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

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                                                            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

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                 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.

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            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.

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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.

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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.

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       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.

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                                         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.

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                                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

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                  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.

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                         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.

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            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).

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                        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:

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           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.

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                          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.

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                                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.

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             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.

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y.-rcd ] iruo.* and return  cruxlge
concentration censors are being
developed to control return
sludge pumping and solids distri-
bution.

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                               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.

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                 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.

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                 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.

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