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N removal Denitrification and Nitrification How is Den and Nit used for N removal? What are the critical process conditions What is the effect of Oxygen on both? To what extent are Den and Nit exclusive? Can they both happen in the same reactor?
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N removal • Denitrification and Nitrification • How is Den and Nit used for N removal? • What are the critical process conditions • What is the effect of Oxygen on both? • To what extent are Den and Nit exclusive? • Can they both happen in the same reactor? • Can they happen in the same reactor at the same time ? How? • How can process conditions be optimised to achieve simultaneous nitrif, denitrif.? • Eutrophication explain
Examinable concepts on Wast Water Treatment (WWT) • Eutrophication, why is dissolved nitrogen harmful? • Biological WWT, biomass recycle, flocculation • Nitrification, denitrification • Nitrification followed by denitrification, why does it not work? • Alternating nitrification denitrification • Simultaneous N and D (SND) • SND via nitrite • Storage capacity as PHB of most bacteria • Parallel nitrification denitrification • Anammox
Eutrophication • COD BOD nutrients • Biomass rec ycle • Flocs (Stokes law) • SRT> HRT • Intermittent high COD supply • High feed COD/biomass ratio • Either plug flow or SBR • Reactor types • Batch • Chemostat • SBR • Plugflow • Fedbatch
Waste Water Treatment Technology Oxygen supply - Major investment (1 M$/y per treatment plant) Fine bubble diffusers Nitrogen Removal How? : Aerobic Nitrification NH3 + O2 NO3 Anaerobic Denitrification NO3 + organics N2 Problems Nitrifiers grow slow and are sensitive and need oxygen Denitrifiers need organics but no oxygen Nitrification can be either sequential or simultaneous:
List Pollutants to be removed • Suspended material (inorganic, bacteria, organic) • Dissolved organics (COD,BOD) • COD = chemical oxygen demand (mg/L of O2) • dichromate as the oxidant • BOD5 = biochemical oxygen demand(mg/Lof O2 in 5 days • microbial O2 consumption over 5 days • N • P • pathogens • odor, colour • ultimate aim: recycle of water for re-use
Why organic pollutant removal? Organic pollutants represent an oxygen demand (COD or BOD) Bacteria in the environment will degrade the pollutants and use oxygen. If oxgygen uptake > oxygen transfer oxygen depletion . Collapse of ecosystem
Why nutrient removal?Simplified Sequence of events of eutrophication Pristine aquatic ecosystems are typically limited by nutrients. Supply of nutrients (N or P) photosynthetic biomass (primary and secondary). More oxygen production and consumption Sedimentation and decay of dead biomass Depletion of oxygen in sediment/water column Collapse of ecosystem
Why nutrient removal?comprehensive Sequence of events of eutrophication (needs understanding of anaerobic respirations) • Pristine aquatic ecosystems are typically limited by nutrients. • Supply of nutrients (N or P) • photosynthetic biomass (primary and secondary). • More oxygen production and consumption • Sedimentation and decay of dead biomass • Depletion of oxygen in sediment/water column • Oversupply of e- donors • Use of other electron acceptors (anaerobic respirations) • Ferric iron reduction to ferrous iron (Fe3+ --> Fe2+) • Sulfate reduction to sulfide (H2S) (poison, oxygen scavenger • Solubilisation of iron and phosphate (ferric phosphate poorly soluble) • Further supply of nutrients cycle back to beginning • O2 depletion, sulfide and ammonia buildup • Upwards shift of chemocline --> Killing of aerobic organisms • Further sedimentation • Collapse of ecosystem
Simplified Principle of of Activated Sludge COD, NH4+, phosphate to ocean Activated Sludge (O2 + X) Clarifyer 100:1 Excess sludge Biomass Recycle (Return Activated Sludge) • After primary treatment (gravity separation of insoluble solids) • Secondary treatment: Oxidation of organic pollutants, (COD and BOD removal, partial N removal • Needed: NH4+ conversion to N2 ? How?
