SEWAGE TR2B&TW2IPf PLANT CEPKKABILITI WitU Special Reference to the ACTIVATED SLUDGE PROCESS by A. W. West, P.E., Chief Sewage Treatment Plant Operation & Design Branch ENVIROKMEinAL PROTECTION AGESGY OFFICE OF ENFORCEMENT & GEITEPAL CCUIISSL CINCIHNATI FIELD IliVES P1GATIOIK CEN2ER IKClNylATj, OHIO OCTOBER 1971 ------- ------- TAF-L3 OF 0 PAGE NO. I INTRODUCTION AND CONCLUSIONS 1 II DESIGN CONSIDERATIONS 1 SELECT PROPER PROCESS TYPES 2 PROVIDE GENEROUS CAPACITY 5 INCLUDE ESSENTIAL FLEXIBILITY 7 Process 7 Aeration Tanks 7 Final Clarifiers 8 Return Sludge Pumps 9 Excess Sludge Wasting 9 Emergency Chemical Treatment 10 Adjunct Facilitie s. . -. 10 MAKE PLANT TRULY CONTROLLABLE 12 Meters 12 Meter-Control Panel lA- Automatic Controllers 1^ III OPERATIONAL CONTROL PERSONNEL 16 DUTY DELEGATION 17 CONTROL TESTS 17 PROCESS RELIABILITY 18 ------- With Special .;cr^.-e ice to the ACTIVATED SLUDGE PROCESS INTRODUCTION & CONCLUSION Operational experience at dozens of sewage treatment plants has shown that, to achieve dependable, consistently satisfactory, muni- cipal and industrial vacte treatment plant performance, the owners should: Retain the best possible consultants for design. Hire the best trained, intelligent, dedicated, imaginative Superintendent and Operators available. II DESIGN CONSIDERATIONS Though this discussion emphasizes activated sludge treatment, the following four general design concepts have been found essential to plant dependability for practically all types of waste treatment plants. Select the Proper Process Type. Provide Generous Capacity. Include Essential Flexibility. Make Plant Truly Controllable. ------- ------- SELECT T'.W?'7"?. F^.OCFSS TIPS A. Early in the design stage, the engineer must exercise hie best experienced professional judgment in selecting the Process Modification most appropriate to the Known characteristics of the incoming wastes, and the effluent quality requirements. The following illustrations, for example, are drawn from personal plant operation experiences: 1. The "Classic Activated Sludge Process" design - conforming generally to "10-States Standards",usually performs satisfactorily for "normal municipal wastes" where domestic sewage predominates. The "Classic Activated Sludge Process'' is defined as the original activated sludge system; where all return sludge and all settled sewage enters the head end of the aeration tanks. 2. The "Complete Mix" modification has been found admirably suited for mixtures of dom- estic sewage and industrial wastes with highly variable characteristics and concentrations. 3. The "Step Aeration" modification permits an operator to select and change his basic pro- cess cycle to accommodate unexpected overloads, to adjust sludge solids distribution and to control mixed liquor sludge characteristics. To be truly effective; the "Step" design must permit controlled measured incremental sewage discharge to each aerator pass or com- partment. It must also permit discharging 100 percent of the sewage into the head end of the aerator or to the last pass. Such an aera- tor can then be operated in either extreme - Classic Mode or Contact Stabilization Mode - or any place in the middle - according to the "Step" percentages that are selected - to meet actual loading and system demands. ------- ------- Be mr-? to consider "T?rtiarv" or "^Il-V"1 "lU-l::"'' J .--:2i:neht "r.yr^er:s that luay je necessary -co meet spec- ial water quality criteria. This paper does not include discussion of these important features that are covered thoroughly by the Ad- vanced Waste Treatment Research Laboratory in Cincinnati. ------- ------- Whqri in ^-oivot - and vbo isn't at times? - Pilot! The terra ''i-'ilot Studies/' having a broad, general meaning, could include: •'•• ftemh Scale. Waste treatability studies can be performed effectively aad economically at Bench Scale. 2. Pilot Scale. A specially constructed small pilot plant (possibly 0.1 MG-D or more capacity) may be needed if addi- tional essential information is required. Such units can validate process suit- ability, relative tank sizes, and system dynamics for the proposed full size plant. 