Waste Water Treatment

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Waste Water Treatment

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Waste Water Treatment

  1. 1. WATER & WASTEWATER TREATMENT <ul><li>Objectives: </li></ul><ul><ul><li>Define primary, secondary, and tertiary treatment </li></ul></ul><ul><ul><li>Define BOD </li></ul></ul><ul><ul><li>Describe the activated sludge process </li></ul></ul><ul><ul><li>Setup and solve a mass balance for an activated sludge system </li></ul></ul>
  2. 2. REVIEW <ul><li>Sorption: </li></ul><ul><ul><li>K d =C s /C L </li></ul></ul><ul><ul><li>C T =(1+K d C ss )C L </li></ul></ul><ul><ul><li>Fraction sorbed vs. fraction remaining in water </li></ul></ul><ul><li>Settling </li></ul><ul><ul><li>Settling velocity: </li></ul></ul><ul><ul><li>Percent of particles removed: (1-C ss /C ss,o ) x 100% </li></ul></ul><ul><ul><ul><ul><li>Where, </li></ul></ul></ul></ul>
  3. 3. WELL-MIXED SETTLING TANK Q, C ss,o Q, C ss C ss V Suspended solids remaining: Define the Overflow Rate: ~ 20 – 100 m/day in treatment plants v s
  4. 4. WASTEWATER TREATMENT <ul><li>POTW – Publicly Owned Treatment Works </li></ul><ul><ul><li>0.4 – 0.6 m 3 /person/day </li></ul></ul><ul><ul><li>15 million people in Los Angeles  7.5 x 10 6 m 3 /day or 2000 MGD (million gallons per day) </li></ul></ul><ul><ul><li>Hyperion – 450 MGD </li></ul></ul><ul><li>Clean Water Act (CWA) – 1977 – Set effluent (what is released by treatment plants into the environment) standards </li></ul>
  5. 5. STAGES OF WATER TREATMENT <ul><li>Primary </li></ul><ul><ul><li>Contaminants (60% of solids and 35% of BOD removed) </li></ul></ul><ul><ul><ul><li>Oil & Grease </li></ul></ul></ul><ul><ul><ul><li>Total Suspended Solids (C ss or TSS) – 60% Removed </li></ul></ul></ul><ul><ul><ul><li>Pathogens </li></ul></ul></ul><ul><ul><ul><li>BOD – 35% removed </li></ul></ul></ul><ul><ul><li>Processes </li></ul></ul><ul><ul><ul><li>Screens </li></ul></ul></ul><ul><ul><ul><li>Grit Settling </li></ul></ul></ul><ul><ul><ul><li>Scum Flotation </li></ul></ul></ul><ul><ul><ul><li>Primary Settling </li></ul></ul></ul>
  6. 6. STAGES (CONTINUED) <ul><li>Secondary </li></ul><ul><ul><li>Contaminants </li></ul></ul><ul><ul><ul><li>BOD – 90% Removed </li></ul></ul></ul><ul><ul><ul><li>TSS – 90% Removed </li></ul></ul></ul><ul><ul><li>Processes </li></ul></ul><ul><ul><ul><li>Trickling Filter – rotating disk </li></ul></ul></ul><ul><ul><ul><li>Activated Sludge – Suspended and mixed </li></ul></ul></ul><ul><ul><ul><li>Oxidation ponds – lagoons </li></ul></ul></ul><ul><ul><ul><li>(promote contact between microbes and contaminants) </li></ul></ul></ul>
  7. 7. STAGES (CONTINUED) <ul><li>Tertiary </li></ul><ul><ul><li>Contaminants </li></ul></ul><ul><ul><ul><li>Nutrients </li></ul></ul></ul><ul><ul><ul><li>Dissolved solids (e.g., salt, other ions, etc.) </li></ul></ul></ul><ul><ul><li>Processes </li></ul></ul><ul><ul><ul><li>Denitrification – bacteria </li></ul></ul></ul><ul><ul><ul><li>Phosphorus removal – precipitation </li></ul></ul></ul><ul><ul><ul><li>Other chemicals – adsorption and precipitation </li></ul></ul></ul>
  8. 8. PRIMARY SLUDGE (CONT’D)
  9. 9. PRIMARY SLUDGE
