Nitro, Denitro, P Removal Flashcards
What is Eutrophication?
Unnatural enrichment of P& N in lake which causes algal bloom
Plants die and sink thus decomposed by bacterias and fungi, O2 is comsumed by the bact to decompose and release N and CO2 and energy. O2 depletion causes anoxic environment which kill fish. Additionally, ammonia and H2s can also be released due to anoxia which is poisonous
Limit for Total P and Total N
Total P <1-2 mg/L
Total N < 10-15 mg/L
Nitrogen present in WW as
Ammonia: 50-80% of Total N in WW
Organic Nitrogen: proteins, DNA, etc will be hydrolysed to ammonia
Nitrogen cycle how to reach from NH3 to N2
Annamox: directly
Nitrification: NH3 -> NO2- -> NO3-
Denitrification: NO3- -> N2
How does nitrification occur?
- Oxidation process
- by nitrifying bacteria (Aerobic Chmolithoautotrophs)
C-Source: CO2
e-donor: NH4+
e-acceptor: O2
Two step bio-oxidation process and bacteria involved
Nitritation: NH4->NO2- (Nitrosomonas)
Nitratation: NO2- -> NO3-
(Nitrobacter)
what are the effect of nitration in terms of pH, Oxygen consumption, and alakalinity?
- pH decrease
- requires O2
ammonia ox: 3.43 gO2/gN-NH4+
Nitrite ox: 1.14 gO2/gN-NH4+ - Consume alkalinity: 7.14 g CaCO3/gN
Nitrification Microbiology Facts
- Growth rate of nitrifying bact is slower than those for organic matter, so SRT should be long enough so that F/M is low to enable development of nitrifiers.
- Nitrite doesn’t usually accumulate in system because the rate to oxidize ammonia to produce NO2 is slower than consuming NO2, So so Ammonia oxidation to NO2 is Rate Limiting reaction.
Yield
Y=Produced biomass/consumed substrate (N) or oxidized N
(g COD/gN) or (g VSS/gN)
What are the environmental factors affecting nitrification?
Temperature
pH: sensitive to pH usually occur at almost neutral pH (6-9)
Alkalinity: consumes alkalinity 2 eq so 2x50/14=7.14 gCaCO3/gNoxidized
DO: <0.5 mg/l greatly inhibit, ideally 1.5-2 mg/l
Alkalinity details for nitrification:
how much consumed, how to maintain pH-7
alkalinity consumed: 7.14 g CaCO3/gNoxidized
To maintain pH=7 Alk>70 mgCaCO3/L
Alk goals = Alk in - alk consumed + alk to be added
What is primary settler effect on Nitrogen
Primary setller increases TKN/COD ratio as it remove 30-40% of COD influent and only remove 10-20% TKN
What is minimum sludge age
results of growth and decay, below minimum SRT, biomass is washed out. Minimum SRT should be computed at worst condition. Usually 5 day.
if nitrification and COD removal in one tank, SRTm depends on nitrifiers growth rate
Mass Balance Nitrogen oxidation
Nitrification
Nitrogen oxidized (QSNO3)=Nitrogen influent (QTKNin) - Nitrogen Effluent (QNe) - Nitrogen in cell mass (0.12 Pxbio)
Cell formula C5H7NO2 biomass contains 0.12 gN/gVSS)
px bio = PxH + PxE + PxA
Increasing TSS what will happen
smaller reactor volume (due to reaction rate)
but cannot be increased above reasonable values because aeration limitation, separation capacity in secondary settling tank
What is denitrification
Biological REDUCTION of nitrate to nitric oxide and nitrogen gas (remove excess nitrogen)
Which microrganism involved in denitrification?
Heterotrophs, facultative aerobic organisms where under anoxic nitrate is final e- acceptor
What is strong inhibitor for denitrification?
O2
What is the difference between anaerobic and aerobic respiration in terms of COD or organic compounds
growth is lower in anoxic condition rate is slightlt lower due to lower energy
What are the conditions required for denitrification?
Availability of organic carbon (COD)
Anoxic Condition (Nitrate Presence, DO absence)
What does denitrification produced?
Alkalinity & N2
How much Alkalinity produced from denitrification?
1 mol alk produced per 1 mol N denitrified
3.57 g CaCO3/gN-NO3
What are advantage of denitrification in terms of Alkalinity
- Reduced 50% alkalinity requirements
- Enhanced operation of secondary sedimentation. Control Filamentous bacteria growth, improve settling characteristics.
- Control Nutrients
O2 Equivalent of nitrate
2.86 gO2/gN
which mean N has much higher capacity to remove COD. 1 g of Nitrate can save 2.86 g o2 for aerobic respiration process
Difference between heterotrophs in aerobic and in anaerobic
Reduced growth yield factor and growth rate.
Nitrate is e-acceptor instead of O2 and might influence kinetics so there’s a correction factor (0.7-0.9) for anoxic growth
What are the two flow diagrams of activated sludge denitrification?
- Pre-Anoxic (substrate driven) Most common
- Post Anoxic (endogenous driven)
Advantages of Pre Anoxic
- Saves energy because COD is removed before aerobic zone
- Alkalinity produced before nitrification
- Already includes selector (avoid filamentous growth and sludge bulking)
- No need for additional COD addition
- For high risk of eutrohication risk, primary settler not used because increase TKN/COD Ratio
Maximum N Removal is related to
- COD availability in WW
- Enough Nitrification rate
- Maximum internal recycle Rate (up to 400%Qin)
Related to ratio of flow across tank
C/Cin = Qin/(Qin+Qr+Qn)
COD removed by aerobic and anaerobic?
aerobic: 3.03 g cod/gO2
anaerobic: 8.6 g COD/gN
O2 consumption in nitrification and denitrification
nitrification consume 4.57 gO2/gN
Denitrification saves/recover 2.86 gO2/gN
So in denitrification 2.86/4.57 = 0.63 of O2 is saved
What is SDNR?
