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Wastewater treatment processes (I)

Wastewater treatment processes (I). ENV H 440/ENV H 541. John Scott Meschke Office: Suite 2249, 4225 Roosevelt Phone: 206-221-5470 Email: jmeschke@u.washington.edu. Gwy-Am Shin Office: Suite 2339, 4225 Roosevelt Phone: 206-543-9026 Email: gwyam@u.washington.edu. Key points.

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Wastewater treatment processes (I)

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  1. Wastewater treatment processes (I) ENV H 440/ENV H 541 John Scott Meschke Office: Suite 2249, 4225 Roosevelt Phone: 206-221-5470 Email: jmeschke@u.washington.edu Gwy-Am Shin Office: Suite 2339, 4225 Roosevelt Phone: 206-543-9026 Email: gwyam@u.washington.edu

  2. Key points • Purpose of the individual unit processes • The typical operating conditions • The outcome of the processes • Microbial reduction of the processes

  3. How much wastewater do we produce each day? These values are rough estimates only and vary greatly by locale. Wastewater Characteristics

  4. Wastewater treatment systems • Decentralized • Septic tank • Waste stabilization ponds • Facultative lagoon • Maturation lagoon • Land treatment • Constructed wetland • Centralized

  5. Sewer systems

  6. Typical composition of untreated domestic wastewater

  7. Microorganism concentrations in untreated wastewater

  8. (Minimum) Goals of wastewater treatment plants • <30 mg/L of BOD5 • <30 mg/L of suspended solids • <200 CFU/100mL of fecal coliforms

  9. Conventional Community (Centralized) Sewage Treatment Secondary Treatment Using Activated Sludge Process Sludge drying bed or mechanical dewatering process Pathogen Reductions Vary from: low (<90%) to Very High (>99.99+%)

  10. Typical Municipal Wastewater Treatment System Preliminary or Pre-Treatment SecondaryTreatment PrimaryTreatment Disinfection Sludge Treatment& Disposal

  11. Preliminary Wastewater Treatment System Preliminary or Pre-Treatment Solids to Landfill

  12. Preliminary Treatment - Bar Racks Bar Racks: are used to remove large objects that could potentially damage downstream treatment/pumping facilities. Preliminary Treatment Facilities Ref: Metcalf & Eddy, 1991

  13. Preliminary Treatment - Grit chamber Grit chamber: used to remove small to medium sized, dense objects such as sand, broken glass, bone fragments, pebbles, etc.

  14. Primary Wastewater Treatment PrimaryTreatment

  15. Primary sedimentation • To remove settleable solids from wastewater

  16. Primary Clarification Scum: Oil, Grease, Floatable Solids PrimaryEffluent PrimarySludge Influent from Preliminary Treatment Section through a Circular Primary Clarifier Primary Treatment

  17. Primary sedimentation • To remove settleable solids from wastewater • Average flow: 800-1200 gpd/ft2 • Retention time: 1.5 - 2.0 hours (at maximum flow) • 50 - 70 % removal of suspended solids • 25 - 35 % removal of BOD5 • ~20 % removal of phosphate • ~50 % removal of viruses, bacteria, and protozoa • 90 % removal of helminth ova

  18. Secondary Wastewater Treatment SecondaryTreatment

  19. Secondary treatment processes • To remove suspended solids, nitrogen, and phosphate • 90 % removal of SS and BOD5 • Various technologies • Activated sludge process • Tricking filter • Stabilization ponds

  20. Secondary Treatment Using Activated Sludge Process SecondaryTreatment Sludge drying bed or mechanical dewatering process Secondary Treatment

  21. Aerobic microbes utilities carbon and other nutrients to form a healthy activated sludge (AS) biomass (floc) The biomass floc is allowed to settle out in the next reactor; some of the AS is recycled Simplified Activated Sludge Description Secondary Treatment

  22. General Microbial Growth • Carbon Source: Dissolved organic matter • Energy Source: Dissolved organic matter • Terminal Electron Acceptor: Oxygen • Nutrients: Nitrogen, Phosphorus, Trace Metals • Microorganisms: Indigenous in wastewater, recycled from secondary clarifier Secondary Treatment

