Week8

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Week8

  1. 1. CE 441 - Wastewater Treatment I Jian Peng, Ph.D. jpeng@fullerton.edu 1
  2. 2. Content for WW Treatment Objective of WW Treatment WW Collection and Sewers Overview of WW Treatment Preliminary Treatment Primary Treatment Secondary Treatment  Activated Sludge Process  Attached Growth process  Other Biological Process Advanced WW Treatment WW Sludge Treatment Effluent Disposal 2
  3. 3. Why Do We Treat Wastewater? Swimable and Fishable!  To Protect the health of the receiving water  Water reuse  Alternative water resources  Aesthetics  To protect the health of humans. 3
  4. 4. Wastewater Collection & Treatment Storm sewer Sanitary sewer  Domestic  Industrial Combined sewer 4
  5. 5. Sanitary Sewer System Wastewater leaves residences and buildings through 4” and 6” diameter building sewers. These sewers are owned by the owner to the point of connection with the publicly owned sewer. 5
  6. 6. Sanitary Sewer System Building sewers empty into larger sewer mains. They then empty the wastewater into trunk sewers. Wastewater from the trunk sewers empties into interceptor sewers. 6
  7. 7. Sewer Corrosion Wastewater turns septic, and H2S is formed. Crown corrosion (odor & sink hole) problems Spray of caustic solution; chlorine disinfection; hydrogen peroxide addition; flushing with NaOH(OCSD) Biotrickling filter for odor control (LA City) 7 Sewer rehab – a “big” project (e.g. 1400 mi. for LACSD)
  8. 8. Manhole and Lift Station – Sewer network Manhole – provide access to sewer for cleaning, repair, sampling, and flow measurements. In some cases, it becomes necessary to pump the sewage up from a low point to a higher elevation, either to reach a treatment plant or to reach another gravity sewer. 8
  9. 9. Gravity Sewer System - Example A gravity sewer system is being designed for a minimum velocity of 2 ft/sec. What is the reason behind the practice? (a) To prevent deposition of solids (b) To release trapped sewer gases (c) To ventilate the water with turbulence (d) TO reduce the length of piping necessary The answer is (a). 9
  10. 10. Hydraulics – Manning equation C 2 3 12 V= R S n V = velocity of flow in feet per second (meters per second) C = Constant = 1.486 for English units (1.00 for metric units) R = Hydraulic Radius in feet (meters) S = channel slope in ft/ft or m/m n = Manning roughness coefficient 10
  11. 11. Hydraulics Manning equation 1.486 2 3 1 2 V= R S n where v is the velocity in cfs R is the hydraulic radius (ft) R= h b Area (A) Wetted perimeter (Pw) What is the hydraulic radius of a full-flow circular pipe with a radius of r? R= bh b+2h If b>>h R~h 11
  12. 12. Sanitary Sewer - Example The average velocity (fps) of a steady uniform flow in a 15”-diameter sewer line with a slope of 0.35%, a depth of 3 in, and a Manning’s roughness coefficient of 0.012 is most nearly: 1.486 2 3 1 2 V= R S n y / d o = 3 / 15 = 0.2; d o = 15in = 1.25ft From Open Channel Hydraulics (by Chow), R/d o = 0.1206 → R = 0.1206(1.25) = 0.151 16 = (0.4644 / 0.012)D 8 / 3 (0.002)1 / 2 2 1 1.486 3 (0.0035) 2 = 2.08fps V= (0.151) 0.012 12
  13. 13. Storm Sewer - Example A proposed storm sewer system will have a slope of 0.20%. The design flow for the line has been determined to be 16 cfs. Assume steady, uniform flow and a Manning’s roughness coefficient of 0.012. What would be the minimum circular pipe size? 1.486 2 3 1 2 V= R S n 1.486 D 2 3 1 2 πD 2 Q = VA = [ ( ) S ]( ) n 4 4 8 1 0.4632 3S 2] =[ ( D) n 16 = (0.4644 / 0.012)D 8 / 3 (0.002)1 / 2 → D = 2.30ft = 27.65in 13
  14. 14. What is Inflow and Infiltration (I&I)? Uncapped Cleanout House Lateral Catch Basin to Sanitary Cracked or Broken Pipe Roof Drain Connection Connected Foundation Drain/ Sump Pump Faulty Manhole Cover or Frame Deteriorated Manhole Storm Sewer Sanitary Sewer Storm CrossConnection Faulty Lateral Connection Inflow is typically rain water that enters publicly owned sewer and manholes, as well as through private property sources such as rain leaders, sump pumps, foundation drains, and leaking house services. Infiltration is typically the seepage of groundwater into the sanitary sewer system through cracks or joints of 14 manholes and pipes; and leaking house services.