Specific Denitrification Rate is rate of how much nitrate is consumed or reduced per day relative to amount of VSS in denitrification tank.
Pre-Anox has higher SDNR than post anox
SDNR related to
organic load to anoxic tank (F/M Ratio)
%of rbcod relative to bcod in influent
temperature
Internal recycle
HRT for denitrification typically
2-10h
How to remove phosphorus?
Biological Process: EBPR
Chemical Process: Alum or iron salts
Hybrid: Phostrip
What is the advantage of EBPR
Reduced chemical costs, less sludge production, recovery more easy
How does Biological Phosphorus Removal works?
By selection of biomass capable to accumulate P inside cell above its P requirement for growth
by PAOs (Phosphorus Accumulating Organisms)
and removed with excess sludge
Explain PAO in Anaerobic conditions
- PAO used energy stored in polyphosphates and assimilate acetate or VFA
- PAO then produce PHB
- Soluble orthophospahe (PO43-)
- PHB content in PAO increase, Polyphosphate decrease
No Growth
Aerobic or Anoxic Condition of PAO (O2 or NO3-)
- Store PHB is metabolized, providing energy from oxidation and carbon from new cell growth
- Energy is used to form polyphosphate in cell storage so soluble ortophosphate is removed from water and stored in cell
- Cell growth occur due to PHB utilization
- New biomass with high polyphosphate storage accounts for phosphorus removal
Sludge P Content
Without PAOs: Approx. 0.02 mg P/mg VSS
or 0.015 mg P/mg TSS
With PAOs: 0.06-0.15 mg P/mg VSS
or 0.05 / 0.10 mg P/mg TSS
PAO can accumulate how much P, how about heteretrophs?
PAOs 0.4 g P/ gVSS
Heterotrophs: 0.02 gP/gVSS
What are PAOs two requisites?
- Alternating anaerobic/aerobic operating conditions (by recirculation)
- Presence of VFAs (can gain from hydrolysis)
Why O2 and Nitrate recycle in anaerobic tank decrease EBPR effectiveness?
Heterotrophic will consume VFAs instead of PAO when O2 and NO3 not available thus competing with PAOs
Explain about PHB production and what it used
Related to fermentable materials (VFA, acetic acid, etc) that is represented as fraction of Ss (rbcod).
In Anaerobic, fraction of Ss is transformed into VFAs
where PAOs assimilate and produce PHB
rbCOD/bCOD ratio meaning in P removal
higher rbCOD/bCOD meaning higher Ss meaning higher formation of VFAs thus greater %P content of Activated sludge, the greater the EBPR
What is phoredox
- Anaerobic tank for P release and the aerobic tank for P assimilation
- Short SRT (2-4d)
- Pros: simple, low BOD/P ratio, good P removal and settling sludge
- Cons: P removal is affected when nitrification occur which might at high T (20c)
in phoredox how is the products like in Anaerobic tank and aerobic tank?
in Anaerobic tank: VFA decrease due to assimilation by PHB, so PHB increase, and Poly-P decrease as it release PO43- to produce energy for PHB to assimilate VFA
in Aerobic tank: PHB use O2 for oxidation and release CO2+H2O and produce energy which poly-P used to uptake PO43-. So, PHB decrease, Poly-P increase, PO43- in water decrease
3-stage modifdied bardenpho is what
A2O
Amaerobic for P release
Anoxic for Denetrification
Aerobic for Nitrification and P Assimilation
Internal recycle from aerobic to anoxic for nitrate
effect of higher anaerobic volume in P removal
Higher anaerobic volume –> higher biomass in sludge anaerobic –> higher VFA –> higher PAO substrate –> higher EBPR
Environmental factors in P removal (SRT and conc of o2 or NO3-)
SRT
SRT < 3d: High P removal due to higher heterotrophic mass & bCOD fermented to VFA
SRT >3d: P removal lower due to extended endogenous phase, lower PAO mass is wasted per day
higher O2 and NO3- presence in recycle, lower P removal due to competition
How to manage sludge EBPR
if sludge undergo another anaerobic condition P will be released again so it the digested sludge/thickened sludge will contain high P concentration.
To prevent: thickening with DAF, gravity belt thickener, stabilization (composting)
for chemical phosphorus removal where to insert the lime or alumunium or ferric iron
Pre-precipitation: before biological treatment line, P removed with primary sludge (lime not used) improve separation in primary settler
Co-precipitation: Just before secondary sedimentation (AS tank) or after it to improve sludge separation. Aeration is used to oxidize Fe(II) to Fe(III)
Post-precipitation: After secondary settler have special tank, will be high efficiency because special for P but expensive. can be combined with other tech like filtration
What are pros and cons of chemical P removal
pros:
1. increased settling characteristics of WW and AS
2. Simple reactors, simple operations
3. Less sensitive to inhibition than biological process
4. rapid interactions, small tank
Cons
1. Very high increase of sludge production
2. Production of chemical sludge (disposal issue)
3. increased water salinity
4. pH neutralization may be required after
5. Cost of chemicals