3. Demonstration Scale. In some cases, full sized plant units must be utilized to demonstrate the relative effectiveness of various facility arrangements. At times certain units in existing plants can be modified for such study before the design of plant additions is initiated. For ex- ample, it is impractical, if not impossible, to prove the suitability of various over- flow weir arrangements for 150 ft. diameter final clarifiern from studies on 10-ft. dia- meter pilot scale models. The type and extent of Pilot Studies will obviously de- pend upon effluent requirements; the specific informa- tion needed; and the size, complexity and cost of the treatment facilities to be constructed. ------- ------- Benign criteria such as, "10-Stateo Standards" and others, should be interpreted ski llfivlly, aid used as intended. In other words, moot suggestions in such Manuals should be considered as minimum, and not maxima, requirement" to achieve plant depend- ability and to provide es',entic,l factors of safety. Generous capacity is always welcomed by plant operators and by plant managers facing the necessity for day-in, day-out, dependable and acceptable plant performance and effluent, quality. Surplu^ capacity, if any, that might be provided will in most cases be used up rapidly as the communities face unexpected rapid population growth and unpredictable development of new waste-producing industries within the area. A. Some design criteria, based on 2k-hour average flow and load, include appropriate diversity factors to accommodate the normal cyclic peaks that occur dur- ing each 24-hour period. Such 24-hour design cap- acities should be based on the averages anticipated during the maximum flow and loading producing week of the year. Obviously, any abnormally high short- term peak load:1, that can be anticipated, must also be included in the design loading. B. Accommodate full load with either one aerator or one clarifier out of service for maintenance. You all know that at times equipment units must be taken out of service for maintenance or repair. Out of dozens of activated sludge plants, I can only recall one or two that did not have a final clarifier down for maintenance at some time dur- ing my stay at the plant. In a four-tank design, for example, this means that clarifiers must be designed so that three of the four can successful- ly handle the estimated total design flow. Simil- arly, aeration capacity must be large enough so that three of the four aerators can handle the total estimated design load. G. Provide multiple aerators and clarifiers; prefer- ably four each for medium to large plants. This is, of course, a corollary to the previously mentioned requirement. ------- ------- Wi-.en aay ono unit is taken out of service. Though discussions of relative aerator to clarifier volumes usually becomes controversial, my experience indicates that properly "balanced operation can "be obtained vhen the total i^rator volume approximates twice the total clariii^r volume. ,-/ixn this relation- ship, for er.a:;>ole_, full plant operation of a four- aerator/fouv-.'j.lc.rifier plaat might require approxi- mately fifty rercont return -:Iudge pumping. With one aerator out o/_' -ervice, the return sludge pump- ing demand rni^hb increase to ICO percent; and con- versely, with oue clarifier out of service, some- thing in the neighborhood of thirty percent return sludge pumping mi ght be required. Other process requirements, that will change when individual units are taken out of service, can usually be accommodated effectively under such circumstances. D. Include the estimated plant recycle (thickener over- flow, filter underflow, dilution flows, etc.) in the design load. In one extreme case, the additional flows imposed by effluent dilution of sludge thick- ener influent, scrubbing water for the furnaces, etc. approximated fifty percent of the incoming sewage flow volume. In most cases, such recycle flows can easily exceed ten percent and possibly approach twenty percent of design flows. These additional flows will influence aeration tank detention times and final clarifier over- flow rates and must be considered in the design of ade- quately sized units. E. Sludge handling facilities must be designed to accom- modate the maximum (not the average) anticipated sludge quantities; and with units down for maintenance. Remem- ber, sludge wasting requirements at activated sludge plants can vary greatly from day-to-day in response to sewage loads, process equilibrium, and mixed liquor sludge quality. At times, extremely high wasting rates, greatly in excess of the anticipated average, will be needed to restore process equilibrium when a system is sliding out of balance. Generous sludge holding-equalizing tank capacity is essential and can reduce substantially the danger of periodic overloading of sludge processing equipment. ------- ------- 7 INCLUDE ESSENTIAL FLEXIBILITY Design considerations that effect, and permit operating flexibility are presented in brief outline form. Though readily understood, and almost universally accepted; omission of one or ------- INCLUDE ESSE3STIAL FLEXIBILITY Design considerations that effect, and permit operating flexibility are presented in brief outline form. Though readily understood, and almost universally