Pipeline Explaining the Activated Sludge Process by ixl26840


									                                                                                                                 Spring 2003
                                                                                                                 Vol. 14, No. 2

                                    Small Community Wastewater Issues Explained to the Public

                           Explaining the Activated Sludge Process
               t is to everyone’s advan-       tank, leaving a relatively clear liquid      The resulting settled solids, the acti-

      I        tage for a community to
               be able to treat its
               wastewater in the most
  economical way. The activated
                                               free of organic material and suspend-
                                               ed solids.
                                               Described simply, screened waste-
                                               water is mixed with varying amounts
                                                                                            vated sludge, are returned to the first
                                                                                            tank to begin the process again.
                                                                                            Initially developed in England in the
                                                                                            early 1900s, the activated sludge
  sludge process has the advantage of
                                               of recycled liquid containing a high         process did not become widespread
  producing a high quality effluent for
                                               proportion of organisms taken from a         in the U.S. until the 1940s. Today a
  a reasonable operating and mainte-
                                               secondary clarifying tank, and it            number of variations of the basic
  nance costs.
                                               becomes a product called mixed               process have been developed. This
  The activated sludge process uses            liquor. This mixture is stirred and          issue of Pipeline includes descrip-
  microorganisms to feed on organic            injected with large quantities of air,       tions of three of the most common
  contaminants in wastewater, produc-          to provide oxygen and keep solids in         variations: Extended aeration,
  ing a high-quality effluent. The basic       suspension. After a period of time,          sequencing batch reactors, and oxida-
  principle behind all activated sludge        mixed liquor flows to a clarifier            tion ditches. A glossary of terms can
  processes is that as microorganisms          where it is allowed to settle. A por-        be found on page 2.
  grow, they form particles that clump         tion of the bacteria is removed as it
                                                                                            The activated sludge plant is the
  together. These particles (floc) are         settles, and the partially cleaned
                                                                                            most popular biological treatment
  allowed to settle to the bottom of the       water flows on for further treatment.
                                                                                            process for larger installations or

                                                                                                                       Photo by Ed Winant
Activated sludge package plant at Mason Dixon Elementary School in Monongalia County, West Virginia.
                                                      Activated Sludge

                                                      small package plants being            water. This sludge is pumped back to
  Activated sludge – sludge particles produced        used today. These plants are          the aeration tank where it is mixed
  in wastewater by the growth of organisms in         capable of producing a high           with the incoming wastewater or
  aeration tanks. The term ‘activated’ comes          quality effluent for the price.       removed from the system as excess,
  from the fact that the particles teem with bac-     Other advantages of the               a process called wasting. The rela-
  teria, fungi, and protozoa. Activated sludge is     activated sludge process are          tively clear liquid above the sludge,
  different from primary sludge in that the           the low construction cost             the supernatant, is sent on for further
  sludge particles contain many living organ-         and the relatively small land         treatment as required. See Figure 1
  isms that can feed on the incoming waste-
                                                      requirement.                          on page 3.
  Activated sludge process – a biological             The activated sludge process
                                                      is widely used by large               Sludge characteristics
  wastewater treatment process which speeds
  up waste decomposition. Activated sludge is         cities and communities                By analyzing the different character-
  added to wastewater, and the mixture is aerat-      where large volumes of                istics of the activated sludge or the
  ed and agitated. After a certain amount of          wastewater must be highly             sludge quality, plant operators are
  time, the activated sludge is allowed to settle     treated economically.                 able to monitor how effective the
  out by sedimentation and is disposed of             Activated sludge process              treatment plant’s process is. Efficient
  (wasted) or reused (returned to the aeration        plants are good choices too           operation is ensured by keeping
  tank)                                               for isolated facilities, such         accurate, up-to-date records; routine-
                                                      as hospitals or hotels, clus-         ly evaluating operating and laborato-
  Aerobic – a condition where oxygen is present
                                                      ter situations, subdivisions,         ry data; and troubleshooting, to solve
  BOD – biological oxygen demand. Measure of          and small communities.                problems before they become seri-
  oxygen organic material in the water requires.                                            ous. A wide range of laboratory and
  Bulking – sludge that forms clouds in the sec-      The process                           visual and physical test methods are
  ondary clarifiers when the sludge does not                                                recommended. Principally, these
                                                      A basic activated sludge
  settle properly, usually caused by filamentous                                            include floc and settleability per-
                                                      process consists of several
  bacteria                                                                                  formance using a jar test, microscop-
                                                      interrelated components:
  F:M – food to microbe ratio                                                               ic identification of the predominant
                                                      • An aeration tank where the          types of bacteria, and analysis of var-
  Floc – clumps of bacteria                           biological reactions occur            ious chemical parameters.
  Flocculation – agitating wastewater to induce       • An aeration source that
  the small, suspended particles to bunch                                                   The treatment environment directly
                                                      provides oxygen and mixing
  together into heavier particles (floc) and settle                                         affects microorganisms. Changes in
                                                      • A tank, known as the clari-
  out.                                                                                      food, dissolved oxygen, temperature,
                                                      fier, where the solids settle
                                                                                            pH, total dissolved solids, sludge age,
  Loading - a quantity of material added to the       and are separated from
  process at one time                                 treated wastewater
                                                      • A means of collecting the        Safety considerations
  MLSS – mixed-liquor suspended solids
                                                      solids either to return them
  MLVSS – volatile mixed-liquor suspended             to the aeration tank, (return          Practice careful personal
  solids                                              activated sludge [RAS]), or               cleanliness
  Mixed liquor – activated sludge mixed with          to remove them from the                Require hard hats, boots, and
  raw wastewater                                      process (waste activated                  gloves
  Package plant – pre-manufactured treatment          sludge [WAS]).                         Ventilate all covered tanks
  facility small communities or individual prop-
  erties use to treat wastewater                      Aerobic bacteria thrive as             Prohibit smoking around the plant
  SRT – solids retention time                         they travel through the aera-          Consider empty tanks as enclosed
                                                      tion tank. They multiply                  spaces and apply the proper entry
  Sludge – the solids that settle out during the      rapidly with sufficient food              procedures
  process                                             and oxygen. By the time the              Keep all hatches closed and
  Supernatant – the liquid that is removed from       waste reaches the end of the              secured
  settled sludge. It commonly refers to the liquid    tank (between four to eight
  between the sludge on the bottom and the            hours), the bacteria has used
                                                                                               Keep tank areas well lighted
  scum on the surface.                                most of the organic matter               Keep walkways clear to prevent
  TSS – total suspended solids                        to produce new cells.                     falling
  Wasting – removing excess microorganisms            The organisms settle to the              Provide lockout protection for all
  from the system                                     bottom of the clarifier tank,             electrical equipment, gates or
                                                      separating from the clearer               valves when working in empty
 PIPELINE – Spring 2003; Vol.14, No. 2                                       National Small Flows Clearinghouse (800) 624-8301 or (304) 293-4191
                                                            Activated Sludge