  10. 10. PRIMARY SLUDGE (CONT’D) <ul><li>Given: </li></ul><ul><ul><li>Q = 4000 m 3 /d </li></ul></ul><ul><ul><li>C ss,o = 200 mg/L and C ss = 100 mg/L </li></ul></ul><ul><ul><li>Sludge density = 0.05 kg/L </li></ul></ul><ul><ul><li>Overflow rate of 50 m/d </li></ul></ul><ul><li>Find </li></ul><ul><ul><li>Population of town served by this unit </li></ul></ul><ul><ul><li>Sludge production rate </li></ul></ul><ul><ul><li>Area of settling tank </li></ul></ul><ul><ul><li>Settling velocity of particles </li></ul></ul><ul><ul><li>Cut-off size of particles (find the particle diameter corresponding to this settling velocity. Assume  s = 2600 kg/m 3 . All particles larger than this size will settle) </li></ul></ul>Q, C ss,o Q, C ss
  11. 11. ACTIVATED SLUDGE
  12. 12. ACTIVATED SLUDGE COMPONENTS
  13. 13. ACTIVATED SLUDGE COMPONENTS
  14. 14. ACTIVATED SLUDGE (CONT’D)
  15. 15. ACTIVATED SLUDGE NOMENCLATURE Q, S o , X o Q+Q R , S, X Q-Q s , S Q s +Q R , X s Q s , X s Q R , X s S stands for conc. of substrate (organic matter, waste, etc.) or BOD X stands for conc. of microorganisms
  16. 16. ACTIVATED SLUDGE NOMENCLATURE (CONT’D) Q, S o , X o Q+Q R , S, X ~Q, S Q s +Q R , X s Q s , X s Q R , X s <ul><li>Assumptions: </li></ul><ul><ul><li>Effluent bacteria concentration is 0 </li></ul></ul><ul><ul><li>Concentration of substrate or BOD in sludge is 0 </li></ul></ul><ul><ul><li>Sludge flowrate (Q s ) is much smaller than Q </li></ul></ul> , V
  17. 17. DECAY OF BOD AND GROWTH OF ORGANISMS <ul><li>Substrate or BOD (S) decays with rate k: </li></ul><ul><li>Microbes (X) grow at rate  : </li></ul>
  18. 18. ACTIVATED SLUDGE EQUATIONS <ul><li>The following equations are derived from conducting mass balances over: </li></ul><ul><ul><li>The entire system </li></ul></ul><ul><ul><li>The aeration tank </li></ul></ul><ul><ul><li>The sedimentation tank </li></ul></ul><ul><li>Any good book on wastewater engineering will have the derivations if you are curious! </li></ul>
  19. 19. ACTIVATED SLUDGE EQUATIONS <ul><li>Biomass (X) balance over entire system: </li></ul><ul><li>Substrate (S) balance over entire system: </li></ul>
  20. 20. MORE AS EQUATIONS <ul><li>Mass balance over sedimentation tank: </li></ul><ul><li>Other equation(s)/rules of thumb: </li></ul><ul><ul><li>F/M = QS o /XV - Food-to-microbe ratio: 0.3 – 0.7 d -1 </li></ul></ul><ul><ul><li>Q R ~ 0.25 – 0.50 x Q </li></ul></ul><ul><ul><li>X ~ 1000 – 2000 mg/L </li></ul></ul><ul><li>Problem types: </li></ul><ul><ul><li>Given Q, S o , and S (target concentration) </li></ul></ul><ul><ul><li>Find Q R , Q s , X,  , V, Y </li></ul></ul>
  21. 21. EXAMPLE <ul><li>Find Q s ,  , V, Y </li></ul><ul><li>Given: </li></ul><ul><ul><li>Q = 1000 m 3 /d </li></ul></ul><ul><ul><li>S o = 150 mg/L </li></ul></ul><ul><ul><li>S = 15 mg/L </li></ul></ul><ul><ul><li>Q R = 240 m 3 /d </li></ul></ul><ul><ul><li>F/M = 0.3 d -1 </li></ul></ul><ul><ul><li>X = 2000 mg/L </li></ul></ul><ul><ul><li>X s = 1% or 10,000 mg/L </li></ul></ul>
  22. 22. DEFINITION OF BOD <ul><li>Microorganisms (e.g., bacteria) are responsible for decomposing organic waste. When organic matter such as dead plants, leaves, grass clippings, manure, sewage, or even food waste is present in a water supply, the bacteria will begin the process of breaking down this waste. When this happens, much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen they need to live. Biological Oxygen Demand (BOD) is a measure of the oxygen used by microorganisms to decompose this waste. If there is a large quantity of organic waste in the water supply, there will also be a lot of bacteria present working to decompose this waste. In this case, the demand for oxygen will be high (due to all the bacteria) so the BOD level will be high. As the waste is consumed or dispersed through the water, BOD levels will begin to decline. </li></ul>

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