accepted, omission of one or more of these elements has been observed in all too many plants. Lack of essential flexibility frustrates operators and degrades effluent quality. Give the operator the tools he needs to modify anticipated control schedules in order to accommodate some of the unforeseen difficulties he will almost certainly face at times. A. Process 1. If the activated sludge process is selected for secondary treat- ment; evaluate the relative ad- vantages of both the "Classic" process and the "Complete Mix" modification with reference to the type and characteristics of the incoming raw wastes, and the plant performance requirements needed to meet final effluent and receiving water quality objectives. 2. Provide for "Step" operation of any "Classic" or conventional activated sludge system. 3. Be sure any "Step" design can be operated in all modes; from Classic, through variable step proportions, and to "Contact Stabilization". k. Where practical, provide each aerator- clarifier combination (or each group of aerators and clarifiers) with its own separate return and waste sludge pumping.facility. B. Aeration Tanks 1. Provide truly effective mixing and oxy- genation. ------- ------- 8 2. Assure arainrt "coreinq;" and compromise by ineffective modifi- cations of inappropriate basic aeration devices. 3. Provide truly controllable, measur- able, variable outputs for mechani- cal aerators pr air blowers. k. Provide separate, controllable, metered air headers for each pass, or compartmented zone of the aera- tion tank. For example, don't con- nect the "A" pass (with its relative- ly high air demand) of one aerator and the "C" pass (with its relative- ly low air demand) of an adjacent aerator to a common air header. 5. Consider provision of recording D.O. meters. (Preferably actuating air blower controllers.) 6. Consider provision of mixed liquor solids concentration sensor and re- corder. (Also similar for return sludge solids concentration.) C. Final Clarifiers 1. Obviously - minimize excessive veloc- ity currents and short-circuiting. 2. Don't skimp on surface area. Contem- porary high capacity designs appear to require considerably less than the con- ventional 800 Gals/Day/Sq. Ft. overflow rate. 3. For large tanks (certainly for 100 feet in diameter and greater) provide appro- priately located and properly spaced multiple effluent weir launders. k. Provide effective surface scum collec- tion and removal devices. ------- ------- 5. Consider 12 feet as a minimum practical side wall depth. (Despite calculable theory - shallow tanks aggrevate floe carry-over characteristics.) 6. Provide combination suction- sciraper sludge collectors to minimize clarifier sludge de- tention time. (Be sure that the actual sludge withdrawal capacity equals at least 100 percent of design waste flow plus recycle.) D. Return Sludge Pumping Facilities Provide multiple, remotely ad- justable, return sludge pumps capable of returning at least 100 percent of design waste plus recycle flow with one pump out of service for maintenance. ^' Excess Sludge Wasting 1. Provide metered, controllable, waste sludge pumps - separate from the return sludge pumps. 2. Be sure they can be operated at the low wasting rates required at times. (A valved interconnec- tion to the return sludge pumping system can accommodate the excep- tionally high wasting demands that occur occasionally.) 3. Provide a suitable valved inter- connection to permit wasting either from the aerator outlet (mixed liq- uor) or from the clarifier sludge withdrawal system (return sludge). ------- ------- 10 F. Emergency Chemical Treatment 1. Provide feeders and piping to per- mit emergency application of chem- icals to aerators or clarifiers ( and primary tanks). 2. For example - application of poly- mers and ferric chloride to the clarifier inlet has solved serious classic bulking; and apparently without destroying other desirable sludge characteristics. G. Adjunct Facilities 1. General - This Section will only highlight general concepts concern- ing certain related plant facilities. 2. Primary Clarifiers - Don't skimp on size and surface overflow rates, es- pecially if excess activated sludge is to be wasted to the primary clari- fiers. 3« Sludge Handling Facilities a) Must be designed to handle maximum (not average) anti- cipated loads,, with units down for maintenance. b) Must be provided with gener- ous storage, or equalizing, tanks to accommodate periodic peak requirements that will at times exceed even estimated maximum loads. c) Then provide means for dis- posing of partially processed sludge during breakdowns. 1) Take a sludge thickening - filtering - burning process, for example: ------- ------- 11 Be sure filtered, sludge can be collected, conveyed out of the building and hauled away in case of serious furnace breakdown. 