BNR1

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Department of Civil Environmental Engineering
Jae K. Park
Biological Nutrient Removal Theories and Design
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Life
1. Matter H, O, C, N, P, S and minor
elements 2. Energy

• Solar radiation Photo-synthetic autotrophs
• Organics Heterotrophs
• Inorganics Chemoautotrophs

3
Photo-Synthetic Autotrophs

• Derive energy from sunlight
• H O ? H O C ? CO
• N, P, S, etc. ? dissolved salts
• Not readily soluble
• Eutrophic (life-giving) substances
• N ?NH NO from natural and manmade
  sources
• P ?PO from human body waste, food waste,
  various household detergents

2
2

-
4
3
3-
4
4
Photo-Synthetic Autotrophs

• Form complex high energy organics (H, O, C)
  and produce O2.
• Algae Oxidation (facultative) pond

O2
Algae CO2 sunlight ??O2
Aerobes Organics O2 ??CO2 H2O
Anaerobes Organics ??CO2 CH4 NH3
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Heterotrophs

• Derive energy by oxidizing organics
• Use high energy organics to form more complex
  biomass constituents including proteins
• Energy - Cell mass production
  - Free
  energy generation
  - Heat loss
• When organic energy reduces to zero,
  heterotrophic life ceases.
• Floc formers in activated sludge

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Chemotrophs

• Derive energy by oxidizing inorganics
• Nitrifying bacteria (obligate aerobes)
• Denitrifiers
• Heterotrophs can be forced to utilize NO3-
  NO2-.

Nitrosomonas
Nitrobacter
Denitrifiers
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Phosphorus Removing Mechanism
Energy
Facultative bacteria
Acinetobacter spp.
Acetate plus fermentation products
Substrate
(Phosphorus removing bacteria, slow grower)
Poly-P
PHB
Anaerobic
Aerobic
Energy
PHB

Poly-P
New biomass
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Redox Reaction
Organic molecule (electron donor)
oxidized

-
CO , H , and e
2
reduced
A molecule (electron acceptor)
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Aerobic condition
O

• Aerobic respiration
• O2 present
• Electron acceptor O2

H O
2
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Anoxic Condition
A

• Anaerobic respiration
• NO and NO present
• Electron acceptor NO and NO

2
3
2
3
N
H O
2
2
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Anaerobic Condition
AN

• Fermentation
• No O , NO , NO , or SO present
• Electron acceptor endogenously generated by
  the microorganism

2-
-
-
2
4
2
3
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COD/VSS ²COD(bac)/²Xa, fcv

• ²COD(sol) ²COD(bacteria) ²O (utilized)
• ²O2 (1 - YCOD) ²COD(sol)
• YCOD ²COD(bacteria)/²COD(sol)
• 
  
• fcv Yh
• (1 - fcvYh) ²COD(sol)

2
Yh
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Empirical Stoichiometric Formulation
Biomass
113 g VSS ?? 32 ? 5 g O ? 160 g COD i.e., 1 mg
VSS ? 160/113 1.42 mg COD COD/VSS (fcv) 1.42
mg COD/mg VSS Measured fcv 1.48 mg COD/mg VSS
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Subdivision of Influent COD
Influent COD (Sti)
100
Unbiodegradable COD (Sui)
Biodegradable COD (Sbi)
80
20
Particulate unbiodeg. COD (Supi)
Soluble unbiodeg. COD (Susi)
Partic. slowly biodegradable COD (Sbpi)
Sol. readily biodegradable COD (Sbsi)
13
7
20
60
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Biodegradable Unbiodegradable Fractions

• 1. Measure carbonaceous oxygen demand (Oc), Yh,
  and bh from a lab-scale exp.
• 2. By trial and error, find Sbi value that
  balances the equation below.
• M(Oc) M(Osynthesis) M(Oendogenous decay)
• (1-fcvYh)M(Sbi) fcv(1-f)bhM(Xa) (mg
  O/d)
• M(Sbi) Q Sbi M(Xa) Q Xa

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Graphical Determination of Carbonaceous Oxygen
Demand
OUR/Xa Yh' SUR bh'
OUR/Xa, 1/day
slope Yh'
bh'
SUR, 1/day
Inhibit nitrification by addition of
thiourea. OUR oxygen uptake rate, mg
O/L/day SUR substrate utilization rate, mg
COD/mg VSS/day
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Graphical Determination of Yield Coefficient
(Yh) and Endogenous Decay Coefficient (bh)
1/Rs Yh SUR - bh
1/Rs, 1/day
slope Yh
0
bh
SUR, 1/day
Rs sludge age, 1/day Yh yield coefficient, mg
VSS/mg COD bh endogenous decay, 1/day
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Graphical Determination of Unbiodeg. Sol. COD
(Susi) and Substrate Removal Rate Const. (n)
SUR n(Ste - Susi)
SUR, 1/day
slope n
Susi
Ste, mg/L
Supi Sti - Sbi - Susi
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Determination of Readily Biodegradable Soluble
COD (Sbsi)

• Consists of simple organic molecules such as
  volatile fatty acids (VFAs) and low molecular
  weight carbohydrates that can pass through the
  cell membrane and be metabolized within minutes.
• Sbsi Total truly sol. CODinf - Non-readily
  sol. CODinf
• Total truly sol. CODinf is determined by
  flocculating with Zn(OH)2 at pH 10.5 and
  filtering with a 0.45 µm filter.
• Non-readily sol. CODinf is determined by
  performing the above test with the effluent of a
  24 hr fill-and-draw activated sludge system (MCRT
  gt 3 days).

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Flocculation Method

• Add 1 mL of a 100 g/L zinc sulfate solution to a
  100 mL wastewater sample and mix vigorously with
  a magnetic stirrer for 1 min.
• Adjust the pH to approx. 10.5 with 6 M sodium
  hydroxide solution.
• Settle quiescently for a few minutes.
• Withdraw clear supernatant (20 30 mL) with a
  pipette and pass through a 0.45 µm membrane
  filter.
• Measure COD on the filtrate.

