Wastewater Treatment (2)

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CE 548 I Suspended Growth Biological treatment
Process
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Activated Sludge Principles
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Activated Sludge Principles

• Wastewater is aerated in a tank
• Bacteria are encouraged to grow by providing
• Oxygen
• Food (BOD)
• Suitable temperature
• Time
• As bacteria consume BOD, they grow and multiply
• Treated wastewater flows into secondary
  clarifier
• Bacterial cells settle, removed from clarifier
  as sludge
• Part of sludge is recycled back to activated
  sludge tank, to maintain
• bacteria population
• Remainder of sludge is wasted

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Applications of activated sludge processes
Process Application Conventional Low-strength
domestic waste, susceptible to shock
loads Complete-mix General application, resistant
to shock loads, surface aerators Step-aeration Gen
eral application to wide range of
wastes Modified-aeration Intermediate degree of
treatment where cell tissue in the effluent is
not objectionable Contact-stabilization Expansion
of existing systems, package plants,
flexible Extended-aeration Small communities,
package plants, flexible, surface aerators Kraus
process Low-nitrogen, high strength
wastes High-rate aeration Use with turbine
aerators to transfer oxygen and control the floc
size, generals application Pure-oxygen General
application, use where limited space is
available, requires expensive oxygen source,
turbine or surface aerators
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Conventional Activated Sludge
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Completely-mixed Activated Sludge
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Step-aeration Activated Sludge
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Contact Stabilization
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Oxidation Ditch/Kraus Process
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Design parameters for activated sludge processes
Process q c (d) q
(d) F/M Qr/Q X (mg/L) Conventional 5-15 4-8 0.2-0
.4 0.25-5 1,500-3,000 Complete-mix 5-15 3-5 0.2-0
.6 0.25-1 3,000-6,000 Step-aeration 5-15 3-5 0.2-
0.4 0.25-0.75 2,000-3,500 Modified-aeration 0.2-0
.5 1.5-3 1.5-5.0 0.05-0.15 200
500 Contact-stabilization 5-15 0.5-1
0.2-0.6 0.25-1 1,000-3,000
3-6

4,000-10,000 Extended-aeration 20-30 18-36 0.05-0
.15 0.75-1.5 3,000-6,000 Kraus
process 5-15 4-8 0.3-0.8 0.5-1 2,000-3,000 High-r
ate aeration 5-10 0.5-2 0.4-1.5 1-5 4,000-10,000
Pure-oxygen 8-20 1-3 0.25-1.0 0.25-0.5 6,000-8,000

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Operational characteristics of activated sludge
processes
Process Flow model Aeration
system BOD5 removal efficiency ()
Conventional Plug-flow Diffused air,
mechanical aerators 85-95 Complete-mix
Complete-mix Diffused air, mechanical
aerators 85-95 Step-aeration
Plug-flow Diffused air 85-95 Modified-aeration
Plug-flow Diffused air 60-75 Contact-stabilizat
ion Plug-flow Diffused air, mechanical
aerators 80-90 Extended-aeration
Complete-mix Diffused air, mechanical
aerators 75-95 Kraus process
Plug-flow Diffused air, mechanical
aerators 85-95 High-rate aeration
Complete-mix Diffused air, mechanical
aerators 75-90 Pure-oxygen
Complete-mix Mechanical aerators 85-95
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Wastewater Characterization

• AS design requires determining 1.) aeration
  basin volume 2.) sludge production 3.) oxygen
  needed and 4.) the effluent concentration of
  important parameters.
• To design AS process, characterization of
  wastewater is required.
• Wastewater characteristics T8-1, p.666 can be
  grouped into the following categories
• carbonaceous substrates,
• nitrogen compounds,
• phosphorus compounds,
• total and volatile suspended solids,
• and alkalinity.

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Wastewater Characterization

• Carbonaceous Constituents. Measured by BOD or
  COD.
• Unlike BOD, some portion of COD is
  nonbiodegradable. COD is fractionalized in F8-4,
  p.668.
• Of interest is whether the COD is dissolved or
  soluble and how much is particulate, comprised of
  colloidal and suspended solids.
• The nonbiodegradable soluble COD, nbsCOD, will be
  found in the AS effluent and the nonbiodegradable
  particulates will contribute to the sludge.
• Because the nonbiodegradable particulate COD,
  nbpCOD, is organic, it will contribute to the VSS
  concentration of the wastewater and mixed liquor
  in the AS and is referred to as the
  nonbiodegradable volatile suspended solids,
  nbVSS.