       Figure 1 Typical Activated Sludge Process

    presence of toxins, and other factors           Food (organic loading) regulates               plant start-up or after an upset, such
    create a dynamic environment for the            microorganism numbers, diversity,              as a shock load (when a stronger
    treatment organisms. The operator               and species unless other factors limit         than usual batch of influent comes
    can change the environment (the                 it. It is important to maintain the prop-      into the plant). Typically, little or no
    process) to encourage or discourage             er ratio of food to microorganisms             sludge forms at this time.
    the growth of specific microorgan-              (F:M) to ensure optimum operation.             Flagellates are free-swimmers and
    isms. See the table below.                                                                     predominate in light mixed liquors
                                                    Activated sludge consists of a mixed
                                                              community of microorgan-             during high food to microorganism
Problem / Effect                                              isms, approximately 95               conditions. Their presence usually
                                                              percent bacteria and 5 per-          indicates poor effluent quality.
 Problem                    Effect/observation
                                                              cent higher organisms                Free-swimming ciliates predominate
 Poor primary               Plugging                          (protozoa, rotifers, and             as the F:M ratio decreases. Stalked
 clarification              Standing water                    higher forms of inverte-             ciliates predominate when there is an
                            Odors                             brates). Particular ones are         abundance of bacteria. Effluent and
                            Reduced efficiency                considered indicator                 sludge quality are typically best
                                                              microorganisms that can be           when these types of microorganisms
                                                              observed using inexpensive           predominate.
 Hydraulic overload         High effluent TSS
                                                              microscopes. Significant
                                                              numbers of a particular              Filamentous bacteria can cause the
 Nitrification              High effluent TSS                 species can indicate the             sludge not to settle properly, a condi-
                            High chlorine demand              condition of the process.            tion called bulking, which causes
                            Low pH                                                                 clouds of billowing sludge rather
                                                              The most predominant                 than settling. These bacteria flourish
                                                              microorganisms are aero-             when the excess sludge is not
 Nutrient shortage          Filamentous bacteria              bic bacteria, but there are          removed at the proper rate. Filamen-
                            Rising sludge                     also substantial populations         tous sludge bulking is a common
                            Pass through of soluble BOD       of fungi and protozoa.               problem at small, extended aeration
                                                              Rotifers and nematodes are           treatment plants.
 Organic overload           Pass through of soluble BOD       most frequently found in
                                                              systems with long aeration           Developing and maintaining good
                            Odors                                                                  floc structure is critical for optimum
                            Low DO                                                                 system performance. A multiple jar
                            Poor effluent quality             Amoeboid forms, the flag-            test is a procedure used to evaluate
                                                              ellates, and the ciliates are        the effectiveness of coagulants, opti-
 Cold weather               Loss in removal efficiency        the most common proto-               mum dosage for coagulation, concen-
                                                              zoans in a working sludge.           tration of the coagulant aid and the
                            Icing problems
                                                              Amoeboids predominate in             most effective order in which to add
                                                              ‘young’ sludges, such as at
 Organic underload          High energy use
  PIPELINE – Spring 2003; Vol.14, No.2                                              National Small Flows Clearinghouse (800) 624-8301 or (304) 293-4191
                                                    Activated Sludge

