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Nutrient Removal in Activated Sludge
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Nutrient Removal in Activated Sludge

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  1. Nutrient Removal in Activated Sludge Chris Groh Wisconsin rural Water Association

  2. Nitrogen Removal Needs • Environmental protection • Meet limits • Mostly done thru normal treatment that is running perfectly • Additional treatment is necessary if loadings are high in nitrogenous compounds

  3. Optimize DO levels for Denitrification • Nitrification is conversion of ammonia, NH3-N (from influent) to nitrate • Denitrificaton converts NO3-N to nitrogen gas, N2 • Nitrification is ammonia removal • Denitrification is nitrate removal • Since nitrogen gas is stripped off, denitrification is total nitrogen removal

  4. Nitrogen Removal and DO • Denitrification can only occur after nitrification (nitrification produces nitrates) • Denitrification only occurs in environments devoid of oxygen but where nitrates are present • A source of carbon must also be present • Carbon can be in the form of BOD • Bugs necessary for the removal are typically present in MLSS

  5. Benefits • Denitrification saves energy costs • During denitrification, part of BOD removal will occur without DO • Denitrification also restores alkalinity which is necessary for other processes in treatment

  6. How to Do It • Creating an anoxic place in a carousel basin is fairly easy • Aerate basin at a level that takes care of your BOD and ammonia oxidation…but not in excess • Trial and error to find this point, but monitor basin to find an area that has a very low DO (<0.5 mg/L)

  7. What It Looks Like Area of Higher DO DO Depletes as flow Travels down channels Anoxic zone RAS Area of Lower DO Effluent

  8. Monitoring • Monitoring DO around the basin can target the anoxic area • Changing aeration can move anoxic areas towards effluent discharges or weirs • Misconception is that DO has to be 1.0 to 2.0 mg/L • DO goes down as water moves down the channel, therefore a lower DO is seen in areas just before aeration zones

  9. Other Strategies

  10. Particulars • NaOH used for pH adjustment and alkalinity • Alum dosed into post anoxic zone for phosphorus precipitation • Methanol could be dosed into post anoxic zone for more nitrogen removal

  11. Other Strategies

  12. Particulars • All influents go thru anoxic zone for nitrogen removal • This system has reached 3.1 mg/L nitrogen consistently

  13. Other Strategies

  14. Particulars • Alum and NaOH dosed in influent for alkalinity and pH control • Uses turbine aerators for aeration, modified (by slowing down) to use as mixers • Resulting separation of anoxic/ aerobic zones provides 0.6 mg/L ammonia consistently

  15. Phosphorus Removal • Most phosphorus removal done thru addition of chemicals • Alum-Hydrated Aluminum sulfate (Al2(SO4)3.14H2O); Al3+ + PO4 AlPO4 reacts on a one to one basis • Ferric chloride (FeCl3)- FeCl3 + PO3- FePO4

  16. Phosphorus Removal • Pickle liquor (ferrous chloride or sulfate) is used rarely, expensive, hard to get and hard to calculate amount of iron in the solution

  17. Phosphorus Removal • Ferric creates a thicker, drier sludge cake, is slightly more expensive, but you use less • Alum creates a “slicker” sludge cake, is less expensive, but more is used • Ferric chloride is very corrosive and produces fumes • Must be stored in non reactive vessels

  18. Phosphorus Removal • Optimum pH range is 4.5-5.0 • Significant removal at higher pHs • If wastewater has a high buffering capacity, little pH change noticed • Wastewater with low buffering cap. Might have problems with ammonia or nitrates in summer

  19. Phosphorus Removal • Alum is moderately corrosive but the piping and storage materials is greater • Optimum pH for phosphorus removal is 5.5-6.5 • There is destruction of alkalinity • At pH above 6.5 more alum is needed to remove the same amount of phosphorus

  20. Biological Phosphorus Removal • Theory: System is best for phosphate-accumulating organisms (PAOs) • PAOs, under anaerobic conditions (absence of oxygen and nitrate), PAOs can break phosphorus bond to create energy to uptake volatile fatty acids (VFAs). These acids are stored in their cells primarily as poly-β-hydroxybutyrate (PHB)

  21. Biological Phosphorus Removal • When these organisms pass into an aerobic environment, they metabolize the PHB and uptake additional phosphorus • By creating an anaerobic zone at the beginning of the treatment process, PAOs gain a competitive advantage. These organisms contain 2-5 times more phosphorus than regular activated sludge

  22. Biological Phosphorus Removal • BOD to phosphorus ratio of 20:1 • As ratio is increased more phosphorus can be removed • Soluble BOD can be a limiting factor • A BOD of 200 mg/L will remove 2 mg/L phosphorus even without enhancing BPR

  23. Biological Phosphorus Removal • 2 important factors that lead to poor BPR • Substrate availability (soluble BOD) and nitrate control • Others • DO & nitrate recycle • Competition w/glygogen accumulating organisms • Return phos loadings • Temp & pH • Effluent TSS

  24. Biological Phosphorus Removal • Variability of wastewater and flow • Inconsistent loadings • Physical restraints (mixing, baffling, detention time) • Control parameters (RAS, WAS, solids retention time) • Secondary release of phos • Most plants control by RAS, WAS and DO concentration

  25. Biological Phosphorus Removal • RAS Control-RAS rate allows solids to remain in the system longer than the wastewater which allows acclimation to pollutants and develop good settling. If there is nitrates present, reducing RAS may help. If pre-anoxic zones are present lowering RAS rates help lengthen detention time

  26. Biological Phosphorus Removal • WAS Control-Phosphorus needs to be wasted from the system to be removed. WAS control allows solids to be removed as bacteria grows. SRT may need to be reduced to accommodate phos removal. Particularly true of aeration systems (like oxidation ditches) that operate at high sludge ages

  27. Biological Phosphorus Removal • DO Control-Optimized aerations includes DO meters or ORP probes. Oxygen and nitrate inhibits the process so control is important if the zone is supposed to be anaerobic. If there is too much DO in the aerobic area there is a better chance to have oxygen to return to anaerobic areas. Aeration rates also affect nitrates. Must avoid low DO bulking

  28. Biological Phosphorus Removal

  29. Biological Phosphorus Removal

  30. Biological Phosphorus Removal • Monitor • Influent BOD, TKN, NH3, O-PO4 • Anaerobic zone NO3, O-PO4 • Aeration basin NO3, O-PO4, TSS • RAS NO3, O-PO4, TSS • Effluent NO3, O-PO4, NH3, TSS • Nitrates are the enemy!!! • Take on soluble BOD as much as you can!!