  15. 15. Smoke Testing (for I/I) Simulated smoke will be injected into the sewer system. As a result smoke may be seen coming from manhole covers, storm drains, roof vents, and building foundations. After each setup the smoke test will last approximately 30 minutes. 15
  16. 16. Sable: Santa Barbara’s Leaky Sewer Detector 16
  17. 17. Significance of Wastewater Contaminants Suspended solids – can cause sludge deposits and anaerobic conditions in the environment Biodegradable organics – can cause anaerobic conditions in the environment Pathogens – transmit disease Nutrients – can cause eutrophication Heavy metals – toxicity to biota and humans Refractory organics – toxicity to biota and humans Dissolved solids – interfere with reuse What are removed by a POTW (publicly-owned treatment works)? 17
  18. 18. Typical Composition of Untreated Wastewater Constituent Weak (mg/L) Strong (mg/L) Alkalinity (as CaCO3) BOD5 (as O2) 50 200 100 300 COD (as O2) TOC (as C) 250 75 1000 300 TSS 100 350 TDS 200 1000 Chloride 30 100 Total Kjeldahl Nitrogen (as N) Total Phosphorous 20 80 5 20 18
  19. 19. Treatment Processes Physical, chemical, or biological processes. What are the things removed in a municipal wastewater treatment? BOD, SS, pathogen, and nutrients (?) 30/30 Rule (BOD5 and TSS) 301H waiver for secondary treatment discharge to the ocean Pretreatment of industrial wastes 19
  20. 20. Pretreatment of Industrial Wastewaters Industrial wastewaters must be pretreated prior to being discharged to municipal sewer system. Approach is to remove materials that will not be treated by municipal system. Local authority must monitor and regulate industrial discharges. Pretreatment requirements set by U.S. EPA. 20
  21. 21. Overview of Municipal Wastewater Treatment Preliminary treatment – removes materials that can cause operational problems, equalization optional. Primary treatment – remove ~60% of solids and ~35% of BOD5. Secondary treatment – remove ~85% of BOD5 and solids (and nutrients now). Advanced treatment – varies: 95%+ of BOD5 and solids, N, P (polishing). 21
  22. 22. Preliminary Treatment – Bar racks Purpose  remove larger objects Solid material stored in hopper and sent to landfill Mechanically or manually cleaned 22
  23. 23. Bar rack (on right) in service. Comminuter (on left) out of service 23
  24. 24. Preliminary Treatment – Grit chambers Purpose: remove inert dense material, such as sand, broken glass, silt and pebbles Avoid abrasion of pumps and other mechanical devices Material is called “grit” 24
  25. 25. (Aerated) Grit Chamber Remove sand, coffee ground, egg shell… Typical retention time = 2 – 5 minutes, based on Q peak. L:W = 3:1 (typ.) Air flow rate = 3 cfm/ft length or 0.3 m3/m/m (typ.) Two tanks in parallel Depth = 6 – 15 ft 25
  26. 26. (Aerated) Grit Chamber Design AGC for Q = 10 MGD, PF = 1.5, and two tanks in parallel, D = 6’. Q peak = 10*1.5/2 = 7.5 MGD per tank V = (7,500,000)(3/1440)/7.48 = 2090 ft3 A = 2090/(6) = 350 ft2 = (3W)(W) W = 11 ft, L = 33 ft Air supply = (33)(3) = 100 cfm for each tank 26
  27. 27. Primary Treatment Primary Sedimentation Tank (PST) Physical process. Remove ~55% SS; ~ 35% BOD τ ~ 2 hours SOR = 1,000 gpd/ft2 Weir loading = 20,000 gpd/ft D ~ 10 ft 27
  28. 28. Primary Treatment (Example 6-4) Q = 0.15 m3/s L x W x D = 40m x 10m x 2m Weir length = 75 m τ = V/Q = 1.5 hr Overflow rate = Q/A = 32 m/d Weir loading rate = Q/L = 173 m3/d/m 28
  29. 29. Fig 6-12: Primary settling tank 29
  30. 30. Secondary Treatment Main Purpose: Provide BOD removal - M/O convert organic wastes into stabilized compounds (similar to self-purification of a stream or river). Secondary Purpose: Additional removal of suspended solids - to have a clarified effluent. Effectiveness depends on availability of high density of microorganisms good contact between organisms and wastes availability of wastes favorable temperature favorable pH absence of toxic compounds 30