Reference A rapid physical-chemical method for
the determi- nation of readily biodegradable
soluble COD in municipal wastewater, Mamais et
al., Water Research, 27(1), 1993.
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Influent Wastewater COD Fractionsfor settled and
unsettled sewage

• Sewage
• Sewage fraction Unsettled Settled
• Soluble unbiodegradable fraction, fus 0.05
  0.08
• Particulate unbiodegradable fraction, fup
  0.13 0.04
• MLVSS/MLSS ratio (fi) 0.75 0.83

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Subdivision of Total Influent TKN
Influent TKN (Nti)
100
Organically bound N (Nti - Nai)
25
75
Biodegrad. N (Nai)
Unbiodegrad. soluble N (Nui)
Unbiodegrad. Particulate N (Npi)
12
3
10
0.1 Xii
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Subdivision of Total Influent P
Influent TP (Pti)
100
Organically bound P (Pti - Psi)
70 90
10 30
10 20 in the activated sludge process
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Effects of Waste Characteristics on Design

• 1. Influent COD Q (mean daily flow)
• Q affects the design of the secondary
  clarifier.

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Effects of Waste Characteristics on Design

• 2. Influent TKN, µnmT, and temp.
• Sludge age will be controlled by the level of
  energy removal.
• e.g. Carbonaceous removal 3 days of sludge
  age
• Nitrogenous removal depends on µnmT
• Nitrifiers are temperature sensitive.
• e.g. A nitrification-denitrification plant,
  µnm 0.3
• At T 20C, 4 days of sludge age
• At T 12C, 15 - 20 days of sludge
  age

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Effects of Waste Characteristics on Design

• 3. Readily slowly biodegradable COD
• COD for denitrification Sbsi Sbpi SBiomass
  lysis
• 4. Influent TP/COD concentration ratio (Pti/Sti)
• TP/CODinf 0.0170.02 mg P/mg CODinf
• If TP/CODinf lt 0.0170.02
  Effl. P 0.5 mg
  P/L possible
• If TP/CODinf gt 0.0170.02 Chemical
  precipitation necessary

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Effects of Waste Characteristics on Design
5. Influent TKN/COD concentration ratio (Nti/Sti)
a
Effluent
A
O
O
A
s
Waste
TKN/COD ratio
a
Effluent
O
A
O
A
s
Waste
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Effects of Waste Characteristics on Design
5. Influent TKN/COD concentration ratio (Nti/Sti)
- cont.

• TKN/COD lt 0.09 Bardenpho process
• TKN/COD gt 0.10 Modified Ludzack-Ettinger
  process
  (MLE)
• TKN/COD lt 0.07 0.08 A/O, A2/O, Phoredox
  process (modified
  Bardenpho)
• TKN/COD lt 0.12 0.14 UCT process
• TKN/COD lt 0.11 Modified UCT process

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Reactor Types
Mixing regime
Completely mixed
Plug flow
  
n CSTR
1
1 CSTR
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Sludge Age, Rs
Waste
q
Aeration basin
Secondary clarifier
Effluent
Influent
Xv, Vp
Q
Xve
s
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Short Sludge Ages (1 5 days)

• COD removal only
  
• BOD/COD reduction 75 90
  
• Predatory activity, which causes turbidity
  and high effluent COD, is relatively low.
  
• No nitrification

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Intermediate Sludge Ages (10 15 days)

• Effluent COD and ammonia are no longer an
  important design factor.
• Sludge age is determined by the requirement for
  nitrification.
• Nitrification causes a significant pH reduction,
  often as low as 5.
• Once denitrification is considered, sludge ages
  longer than 10 15 days are required.
• Oxygen demand per kg COD is doubled and the
  process volume is 3 4 times larger.

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Intermediate Sludge Ages (10 15 days)

• Denitrification in the secondary clarifier
  takes place, causing sludge flotation by nitrogen
  gas bubbles.
• The secondary clarifier may not serve the dual
  purpose of solid-liquid separation and
  thickening.
• Sludge residence time must be minimized by
  increasing the underflow recycle ratio to 1 to
  21.

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Long Sludge Ages (20 days or more)
Aerobic process

• Called "extended aeration plants"
• Compared to intermediate sludge age plants, the
  total oxygen demand is about equivalent and the
  process volume is 50 60 larger.
• When treating low alkalinity wastewater, the
  problem of low pH is expected.
• Problem of rising sludge is expected.
• A low COD effluent but with high nitrate and
  phosphate is expected.
• An anoxic zone will prevent low pH and reduce
  nitrate concentration.

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Long Sludge Ages (20 days or more)
Anoxic-aerobic process

• Nitrification/denitrification occur.
• Effluent nitrate conc. is reduced.
• Total oxygen demand can be reduced to 15 25
  compared with nitrification process.
• Problem of rising sludge is eliminated.
• Problem of low pH effluent is eliminated.

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Long Sludge Ages (20 days or more)
Anaerobic-anoxic-aerobic process

• Nitrification/denitrification and P removal
  occur.
• Aeration (oxygen) control is a problem under
  cyclic load and flow conditions.
• Load and flow equalization may be required.
• When the sludge becomes anaerobic or is
  anaerobically digested, P will be released from
  the sludge mass to the bulk liquid.