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Wastewater Characterization

• The influent wastewater will also contain
  nonvolatile influent suspended inert solids,
  iTSS, that add to the MLSS.
• For biodegradable COD, understanding the
  fractions that are measured as soluble, soluble
  readily biodegradable (rbCOD), and particulate is
  important for AS process design.
• The rbCOD is quickly assimilated by the biomass,
  while the particulate, must first be dissolved by
  extracellular enzymes and are thus assimilated at
  much slower rates.
• The rbCOD is of particular interest, T8-3, p.669,
  and has a direct effect on the AS biological
  kinetics and process performance.

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• A Oxygen used for rbCOD
• B Oxygen used for nitrification
• C Oxygen used for particular COD
• D Oxygen used for endogenous decay

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Wastewater Characterization

• COD and BOD may be correlated as the following
• bCOD consumed in the BOD test is equal to the
  oxygen consumed (UBOD) plus the oxygen equivalent
  of the remaining cell debris
• bCOD UBOD 1.42 fd (YH) bCOD
• bCOD/BOD ratio varies between 1.6-1.7.

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Wastewater Characterization

• Nitrogenous Compounds. F8-5, p.670
• Alkalinity Adequate alkalinity is needed to
  achieve complete nitrification, about 7.07 g
  CaCO3/gNH4-N.
• Additional alkalinity must be available to
  maintain the pH in the range 6.8-7.4.
• Typically the amount of residual alkalinity
  required to maintain the pH near neutral is
  between 70 and 80 mg/l as CaCO3.

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Wastewater Characterization

• Summary Tabulation. P. 673.
• COD bCOD nbCOD
• bCOD ?1.6BOD
• nbCOD nbsCOD nbpCOD
• bCOD sbCOD rbCOD
• TKN NH4-N ON
• ON bON nbON
• nbON nbsON nbpON
• Where terms are defined in T8-2, p.667.
• Study example 8-1 p 674

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Fundamentals of Process Analysis and Control

• Process design considerations
• Reactor type
• Kinetics
• SRT
• Sludge production
• Oxygen requirements
• Others
• Reactor type selection considerations. T8-4, p.
  678.
• Kinetics, summary of equations. T8-5, p.679.
• SRT The SRT in effect represents the average
  period of time during which the sludge has
  remained in the system and used to be called the
  mean cell residence time. In AS sludge design it
  is the MOST critical parameter as it affects just
  about every element of design. The SRT is
  typically 3-5 days, T8-6, p.680.

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Fundamentals of Process Analysis and Control

• Sludge production Excess solids are produced in
  the AS process and must be properly disposed of
  or they will accumulate and exit in the effluent.
• PX,VSS YobsQ(S0-S)(1kg/103g) eq.
  8-14, p.681
• The Yobs term is illustrated in F8-7, p.682.
• Oxygen Requirements If all of the bCOD were
  oxidized, the oxygen demand would equal the bCOD
  concentration. However, bacterial oxidize a
  portion of the bCOD to provide energy and use the
  remaining portion of the bCOD for cell growth.
  Oxygen is also used for endogenous respiration
  which is a function of the SRT.
• The total oxygen requirement including
  nitrification is
• R0 Q(S0-S) 1.42PX,bio 4.33Q(NOx) eq.
  8-17, p.683
• The last term deals with the effects of nitrogen.

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Fundamentals of Process Analysis and Control

• Nutrient requirements Based on cell mass, 12.4
  by weight of nitrogen is required and phosphorus
  is usually assumed to be about 1/5 of the
  nitrogen. As a general rule, for SRT values gt
  7d, about 5g of N and 1g of P will be required
  per 100g of BOD.
• ML Settling Characteristics In the final
  clarifier, the MOs must be separated. A commonly
  used measure of settling characteristics is the
  SVI, the sludge volume index. The SVI is the
  volume of 1g of sludge after 30 minutes of
  settling. The numerical value is calculated from
  the test as follows
• SVI
  ml/g eq.
  8-19

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Fundamentals of Process Analysis and Control

• Example
• Given A ML has a TSS of 3500mg/l and settles to
  a volume of 275 in 30 minutes in a 1L cylinder.
• Find SVI
• SVI
  78.6 ml/g
• SVI 78.6 ml/g
• A value of 100 mL/g is considered a good
  settling sludge and SVI values below 100 are
  desired. SVI values above 150 are typically
  associated with a problem, filamentous growth.