  31. 31. Secondary Treatment Biological Treatment Activated Sludge Trickling Filter Rotating Contactor Beds Secondary Clarification Disinfection 31
  32. 32. Secondary Treatment Biological process. Remove >80% BOD Organics removal and clarified effluent. Suspended growth Attached growth Aerobic Dispersed Growth  Activated sludge  Oxidation ditches/ponds  Aerated lagoons, stabilization ponds Fixed Growth  Trickling filters  Rotating Biological Contactors (RBCs) 32
  33. 33. Biology of Wastewater - Classification Electron Acceptor  Obligate aerobes  Facultative anaerobes  obligate anaerobes  anoxic conditions Carbon source  Heterotrophs (organics)  Autotrophs (CO2) Energy source  Phototrophs (light)  Chemotrophs (organics) 33
  34. 34. Biology of Wastewater - Classification Important Organisms in Wastewater Treatment  Bacteria  Fungi  Algae  Protozoa  Rotifers Temperature Ranges  psychrophiles (0 - 20°C)  mesophiles (20 - 40°C)  thermophiles (45 - 60°C) 34
  35. 35. Heterogeneous microorganism population employed in wastewater treatment 35
  36. 36. Fig 6-19: Conventional activated sludge plant 36
  37. 37. Activated Sludge Q So MLVSS = 0.8 MLSS MCRT, θ X, V Qe, Xe, S RAS, QR, XR WAS, Qw, XR 37
  38. 38. Activated Sludge …. Typical Values  Mixed liqueur aerated 4-8 hours  8 m3 of air per m3 of wastewater treated  Sludge return can be 30 -100% of wastewater flow  MLSS = 3,000 mg/L  F/M = 0.4/d Sludge return  Ideally enough sludge should be returned to keep the mass of microbes in reactor constant.  θc = mean cell residence time (MCRT).  Typical (3 - 15 days) 38
  39. 39. Primary settling tanks in foreground followed by aeration tanks and circular secondary settling tanks 39
  40. 40. Activated sludge aeration tank 40
  41. 41. Activated Sludge - F/M parameter Low F/M (low rate of wasting)  starved organisms  more complete degradation  larger, more costly aeration tanks  more O2 required  higher power costs (to supply O2)  less sludge to handle High F/M (high rate of wasting)  organisms are saturated with food  low treatment efficiency 41
  42. 42. Q So Activated Sludge …. MLVSS = 0.8 MLSS MCRT, θ X, V Qe, Xe, S RAS, QR, XR WAS, Qw, XR F QSo So = = M θX VX QSo Volumetric Loading Rate = V VX VX θ c (or MCRT ) = ≈ Q w X w + Qe X e Q w X w Qr X = Q + Qr X r θ c Y ( So − S ) X= θ 1 + k dθ c and Qr R= Q K s (1 + θ c k d ) S= θ c ( µ m − kd ) − 1 42
  43. 43. Activated Sludge (Examples 6-5 & 6-7) Q = 0.15 m3/s; BOD5 = 84 mg/L Ks = 100 mg/L; µm= 2.5/d; kd = 0.05/d; Y = 0.5 mg VSS/mg BOD5 removed; MLVSS = 2000 mg/L BOD5 of the effluent SS = 63% of SS 30/30 requirements S = BOD5 allowed – BOD5 in SS = 30 - (30)(63%) = 11.1 Q K s (1 + θ c k d ) S= θ c (µ m − kd ) − 1 θc = 5 days So MLVSS = 0.8 MLSS MCRT, θ X, V Qe, Xe, S RAS, QR, XR WAS, Qw, XR 43
  44. 44. Activated Sludge (Examples 6-5 & 6-7) Q So MLVSS = 0.8 MLSS MCRT, θ X, V θ c Y ( So − S ) X= θ 1 + k dθ c Qe, Xe, S RAS, QR, XR WAS, Qw, XR θ = 0.073 d = 1.8 hr (short) V = τQ = 970 m3  F/M = 0.56 mg BOD5/mg MLVSS/d F QSo So = = M θX VX 44
  45. 45. Activated Sludge (Ex. 6-8) Q = 0.15 m3/s; BOD5 = 84 mg/L V = 970 m3, X = 2,000 mg/L, and θc = 5 days (20% of MLVSS in aeration tanks is wasted every day) Assume XR (same as Xw) = 3,986 mg/L θ c (or MCRT ) = VX VX ≈ Q w X w + Qe X e Q w X w  Qw = 97.3 m3/d = 0.0011 m3/s ( ~0.5%Q) Volumetric Loading Rate = Q S o QS o = V [(0.15)(86400)](84) = 1,122 mg / m 3 / d 970 MLVSS = 0.8 MLSS MCRT, θ X, V Qe, Xe, S RAS, QR, XR WAS, Qw, XR 45
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