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Lecture 11

Lecture 11. Domestic Waste and Waste Treatment Text Chapter 21. History. Middle of 19 th century saw increase in waterborne diseases in densely-populated areas (London, England) Cholera Modern sewage treatment practices began in early 20 th century

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Lecture 11

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  1. Lecture 11 Domestic Waste and Waste Treatment Text Chapter 21

  2. History • Middle of 19th century saw increase in waterborne diseases in densely-populated areas (London, England) • Cholera • Modern sewage treatment practices began in early 20th century • Treatment of organic matter in domestic waste • Recently, focus has been on reduction of pathogenic microbes and removal of toxic substances

  3. History • Today, more than 15,000 treatment plants treat approximately 150 billion liters of wastewater/day in the U.S. • Today 25% of the U.S. population, mainly in rural areas, use septic tanks to treat domestic sewage

  4. Composition of domestic sewage • Human feces and urine • 100-500 g of feces and 1-1.3 liters of urine/person/day • “graywater” • Water from sinks, bathtubs, yard sprinklers • Assessment of amount of organic matter in sewage • Total organic carbon (TOC) • Biochemical oxygen demand (BOD) • Chemical oxygen demand (COD)

  5. Objective of wastewater treatment • To reduce BOD • BOD is the amount of dissolved oxygen consumed by microorganisms during the biochemical oxidation of organic and inorganic matter to carbon dioxide • 5-day BOD test • Purpose of BOD test • Determine amount of oxygen required to oxidize the organic matter in the wastewater • Determine size of treatment system needed • Assess the efficiency of the treatment process • Determine compliance with wastewater discharge permits

  6. BOD bottles BOD as a function of time

  7. P is decimal volumetric fraction of wastewater used in test D is dissolved oxygen concentration at Time=0 and Time = 5 days D0 – D5 BOD (mg/l) = __________ P 5-day BOD test

  8. Sample calculation Determine the 5-day BOD for a 15 ml sample that is diluted with dilution water to a total volume of 300 ml when the initial DO concentration is 8 mg/l and after 5 days, has been reduced to 2 mg/l. D0 = 8 D5 = 2 P = 15 ml/300ml = 0.05 BOD (mg/l) = _______ = 120 0.05 8 - 2

  9. Table 21.2

  10. Modern Wastewater Treatment • Primary treatment • Separation of large debris following sedimentation • Gravel, sand, twigs. leaves Grit Chamber Primary clarifiers Bar Screen Primary Settling Tank Anaerobic sludge digestor To land application sludge

  11. Primary Clarifiers • Separate liquids from solids • Skimmer removes grease at the surface and sends it to anaerobic digestor

  12. Aeration tank or Trickling filter Secondary Treatment • Remaining suspended solids are decomposed and number of pathogens are reduced Final settling tank or clarifier Primary settling tank sludge To anaerobic sludge digester Sludge digester Gravity thickener plant 1% 6% solids content Land application

  13. Activated Sludge Process (aerobic microbial metabolism) • Mixed Liquor Suspended Solids (MLSS) • Air is pumped through the wastewater • Sludge is removed from the bottom and sent to the anaerobic sludge digestor • Some of the sludge is used to inoculate the fresh, incoming wastewater entering the aeration tank Q = flow rate of sewage in millions of gallons per day (MGD) MLSS is in mg/l V is volume of aeration tank (gallons) Q x BOD Food/Microbes Ratio = _____________ MLSS x V

  14. Food/Microbe Ratio The higher the waste rate, the higher the ratio. 0.2-0.5 lb/BOD5/day/lb MLSS is normal A low ratio means that the microbes are starving. Computers keep track of properties of sewage and operating parameters of wastewater treatment process

  15. Important Operating Parameters • Organic loading rate • Oxygen supply • Control and operation of the final settling tank Functions: Clarification Thickening Sludge settleability is determined by sludge volume index (SVI) Final settling tank sludge V x 1000 where V is volume of settled sludge after 30 min SVI (ml/g) = ___________ MLSS

  16. Filamentous Bulking • Defined as slow settling and poor compaction of solids in the clarifier • Caused by excessive growth of long-chain filamentous bacteria (Nocardia spp., actinomycetes) • A high SVI (>150 ml/g) indicates bulking • Causes • Low F/M (food/microbe) • Low dissolved oxygen • Low nutrient • High sulfide concentrations • Treatment • Treat return sludge with chlorine or hydrogen peroxide to kill filaments

  17. Using biological indicators of health and efficiency of plant operation • An abundance of protozoans such as rotifers indicates healthy situation

  18. Energy to run plant CH4 Anaerobic Sludge Digestor • Maintain temp at 37C • 30-day retention time • Kills pathogens • Produces methane used to run facility from settling tanks sludge Gravity thickener plant 1% 6% solids content

  19. Trickling Filter-alternative to activated sludge tank

  20. Organic matter CO2 + H2O Trickling Filter Porous media Microbial biofilm

  21. Disinfection • Addition of chlorine • 24-h contact time needed for chlorine to kill bacteria in water before release into the environment • Only in summer in Bozeman • Assume low water temps in receiving water kills pathogens • Sulfur dioxide is added to water to remove chlorine after sufficient contact time to kill pathogens before discharge of water into environment • In future, uv-treatment to kill microbes will replace chlorine • Ultraviolet radiation of water allows less chlorine to be used, and reduces contact time.

