Nutrient Removal 101 - Process Fundamentals and Operation

JTAC Presentation – May 18, 2017

Steve Polson, P.E. Presentation Goals

• Develop understanding of: – Why to remove nutrients – How to remove nutrients using biological processes – Reasons for facilities configuration • This is a presentation on basics What are Nutrients?

• Inorganic constituents in wastewater that can cause problems when discharged • Fortunately, they are also elements and compounds that micro-organisms can utilize, and in the process, remove from the wastewater Nutrients of Concern in Wastewater • Nitrogen

– Ammonia (NH3) - – Nitrate (NO3 ) – Organic nitrogen – Sources contributing to wastewater? • Human waste • Industrial sources (refrigeration, pulp and paper, mining, food processing and refining) Nutrients of Concern in Wastewater • Phosphorus – Dissolved & particulate – Chemical categorization • Orthophosphate (soluble) • Condensed phosphate (complex) • Organic phosphates (complex) – Total phosphorus (TP) is reported • Complex forms must be converted to orthophosphate for measurement – Sources contributing to wastewater? • Human waste • Food waste • Detergents and cleaners • Industrial sources (industrial cleaners, steel production, metal finishing, food and beverage processing, pharmaceuticals, and fertilizer production) Nutrients of Concern in Wastewater • Selenium – At normal wastewater pH ranges there are four forms (oxidation states) of selenium: 1. Selenate (Se+6): Very Soluble and difficult to precipitate 2. Selenite (Se+4): Soluble and can co-precipitate with iron 3. Elemental Selenium (Se0): A solid precipitant 4. Selenide (Se-2): Readily Precipitates – Sources: • Discharges from coal‐fired power plants using selenium‐rich coal • Effluent from oil refineries • Infiltration/Inflow (I/I) Why Include Nutrient Removal Capabilities?

• Ammonia – Toxicity; oxygen demand • Nitrate – Groundwater contamination (blue baby syndrome); algae growth; reduce operating costs • Phosphorus – Eutrophication in lakes and reservoirs (algae growth) • Selenium – Negative effects on the growth and survival of juvenile fish – Birth deformities in the larval offspring of adult fish Nitrification Nitrification Biologically Transforms Ammonia to Nitrate

Raw Sewage Organic Nitrogen

Bacterial Decomposition

Ammonia Nitrogen Bacterial Cells Net Growth

Decomposition O2

Nitrite (NO -) Nitrification 2

- Nitrate (NO3 ) Nitrification Converts Ammonia to Nitrate in Two Steps

“Nitrosomonas” + - + Step 1: 2NH4 + 3O2 2NO2 + 2H2O + 4H + Cells

“Nitrobacter” Step 2: 2NO - + O - 2 2 2NO3 + Cells

Nitrifiers are “autotrophic”: • Carbon dioxide carbon source • Oxidize ammonia for energy 4H+ is acidic For Each Gram of Ammonia Nitrified:

• 4.6 gm O2 required – Increases aeration requirements

• 7.2 gm alkalinity (as CaCO3) destroyed – Can cause drop in pH Nitrification Can Be Accomplished in Several Ways

• Basic Process – Suspended growth (activated sludge) – Attached growth (BAF) • Configuration – Combined with carbon oxidation – Separate stage (following carbon oxidation) • Nitrifying TF • Focus of presentation is on suspended growth Primary Control Parameter is Sludge Age (SRT)

• Activated sludge process • SRT and wastewater characteristics determine MLSS concentration • SRT must accommodate growth rate – Slower for nitrifiers • Nitrification SRT is sensitive to: – Temperature – Dissolved oxygen (DO) – Mixed liquor pH Nitrifier Minimum Aerobic SRT Varies With Temperature.

Nitrification

No Nitrification

Assumes DO = 2.0 mg/L Dissolved Oxygen Also Affects Minimum SRT

• As DO decreases, min SRT increases – SRT at 0.3 mg/L double that at 2.0 mg/L • DO in nitrifying system should be at least 2.0 mg/L • However, DO above 3.0 mg/L is unnecessary and wastes energy Alkalinity Consumption Can Reduce Mixed Liquor pH

• Reduction in pH dependent on: – Alkalinity of raw wastewater – Extent of nitrification – Upstream processes (chemical addition, denitrification) • ML pH less than 7.0 increases minimum SRT for nitrification • Supplemental alkalinity may be needed – Caustic soda (sodium hydroxide) – Soda ash (sodium bicarbonate) –Lime Operating SRT Must be Greater than Minimum SRT

• Accounts for fact that plant is not ideal reactor, diurnal variations • Ratio of operating SRT to minimum is called “Operating Factor” (OF) – Also called “safety factor” • OF is typically 1.5 to 2.5 • Determined through experience Operating Factor Determines Effluent Ammonia

Source : MWRD Robert W. Hite Treatment Facility SRT Based on Operating Factor is Aerobic SRT