  various chemicals. It consists of a        This process operates at a high solids       highly treated effluent with low
  multiple stirring apparatus with a         retention time (low F:M), resulting in       solids production.
  variable-speed drive. Samples are          a condition where nitrification may
                                                                                          The wastewater is screened to
  held in one- or two-liter jars or          occur. The microorganisms compete
                                                                                          remove large suspended or floating
  beakers.                                   for the remaining food. This highly
                                                                                          solids before entering the aeration
                                             competitive situation results in a
  The activated sludge samples are
  mixed and agitated for varying
  lengths of time, and then allowed to
  settle. The nature and settling charac-             Percent of:                                        Purpose / Operation
  teristics of the floc are noted, as well           Max     Cycle
  as the clarity of the supernatant.                Vollume Time
  Chemical testing reveals sludge con-
  ditions and can warn of impending                    25
  process problems. Compliance with                    to       25
  the plant’s National Pollutant                      100
  Discharge Elimination System
  (NPDES) permit requires specific
  chemical analyses. Alkalinity, solids
  (total, suspended and dissolved), bio-
  logical oxygen demand, chemical
  oxygen demand, nitrogen and phos-                                                                            Air On
  phorus are some of the parameters                   100       35
  that plant operators must monitor.

  Variations of the Activated
  Sludge Technology
  Package plants are pre-manufactured
  treatment facilities used to treat
  wastewater. Usually designed to treat
  flows between 10,000 and 250,000                    100       20                                             Air Off
  gallons per day, these are good
  choices for areas with a limited num-
  ber of people and small wastewater
  flows. These plants are options for
  small communities or in such isolat-
  ed locations as trailer parks, highway
  rest areas, hospitals and prisons.
  Some of the most common types of                                                                             Air Off
  package plants use biological aera-                 100
  tion processes: extended aeration,                   to       15
  sequencing batch reactors and oxida-                 35
  tion ditches.
  Extended aeration
  The extended aeration process holds
  wastewater in an aeration tank for 18
  hours or more and the organic wastes                                                                         Air Off
  are removed under aerobic condi-                     35
  tions. Air may be supplied by                        to       5
  mechanical or diffused aeration.                     25
  Mixing is by aeration or mechanical

                                                            Typical sequencing batch reactor
                                                            operation for one cycle
                                                Figure 2
PIPELINE – Spring 2003; Vol.14, No. 2                                      National Small Flows Clearinghouse (800) 624-8301 or (304) 293-4191
                                                     Activated Sludge