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Fundamentals of Process Analysis and Control

• Secondary Clarification The design is typically
  based on the surface overflow rate and solids
  loading rate, T8-7, p.687.
• Overflow rates are based on wastewater flow
  rates instead of ML flowrates.
• Solids loading rate

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Fundamentals of Process Analysis and Control
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Fundamentals of Process Analysis and Control

• Effluent Characteristics The major parameters of
  interest are
• organic compounds, sBOD usually less than 3 mg/l
• suspended solids, 5-15 mg/l
• and nutrients.
• Process Control.
• Maintaining DO in the aeration tanks.
• Regulating RAS
• Controlling WAS
• The most commonly used parameter for controlling
  the AS process is SRT. The waste AS flow from the
  recycle line is usually used to maintain the
  desired SRT. The MLSS is also used as a control.

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Fundamentals of Process Analysis and Control

• The DO should be 1.5-2 mg/l in all areas of the
  aeration tank. Values above 2 mg/l may improve
  nitrification (when BOD is high). Values above 4
  mg/l do not improve operations but significantly
  increase aeration costs.
• RAS Control
• The RAS is returned from the final clarifier to
  the inlet of the aeration tank.
• The solids form a sludge blanket in the bottom of
  the clarifier.
• Return sludge pumping rates of 50-75 of the
  average design wastewater flowrates are typical.
  However, the design average capacity is typically
  100-150 of the average design flowrate.
• Return AS concentrations from the secondary
  clarifier range typically from 4000-12,000 mg/l.

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Fundamentals of Process Analysis and Control

• Settleability To calculate return-sludge
  flowrate, several techniques are used
• Settleability test
• In a 1000 ml graduated cylinder the volume of
  settleable solids after 30 minutes is divided by
  the volume of clarified liquid (supernatant).
• SVI (Sludge Volume Index) test

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Fundamentals of Process Analysis and Control

• Sludge Wasting To maintain a given SRT, the
  excess AS produced each day must be wasted, WAS.
• The sludge can be wasted from the RAS line or the
  aeration tank.
• The RAS is more concentrated thereby requiring
  smaller pumps.
• The WAS is discharged to the primary
  sedimentation tanks for co-thickening or to
  sludge thickening facilities prior to digestion.
• If wasting is from the RAS line
• If wasting is done from the aeration tank

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Fundamentals of Process Analysis and Control

• Operational Problems
• Bulking sludge The MLSS floc does not compact or
  settle well and floc is discharged in the
  clarifier effluent. The principal cause is
  filamentous bacteria which are very competitive
  at low substrate, nutrient or DO conditions.
• Rising sludge
• The sludge has good settling characteristics but
  rises to the surface.
• The most common cause is denitrification in which
  nitrites and nitrates are converted to nitrogen
  gas, N2 which makes the mass buoyant.
• Rising sludge is differentiated from bulking
  sludge by the presence of small gas bubbles and
  floating sludge in the secondary clarifiers.
• Rising sludge problems may be overcome by
  reducing the detention time in the clarifier by
  increasing the RAS rate.

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Fundamentals of Process Analysis and Control

• Operational Problems
• Foaming
• Nocardia can be responsible for excessive
  foaming.
• The bacteria have hydrophobic cell surfaces and
  attach to air bubbles where they stabilize the
  bubbles to cause foam.
• Usually found above the ML.
• Nocardia can by controlled by avoiding trapping
  foam in the secondary treatment process and using
  chlorine spray.

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Processes for BOD Removal and Nitrification

• Three Activated-Sludge process design examples
  are provided in this section (8-4) to demonstrate
  the application of the fundamental principles to
  BOD removal and nitrification.
• The examples are
• A single sludge complete-mix activated-sludge
  process without and with nitrification. Example
  8-2
• A sequencing batch reactor (SBR) with
  nitrification. Example 8-3
• A staged nitrification process. Example 8-4

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Processes for BOD Removal and Nitrification

• Sequencing Batch Reactor
• (SBR) is a fill-and-draw activated-sludge
  treatment system. In SBR aeration and
  sedimentation are carried out sequentially in the
  same tank. The process takes place in five steps
• fill
• addition of wastewater to reactor
• liquid level rises from 25 to 100
• normally lasts 25 of full cycle time
• react
• complete the reaction
• Lasts 35 of cycle time.