• Sludge mass under aeration Aer SRT, days = (MLSS x Aer Vol x 8.34)/(Sludge Wasting Rate) =(Aerobic Sludge Inventory, lb)/(Sludge Wasting Rate, lb/day) Note: • MLSS in mg/L • Aer Vol in mgal Aerobic SRT Can be Converted to Overall Operating SRT

• SRT based on OF must be divided by percent of basin volume that is aerated for overall operating SRT • SRT managed by sludge wasting (typical activated sludge control) • Wasting rate (lb/day) = WAS flow (mgd) x Conc (mg/L) x 8.34; or • WAS flow (mgd) = Wasting rate (lb/day)/[Conc (mg/L) x 8.34] Denitrification Denitrification Completes the Nitrogen Conversion Process

Raw Sewage Organic Nitrogen

Bacterial Decomposition

Ammonia Nitrogen Bacterial Cells Net Growth

Decomposition O2

Nitrite (NO -) Nitrification 2

- Nitrate (NO3 )

Org Carbon Denitrification

Nitrogen Gas (N2)

Nitrogen gas is harmless byproduct Denitrification Converts Nitrate to Nitrogen Gas (cont)

Typical reaction:

- 6NO3 + 5CH3OH 3N2 + 5CO2 + 7H2O + 6OH- (methanol) Denitrifiers are “facultative/heterotrophic”: • Oxygen obtained from dissolved oxygen or nitrate • Organic carbon serves as carbon source

Denitrification occurs under “anoxic” conditions • Nitrate present • No dissolved oxygen OH- is basic For Each Gram of Nitrate Denitrified:

• 2.9 gm BOD consumed – Reduces downstream aeration requirements

• 3.6 gm (as CaCO3) alkalinity produced – Partially offsets nitrification reduction Denitrification Efficiency Can Vary

• Enhanced by wastewater biodegradability – Readily available food for denitrification • Adversely affected by DO Denitrification Can Be Added in Several Ways

• Basic Process – Suspended growth (activated sludge) – Attached growth • Configuration – Combined with carbon oxidation/nitrification – Separate stage (following carbon oxidation/nitrification) • Focus of presentation is on suspended growth Nitrification and Denitrification are Complimentary Reactions.

But, Denitrification Must Precede Nitrification for Benefit. Typical Activated Sludge Process Configuration:

Secondary Activated Sludge Clarifier Reactor

Oxic SE

Carbon Oxidation Nitrification

WAS RAS Two-Stage Activated Sludge Process Configuration for Denitrification:

BNR Reactor Secondary Clarifier MLR

Anoxic Oxic SE

Denitrification Carbon Oxidation Nitrification

WAS RAS Modified Ludzack-Ettinger (MLE) Process Denitrification is Controlled by Mixed Liquor Recirculation.

% Denite = R/(R+Q) * 100 4-Stage Activated Sludge Process Configuration for Denitrification:

BNR Reactor Secondary MLR Methanol Clarifier

Anoxic Aerobic Anoxic Oxic SE

Achieves Lower Effluent Nitrate Concentrations

WAS RAS 4-Stage Bardenpho Process Deammonification

• Sidestream Treatment – e.g., Centrate from anaerobic sludge dewatering • Key organism – deammonification (annamox) bacteria • Two stage process – Ammonia oxidizing bacteria (AOB) – half the available ammonia is oxidized to nitrite (nitritation, not nitrification) – Annamox bacteria -- Residual ammonia combined with nitrite is anaerobically transformed to nitrogen gas.

(Source: Demon® literature) Deammonification (cont)

• Notable properties of anammox bacteria: – Very low growth rate (1/10th that of nitrifiers!) – Inhibited by oxygen even at very low levels • Processes are proprietary • Can be difficult to control • Claims: – 80+ percent ammonia removal – 60 percent less energy required – Eliminates need for supplemental carbon (methanol) for denitrification – 90 percent less sludge production Annamox Organisms

Not a bad rash… Deammonification Systems

(Proprietary)

Greeley has Demon® process (first in CO) Biological Phosphorus Removal (Bio-P) Phosphorus Removal May Be Biological and/or Chemical

• Biological treatment – Incorporation into activated sludge cell mass; settle/waste to remove • Chemical treatment – Conversion from soluble to settleable particulate form; settle/waste and/or filter to remove • Focus today is on biological Biological Phosphorus Removal Fundamentals

• Standard primary/secondary treatment removes some phosphorus – Example – Influent TP = 5.0 mg/L: • 1.0 mg/L removed in primary clarifiers – Particulate removal with primary sludge • 1.0 mg/L removed in secondary clarifiers – Incorporation into activated sludge cell mass; waste from system • 3.0 mg/L +/- remaining in secondary effluent Biological Phosphorus Removal Fundamentals (cont)

• Bio-P treatment improves phosphorus uptake – Grow microorganisms that store P (Bio-P organisms) – Improved removals: • 5.0 mg/L influent TP • 1.0 mg/L removed in primary clarifiers • 3-3.5 mg/L removed in secondary clarifiers • 0.5-1.0 mg/L remaining in secondary effluent PAO’s Have Unique Anaerobic/ Aerobic Metabolism

