JTAC Presentation May 18, Nutrient Removal Process Fundamentals and Operation

Transcription

1 JTAC Presentation May 18, 2017 Nutrient Removal Process Fundamentals and Operation Steve Polson, P.E.

2 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

3 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

4 Nutrients of Concern in Wastewater Nitrogen Ammonia (NH 3 ) Nitrate (NO 3- ) Organic nitrogen Sources contributing to wastewater? Human waste Industrial sources (refrigeration, pulp and paper, mining, food processing and refining)

5 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)

6 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 (Se 0 ): 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)

7 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

8 Nitrification

9 Nitrification Biologically Transforms Ammonia to Nitrate Raw Sewage Organic Nitrogen Bacterial Decomposition Ammonia Nitrogen Bacterial Cells Net Growth O 2 Decomposition Nitrification Nitrite (NO 2- ) O 2 Nitrate (NO 3- )

10 Nitrification Converts Ammonia to Nitrate in Two Steps Step 1: 2NH O 2 Nitrosomonas 2NO H 2 O + 4H + + Cells Step 2: 2NO 2- + O 2 Nitrobacter 2NO 3- + Cells Nitrifiers are autotrophic : Carbon dioxide carbon source Oxidize ammonia for energy 4H + is acidic

11 For Each Gram of Ammonia Nitrified: 4.6 gm O 2 required Increases aeration requirements 7.2 gm alkalinity (as CaCO 3 ) destroyed Can cause drop in ph

12 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

13 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

14 Nitrifier Minimum Aerobic SRT Varies With Temperature. Nitrification No Nitrification Assumes DO = 2.0 mg/l

15 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

16 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

17 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

18 Operating Factor Determines Effluent Ammonia Source : MWRD Robert W. Hite Treatment Facility

19 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

20 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]

21 Denitrification

22 Denitrification Completes the Nitrogen Conversion Process Raw Sewage Organic Nitrogen Bacterial Decomposition Ammonia Nitrogen Bacterial Cells Net Growth O 2 Decomposition Nitrification Nitrite (NO 2- ) O 2 Nitrate (NO 3- ) Denitrification Org Carbon Nitrogen Gas (N 2 ) Nitrogen gas is harmless byproduct

23 Denitrification Converts Nitrate to Nitrogen Gas (cont) Typical reaction: 6NO CH 3 OH (methanol) 3N 2 + 5CO 2 + 7H 2 O + 6OH- 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

24 For Each Gram of Nitrate Denitrified: 2.9 gm BOD consumed Reduces downstream aeration requirements 3.6 gm (as CaCO 3 ) alkalinity produced Partially offsets nitrification reduction

25 Denitrification Efficiency Can Vary Enhanced by wastewater biodegradability Readily available food for denitrification Adversely affected by DO

26 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

27 Nitrification and Denitrification are Complimentary Reactions. But, Denitrification Must Precede Nitrification for Benefit.

28 Typical Activated Sludge Process Configuration: Activated Sludge Reactor Secondary Clarifier Oxic SE PE Carbon Oxidation Nitrification RAS WAS

29 Two-Stage Activated Sludge Process Configuration for Denitrification: BNR Reactor MLR Secondary Clarifier Anoxic Oxic SE PE Denitrification Carbon Oxidation Nitrification RAS Modified Ludzack-Ettinger (MLE) Process WAS

30 Denitrification is Controlled by Mixed Liquor Recirculation. % Denite = R/(R+Q) * 100

31 4-Stage Activated Sludge Process Configuration for Denitrification: MLR BNR Reactor Methanol Secondary Clarifier Anoxic Aerobic Anoxic Oxic SE PE Achieves Lower Effluent Nitrate Concentrations RAS 4-Stage Bardenpho Process WAS

32 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)

33 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

34 Annamox Organisms Not a bad rash

35 Deammonification Systems (Proprietary) Greeley has Demon process (first in CO)

36 Biological Phosphorus Removal (Bio-P)

37 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

38 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

39 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 mg/l removed in secondary clarifiers mg/l remaining in secondary effluent

40 PAO s Have Unique Anaerobic/ Aerobic Metabolism Aerobic Conditions Energy PO 4-3 CO 2 + H 2 O Anaerobic Conditions O 2 PO 4-3 Energy Acetate

41 PAOs Grow Slow But Faster Than Nitrifiers PAOs Nitrifiers

42 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)

43 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

44 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)

45 Typical Process for Bio-P (and Nitrogen) Removal: BNR Reactor MLR Secondary Clarifier Anaerobic Anoxic Oxic SE PE Phosphorus Release Denitrification A2O Process RAS Carbon Oxidation Nitrification Phosphorus Uptake WAS P-removal

46 Alternate Process for Bio-P (and Nitrogen) Removal: PE BNR Reactor MLR Secondary Clarifier Anx Ana Anx Oxic SE Key: RAS ANX Zone RAS Johannesburg (JHB) Process WAS P removal

47 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

48 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

49 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

50 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 Pearl TM Aquatec Maxon Crystalactor

51 AirPrex Piloting at Metro Anaerobic digester effluent treated to form struvite AirPrex reactor Strip CO2 to raise ph Add magnesium to form struvite Struvite settles and is removed from digested sludge prior to dewatering Struvite Product

52 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

53 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 coprecipitate with iron 3. Elemental Selenium (Se 0 ): A solid precipitant 4. Selenide (Se -2 ): Readily Precipitates

54 Addressing Selenium Reduce I/I Treatment Incorporate into other processes Separate stage

55 Treatment Considerations Reduce Se to Se 0 (elemental) Microorganisms have hierarchy of preferred electron sources: Oxygen O 2 H 2 Nitrate/nitrite NO 3 N 2 Selenate/Selenite Se +6 Se +4 Se 0 Sulfate SO 4 S - Methanogenesis CO 2 CH 4 Therefore anoxic and anaerobic conditions will reduce/precipitate selenium

56 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?

57 Selenium Treatment More Exotic Approaches Downflow Packed Granular Filtration Beds (GE ABMet) Wetlands Reverse osmosis Watch for future developments

58 Questions?