    chamber, where it is mixed, and oxy-     able components, and then dis-                 cost effective if treatment beyond
    gen is added. The solids settle out      charged.                                       biological treatment, such as filtra-
    and are returned to the aeration                                                        tion, is required. SBRs also offer a
                                             SBR systems consist of five common
    chamber to mix with incoming                                                            potential capital cost savings by
                                             steps carried out in sequence: (1) fill,
    wastewater. The clarified wastewater                                                    eliminating the need for clarifiers.
                                             (2) react (aeration), (3) settle (sedi-
    flows to a collection channel before
                                             mentation/clarification), (4) draw             SBRs require a sophisticated level of
    being diverted to the disinfection
                                             (the effluent is decanted), and (5)            maintenance due to the timing units
                                             idle. Sludge wasting usually occurs            and controls. Depending upon the
    This is the process many package         during the settling phase. The SBR             downstream processes, it may be
    plants that schools, housing develop-    acts as an equalization basin when             necessary to equalize effluent after
    ments, and small communities use.        filling with wastewater, enabling the          leaving the SBR.
    Due to the light food to microorgan-     system to tolerate peak flows or
    ism loading, extended aeration plants    loads.                                         Oxidation ditches
    are considered one of the most stable                                                   The oxidation ditch is an extremely
                                             After passing through a screen to              effective variation of the activated
    wastewater treatment processes.
                                             remove grit, the effluent enters a par-        sludge process, consisting of a ring
    The extended aeration process can        tially filled reactor. Once the reactor        or oval shaped channel equipped
    accept periodic (intermittent) load-     is full, it performs like a convention-        with mechanical aeration devices,
    ings without upsetting the system.       al activated sludge system without a           such as brush rotors or disc aerators.
    Extended aeration does not produce       continuous influent or effluent flow.          See Figure 3 below.
    as much waste sludge as other            Aeration and mixing are discontin-
    processes; however, wasting still is     ued after the biological reactions are         Oxidation ditches typically operate in
    necessary to maintain proper control     complete, the solids are allowed to            an extended aeration mode with long
    of the process.                          settle, and the treated effluent (super-       solids retention times (SRTs). Solids
                                             natant) is removed. Excess solids are          are maintained in suspension as the
    Sequencing batch reactors                removed at any time during the                 mixed liquor circulates around the
    The sequencing batch reactor (SBR)       cycle. See Figure 2 on previous page.          ditch.
    is considered a fill-and-draw activat-
                                             SBRs are typically used where                  Preliminary treatment involves bar
    ed sludge system. The processes of
                                             flowrates are five million gallons per         screens and grit removal. Secondary
    equalization, aeration, and clarifica-
                                             day or less. Due to their relatively           sedimentation tanks are used for
    tion are all achieved in the same
                                             small footprints, they are useful in           most applications. Tertiary filters
    tank, unlike a conventional activated
                                             areas where available land is limited.         may be required after clarification
    sludge system, in which the same
                                             In addition, it is easy to modify              and disinfection. Re-aeration may be
    processes are accomplished in sepa-
                                             cycles within the system for nutrient          necessary prior to final discharge.
    rate tanks. Wastewater is added to
    the tank, treated to remove undesir-     removal if necessary. SBRs are also

Figure 3
            Oxidation Ditch

  PIPELINE – Spring 2003; Vol.14, No.2                                       National Small Flows Clearinghouse (800) 624-8301 or (304) 293-4191
                                                          Activated Sludge

                                        Comparisons of the advantages and disadvantages of
  Oxidation ditch process               extended aeration plants, SBRs, and oxidation ditches
  plants can be designed to
  achieve specific objectives             Type                 Advantages                                Disadvantages
  including nitrification, den-
  itrification, and/or biologi-
  cal phosphorus removal.                 Extended aeration    Easy to operate                           Unable to achieve denitrifi-
  And due to the constant                                      Easy to install                             cation or phosphorus
  water level and continuous                                   Odor free                                   removal
  discharge, oxidation ditch                                   Small footprint                           Limited flexibility in
  technology is very reliable                                                                              response to changing
                                                               Low sludge yield
  and does not cause an                                                                                    effluent requirements
  effluent surge common to                                                                               Large energy requirement
  other biological processes,
  such as SBRs.
                                          SBRs                 Able to achieve nitrification,            High energy consumption
  Oxidation ditches are more                                     denitrification, and phospho-           Difficult to adjust cycle
  energy efficient than other                                    rous removal                              times for small commu-
  similar processes, so this                                   Large operation flexibility                 nities
  technology can be a better
                                                               Minimal sludge bulking                    Frequent sludge disposal
  choice for small communi-
  ties and isolated institu-                                   Few operation and maintenance
  tions over conventional                                        problems
  treatment plants. But oxi-                                   Able to be operated remotely
  dation ditches require a
  larger land area which                  Oxidation ditches    Moderate energy requirements              Noisy and odiferous if not
  sometimes limits their use                                                                               operated correctly
                                                               Unaffected by weather
  in areas where land costs
                                                               Provides high quality effluent in         Unable to treat toxic waste
  are high.
                                                                 terms of TSS, BOD, and                    streams
                                                                 ammonia                                 Relatively large footprint
                                                               Low sludge yields
                                                               Capable of handling shock

  Package plant servicing an apartment complex in rural West Virginia                                                  Photo by Charles C. Metzgar

PIPELINE – Spring 2003; Vol.14, No. 2                                           National Small Flows Clearinghouse (800) 624-8301 or (304) 293-4191
                                                        Activated Sludge