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Nitrification
Nitrosomonas
Nitrobacter
The conversion of ammonia to nitrate (as N)
requires 4.57 mg O/mg NH3-N.
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Nitrosomonas Kinetics
Growth
Monod eq. for nitrifiers
Growth rate
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Nitrosomonas Kinetics
Endogenous respiration
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Minimum Sludge Age for Nitrification
Applicable for all Nai gt 5 mg N/L
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Factors Influencing Nitrification

• 1. Influent source
• µnm specific to the source of the waste and
  even different from batch to batch from the same
  source should be classified as a
  wastewatecharacteristic. Ranges 0.30 to 0.65
  1/day
• The test is performed in a single completely
  mixed reactor at about 6 to 10 day sludge age
  with alternating cycles of anoxic and aerobic
  periods of 2 to 3 hours each.

bnT empirically taken as constant for all waste
flows
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Factors Influencing Nitrification

• 2. Temperature
• For every 6C drop, the value will halve.
• Design for nitrification plant should be
  based on the minimum expected temperature.

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Factors Influencing Nitrification

• 3. pH
• Optimal nitrification pH 7.2 lt pH lt 8.5
• For 7.2 lt pH lt 8.5
• For 5 lt pH lt 7.2

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Factors Influencing Nitrification

• 4. Alkalinity (as CaCO3)
• 2 moles of hydrogen ion 1 mole of alkalinity
• If alkalinity lt 40 mg/L as CaCO3 then
  dangerous.
• Ex. alk. 200 mg/L as CaCO3
  nitrate production 24 mg
  N/L.
• Expected alk. 200 - 7.1424 29 mg/L as
  CaCO3.

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Factors Influencing Nitrification

• 5. Unaerated zones
• Assumptions nitrifiers grow only in the aerobic
  zone. Endogenous decay occurs under both aerobic
  and anoxic conditions. The nitrifier
  concentrations in the aerated and unaerated zones
  are equal.

Na ammonia concentration, mg NH3-N/L
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Factors Influencing Nitrification

• 5. Unaerated zones - cont.
• Minimum sludge age
• Minimum aerobic sludge mass fraction
• Maximum allowable unaerated mass fraction

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Factors Influencing Nitrification

• 6. Dissolved oxygen concentration
• O oxygen conc. in bulk liquid (mg O/L)
• Ko half saturation const. (mg O/L)
• µnmo maximum specific growth rate (1/day)
• µno specific growth rate (1/day)
• Ko 0.3 2 mg O/L
• Minimum oxygen conc. 2 mg O/L

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Factors Influencing Nitrification

• 7. Stimulation of Nitrifying Bacteria

Compound Concentration, mg/L Calcium 0.5
Copper 0.005 - 0.03 Iron 7.0 Magnesium 0.03
- 12.5 Molybdenum 0.001 - 1.0 Nickel 0.1
Phosphorus 310.0 Zinc 1.0
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Factors Influencing Nitrification

• 8. Nitrification Inhibition and Toxicity
• Susceptible to toxic chemicals discharged into
  municipal and industrial wastewater treatment
  plants
• Complete inhibition to Nitrosomonas
• Nickel 0.25 3.0 mg/L
• Chromium 0.25 mg/L
• Copper 0.1 0.5 mg/L
• Zinc 3.0 mg/L
• Many organic compounds are also toxic.
• NH4 and NO2- concentration range for Nitrobacter
• pH NH4-N, mg/L NO2--N,
  mg/L
• 6.0 210 2100 30
  330
• 6.5 70 700 88
  1050
• 7.0 20 210 260
  3320
• 7.5 7 70
• 8.0 2 20

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Factors Influencing Nitrification

• 9. Cyclic flow and loading
• A conservative estimate of µnm is essential for
  a safe design. Otherwise, even with a safety
  factor, nitrate concentration in the effluent
  will fluctuate.

Kinetic constants for nitrosomonas
Constant Symbol Value Temp. Eq. no. Specific
yield coef. Yn 0.01 1.000
- Endogenous respiration rate bn
0.04 1.029 5.16 Half saturation coef. Kn
1.00 1.123 5.15b
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Specific Growth Rate
EPA Method
South African Method
for pH lt 7.2
for 7.2 pH lt 8.5
The EPA method gives a higher µnmT value,
resulting in a shorter sludge age and a greater
unaerated mass fraction than the South African
method.
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Biological Denitrification

• Nitrate Reduction in biological systems
• Assimilation Nitrate to ammonia
• Dissimilation or denitrification
• NO3- ??NO2- ??NO ??N2O ??N2
• Bacteria capable of denitrification are both
  heterotrophic and autotrophic. Achromobacter,
  Acinetobacter, Agrobacterium, Alcaligenes,
  Arthrobacter, Bacillus, Chromobacterium,
  Corynebacterium, Flavobacterium, Hypomicrobium,
  Moraxella, Neisseria, Paracoccus,
  Propionibacterium, Pseudomonas, Rhizobium,
  Rhodopseudomonas, Spirillum, and Vibrio
• Thiosphaera pantopropha (heterotroph) is known to
  nitrify and denitrify simultaneously under
  aerobic conditions using acetate as a carbon
  source.

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Biological Denitrification

• When nitrate serves as the electron acceptor, the
  equivalent mass of oxygen (as O) is
• 1 mg NO3-N ? 2.86 mg O as O
• Thus, for nitrification, 4.57 mg O/mg N are
  required, but in denitrification, 2.86 mg O/mg N
  can be recovered, i.e., with denitrification,
  2.86/4.57100 63 of the oxygen demand for
  nitrification can be recovered.

Overall nitrate removal (empirical)
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Requirement for Denitrification

• 1. Presence of nitrate (or nitrite)
• 2. Absence of dissolved oxygen
  When DO 0 mg/L, 100 denitrification
  When DO 0.2 mg/L, no significant
  denitrification.
• 3. A facultative bacterial mass
• 4. Presence of a suitable electron donor (energy
  source)
  Addition of readily biodegradable COD increases
  the denitrification potential.