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Processes for BOD Removal and Nitrification

• Sequencing Batch Reactor
• settle
• to allow solid separation to occur
• more efficient than continuous flow systems.
• Lasts 20
• draw
• to remove clarified treated waste lasts from
• 5 - 30 of cycle time, typically 45 minutes
• idle
• to provide time for one reactor to complete its
  fill cycle before switching to another unit.
• Sometimes omitted.

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Processes for BOD Removal and Nitrification

• Sequencing Batch Reactor
• sludge wasting usually occurs during settle or
  idle phases.
• no need for recycling both aeration and settling
  occur in the same chamber
• Process kinetics
• Accumulation inflow outflow reaction

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Processes for BOD Removal and Nitrification

• Staged activated-sludge process
• Consists of a series of complete-mix reactors.
• For the same reactor volume, rectors in series
  can provide greater treatment efficiency than a
  single complete-mix reactor, or provide a greater
  treatment capacity.
• The oxygen uptake is higher in the first stage
  and decreases gradually.

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Processes for BOD Removal and Nitrification

• Overview of biological nitrogen removal processes
• All biological nitrogen removal processes include
  aerobic zone (nitrification) and anoxic zone
  (denitrification).
• Categories of suspended growth biological
  nitrogen removal processes include (1)
  single-sludge or (2) two-sludge.
• Single-stage processes (three types)
• preanoxic initial contact of influent and
  return activated sludge is in the anoxic zone.
  (commonly used)
• Postanoxic anoxic zone follows the aerobic zone.
• Simultaneous nitrification-denitrification
  (SNdN) both zones exisis in a single reactor.
  Requires DO control.
• Two-sludge processes consists of two separate
  stages for nitrification followed by
  denitrification. (not commonly used)

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Preanoxic
Postanoxic
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Simultaneous
Two-sludge
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Processes for Phosphorous Removal

• Process for biological phosphorous removal
• Three biological phosphorous removal (BPR)
  configuration are commonly used
• Phoredox (A\O) represent any process with an
  anaerobic/aerobic sequence to promote BPR.
  Nitrification does not take occur.
• A2O? process sequence, anaerobic/anoxic/aerobic.
  Nitrification takes place.
• UCT (University of Cape Town) used for weak
  wastewater where the addition of nitrate would
  have significant effect on the BPR performance.
• The PhoStrip? process combines biological and
  chemical processes for phosphorous removal.

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Design of Physical Facilities for AS Process

• Design of Aeration Tanks
• After selecting the activated sludge process and
  the aeration system, the next step is to design
  the aeration tanks and support facilities.
• Aeration Tanks
• constructed of reinforced concrete
• capacity is determined from process design
• for plants in a capacity range of
• 0.5 10 Mgal/d minimum two tanks
• 10 15 Mgal/d 4 tanks
• gt50 Mgal/d gt 6 tanks
• Some large plans have 30 to 40 tanks

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Design of Physical Facilities for AS Process

• Aeration Tanks
• wastewater depth in the tank should be 15 25 ft
  for diffusers to work efficiently.
• free board from 1 2 ft above waterline should
  be provided
• width to depth ratio 11 2.21 (1.51 is
  common)
• for large plants channel length can exceed 500 ft
  per tank
• tanks may consist of one to four channels
• length-to-width ratio of each channel should be
  at least 51
• for mechanical aeration system, one aerator per
  tank is commonly used with a free board 3.5 5 ft

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Suspended Growth Aerated Lagoons

• Consists of shallow earthen basins varying in
  depth from 2-5m provided with mechanical
  aerators.
• mechanical aerators provide oxygen and mixing
• Suspended growth aerated lagoons are operated on
  a flow-through basis or with recycle.
• Lagoons with solid recycle are essentially the
  same as the activated sludge process.
• Types of Suspended growth aerated lagoons
• Facultative partially mixed
• Aerobic flow-through with partial mixing
• Aerobic with solids recycle and nominal complete
  mixing
• The general characteristics of these lagoon
  systems are summarized in Table 8-29

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Suspended Growth Aerated Lagoons

• Facultative partially mixed
• The energy input is sufficient to meet oxygen
  requirement but not sufficient to maintain all of
  the solids in suspension.
• A portion of incoming solids will settle a long
  with a portion of the biological solids (AS)
• Settled solids will undergo anaerobic
  decomposition
• The term facultative is derived from the aerobic
  and anaerobic processes that occur in the lagoon
• Facultative lagoons must be dewatered and the
  accumulated soilds removed.
• Not commonly used.