2) Consider, for example, sludge digestion: - Provide facilities to collect, pump, and haul supernatant and partially digested sludge to prescribed land disposal if di- gestion or drying facilities are seriously overloaded. 3) Plan an "out" so that the secon- dary process and effluent quality need not be degraded by break- down of other plant facilities. k. Equalizing Tanks - In special situations "(separate interceptors collecting slug-flow strong wastes) provide adequate raw waste holding tanks to permit uniform process loading throughout the 24-hour cycle. 5- Holding Ponds - Consider holding ponds for effluent polishing, or for storage and re- cycle of primary effluent, during periodic severe plant overloading. ------- ------- 12 A. Meters svrtern is a controllable process that, rnur.t include appropriate meters and accurately controllable equipment,gates, valves, pumns and blowers for optimum perform- ance. More importantly; it should be developed and run by intelligent competent designers and operators. 1. Obviously - the most reliable, proper type, meters should be specified. 2. Throughout the plant, meters can range from the most simple elementary type to the highly sophisticated system; depend- ing upon the specific output needed. 3. A single subcontractor should supply, and and be fully responsible for satisfactory performance of, the entire meter-controller package. k, A qualified instrument technician should be included on the staff of all large plants. A capable technician, from within the com- munity if possible, should be retained for periodic meter maintenance and emergency repair at smaller plants. 5. Separate independent meters are needed at each plant unit requiring control adjust- ment. A summator, in addition to read-out from individual meters, is helpful for multi- unit plants. But beware of a subtracter as the sole means of obtaining an essential third flow measurement from two other inde- pendent meters. 6. Beware of relying too greatly on so-called hydraulic similitude for balancing flows between multiple units. Individual meters and control gates are usually needed in critical areas. ------- ------- 13 7. When der.i^n contemplates a phased series enougn) to permit accurate measurement of the relatively low initial flow rates. When pipc:3 are sized for future additions, it Tr.ay be necessary to install replaceable meters in temporarily reduced pipe sections. 8. Be certain that metering is adequate to per- mit accurate control adjustments, maintain essential balance in multiple parallel plant units, to document plant performance, and to evaluate process and effluent quality require- ments. a) Measure either plant influent or effluent separately. A back-up sensor and indicator on the other will be useful. Don't depend on mechanical addition of other inter- nal meters for this value. b) Be sure that waters recycled within the plant are metered and can be accounted for. (Thickener influent and effluent, dilution water, fur- nace condenser spray, etc.) c) Provide individual meters for each of the following similar parallel plant features. (i.e. Four meters for influent flow to four parallel aerator s.) l) Return Sludge Flow - To each Aerator. From each Glarifier. 2) Waste Water Flow - To each Aerator. To each pass in "Step"(*). From each Clarifier(**). 3) Mixed Liquor Flow To each Clarifier. U) Air Discharge - To each Aerator. To each "pass" of each Aerator(*). 5) Waste Sludge - One meter for each individual aerator-clarifier battery (*) Can be calibrated manometers or simple indicating meters, (**) Desirable, but can be eliminated if all other meters are provided. ------- ------- 1. General In large multi-unit plants it is utterly impractical to adjust or bal- ance flown manually at valve or pump locations that are almost always far removed fron the meter panels. A centrally located meter-control panel, wired to mechanical valve and puno actuc.to.-s, permit:; accurate ad- justment of critical flows while ob- serving; the restored response. This applies in principle, though not in degree, to small as well as large plants. * 2. Remotely actuated controllers should be provided for: a) Return and waste sludge pumps. b) Proportioning waste water and return sludge flows to individ- ual aerators. c) Proportioning mixed liquor flow to, and return sludge withdrawal from, individual clarifiers. C. Automatic Controllers 1. General Density sensors coupled to automatic controllers should be provided as part of the adjunct sludge handling and disposal facilities. Such control, according to the operator's "set point" requirements greatly improves sludge thickener, digester, etc. performance and minimises the supplementary recycle load on the secondary treatment facilities. 