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Denitrification Reaction

• Nn nitrate conc. (mg N/L)
• K specific denitrification const. (mg N/mg
  VSS/day)

Difference between inf. eff. nitrate conc.
²Na Nni - Nno K Xa Ra System
removal ²Nns (a 1) ²Na
Ra actual retention time
Rn nominal retention time
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Nitrate Reduction in Primary Anoxic Reactor
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Primary Anoxic Reactor
Nitrate reduction

• ²Nnps ²Nn1s ²Nn2s
• K1Xat1 (a 1) K2XaRap (a 1)
• K1Xat1 (a 1) K2XaRnp
• t1 duration of the first denitrification phase.

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Primary Anoxic Reactor
Nitrate reduction

• ²Nn1s ?Sbi
• ? 0.028 mg N/mg biodegradable COD
• fbs (1 - fcv Yh)/2.86

²Nnps K1Xat1 (a 1) K2XaRnp
?Sbi K2XaRnp
Duration of the 1st denitrification phase
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Secondary Anoxic Reactor
Nitrate reduction
²Nnss K3XaRns
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Nitrate Removal and Substrate Utilization

• For the utilization of 1 mg COD under aerobic
  conditions, an amount of 0.33 mg oxygen (1 -
  fcvYh 1 - 1.48 0.45) is required. As the
  oxygen equivalent of nitrate is 2.86 mg O/mg N,
  the nitrate consumption per mg COD utilized is (1
  - fcvYh)/2.86 0.116 mg N/mg COD or conversely
  8.6 mg COD are required to reduce 1 mg nitrate
  nitrogen.
• The first phase of denitrification rate is
  associated with the utilization of the readily
  biodegradable COD of the influent.

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Alkalinity Change

• Biological denitrification is accompanied by
  an increase in alkalinity.
• 3.57 mg/L as CaCO3 alkalinity recovered
  per 1 mg NO3-N denitrified
• During nitrification, 7.14 mg/L as CaCO3
  alkalinity is consumed per 1 mg NH3-N nitrified.
  Hence, for low alkalinity waste, denitrification
  is strongly recommended to prevent the pH drop.

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Biological Phosphorus Removal (BPR)
Return sludge
Influent
Anaerobic
Aerobic
To clarifier
Orthophosphorus
Conc.
Sol. BOD
Time
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Phosphate Release and Uptake -Secondary Release
70
Biological Phosphorus Removal (BPR)

• Phosphorus Removal Mechanism
• Entirely by disposal of sludge containing larger
  portions of phosphorus in the biomass than
  conventional activated sludge

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Phosphate Release and Uptake -Secondary Release
Design Implications

• For fairly fresh, weak sewage, any effort at acid
  fermentation in the anaerobic basin (by
  increasing HRT in the anaerobic zone) is
  counterproductive.
• For strong, partially fermented sewage, a longer
  anaerobic retention time may be useful.
• With prefermentation, the designer can make sure
  that conditions will be optimal for the growth of
  acinetobacter even with weak wastewater and low
  temperatures.

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Phosphate Release and Uptake -Secondary Release
Design Implications

• Without prefermentation, the five-stage Bardenpho
  performed reasonably, but when the plant was
  switched to the UCT configuration, phosphate
  removal was lost until the size of the anaerobic
  basin configuration was doubled (Daspoort,
  Pretoria) - longer anaerobic mass fraction
  increased the VFA production.
• Phosphate removal was improved by switching to
  the UCT mode even though nitrate were not present
  in the RAS underflow (Westbank, British Columbia)
  - reducing anaerobic mass fraction improved the
  plant performance.

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Importance of Nitrate for P Removal

• Nitrates constitute the single most important
  factor that must be controlled to ensure good
  performance by BNR plants.
• Exception Some wastes containing sufficientn the
  RAS. In situations whether most wastewater is
  pumped to the plant and the temperatures are
  favorable, P removal will take place without a
  serious attempt at reducing nitrates.
• When high percentage removal of both N and P is
  required, the plant should be biased towards
  nitrogen removal, since augmenting P removal with
  chemicals is much less costly and easier to
  control.

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Importance of Nitrate for P Removal

• VFAs as well as other easily degradable materials
  are responsible for the first high rate of
  denitrification. However, the idea is to reserve
  the SCVFA for the BioP organisms. If the
  available carbon is not sufficientan be augmented
  by prefermentation.
• The major portion of the BioP organisms removes
  little or no nitrates. Thus, the food they
  remove preferentially is not available for
  denitrification.
• Although some of the VFAs needed for the rapid
  denitrification in the anoxic zone has been
  sequestered by the BioP organisms, the stored COD
  more than makes up for the shortfall and allows a
  high percentage denitrification.

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Biological Phosphorus Removal (BPR)
Technical Feasibility

• With BPR processes alone, an effluent total P
  limit of 1.0 to 2.0 mg/L can be achieved if the
  practice of anaerobic digester supernatant
  recycle were terminated or if digester
  supernatant were treated chemically to remove
  released phosphorus prior to recycle to the
  biological process.
• To achieve an effluent total P limit of 1.0 mg/L,
  effluent filtration or chemical precipitation
  will be required.
• To consistently achieve an effluent total P limit
  of 0.3, all the BPR processes require final
  effluent filtration.

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Biological Phosphorus Removal (BPR)
Technical Feasibility - Cont.

• BPR facilities should be designed to biologically
  reduce the phosphorus content of the wastewater
  to a practical minimum and the residual
  phosphorus should be removed chemically to the
  prescribed effluent limit. Chemical back-up to
  the BPR process is recommended.
• More attention needs to be devoted to sludge
  management practices.