  Aeration serves two
  important purposes: sup-
  plying the required oxy-
  gen to the organisms to
  grow and providing opti-
  mum contact between the
  dissolved and suspended
  organic matter and the
  microorganisms. The aer-
  ation system consumes
  approximately 50 to 65
  percent of the net power
  demand for a typical
  activated sludge waste-
  water treatment plant,
  therefore the efficiency
  of different aeration sys-
  tems is an important con-
  sideration. The time that
  the mixed liquor is aerat-
  ed varies from as little as
  30 minutes to as much as
  36 hours depending upon
  the treatment process
  used. Aeration can be          An example of mechanical aeration, provided by rotary brushes, at an oxidation ditch in in Jane Lew, WV.
  performed mechanically or
                                              Porous (fine bubble) diffusers are            air and liquid through a nozzle, and
  by using a diffused system.
                                              attached to the bottom of the tank or         aspirator aerators that use a propeller
  Mechanical aerators physically              positioned just below the surface.            on the end of a hollow shaft, creating
  splash the wastewater into the atmos-       They are available in various shapes          a vacuum as the propeller draws air
  phere above the tank and create tur-        and sizes, such as discs, tubes,              from the atmosphere and disperses it
  bulence assuring effective waste-           domes, and plates. Fine pore dif-             into the wastewater.
  water mixing. Mechanical aerators           fusers introduce air in the form of
  include brushes, blades or propellers       very small bubbles, maximizing the
  that introduce air from the atmos-          contact time the air bubbles have
  phere. Surface aerators float at the        with the mixed liquor and encourag-
  surface or are mounted on supports          ing mixing while at the same time,
  in or above the basin. Mechanical           discouraging deposits on the tank
  aerators tend to incur lower installa-      bottom. These fine pore diffusers                                Info
  tion and maintenance costs.                 produce a high oxygen transfer effi-
                                                                                                    Readers are encouraged to
                                              ciency, but they are susceptible to                 reprint Pipeline articles in local
  A diffused air system introduces
                                              chemical or biological fouling and as             newspapers or include them in fly-
  compressed air through a perforated
                                              a result, require routine cleaning.              ers, newsletters, or educational pre-
  membrane into the wastewater.                                                                sentations. Please include the name
  Diffusers are classified by the physi-      Nonporous (course bubble) diffusers            and phone of the National Small Flows
  cal characteristics of the equipment,       usually have fixed or valved orifices.         Clearinghouse (NSFC) on the reprinted
  or by the size of the air bubble. The       Due to the larger bubble size, non-            information and send us a copy for our
  choice of bubble size, diffuser type,       porous diffusers produce lower oxy-             files. If you have any questions about
                                                                                              reprinting articles or about any of the
  and diffuser placement can have a           gen transfer efficiencies.                        topics discussed in this newsletter,
  great effect on the efficiency of the                                                             please contact the NSFC at
                                              Other diffusion devices include jet
  aeration process.                                                                                       (800) 624-8301.
                                              aerators, which discharge a mix of

PIPELINE – Spring 2003; Vol.14, No.2                                            National Small Flows Clearinghouse (800) 624-8301 or (304) 293-4191
NSFC RESOURCES AVAILABLE                                                                                            PIPELINE
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                                                                                                 Pipeline is funded through a grant from the
 Related EPA Fact Sheets                                                                           U.S. Environmental Protection Agency
 These wastewater technology fact sheets can be read or                                                        Washington, D.C.
                                                                                                        Steve Hogye—Project Officer
 downloaded from www.epa.gov.                                                                             Municipal Support Division
                                                                                                     Office of Wastewater Management
 Wastewater Technology Fact Sheet: Package Plants                                                     National Small Flows Clearinghouse
 EPA 832-F-00-016, September 2000 (# WWFSGN194. Cost is $2.40)                                               West Virginia University
                                                                                                      Peter Casey — Program Coordinator
 Wastewater Technology Fact Sheet: Fine Bubble Aeration                                                      Marilyn Noah — Editor
                                                                                                      Jennifer Hause — Technical Advisor
 EPA 832-F-99-065, September 1999 (# WWFSGN187. Cost is $1.40)                                         Ed Winant PE — Technical Advisor
                                                                                                       Chris Metzgar — Graphic Designer
 Wastewater Technology Fact Sheet: Oxidation Ditches                                             Permission to quote from or reproduce articles in
                                                                                            this publication is granted when due acknowledgement is
 EPA 832-F-00-013, September 2000 (# WWFSGN195. Cost is $1.20)                           given. Please send a copy of the publication in which informa-
                                                                                           tion was used to the Pipeline editor at the address above.

 Wastewater Technology Fact Sheet: Sequencing Batch Reactors
 EPA 932-F-99-073, September 1999 (# WWFSGN179. Cost is $1.80)                                                an equal opportunity/affirmative action institution

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