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Suspended Growth Aerated Lagoons

• Aerobic flow-through with partial mixing
• The energy input is sufficient to meet oxygen
  requirement but not sufficient to maintain all of
  the solids in suspension.
• t SRT
• Effluent solids are removed in an external
  sedimentation facility
• Aerobic flow-through with partial mixing
• Same as extended aeration AS process, with the
  exception that an earthen basin is used in place
  of reinforced concrete reactor.
• Hydraulic detention time (up to 2 days) is longer
  than conventional extended aeration process.
• Higher aeration requirement than aerobic
  flow-through lagoons to maintain solids in
  suspension.

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Suspended Growth Aerated Lagoons

• Process design for flow-through lagoons
• BOD removal the basis of design is SRT ,
  typical values of SRT range from 3 6 days.
  Once SRT is selected S can be calculated using
  equations from Ch. 7.
• An alternative approach is to assume that removal
  can be described by first-order function. (rsu
  -kS). The pertinent equation for a single aerated
  lagoon is
• k first-order removal-rate const. d-1
• (k varies from 0.5 1.5 d-1)

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Suspended Growth Aerated Lagoons

• Process design for flow-through lagoons
• For lagoons in series, the following equation can
  be used
• Oxygen requirements
• Can be computed in the same way as for activated
  sludge process.
• Oxygen requirements have been found to vary from
  0.7 1.4 the amount of BOD5 removed.

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Suspended Growth Aerated Lagoons

• Process design for flow-through lagoons
• Temperature
• Temperature effect include
• reduced biological activity and treatment
  efficiency.
• formation of ice.
• Temperature can be estimated using
• the proportionality factor incorporates
• heat transfer coefficients
• effect of surface area increase due to aeration
• effect of wind and effect of humidity

Study example 8-13
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Biological Treatment with Membrane Separation

• Overview of membrane bioreactor (MBR) technology
• The Membrane Bioreactor (MBR) process is an
  emerging advanced wastewater treatment technology
  that has been successfully applied at an ever
  increasing number of locations around the world.
• In addition to their steady increase in number,
  MBR installations are also increasing in terms of
  scale. Over 1500 installation in more than 1000
  cities world-wide for municipal and industrial
  application have been reported to range in
  capacity from few hundreds of cubic meters per
  day to over 50,000 cubic meters per day.
• New large plants under construction include the
  new Brightwater municipal wastewater treatment
  plant in King County in the State of Washington
  which will treat approximately 144,000 cubic
  meters of municipal sewage with peak flows up to
  204,000 cubic meters, serving over 100,000
  households.

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Biological Treatment with Membrane Separation

• MBR Process Description
• Membrane bioreactors (MBRs) combine the use of
  biological processes and membrane technology to
  treat wastewater.
• As shown in figure 1, within one process unit, a
  high standard of treatment is achieved, replacing
  the conventional arrangement of aeration tank,
  settling tank and filtration that generally
  produces what is termed as a tertiary standard
  effluent.
• The dependence on disinfection is also reduced,
  since the membranes with pore openings, generally
  in the 0.01-0.5 µm range, trap a significant
  proportion of pathogenic organisms (Figure 2).
• Operating at a mixed liquor suspended solids
  (MLSS) concentration of up to 20,000 mg/L and a
  sludge age of 30-60 days, MBRs offer additional
  advantages over conventional activated sludge
  plants, including a smaller footprint.

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Biological Treatment with Membrane Separation

• MBR Process Advantages
• The ability to eliminate secondary clarifier and
  operate at higher MLSS concentrations provide the
  following advantages
• Higher volumetric loading rate resulting in
  shorted hyd. detention time.
• Longer SRT resulting in less sludge production.
• Operate at lower DO concentration.
• High-quality effluent (TSS, BOD, bacteria,
  turbidity, etc.) Table 8-30
• Less space required for wastewater treatment.
• MBR Process disadvantages
• High capital cost and energy cost.
• Limited data on membrane life, (high cost for
  membrane replacement)
• Membrane fouling

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