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Biological Phosphorus Removal (BPR)
Performance Improvement

• Flow and load equalization to the BPR process
• Exclusion of recycle streams containing high
  phosphate (soluble and/or particulate)
• Compartmentalization of the BPR basins
• Flexibility in use of BPR basin volumes between
  anaerobic, anoxic, and aerobic environments
• Ability to provide a source of readily
  metabolizable soluble carbon in the influent to
  the BPR process
• Maintenance of an oxygen-free and nitrate-free
  environment in the anaerobic basin

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Biological Phosphorus Removal (BPR)

• The minimum readily biodegradable COD
  concentration in the anaerobic reactor (Sbsa) to
  simulate phosphorus release in the reactor is
  about 25 mg COD/L.
• The degree of P release appears to increase as
  Sbsa increases above 25 mg COD/L, i.e., P release
  increases as (Sbsa-25) increases.
• Excess phosphorus uptake is obtained only when
  phosphorus release takes place, and tends to
  increase with (Sbsa-25).

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Determination of Sbsa

• The UCT Process

• The Phoredox Process

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Biological Phosphorus Removal (BPR)
Hypotheses on P removal

• 1. Excess P removal is obtained only when Sbsa gt
  25 mg COD/L.
• 2. As Sbsa increases above 25 mg COD/L, so the P
  removal increases.
• 3. The longer the actual anaerobic retention time
  (Ran), the higher the P removal.
• 4. The larger the mass of sludge recycled through
  the anaerobic reactor per day expressed as a
  fraction of the mass of sludge in the process, n,
  the higher the P removal.

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Biological Phosphorus Removal (BPR)
Hypotheses on P removal - Cont.

• When any one of the factors (Sbsa-25), Ran, or n
  is zero, excess P removal will be zero.
• Excess P removal propensity factor, Pf, can be
  expressed as follows
• Pf (Sbsa - 25) Ran n
  
• The P removal due to excess uptake in the sludge,
  Ps, is
• Ps f(Pf)

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Biological Phosphorus Removal (BPR)
Hypotheses on P removal - Cont.

• Ran n fxa (anaerobic mass fraction). Then,
• Pf (Sbsa - 25) fxa when Sbsa gt 25
  Pf 0.0 when Sbsa 25
• Semi-empirical model for Ps (mg P/L)

fp P content in the biomass (mg P/mg VSS)
? 0.35 - 0.29 exp (-0.242Pf) (mg P/mg VASS)
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?vs Pf Observed in BPR Processes
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Biological Phosphorus Removal (BPR)
Conditions for the Empirical Model

• Influent COD conc. 250 - 800 mg COD/L
• Readily biodegradable COD 70 - 220 mg COD/L
  i.e., fraction fts 0.12 - 0.27
• TKN/COD ratio 0.09 - 0.14
• Sludge age 13 - 25 days
• Temperature 12 - 20C
• The use of this model must be limited strictly to
  within the ranges of process parameters and
  wastewater characteristics listed herein.

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P Removal per mg COD Load
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Pf, Sbsa, and ?vs Nitrate Concentration
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Case HistoriesLargo WWTP, Largo, Fl

• Facility Description
• Preliminary, primary, and secondary treatment
  plus effluent filtration and disinfection
• 15 MGD lt 10 days of sludge age gt 20C
• The A2/O process to remove both N and P
• HRT 0.8 hr in the anaerobic zone, 0.5 hr in the
  anoxic zone, and 2.9 hrs in the aerobic zone
• Effluent Limits
• Effluent TBOD5 and TSS 5 mg/L
• N limitations Total N - Annual avg. 8 mg/L
  Monthly avg. 12 mg/L Weekly avg. 18 mg/L
• Effluent ammonia-nitrogen
• Monthly avg. 2 mg/L Weekly avg. 3 mg/L

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Largo WWTP, Largo, Fl

• Wastewater Characteristics
• A typical medium strength
• Operating Results
• Avg. plant Q 9.9 MGD MLSS lt 3000 mg/L
• Avg. TBOD5 and TSS 5 and 4 mg/L
• Avg. monthly TN 7.7 mg/L
• RAS 0.5 Q Recycle from A to AX 1 2 Q
• Provided partial nitrogen removal

Parameter Average, mg/L Range, mg/L TBOD5
200 113 375 TSS 325 143 511 TKN,
max. 30 - NH3-N, max. 20 -
89
Palmetto WWTP, Palmetto, Fl
Primary Tanks
Mixers
Influent
Effluent
Aerators
Sludge
Secondary Clarifiers
Chlorine
R lt 4 Q
P
Sand Filters
1 hr
1 hr
4.7 hr
P
2.7 hr
2.2 hr
Pump Station
Return Sludge
To Drying Beds
90
Palmetto WWTP, Palmetto, Fl

• Effluent Limits
• TBOD5 5 mg/L
• TSS 5 mg/L
• TN 3 mg/L
• TP 1 mg/L
• Wastewater Characteristics
• A typical medium strength

Design value, mg/L Observed value,
mg/L Average Range TBOD5 270 1
58 87 232 TSS 250 135 70
224 TKN 43 33.1 15.1 45.9 Temp. -
- 18 25C
91
Palmetto WWTP, Palmetto, Fl

• Operating Results
• During the operation, the plant was loaded at and
  above its design hydraulic capacity (1.4 MGD),
  but it was underloaded with respect to organic
  and nutrient loadings.
• Q 0.74 2.44 MGD TBOD5 54 of design
• Sludge age 14 (summer) to 20 (winter) days
• Avg. MLSS 4090 mg/L design MLSS 3500 mg/L
• Met its effluent permit limitations
• Summary
• A successfully operating Bardenpho nutrient
  removal plant
• The need to provide an adequate sludge age
  capacity for extensive N removal.