2. Other automatic controllers that can im- prove plant performance are: a) Blower control by D.O. sensors. b) Meter actuated controllers to pro- portion return sludge punpage according to the cyclic incoming waste water flow rates. ------- ------- y.-rcd ] iruo.* and return cruxlge concentration censors are being developed to control return sludge pumping and solids distri- bution. ------- ------- 16 III OFT'T'ATJCTAL "CXriOL operator or :ie tidier qualiiieatioas would be academic at "bent. It is unquestionably true that oualified operators are required to achieve the high quality effluent that can be produced by properly designed waste treatment plants. Cf even greater importance; dedicated, experienced,•operator ingenuity is needed to get the best out of plant3 that may suffer from certain design defects. Though some of the requirements for proper operational control discu'red here may reem repetitious to a few, all of the EC elements have been observed again and attain where coafomance to such principles has enhanced pollution abatement immeasurably or, conversely, where neglect has degraded final effluent quality. FERSONTIEL 1. Hire the best qualified people available. 2. Exert your greatest, and most effective, support to Operator Certification programs. 3. Send selected personnel to training courses and to similar treatment plants to upgrade their knowledge. k. Conduct continuing in-house training. 5. Inspire all operators to recognize that consistent production of the best possible final effluent quality is their foremost job responsibility. 6. In medium to large activated sludge plants; be sure there is a staff position - somewhere between the Superintendent and crew chief positions - with the specific responsibility to evaluate plant performance, cause and effect relationships, and to direct process control operations. 7. Obviously; provide tools and facilities the staff needs to accomplish their objective. ------- ------- 17 1. Practically every one is, or should be, aware of the need for properly planned safety, preventive maintenance, and em- ergency repair duty delegation. 2. Conduct critical reviews of routine opera- ting procedures. Minimize less important activities to provide additional time to beef-up the more essential and productive operational tasks. 3. Be sure crew chiefs are fully aware of their specific responsibility and author- ity. Excess sludge wasting schedules, for example, are dictated by process and efflu- ent quality requirements. Wasting adjust- ments should, therefore, be directed by the secondary process crew chief; not by (ex- cept in extreme emergency) the crew chief in charge of sludge handling. k. Written Standing Orders and Special Instruc- tions should be posted in the control office to avoid confusion among shift operators coming on duty aro\md-the clock. CONTROL TESTS 1. Again; all of us are aware of the need for conscientious, timely and proper sample collection. 2. Essential control tests should be run at least once every 8-hour shift; and more fre- quently when needed during troublesome times. 3. The control test series for activated sludge should include the all too frequently neg- lected following routines: Aeration Tank D.O. Clarifier sludge blanket depth. Final effluent turbidity. Mixed liquor sludge settlometer test. Sludge concentration by centrifuge tests. ------- ------- 18 d;.r.:and.~ from c,er,T; series 'trill c'e scribe process status and dictate the type and magni- tude of control adjustments needed to maintain, or restore, proper process performance. PROCESS RELIABILITY 1. The senior staff member directing process control operations snould: a. Summarize and evaluate all essential control terrc. data, results of demand calculations, and extent of control adjustments daily. b. Develop (and keep up-to-date) running trend charts illustrating significant features of plant performance. For example: Plant Loading Sludge Settling Characteristics Sludge Concentration Characteristics Sludge Blanket Depth Final Effluent Turbidity, etc. c. Study trend charts and determine cause- effect relationships between process con- trol and plant performance to: l) Document loadings or control pro- cedures that have caused trouble and must be avoided in the future. 2) Identify procedures that have proved successful and should be continued. 2. By conscientious, intelligent application of the basic operational requirements, discussed prev- iously, the Director of a properly designed treat- ment plant will achieve consistently reliable plant performance and excellent final effluent quality. ------- ------- |