  22. Disinfection tank Tertiary treatment • Involves a series of steps to further reduce organic concentration, turbidity, N, P, metals, and pathogens Settling tank Sand or mixed media filter Discharge to environment filters out protozoans & pathogenic bacteria Sludge digestor

  23. Tertiary Treatment • Process used when water is to be used for irrigation, recreation, drinking water • Involves • Filtration • Very effective in removing Crytosporidium and Giardia • 90% removal of enteric bacteria and viruses • Coagulation (iron and aluminum salts, pH>11 • 99% removal of enteric viruses • Activated carbon adsorption • Additional disinfection stopped

  24. Nitrogen Removal During Activated Sludge Process • Encourage nitrification followed by denitrification • Growth rate of nitrifying bacteria must be greater than the heterotrophic bacteria in system • Nitrification requires a long (>4 days) sludge retention time

  25. Denitrification Denitrification Bardenpho process

  26. Phosphorus Removal • Uptake of phosphate by microbes during aerobic stage followed by release of phosphate during anaerobic stage

  27. Primary settling tank Aeration tank sludge Aerobic/oxidation process for phosphate removal Anaerobic stage Final settling tank Sludge digestor To sludge digestor

  28. energy Luxury Phosphorus Uptake Aeration tank Settling tank aerobic anaerobic anaerobic influent Poly-P Pi Poly-P PHAs Poly-P PHAs 7-14 mg/L Pi Pi BOD removal Detrital organic-P Porg = 1/3 to 1/2 Ptotal Lime treatment air Pi precipitation

  29. Reverse Osmosis Membrane Filtration • >99.9% removal of enteric viruses • Used for water that will be reinjected into aquifer for protection and storage

  30. Numbers per liter Enteric viruses Salmonella Giardia Cryptosporidium Concentration in raw sewage 105-106 5K-80K 9K-200K 1-4,000 Primary treatment 1.7K-500K 160-3300 72K-146K Secondary treatment 80-470K 3-1000 6.5K-110K Advanced secondary treatment 0.007-170 4x10-6-7 0.099-3000 Pathogen removal

  31. Pathogen analysis • Sample effluent 3 times/week • Total coliforms • Fecal coliforms • protozoans • viruses (not yet, too expensive, not yet regulated)

  32. Sludge Processing • Sludge from primary settling tank contains 3-8% solids • Sludge from secondary settling tank contains 0.5-2% solids • Purpose of sludge processing • Reduce water content • Stabilize organic matter

  33. Anaerobic Sludge Digestor

  34. Sludge Processing • Thickening • Settling or centrifugation • Digestion • Microbial process • Stabilization of solids, removal of pathogens, production of methane • Takes 2-3 weeks in large covered tanks • Conditioning • Addition of alum, ferric chloride, lime to aggregate solids • Dewatering to remove water • Air drying, spreading basins, centrifugation, vacuum filtration • All of above results in reduction of pathogens in solids

  35. Sludge Processing Steps

  36. Residence time forwater in treatmentplant is 16-20 hrs stopped

  37. Land disposal of biosolids • Application of biosolids on agricultural land

  38. Quality of biosolids • EPA has established 2 classes of biosolids • Class A • Solids sold in bags to be applied to lawns, gardens • Class B • Solids applied to agricultural land • No food crops should be grown on land for 18 months

  39. Alternatives to conventional tertiary treatment • Infiltration basins containing coarse sands with clean water infiltration • Alternating 1 day of flooding and 1 day of drying • Aquifer residence time of 20-45 days • Still required disinfection before release into river Secondary treated water To river Land surface 100 ft 100 ft

  40. Constructed Wetlands • Typically less than 1 meter in depth • support growth of aquatic vegetation • being used more to treat secondary wastewater effluents • vegetation provides surfaces for microbial attachment and aids in filtration and removal of wastewater contaminants rhizosphere

  41. Types of constructed wetlands Free water surface Subsurface flow systems Water hyacinths Benefits: surface can be used for other purposes no odors Cryptosporidium reduce 53% Giardia reduced 58% enteric viruses reduced 98% fecal coliforms reduce 98% Substituting duck weed for hyacinths Crypto and Giardia removal 98% fecal coliform removal 57%

  42. Summary • Municipal wastewater treatment plant is engineered to reduce area/volume normally required in nature to remove nutrients and pathogens from wastewater • Primary treatment • Physical removal of large debris • Secondary treatment • Microbiological conversion of organic-C to CO2 and H2O • Tertiary treatment • Inactivate pathogens, remove, N, P, toxins from water before release to environment

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