92
Guidelines for Biological Nutrient Removal (BNR)
Process Selection

• Nitrogen Removal
• Four Stage Bardenpho Process
• Modified Ludzack-Ettinger (MLE) Process
• Phosphorus Removal Only
• A/O Process
• Nitrogen and Phosphorus Removal
• Five Stage Bardenpho (Phoredox) Process
• University of Cape Town (UCT) Process
• Modified UCT Process
• Virginia Initiate Process (VIP)

93
Process Selection Based on TKN/COD ratio
(Initial Screening)

• Nitrogen Removal
• TKN/COD lt 0.09 Bardenpho process
• TKN/COD gt 0.10 MLE process
• Nitrogen and/or Phosphorus Removal
• TKN/COD lt 0.07 0.08 A/O, A2/O, Phoredox
  process (modified Bardenpho)
• TKN/COD lt 0.12 0.14 UCT process
• TKN/COD lt 0.11 Modified UCT process

94
BPR process requirements for variouseffluent
total phosphorus limits(CANVIRO Consultants Ltd,
1986)

• Eff. TP, mg/L Phoredox UCT A/O
  Phostrip
• 1.0 - 2.0 BPR BPR BPR BPR
• 1.0 BPR filt. BPR filt. BPR
  filt. BPR or or
  or BPR chem. BPR chem. BPR chem.
• 0.3 BPR filt. BPR filt. BPR
  filt. BPR filt. chem. chem.
  chem.

CANVIRO Consultants ltd (1986). Retrofitting
Municipal Wastewater Treatment Plants for
Enhanced Biological Phosphorus Removal,
Environmental Canada, Report EPS 3/UP/3.
95
Biological Nutrient Removal (BNR)Process
Comparison
(Daigger et al., 1986)

• Nutrient removal Sludge
  Effluent
• capability
  disposal Chem. filtration Oper. Oper.
  Cost
• Process P N impact
  req. req. flex. reliability
  impact
• Phostrip Best Little M M Least G G M
• Bardenpho Least B L L Most L M H
• Oxidation ditch NA G L L NA M M L
• A2/O M M L L Most L L L
• UCT G M L L Most M M M
• Chemical treat. Best Little H H Least B B HH

Daigger, G.T., Smith, J.J., and Simpkin, T.J.
(1986). Removal of Nutrients from Wastewater
Using Biological Processes, Presented at the 59th
Annual Conference of the Central States Water
Pollution Control Association.
96
Comparative Mass Balances
(Daigger et al., 1986)

• Item
  Conventional ENR
• Total P (mg P/L)
• Influent 6 6
• Equiv. P-conc. in waste sludge 1.5 4.5
• Effluent (by difference) 4.5 1.5
• Removal efficiency () 25 75
• Alum required (lb/mgal) 584 83
• Cost of alum (/mgal)
  39.4 5.6
• Nitrogen (mg P/L)
• Influent TKN 30.0 30.0
• Equiv. N-conc. in waste sludge 8.9 8.9
• Sol. nonbiodeg. TKN 1.5 1.5
• Nitrate before denitrification 22.6 7.9
• Denitrified 0 14.7
• Effluent nitrate (by difference) 22.6 7.9
• Effluent total nitrogen 24.1 9.4
• Removal efficiency () 20 69

97
Process Oxygen and Alkalinity Requirement
(Daigger et al., 1986)

• Item
  Conventional ENR
• Oxygen demand (mg/L)
• Carbonaceous 140 140
• Nitrogenous 104 104
• Credit for denitrification 0 42
• Net (by difference) 244 202
• Savings () 0 17
• Alkalinity (mg/L as CaCO3)
• Consumed by nitrification 163 163
• Produced by denitrification 0 53
• Net consumption (by difference) 163 110
• Savings () 0 32

98
Process Selection Based on CODinf/P ratio
(Initial Screening)

• If TCODinf/TP gt 50 mg CODinf/mg P
  Effl. sol.
  P 0.5 mg P/L is possible with biological
  phosphorus removal processes
• If 40 TCODinf/TP 50 mg CODinf/mg P Effl.
  sol. P 1 mg P/L is possible with biological
  phosphorus removal processes
• If TCODinf/TP lt 40 mg CODinf/mg P
  Prefermentation or effluent polishing by chemical
  precipitation is necessary

99
Phosphorus vs COD Limitation

• If phosphorus is limiting, the available organics
  will not be completely removed in the anaerobic
  stage and soluble organics will enter the aerobic
  stage. Thus, the aerobic zone size should be
  enlarged.
• If COD is limiting, the P removal will be limited
  and the desired effluent P concentration may not
  be achievable without prefermentation or
  supplemental chemical addition.
• If biodeg. CODTP ratio is considerably higher
  than 401 whereas the BOD5TP ratio is
  considerably lower than 201, then the wastewater
  has not undergone substantial fermentation and
  thus make the anaerobic zone larger.

100
Aeration Requirements

• In general, oxygen requirements are reduced by
  BPR processes.
• It is recommended that the theoretical
  require-ments be reduced by 10 for design
  purposes.
• During aeration with draft tubes, there exists no
  or very low DO in the mixed liquor. This results
  in denitrification in the aeration basin varying
  from 30 to 100. Assume 10 to 20 of the total
  nitrogen nitrified will be lost through
  simultaneous denitrification in the diffused-air
  aeration basin.

101
Modified Ludzack-Ettinger Process (MLE)
a
A Anoxic O Aerobic
Effluent
A
O
s
Waste

• Nitrogen removal only.
• First biological nitrification-denitrification
  process.
• Complete denitrification is not possible.

102
Bardenpho Process
a, 4Q
Effluent
A
A
O
O
s, 0.5Q
Waste

• Nitrogen removal only. P removal incidental.
• Introduced a flash aeration basin between the
  secondary anoxic reactor and the clarifier to
  strip N2.
• Maintain thin sludge blanket to prevent sludge
  flotation due to denitrification of residual
  nitrate.

103
Modified Bardenpho (Phoredox) Process
a, 4Q
AN Anaerobic
Effluent
A
O
A
O
AN
s, 0.5Q
Waste

• Nitrogen/phosphorus removal.
• Maintain thin sludge blanket to prevent sludge
  flotation due to denitrification of residual
  nitrate.