What is Nitrification? Microbial oxidationof reduced nitrogen compounds (generally NH4+). Autotrophic ammonium oxidising bacteria (AOB) (Nitrosomonas, Nitrosospiraetc.): NH4+ + 1.5 O2 NO2- + H2O + 2 H+ Autotrophic nitrite oxidisers (Nitrobacter, Nitrospira etc.) NO2- + 0.5 O2 NO3- Aerobic conversion of NH4+ to NO3 + removes NH4+ toxicity tofish and odor from wastewater - does not accomplish nutrient removal
What is denitrification? • Microbial reductionof oxidised nitrogen compounds (generally NO3-). • Anoxic process using nitrate as an alternative electron acceptor to oxygen (anaerobic respiration) • Catalysed by non- specialised factultative aerobic heterotrophic bacteria. • A series of reduction steps leading to potential accumulation of intermediates • Electron donor: organic substances (BOD, COD) • NO3- + 2 H+ + 2 e- NO2- + H2O (nitrate reductase) • NO2- + 2 H+ + e- NO + H2O (nitrite reductase) • 2 NO + 2 H+ + 2 e- N2O + H2O (nitric oxide reductase) • N2O + 2 H+ + 2 e- N2 + H2O (nitrous oxide reductase)
Review of Terms: Ana-Cata • Metabolic processes can be differentiated between: • Processes that make use of exergonicredox reactions, conserve the energy of the reaction as ATP • Catabolism or Dissimilation or Respiration • typically oxidative process (degradation or organics to CO2) • Processes that drive endergonic reactions by using the ATP generated from Dissimilation • Anabolism or Assimilation or Biomass Synthesis • typically reductive processes (synthesis of complex organics from small building blocks • If the building block is CO2 autotrophic
The Nitrogen cycle Ox State -3 CNH2 NH4+ -2 -1 0 N2 +1 +2 NO +3 NO2- +4 +5 NO3-
The Nitrogen cycle Ox State -3 CNH2 NH4+ -2 -1 0 N2 +1 +2 NO +3 NO2- +4 +5 NO3- Dotted lines are assimiliative paths
The Nitrogen cycle Ox State -3 CNH2 NH4+ -2 -1 0 N2 +1 +2 NO +3 NO2- +4 +5 NO3- Nitrogen fixation: Atmospheric N2 reduction to ammonium and amino acids. Syntrophic Rhizobia types, free living bacteria and cyanobacteria. Reactions serves assimilation.
The Nitrogen cycle Ox State -3 CNH2 NH4+ -2 -1 0 N2 +1 +2 NO +3 NO2- +4 +5 NO3-
The Nitrogen cycle Ox State -3 CNH2 NH4+ -2 -1 0 N2 +1 +2 NO +3 NO2- +4 +5 NO3- • Nitrification step 1 Nitritification: • Ammonium as the electron donor for aerobic respiration. • Chemo-litho-autrophic. • Nitrosomonas type species.
The Nitrogen cycle Ox State -3 CNH2 NH4+ -2 -1 0 N2 +1 +2 NO +3 NO2- +4 +5 NO3- • Nitrification step 2 Nitratification: • Nitrite as electron donor for aerobic oxidation to nitrate • Chemo-litho-autrophic • Nitrobacter type species.
The Nitrogen cycle Ox State -3 CNH2 NH4+ -2 -1 0 N2 +1 +2 NO +3 NO2- +4 +5 NO3- • Denitrification • using either nitrate (NO3-) or nitrite (NO2-) as the electron eacceptor for anaerobic respiration. • Most COD can serve as electron donor. • Non-specific bacteria replacing O2 with Nitrate as e- acceptor when oxygen is depleted.