  23. Activated Sludge Aeration Basins Empty basin, airdiffusers on bottom Same basin,in operation Secondary Treatment

  24. The Oxidation Ditch Secondary Treatment Ref: Reynolds & Richards,1996, Unit Operations and Processes in Environmental Engineering

  25. The Oxidation Ditch Secondary Treatment Ref: Reynolds & Richards,1996, Unit Operations and Processes in Environmental Engineering

  26. Circular Secondary Clarifier SecondaryEffluent Influent from Activated Sludge Aeration Basin or Trickling Filter Return (Secondary) Sludge Line Section through a Circular Secondary Clarifier Secondary Treatment

  27. Activated sludge process • To remove suspended solids, nitrogen, and phosphate • Food to microorganism ratio (F:M ratio): 0.25 kg BOD5 per kg MLSS (mixed liquor suspended solids) per day at 10 oC or 0.4 kg BOD5 per kg MLSS per day at 20 oC • Residence time: 2 days for high F:M ratio, 10 days or more for low F:M ratio • Optimum nutrient ratio: BOD5:N:P =>100:5:1 • 90 % removal of SS and BOD5 • ~20 % removal of phosphate • > 90 % removal of viruses and protozoa and 45 - 95 % removal of bacteria

  28. Secondary Treatment Using Trickling Filter Process SecondaryTreatment TricklingFilter Secondary Treatment

  29. Trickling Filter Rotating arm todistribute water evenly over filter Primary effluent drips onto rock orman-made media Rock-bed with slimy (biofilm) bacterial growth Treated waste to secondary clarifier Primary effluent pumped in http://www.rpi.edu/dept/chem-eng/Biotech-Environ/FUNDAMNT/streem/trickfil.jpg

  30. Trickling Filter http://www.eng.uc.edu/friendsalumni/research/labsresearch/biofilmreslab/Tricklingfilter_big.jpg

  31. Tricking filter process • To remove suspended solids, nitrogen, and phosphate • Organic loading (BOD5 X flow/volume of filter): 0.1 kg BOD5 per m3 per day • Hydraulic loading: 0.4 m3 per day per m3 of plan area • 90 % removal of SS and BOD5 • ~20 % removal of phosphate • Variable removal levels of viruses, 20-80 % removal of bacteria and > 90 % removal of protozoa

  32. Stabilization Ponds • The oldest wastewater treatment systems • Requires a minimum of technology • Relatively low in cost • Popular in developing countries and small communities in the US (90 % communities with population <10,000) • Used for raw sewage as well as primary‑ or secondary‑treated effluents. • Facultative ponds and aerated lagoons Ponds and Lagoons

  33. Facultative Pond Ponds and Lagoons

  34. Facultative ponds • 3 zones: upper photic (aerobic) zone, facultative (aerobic and anaerobic) zone and lower anaerobic zone. • Upper aerobic zone: algae use CO2, sunlight and inorganic nutrients (photosynthesis) to produce oxygen and algal biomass. • Facultative zone: bacteria and other heterotrophs convert organic matter to carbon dioxide, inorganic nutrients, water and microbial biomass. • Lower anaerobic zone: anaerobic bacteria degrade the biomass from upper zones • Influence by many factors • Sunlight • Temperature • pH • Biological activities • Characteristics of wastewater Ponds and Lagoons

  35. Facultative ponds • To remove suspended solids, nitrogen, phosphate, and pathogens • Operating water depth: 1-2.5 meters • (maximum) BOD loading: 2.2-5.6 g/m3 /day • Retention time: 3-6 months • >90 % SS and BOD removal (warm and sunny climates) • Microbe removal may be quite variable depending upon pond design, operating conditions and climate. • 90-99% removal of indicator and pathogenic bacteria • 99 % removal of PV1 • 99.9 reduction of Giardia and Cryptosporidium