104
The Phoredox Process

• An anaerobic fraction (fxa) of the total
  unaerated sludge mass fraction (fxt) is set aside
  to establish the prerequisites for excess
  phosphorus removal.
• If no nitrate is to be recycled to the anaerobic
  reactor, complete denitrification must be
  achieved in the anoxic sludge mass fraction
  (fxdt fxt-fxa).
• Complete denitrification is achieved only when
  the TKN/COD ratio lt 0.085. As a safety, the
  ratio should not exceed 0.07 to 0.08 at 14C for
  sludge ages 20 to 30 days. This restricts
  application of this process for municipal
  waste-water treatment having higher TKN/COD
  ratios.

105
Modified Bardenpho (Phoredox) Process

• Five-stage Bardenpho basin hydraulic retention
  times (hrs)
• Basin Typical range Palmetto, Fl. Kelowna,
  B.C.
• Anaerobic 1 - 2 1.0 2.9
• First anoxic 2 - 4 2.7 2.9
• First aerobic 3 - 8 4.7
  8.6
• Second anoxic 2 - 4 2.2
  3.8
• Second aerobic 0.5 - 1 1.0
  1.9
• Total 8.5 - 19 11.6 20.1

106
Three Stage Phoredox Process
a, 12Q
Effluent
O
AN
A
s, 0.5Q
Waste

• Modified for partial denitrification.
• Basically identical to A2/O process.
• For A2/O, basins are tightly compartmentalized.

107
Two Stage Phoredox Process
Effluent
O
AN
s, 0.5Q
Waste

• No nitrification.
• Basically identical to A/O process.
• Greater degree of compartmentalization of the
  basins in A/O system.
• A/O process uses high purity oxygen while this
  process uses air for aeration.

108
A/O Design Considerations

• Size of the anaerobic zone, prefermentation,
  sludge age subdivision of anaerobic zone, mixing
  requirements
• Incorporate sufficient flexibility for the
  operator to adjust the system to varying
  conditions.
• Prevent significant entrainment of DO into the
  anaerobic mixed liquor phosphorus during
  clarification and recycle of phosphorus from
  sludge processing.

109
A/O and A2/O Processes

• Typical A/O and A2/O design and operating
  parameters
• Variable Units Range
• Influent retention time
• Anaerobic section hrs 0.5 - 1.0
• Anoxic section hrs 0.5 - 1.0
• Aerobic section
• Non-nitrifying (A/O) hrs 1.8 - 2.5
• Nitrifying (A2/O) hrs 3.5 - 6.0
• F/M ratio kg
  BOD5/kg MLVSSday 0.15 - 0.7
• Soluble BOD5/soluble P (influent) -
  10
• Mixed liquor suspended solids mg/L 2000 - 4000
• Temperature C 5 - 30
• RAS recycle rate of
  influent flow 25 - 75
• Internal mixed liquor recycle rate of influent
  flow 50 - 250
• Basin configuration type -
  Staged system
• Number of stages
• Anaerobic/anoxic/aerobic - 3/3/4

110
Virginia Initiate Plant (VIP) Process
r, 12Q
a, 12Q
Effluent
A
O
AN
S, 0.5Q
Waste

• Similar to the UCT process.
• Multiple complete mix cells are used for the
  anaerobic, anoxic and aerobic treatment zones to
  increase the phosphorus uptake rate by virtue of
  a higher concen-tration of residual organics in
  the first aerobic cell.
• The VIP process is designed for a total sludge
  age of 5 to 10 days while the UCT process is
  generally designed for an sludge age of 13 to 25
  days.

111
University of Cape Town (UCT) Process
r, 12Q
a, 12Q
Effluent
A
O
AN
S, 0.5Q
Waste

• RAS passes through "A" basin prior to entering
  "AN" basin for residual NO3- removal thus,
  provides an additional barrier to the entry of
  NO3- into the anaerobic basin.
• Full-scale confirmation of design and performance
  data are presently lacking.

112
The UCT Process

• For TKN/COD ratios gt 0.14, nitrate will be
  present in the primary anoxic reactor and a
  discharge of nitrate to the anaerobic reactor
  cannot be avoided leading to a decline in excess
  P removal. As a safety, the upper limit is 0.12
  to 0.14. This limit is above that for most
  settled and raw municipal wastewaters.
• Problems associated with the UCT process
• 1. Process control 2. Sludge settleability

113
University of Cape Town (UCT) Process

• The a-recycle must be carefully controlled to
  just underload the primary anoxic basin with
  nitrate to avoid a nitrate discharge to the
  anaerobic basin. Under full-scale operation such
  careful control of a-recycle is not possible due
  to uncertainty in the TKN/COD ratio.
• As the TKN/COD ratio increases, the a-recycle
  ratio needs to be decreased to avoid a nitrate
  discharge to the anaerobic basin, which in turn
  causes an increase in the actual anoxic retention
  time. For inf. COD gt 500 mg/L and TKN/COD ratio
  gt 0.11, the actual anoxic retention time exceeds
  1 hr, causing the decline of sludge
  settleability.

114
Modified UCT Process
a, 12Q
r, 12Q
Effluent
A
O
A
AN
s, 0.5Q
Waste

• Avoids careful control of a-recycle.
• Limit the anoxic retention time to 1 hr to
  improve sludge settleability.

115
The Modified UCT Process

• Process control The UCT process requires a
  careful control of a-recyle to the primary anoxic
  reactor to avoid a nitrate discharge to the
  anaerobic reactor, which is impossible due to
  uncertainty in the TKN/COD ratio, particularly
  under cyclic flow and load conditions.
• Sludge settleability When the actual retention
  time exceeded 1 hr, the sludge settleability
  declined. To preserve good settleability of the
  sludge, the actual anoxic retention time should
  be limited at 1 hr.

