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Secondary Treatment of Sewage

Secondary Treatment of Sewage. Objective – To remove non- settleable colloidal solids & to stabilize organic matter Effluent from primary sedimentation tank contains about 60 o 80% of unstable OM Sewage supplies biological food and growth nutrients

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Secondary Treatment of Sewage

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  1. Secondary Treatment of Sewage

  2. Objective – To remove non-settleable colloidal solids & to stabilize organic matter • Effluent from primary sedimentation tank contains about 60 o 80% of unstable OM • Sewage supplies biological food and growth nutrients • Unstable OM changes to stable forms, which are then removed in secondary settling tanks

  3. Biological process (Oxygen Dependance) Aerobic Anaerobic Aerobic-anaerobic • Aerobic preferred because of no bad smell & 3 times more active than anaerobic bacteria @ 30oC Trickling Filters Activated Sludge Processes Aerobic stabilization ponds Aerated Lagoons Anaerobic sludge digestion Anaerobic contact processes Anaerobic filters Anaerobic lagoons & ponds Combination

  4. Biological Treatment Techniques • Attached Growth Processes (fixed film processes) • Bacteria attached to inert material like rock, plastic etc • Intermittent sand filters • Trickling filters • Rotating biological contactors • Packed bed reactors • Suspended Growth Processes • Bacteria maintained in suspension within the liquid by natural/ mechanical mixing • Activated sludge process • Aerated lagoons • Sludge digestion systems • Combined Processes • Consists of both • Facultative lagoons • Activated sludge, Trickling filter • Trickling filter, Activated sludge

  5. Sewage Filtration • Activated Sludge Process • Miscellaneous Methods Oxidation ditches Stabilisation ponds (Oxidation ponds) Aerobic ponds (Algae ponds) Anaerobic ponds Aerated lagoons Rotating Biological Contactors

  6. Sewage Filtration • Filter units consists of open beds of coarse aggregates • Organic film will form around each aggregate • Sewage will oxidise when it passes through organic film since it contains aerobic bacteria

  7. Trickling Filters

  8. Trickling Filter Process

  9. Merits • Lesser land area • Remove 75% BOD & 80% SS • Simple working • Self-cleaning • Less mechanical wear & tear Demerits • Head loss is high • Cost is high • Cannot treat raw WW, primary treatment is must • Fly & odour nuisance • Ponding trouble

  10. Types of Trickling Filters • Conventional/ Ordinary/ Standard rate/Low rate Trickling filters • High rate Trickling filters (Re-circulation) --- allows continuous dosing of the filters --- increases efficiency of the filter --- fly nuisance is less --- influent remains fresh, so less odour

  11. Low rate Trickling Filter

  12. Single stage High rate trickling filter

  13. Two stage high rate Trickling Filter

  14. Types of high rate filters • Bio-filters (1.2 to 1.5m depth) • Accelo-filters (1.8 to 2.4m depth) • Aero-filters (special rain-drop distribution)

  15. Secondary Settling Tanks / Humus Tanks • Detention period : 1.5 to 2 hours • Circular tanks with dia & depth ranges between 7.5 to 9m • Overflow rate : 40,000 to 70,000 l/m2/day

  16. Pri.Eff. Sec. Eff. MLSS Activated Sludge System Air →Provides Oxygen and Mixing Biomass (suspended) Secondary Clarifier Aeration Tank Return Activated Sludge (RAS) Waste Activated Sludge (WAS)

  17. Aeration Tanks of an Activated Sludge Plant i) Diffused air aeration/ Air diffusion

  18. Mechanical Aeration Systems

  19. Combined Aeration Systems

  20. Secondary Sedimentation Tank of an Activated Sludge Plant • No provision for removal of scum • Detention period : 1.5 to 2 hours • Depth : 3.5 to 4.5m • Length to depth ratio : 5 for circular; 7 for rectangular

  21. Sludge Thickener & Sludge Digestion Tank of an Activated Sludge Plant • Gravity Thickeners • Floatation Thickeners • Centrifugal Thickeners

  22. Problems encountered • Bulking of sludge: sludge that exhibits poor settling characteristics – filamentous micro-organisms • Rising sludge: Denitrification in secondary clarifier – formation of nitrogen gas bubbles

  23. Merits --- lesser land area --- low head loss --- no fly or odour nuisance --- less capital cost • Demerits --- difficult handling --- high operating cost --- bulking of sludge Activated Sludge Process is more efficient than Trickling Filter

  24. Rotating Biological Contactors • RBC – cylindrical media made of closely spaced plastic sheets of 3 to 3.5m in diameter, 10mm thick & placed @ 30 to 40mm spacings • They are immersed in WW by about 40% of their diameter • Rotated @ a speed of 1-2 rpm • Microbes from WW adhere to the rotating surfaces & form 1 to 3mm layer of biological slime

  25. Rotating Biological Contactors

  26. RBC Schematic Film of Microorganisms Rotation Leonard W. Casson, Ph.D., P.E., DEE Wastewater

  27. Aerobic Stabilization Units Waste Stabilization Ponds Algal-symbiosis

  28. 1. Oxidation ponds

  29. 2. Oxidation ditches/ Extended Aeration Lagoons

  30. 3. Mechanically aerated lagoons

  31. Anaerobic stabilization units • Deeper stabilization ponds • Stages • Acid fermentation: complex OM are broken down to short chain acids & alcohols • Methane fermentation: these materials are converted to gases primarily, CH4 & CO2

  32. High Rate Anaerobic Systems 1. Anaerobic Contact Process

  33. 2. Anaerobic Filter (Upflow packed bed)

  34. 3. Anaerobic Fixed Film Reactors (Downflow packed bed)4. Fluidized/Expanded bed reactor

  35. 5.Upflow Anaerobic Sludge Blanket (UASB)

  36. Chlorination of Sewage • Disinfection by using Chlorine • Post-chlorination: reduce bacterial count in effluent • Pre-chlorination: control odours, removal of greases • Breakdown of sulphur compounds and reduce H2S production • Split-chlorination: both at beginning & end of the treatment process

  37. Reclamation & reuse of WW • Natural evaporation • Ground water recharge • Irrigation • Recreational • Municipal uses • Industrial uses • Discharge into natural sources

  38. Benefits of Agricultural Reuse • High concentrations of nutrients • May eliminate need for fertilizer • Long-term soil enrichment • Decreases demand on potable water supply • Additional treatment in soil • Water not discharged to receiving waters

  39. Disadvantages of Agricultural Reuse • Health risk from associated pathogens • Health risk from other contaminants (e.g. metals, chemicals, pharmaceuticals) • Decrease in soil quality from accumulation of metals and acidification • Infiltration of groundwater

  40. Urban Wastewater Reuse • Recycled urban wastewater or reclaimed water • Urban wastewater that has undergone additional treatment following secondary treatment in order to be reused rather than discharged into the environment • Can reduce strain on potable water supply

  41. Urban Wastewater Reuse • What can urban reclaimed water be used for? • Irrigation - public parks, schools, road medians, any landscaped areas, golf courses • Commercial - vehicle washing facilities, laundry facilities, window washing, mixing pesticides and herbicides • Construction - dust control, concrete production • Toilet and urinal flushing • Fire protection • Drinking water?? in Australia, not yet in the U.S.

  42. Urban Wastewater Reuse • Major considerations are public health and reliability of the system • Water must be of acceptable quality for intended uses • System must be maintained and operated properly • Reclaimed water pipes must be clearly marked

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