How to accomplish overall N-removal? Nitrification typically occurs during the aerobic treatment of wastewater: COD + O2 CO2 Ammonium + O2 Nitrate In addition to the aerobic activated sludge treatment an anaerobic treatment step is included aiming at N-removal (tertiary treatment) Insufficient N removal is typically achieved. why? Clarifier Anaerobic Treatment Aerobic Treatment Effluent Recycled sludge
How to accomplish overall N-removal? • N removal by the anaerobic step requires an electron donor to reduce NO3- to N2. • This electron donor is organic material. • Solution A: Add organic material to the anaerobic treatment step. • Example: Methanol • Problems: costs, contamination • Alternative solutions? N2 CO2 NO3- CO2 NH4+ COD Clarifier Anaerobic Treatment Aerobic Treatment Effluent Recycled biomass (sludge)
How to accomplish overall N-removal? • The obvious solution to successful N removal: • Use the COD as electron donor for denitrification • How to allow anaerobic denitrification to occur in the presence of oxygen? N2 CO2 NO3- CO2 NH4+ COD Clarifier Anaerobic Treatment Aerobic Treatment Effluent Recycled biomass (sludge)
How to accomplish overall N-removal? • Observations in the laboratory have shown that aerobic nitrification and anerobic denitrification can sometimes occur at the same time. • This simultaneous nitrification and denitrification (SND) has been the focus of many R&D projects for improved N-removal. N2 CO2 NO3- CO2 NH4+ COD Clarifier Anaerobic Treatment Aerobic Treatment Effluent Recycled biomass (sludge)
Idea for SND • Q: How to allow anaerobic denitrification at the same time as aerobic nitrification? • A: Intelligent oxygen control, not straightforward: • Aerobic: COD + O2 CO2 • Ammonium + O2 Nitrate • Anaerobic: COD + Nitrate N2 + CO2 • COD should be e-donor for nitrate reduction, not oxygen reduction. • Oxygen supply will burn COD faster than ammonium • No COD No denitrification NO3- pollution • Goal for improved N removal: Slow down aerobic COD oxidation, to leave electron donor for denitrif.
Ideas for SND • 1: Alternating aeration • 2: Limiting aeration • 3: SBR technology: Slowing down COD oxidation by conversion to PHB • Intelligent aeration control
Plug flow allows alternating aerobic / anaerobic conditions without time schedule Clarifier Influent Effluent Waste Sludge Return Activated Sludge Air Line Biomass Retention in WWTP
Alternating Aeration in Batch Systems • Aerobic: COD + NH4+ + O2 NO3- + residual COD • Anoxic: Residual COD + NO3- N2 • There is always substantial COD + O2 CO2 wastage. Effective N removal is limited Which phase is anaerorobic, which lines are COD, NO3- and NH4+ ?
Alternating Aeration in Batch Systems • Aerobic: COD + NH4+ + O2 NO3- + residual COD • Anoxic: Residual COD + NO3- N2 • There is always substantial COD + O2 CO2 wastage. Effective N removal is limited COD anoxic aerobic NH3 NO3-
Alternating Aeration in Batch Systems • Aerobic: COD + NH4+ + O2 NO3- + residual COD • Anoxic: Residual COD + NO3- N2 • There is always substantial COD + O2 CO2 wastage. Effective N removal is limited COD anoxic COD oxidation with NO3- aerobic COD and NH3 oxidation NH3 NO3-
What is SND (Simultaneous Nitrification and Denitrification) ? • Compromise with DO to go so low that ammonium oxidation is still working and denitrification is enabled. • Basically: Run nitrification and denitrification at same speed sophisticated control needed.
Oxygen dependency of Nitrification • Nitrification is not only limited • by the substrate concentration (nitrate) but also by the oxygen concentration • double limitation • \ Rate Nitrif. DO (mg/L)
Oxygen dependency of Denitrification Oxygen inhibition constant (ki) can be measured and used for modeling Similar to half saturation constant half inhibition constant Rate Denitri. DO (mg/L)
Oxygen dependency of SND Under-oxidation Over-oxidation Underoxidation: NH3 build- up Over-oxidation: NO3- build-up To match Nitrif. and Denitri.: Flux of reducing power (NH3, COD) should match flux of oxidation power. But how? What is the magical DO level that enables max SND? How does the SND curve change with different loading rates, biomass levels and N:C levels? Rate Nitrif. Denitri. SND DO (mg/L)
