.3 1.3 (I) 5 0.85 0.41 12.1 2.1 6 1.04 0.66 11.6 2.5 7 0.39 1.12 11.5 2.6 8 0.82 0.69 12.2 3.8 ¯¯¯¯¯¯¯¯¯¯ 9 0.61 0.97 13.8 4.0 10 0.75 0.67 13.1 4.6 11 .81 0.70 15.8 3.8 12 .44 1.26 14.6 4.6 (Il) 13 .85 .89 22 2.7 14 .82 .63 23 4.2 15 .43 1.65 15.7 3.0 16 .89 .72 21.7 2.3 ¯¯¯¯¯¯¯¯¯¯ TABLE 5 (Cont'd.) Ib O, Net P5 lb BOD, X Removed Week F/M Overall (U-F) P5 ppm Configuration 17 .39 1.73 18.9 3.2 18 .38 1.27 16.5 3.0 19 .32 1.31 17.9 2.7 20 .24 .85 7.6 1.3 (III-) 21 .31 1.12 11.3 3.6 22 .49 .80 14.7 4.6 23 .12 1.34 1.92 .89 24 .21 .78 2.06 .72 ¯¯¯¯¯¯¯¯¯¯ (I) Anaerobic zone of three stages, 1.2 volumes each. Oxidation zone of five stages, 1.5 volumes each. (II) Anaerobic zone of three stages, 1.8 volumes each. Oxidation zone of five stages, 2.4 volumes each. (III) Anaerobic zone of three stages, 1.4 volumes each. Oxidation zone of five stages, 2.4 volumes each. EXAMPLE 5 The following is a preferred operation according to the invention illustrating excellent operation of an A/O system with the DO level maintained in the oxidation zone at 0.3 ppm, with consequent large savings in required aeration power. The pilot plant comprised a BOD sorbtion zone having three sections of 58 gallons each and an oxidation zone of 4 sections of 147 gallons each. The operating conditions and results are reported in Table 6. TABLE 6 Influent detention time (hrs) 1.6 Sludge Recycle, ratio to influent (vol/vol) 0.20 MLVSS avg. ppm 2700 Temperature, OC 23 TABLE 6 (Cont'd) F/lYl overall 0.63 BOD total, ppm Infl. 112 Effl. 9.5 BOD soluble, ppm Infl. 55 Effl. 2 Total suspended solids, ppm Infl. 104 Effl. 21 Soluble phosphates, ppm (influent 4.9 (effluent 1.9 BOD5/P5 in influent 11 % sorption BOD5 soluble in BOD sorption zone 57 DO avg. in BOD sorbtion zone, ppm 0.2 DO avg. in oxidation zone, ppm 0.3 From the above Table 6 it is apparent that an A/O system can perform satisfactorily when operating with relatively low DO in the oxidation zone. In such low DO operation, considerable savings can be had in the power requirements for oxygen transfer from the air to the liquid (at the same oxygen consumption). The calculated savings that can be achieved at the lower DO levels as compared to an operation at DO of 3 will be appreciated from the tabulation below calculated on the basis of using air and assuming that the oxygen saturation level in the liquid is at a concentration of 8 ppm. D.O. in BOD Oxidation Zone, ppm 0.3 1.0 2.0 3.0 Power saving, % 65 29 17 0 While the invention has been described in connection with systems wherein the initial BOD sorption zone is followed by an oxidation zone (A/O system), it is equally applicable to systems designed for NOx removal wherein an anoxic zone is provided between the BOD sorption zone and the oxidation zone. It should be noted that less than 1% of the total influent BOD5 was oxidized in the sorption zone.

Claims: What is Claimed. 1. In the operation of an activated sludge system, the method which comprises: a) producing a mixed liquor by initially mixing activated biomass with BOD-containing wastewater influent in a BOD sorption zone; said mixing being effected under selected conditions such that less than 5% of the total BODS is oxidized by oxygen or other oxidizing agents and that at least 25% of the soluble BOD5 is sorbed by the biomass in said sorption zone; b) in a subsequent oxidation zone oxidizing BOD contained in the mixed liquor, including at least part of the BOD sorbed in said biomass, under controlled aeration conditions such that at least 30% of the total influent BOD5 is oxidized, said conditions including a dissolved oxygen content in said oxidation zone of less than one ppm; ; c) settling the thus oxidized mixed liquor so as to separate the supernatant liquor from the more dense sludge including biomass; and d) recycling at least a portion of said more dense sludge to provide activiated biomass in said initial BOD sorption zone. 2. The method as defined in Claim 1 wherein at least 50% of the soluble BOD5 of said influent wastewater is sorbed by said biomass in said sorption zone. 3. The method as defined in Claim 1 wherein said BOD sorption zone comprises a series of at least two hydraulically distinct consecutive stages. 4. The method as defined in Claim 1 wherein said oxidation zone comprises a series of at least two hydraulically distinct consecutive stages. 5. The method as defined in Claim 1 wherein said initial BOD sorption zone is maintained under anaerobic conditions such that less than 2% of the influent total BOD5 is oxidized in said zone. 6. The method as defined in Claim 1 wherein said wastewater influent contains phosphate values and wherein a major portion of said phosphate is removed from solution in the oxidation zone liquor and stored as polyphosphate in the biomass. 7. The method as defined in Claim 1 wherein the rate of introduction of the wastewater influent is so related to the total biomass in said sorption zone and said oxidation zone to provide an FFI overall in excess of 0.3, wherein F is the weight of the total BOD5 introduced by the wastewater influent per day and M is the weight of biomass volatile suspended solids contained in said BOD sorption zone and said oxidation zone. 8. The method as defined in Claim 1 wherein said system comprises an anoxic zone intermediate said BOD sorption zone and said oxidation zone and wherein the rate of introduction of the wastewater influent is so related to the total biomass contained in said BOD sorption zone, said anoxic zone and said oxidation zone, to provide an overall F/M ratio in excess of 0.3 wherein F is the weight of the total BOD5 introduced by the wastewater influent per day and M is the weight of biomass, the volatile suspended solids, contained in the BOD sorption zone, the anoxic zone and the oxidation zone. 9. The method as defined in Claim 1 wherein conditions in said BOD sorption zone are controlled to provide an F/M ratio in said zone of less than 10, wherein F is total BOD5 introduced by the wastewater influent per day and M is the weight of biomass volatile suspended solids contained in said BOD sorption zone. 10. The method as defined in Claim 9 wherein the F/## ratio in said BOD sorption zone is maintained at less than 5. 11. The method as defined in Claim 1 wherein said BOD sorption zone is operated under conditions such that less than 1% of the total influent BOD, is oxidized by oxygen or other oxidizing agents in said zone. 12. The method as defined in Claim 1 wherein at least part of the biomass in said system is obtained by seeding with biomass obtained from a system which has achieved steady state operation under conditions as defined in Claim 1.

Inventors: CHEN, MICHAEL S.K., SPECTOR, MARSHALL L.
Applicants: AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1982
Main classification: C02F3
IPC codes: C02F3