116
Schematic Process Configurationfor Optional
Operations
Mixed liquor recycle, r
Mixed liquor recycle, a
Secondary clarifier
Anoxic
Anaerobic
Aerobic
Influent
Effluent
Sludge recycle, s
Phoredox process
UCT process
Modified UCT process
117
Retrofit of Existing Plants
Considerations

• Aeration basin size and configuration
• Clarifier capacity
• Aeration requirements
• Type of aeration system
• Sludge processing units
• Operator skills

118
Aeration Basin Size and Configuration

• No need to increase the size because the removal
  of substrate in the anaerobic zone is more rapid
  than in the aerobic zone of equal size.
• A plug flow basin is the easiest type to retrofit.

Anaerobic zone
Aerobic zone - no change
119
Clarifier Modification

• Usuallythan centerfeed clarifiers because the
  flow is usually up through the sludge blanket.
• Some phosphorus release typically occurs in the
  clarifier sludge blanket of a BPR plant but in a
  properly operated centerfeed clarifier the entire
  sludge blanket plus the released phosphorus is
  drawn off the bottom of the clarifier and
  recycled to the anaerobic zone.

120
Aeration Requirements and Type of Aeration System

• The aeration equipment is usually removed from
  any zone that will permanently become a part of
  the anaerobic zone.
• There is no need to add additional aeration
  equipment because the processes in the anaerobic
  zone reduce the oxygen transfer requirements by
  10 to 20.
• The primary concern should be the protection of
  the anaerobic zone from the recycle of too much
  dissolved oxygen.

121
Sludge Processing Units

• The inclusion of BNR results in a 5 to 15
  reduction in WAS while the inclusion of BPR will
  increase the WAS production slightly.
• The sludge processing units are of primary
  concern.
• The recycle of any soluble P changes the CODP
  ratio entering the activated sludge process.
• The use of anaerobic digesters, gravity
  thickeners for waste activated sludge (WAS), and
  the recycle of the WAS for settling with the
  primary sludge in the primary clarifier are
  detrimental if not properly managed.

122
Sludge Treatment Alternatives for BNR WWTPs
BNR activated sludge
Effluent
Influent
Primary settler
Final settler
Flotation thickening
Gravity thickener
Centrifuge
Linear screens
Lime
Belt press
Centrifuge
Anaerobic digestion
Chemical disinfection
Dewatering
Composting
Landfill
Incineration
Land
123
Sludge Processing Units - continued

• Sludge dewatering
• Separate the thickening of primary sludge and
  WAS.
• Flotation thickening is ideal. After thickening,
  the sludge may be further dewatered by belt press
  with the addition of polymers.
• Note that some polymers inhibit nitrification.
• After thickening or dewatering the sludge may be
  treated by composting, digestion, landfill,
  incineration, heat treatmenttreatment

124
Sludge Processing Units - continued

• Composting
• Primary sludge can be dewatered to 22 solids and
  WAS to 16. No phosphates will be released.
• Digestion (aerobic and anaerobic)
• This will lead to the release of phosphates from
  the microbial cells. In some instances,
  phosphates may be precipitated during anaerobic
  digestion. If the liquid is to be returned

125
Sludge Processing Units - continued

• Landfill
• Phosphates will be bound by the heavy metals in
  the leachate. Usually no problem.
• Incineration
• Separate the primary and secondary sludges. No
  problem.
• Heat treatment
• It may return unwanted (nondegradable) compounds

126
Operator Skills

• Greater operatort the necessary skills are easily
  learned and applied.
• A retraining program for the operators should be
  part of any retrofit project.

127
Case Histories The York River, Virginia,
Wastewater Treatment Plant

• Simultaneousf 7.4 hrs primary sludge gravity
  thickeners, secondary sludge dissolveds, and belt
  filter presses no supernatant recycle from the
  anaerobic digesters but the recycle of the
  filtrate from the belt presses and the
  supernatants from the thickeners.
• The projected retrofit cost is 2 million
  (133,000 per 1 MGD).

128
Case Histories The York River, Virginia,
Wastewater Treatment Plant
Primary clarifier
Influent
Gravity thickener
Anaerobic
Secondary digester
Variable zone
RAS
Primary digester
Aerobic
Belt filter press
WAS
DAF
Nitrate recycle
Filter cake
Effluent
129
Limitations

• There were low concentrations of organics in the
  raw wastewater compared to TKN and P.
• The organic strength of the wastewater was
  reduced by preaeration and primary settling
  before entering the biological process
• Phosphorus was recycled back to the headworks
  from the two sludge thickening processes and from
  anaerobic digestion.
• BOD5P ratio - raw 181 to 271, combined flow
  151, primary effluent 12.51.

130
Process Start-Up
Effluent flow (ML/day) 23.0 Target MCRT
(day) 4 Average HRT (hr) 5.25 Number of Anaerobic
cells in use 4 Number of anoxic cells in
use 0 Anaerobic sludge mass fraction
() 33.3 Aerobic sludge mass fraction
() 66.7 RAS recycle rate () 70
The average effluent P 2.95 mg/L TP or
2.15 mg/L sol. P.
The P in the activated sludge changed from 3
4 to 10 11 during the A/O operation.
131
Sludge Production

• FM 0.47 (BOD5) 0.89 (COD)
• Sludge production 0.48 kg TSS/kg BOD5 removed
  (MCRT 3.2 7.9 days)
• A nutrient removal pilot plant 0.77 kg TSS/kg
  BOD5 removed
• Sludge production 0.26 kg TSS/kg BOD5 removed
  (MCRT 10 14 days)
• 22 mg/L of BOD5 consumed for 1 mg/L of P removed
  by BPR for anaerobic mass fraction of 33.