Why Simultaneous nitrification and denitrification(SND) ? • Minimise aeration costs by running at low DO • Avoid external COD addition to • (a) lower costs • (b) encourage (AOB) rather than heterotrophs • adapt high N-removal performance sludge • Avoid costs for pH corrections (nitrification uses acid while denitrification produces acid (can you show this with stoichiometric equations?) • Save further O2 and COD by SND via nitrite • Simplified operation
SND pathway O.S -3 -2 -1 0 1 2 3 4 5 If nitrification and denitrification can occur simultaneously there is a possibility of by-passing nitrate formation and nitrate reduction SND via nitrite. Has the advantage of oxygen savings and COD savings. NH3 NH2OH N2 O2 N2O COD NO2- NO2- NO3-
DO Effect on Nitrification and Denitrification SND via NO2- can operate more easily than SND via NO3- as oxygen has a stronger inhibition effect on nitrate reduction than nitrite reduction If SND proceeds via nitrite, then: how much savings are generated? Rate Nitrification NO2- reduction NO3- DO (mg/L)
Under-oxidation Over-oxidation Nitrif. Nitrif. NH3 Rate [N] in outflow Denitri. Denitri. DO (mg/L) DO (mg/L) Conclusion: For best N-removal in the outflow of the treatment process, a low DO should be chosen
Tenix / Murdoch University SND SBR pilot plant (Woodman Pt. 03-12-24) Labview control Bioselector, Online OUR monitoring, N2O emission, O2 minimisation
Return activated sludge ready to be contacted with incoming feed to allow “feast time” and enhance floc formation
Why Storage Driven Denitrification? • Idea: Making use of bacteria’s behaviour of taking up organic substances for storage as PHB. • Denitrification needs organic reducing power: • Either sufficient COD or PHB storage • Problem with COD: degrades quicker than NH3 • no COD left for denitrification • Advantages of bacterial Storage of COD as PHB as PHB: • Oxidises slower lasts longer important for SBR • Reducing power inside the floc rather than outside • Reducing power can be settled and build up. PHB
Fill Decant Aeration Cycle Settle Use of Sequencing Batch Reactor (SBR) for a) Biomass Retention via internal biomass feedbackb) floc formation by oxposing biomass to a sudden high inflow of biomass Influent Effluent Waste Sludge Biomass Retention in WWTP
Why Storage Driven Denitrification? • Denitrification needs organic reducing power: • Either sufficient COD or PHB storage • Problem with COD: degrades quicker than NH3 • no COD left for denitrification • Advantages of bacterial Storage of COD as PHB as PHB: • Oxidises slower lasts longer important for SBR • Reducing power inside the floc rather than outside • Reducing power can be settled and build up. PHB
2 Acetate 2 CoA 4 ATP 2 Acetyl-CoA (16 e-) PHB (18 e-) 1 NADH (2 e-) Bio-mass 2 CoA 24 ATP TCA cycle 8 NADH (16 e-) H2O ETC O2 2 CO2 BOD storage as PHB needs ATP • Mechanisms for ATP generation: • O2 respiration • Nitrate respiration • Glycogen fermentation • Poly-P hydrolysis • Our results: • Storage under some O2 supply • Glycogen, P complicated • NO3- too low. • Aerobic bioselector? PHB
Expected Benefit of Storing Reducing Power Inside the Floc • PHB physically separated from O2 • Selective availability of O2 to AOB. • PHB may be more readily oxidised by nitrate or nitrite being formed by the aerobic reaction N2 COD O2 NO2- PHB anoxic NH3 aerobic CO2 PHB
A B C D Increasing PHB (dark) buildup in bacterial biomass (red) during early phase of SBR PHB
Time (min) 0 50 100 250 300 350 10 4 • Three phases could be observed • 1st: COD PHB • 2nd : PHB driven SND (60%) • OUR indicates NH3 depletion • 3rd : wastage of reducing power Aerobic Anoxic 8 3 6 Carb. comp. (CmM) PHB Nitrog-comp. (mM) 2 4 NO3- 1 2 0 0 50 40 • 69 % N-removal, no reducing power left • Needed: Automatic stopping of aeration when ammonia is oxidised to prevent PHB oxidation with oxygen • Could be detected from OUR monitoring SOUR (mgO2/g/h) 30 20 NH3 OUR 10 0 0 50 150 200 Time (min)
10 4 Anoxic Aerobic Settle 8 3 6 Carb. comp.(mM) Nitrog. comp. (mM) 2 4 1 2 0 0 0 50 100 150 200 250 300 350 Time (mins) Effect of auto-aeration cut-off onPHB levels and N-removal 10 4 • Aim: Avoid wastage of reducing power • by: auto-aeration cut-off • Outcomes: • More PHB preserved • N-rem 6986% • Less air • Shorter treatment Aerobic Anoxic 8 3 6 Carb. comp. (CmM) PHB Nitrog-comp. (mM) 2 4 NO3- 1 2 0 0