Aerobic Conditions

-3 PO4 Energy

CO2 + H2O

O2 Anaerobic Conditions

-3 PO4 Energy Acetate PAOs Grow Slow But Faster Than Nitrifiers

Nitrifiers

PAOs Bio-P Requires Anaerobic/Aerobic Basins and Clarifiers

• Anaerobic basin – Devoid of oxygen and nitrate – Fermentation breaks down complex organic materials to volatile fatty acids (VFAs) • Acetic acid (vinegar) – Bio-P organisms store VFAs and release phosphorus (provides energy) Bio-P Requires Anaerobic/Aerobic Basins and Clarifiers (cont)

• Aerobic basin – Basin contents aerated – Bio-P organisms oxidize stored VFAs and use energy gained to store phosphorus • VFA oxidation provides energy for P-storage • Higher concentration in cells than typical • Most of soluble phosphorus removed – Carbon oxidation, nitrification, and phosphorus uptake occur simultaneously • P release and uptake simulates rechargeable battery – Stored P -- high energy bonds – Bugs break bonds and gains energy Bio-P Requires Anaerobic/Aerobic Basins and Clarifiers (cont)

• Secondary Clarifiers – Activated sludge settles for recycle and wasting (removes phosphorus) – Effluent phosphorus concentration dependent on solids removal efficiency – “Secret” of bio-P removal – Remove the bugs at the point where they have stored P (i.e., after aerobic treatment) Typical Process for Bio-P (and Nitrogen) Removal:

BNR Reactor Secondary Clarifier MLR

Anaerobic Anoxic Oxic SE

Carbon Oxidation Phosphorus Denitrification Release Nitrification Phosphorus Uptake WAS RAS P-removal A2O Process Alternate Process for Bio-P (and Nitrogen) Removal:

BNR Reactor Secondary Clarifier PE MLR

Anx Ana Anx Oxic SE

Key: RAS ANX Zone

WAS RAS P removal Johannesburg (JHB) Process Bio-P Removal is Sensitive to Raw WW and System Operation

• Readily degradable BOD • Recycle of DO and nitrate to anaerobic zone • Excellent secondary clarifier performance How to Improve Bio-P Performance?

• Add supplemental carbon source – Acetic acid – High fructose corn syrup –Dairy waste – Brewery waste • Create VFAs “in-house” – Primary sludge fermentation • Hold sludge in separate fermenter • Often coupled with gravity thickening • JHB process Bio-P Comes with a Price

• Struvite – Ammonium-magnesium-phosphate (NH4MgPO4·6H2O) – Forms in solids processing – Creates problems with pipe plugging, damage to pumps, centrifuges Phosphorus Recovery

• Sludge from EBPR plants is high in phosphorus and ammonia – Can cause struvite problems in digesters and piping • Struvite is ammonium-magnesium-phosphate • Intentional creation of struvite results in high-P fertilizer – Replaces mined phosphorus • Limited resource – Generates income • Patented processes: – Ostara PearlTM – Aquatec Maxon Crystalactor® AirPrex Piloting at Metro

• Anaerobic digester effluent treated to form struvite Struvite Product – AirPrex reactor • Strip CO2 to raise pH • Add magnesium to form struvite • Struvite settles and is removed from digested sludge prior to dewatering Selenium – The Latest Challenge

• Effluent permits starting to limit selenium • Why? – Negative effects on the growth and survival of juvenile fish – Birth deformities in the larval offspring of adult fish Selenium Chemistry

• At normal wastewater pH ranges there are four forms (oxidation states) of selenium: 1. Selenate (Se+6): Very Soluble and difficult to precipitate 2. Selenite (Se+4): Soluble and can co- precipitate with iron 3. Elemental Selenium (Se0): A solid precipitant 4. Selenide (Se-2): Readily Precipitates Addressing Selenium

• Reduce I/I • Treatment – Incorporate into other processes – Separate stage Treatment Considerations

• Reduce Se to Se0(elemental) • Microorganisms have hierarchy of preferred electron sources:

– Oxygen O2 → H2

– Nitrate/nitrite NO3 → N2 – Selenate/Selenite Se+6 → Se+4 → Se0 - – Sulfate SO4 → S

– Methanogenesis CO2 → CH4 • Therefore anoxic and anaerobic conditions will reduce/precipitate selenium Selenium Treatment

• Not well studied or documented • Biological and chemical options – Ferric chloride can precipitate Se • Success depends on how reduced – ANA/ANX zones in BNR systems • Potential to reduce oxidation state • Need to limit final OXIC zone – Combine biological and chemical? Selenium Treatment

• More Exotic Approaches – Downflow Packed Granular Filtration Beds (GE ABMet) – Wetlands – Reverse osmosis • Watch for future developments Questions?

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