  36. Aerated Lagoons Stabilization Lagoon Aerated Lagoons http://www.lagoonsonline.com/marshill.htm Ponds and Lagoons

  37. Aerated lagoons • Biological activity is provided by mainly aerobic bacteria • Influence by many factors • Aeration time • Temperature • pH • Biological activity • Characteristics of wastewater

  38. Aerated lagoons • To remove suspended solids, nitrogen, phosphate, and pathogens • Operating water depth: 1-2 meters • Retention time: <10 days • 85% BOD removal (at 20oC and an aeration period of 5 days) • 65% BOD removal (at 10oC and an aeration period of 5 days) • Microbe removal may be quite variable depending upon pond design, operating conditions and climate

  39. Wastewater Disinfection Disinfection

  40. Typical Municipal Wastewater Treatment System Preliminary or Pre-Treatment SecondaryTreatment PrimaryTreatment Disinfection Sludge Treatment& Disposal

  41. Sludge processing • Thickening • Digestion • Dewatering • Disposal

  42. Sludge thickening • To reduce the volume of sludge • to increase sludge solids at least 4 % • Gravity thickening and mechanical thickening • Gravity thickening • Used for primary and tricking filter solids • Without chemical flocculants • loading rate: 30-60 kg/m2 per day • Mechanical thickening • Used for activated sludge solids • With chemical flocculants • dissolved air flotation, gravity belt thickeners, and centrifuge thickening • loading rate: 10-20 kg/m2 per day (dissolved air flotation), 400-1000 L/m (gravity belt thickeners), 1500-2300 L/m (centrifuge thickening) • The concentration of pathogens increased during this process

  43. Gravity belt thickener

  44. Regulatory requirement for disposal of sewage sludge • Class B biosolids (agriculture land) • < 2 million MPN/g of fecal coliforms • Seven samples over 2-weeks period • ~2 log removal • Class A biosolids (home lawn and garden) • < 1000 MPN/g of fecal coliforms • < 3 MPN/4g of Salmonella sp. • < 1 PFU/4g of enteric viruses • < 1/4g of Helminth ova • ~ 5 log removal

  45. Processes to significantly reduce pathogens (PSRP) for a Class B biosolids • Aerobic digestion • Anaerobic digestion • Air drying • Composting • Lime stabilization

  46. Digestion • To stabilize organic matter, control orders, and destroy pathogens • Aerobic digestion and anaerobic digestion • Aerobic digestion • Sludge is agitated with air/oxygen • loading rate (maximum): 640 g/m2 per day • Temperature and retention time: 68 oF for 40 days or 58 oF for 60 days • Solids and BOD reduction: 30-50 %

  47. Anaerobic digestion • Sludge is treated in the absence of air • Operation conditions (optimum) • Temperature: 85-99 oF (98 oF) • pH: 6.7-7.4 (7.0-7.1) • Alkalinity: 2000-3500 mg/L • Solid loading: 0.02-0.05 lb/ft3/day • Retention time: 30-90 days • Treatment outcome • Solid reduction: 50-70 % • Significant reduction of most pathogens • Gas production: methane and carbon dioxide

  48. Anaerobic digester

  49. Air drying, composting, and lime stabilization • Air drying • Sludge is dried on sand beds/(un)paved basins • Retention time: minimum of 3 months • Composting • Various methods: in-vessel, static aerated file, and periodically mixed windrows • File temperature should be raised > 40 oC for 5 days • For 4 hours during the 5 days, the file temperature should be > 55 oC • Lime stabilization • Sufficient lime should be added to raise the pH 12 after 2 hour contact • 4 log inactivation of enteric viruses, 2-7 log inactivation of indicator bacteria, no inactivation of Acaris ova

  50. Processes to further reduce pathogens (PFRP) for a Class A biosolids • Heat drying • Sludge is dried by contact with hot gases • The temperature of gas is >80 oC • Thermophilic aerobic digestion • Sludge is agitated with air/oxygen • 132-149 oF for 4-20 hours • Pasteurization • 158 oF for 30 minutes • Beta- or gamma ray irradiation • Sludge is irradiated with either beta- or gamma ray • > 1.0 Mrad at room temperature

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