Presentation on theme: "Previously Microbial survival in environmental media –Water, land, air Small scale sanitation, composting latrines, etc Conventional wastewater treatment."— Presentation transcript:
Previously Microbial survival in environmental media –Water, land, air Small scale sanitation, composting latrines, etc Conventional wastewater treatment in developed countries –Screening/settling/”activated sludge”/disinfection –Overview of processes, chemical and physical
Outline But what about developing countries? –In many cases, no wastewater treatment –“Natural attenuation” What are some practical options for wastewater treatment in poor countries? –(& see handouts) More about microbes and how they are reduced in wastewater
Alternative Biological Treatment of Wastewater: Alternatives for Small and Rural Communities Letting nature do its thing Lagoons, Ponds and Ditches –aerobic, anaerobic and facultative; for smaller communities and farms –enteric microbes are reduced by ~90-99% per pond multiple ponds in series increases microbe reductions Constructed Wetlands –aerobic systems containing biologically active, oxidizing microbes and emergent aquatic plants Lagoons and constructed wetlands are practical and economical sewage treatment alternatives when land is available at reasonable cost (Phnom Penh)
Stabilization Ponds or Lagoons Aerobic and Facultative Ponds: Biologically Rx by complementary activity of algae and bacteria. Used for raw sewage as well as primary ‑ or secondary ‑ Rx’d. effluent. Bacteria and other heterotrophs convert organic matter to carbon dioxide, inorganic nutrients, water and microbial biomass. Algae use CO 2 and inorganic nutrients, primarily N and P, in photosynthesis to produce oxygen and algal biomass. Many different pond designs have been used to treat sewage Used here in the US in decentralized plants and CAFOs
Stabilization Ponds or Lagoons, cont Facultative ponds: upper, aerobic zone and a lower anaerobic zone. Aerobic heterotrophics and algae proliferate in the upper zone. Biomass from upper zone settles into the anaerobic, bottom zone. Bottom solids digested by anaerobic bacteria Also used with aquaculture systems
Enteric Microbe/Pathogen Reductions in Stabilization Ponds BOD and enteric microbe/pathogen reductions of 90%, esp. in warm, sunny climates. Even greater enteric microbe /pathogen reductions by using two or more ponds in series Better BOD and enteric microbe/pathogen reductions if detention (residence) times are sufficiently long (several weeks to months) Enteric microbes reduced by 90% in single ponds and by multiples of 90% for ponds in series. Microbe removal may be quite variable depending upon pond design, operating conditions and climate. –Reduction efficiency lower in colder weather and shorter retention times
Constructed Wetlands and Enteric Microbe Reductions Surface flow (SF) wetlands reduce enteric microbes by ~90% Subsurface flow (SSF) wetlands reduce enteric microbes by ~99% Greater reduction in SSF may be due to greater biological activity in wetland bed media (porous gravel) and longer retention times Multiple wetlands in series incrementally increase microbial reductions, with 90-99% reduction per wetland cell.
Septic Tank-Soil Absorption Systems for On-Site Sewage Rx Used where there are no sewers and community sewage treatment facilities: ex.: rural homes Septic tank: solids settle and are digested Septic tank effluent (STE) is similar to primary sewage effluent Distribute STE to soil via a sub-surface, porous pipe in a trench Absorption System: Distribution lines and drainfield Septic tank effluent flows through perforated pipes located 2-3 feet below the land surface in a trenches filled with gravel, preferably in the unsaturated (vadose) zone. –Effluent discharges from perforated pipes into trench gravel and then into unsaturated soil, where it is biologically treated aerobically. Enteric microbes are removed and retained by the soil
More on sanitation technologies: www.wateraid.co.uk Handouts For more on small-scale sanitation, check out The Humanure Handbook
Log 10 Reduction of Pathogens by Wastewater Rx Processes
Disinfection of Wastewater Intended to reduce microbes in 1 o or 2 o treated effluent – Typically chlorination –Alternatives: UV radiation, ozone and chlorine dioxide Good enteric bacterial reductions: typically, 99.99+% –Meet fecal coliform limits for effluent dicharge Often 200-1,000 per 100 ml geometric mean as permitted discharge limit Less effective for viruses and parasites: typically, 90-99% reduction Toxicity of chlorine and its by ‑ products to aquatic life now limits wastewater chlorination; may have to: –Dechlorinate –Use an alternative, less toxic chemical disinfectant or –Use an alternative treatment process to reduce enteric microbes
When Wastewater Disinfection is Recommended or Required Discharge to surface waters: –near water supply intakes –used for primary contact recreation – used for shellfish harvesting –used for irrigation of crops and greenspace – other direct and indirect reuse and reclamation purposes Discharge to ground waters waters: –used as a water supply source –used for irrigation of crops and greenspace –other direct and indirect reuse and reclamation purposes
Wastewater Reuse: reading for Thursday Wastewater is sometimes reused for beneficial, non-potable purposes in arid and other water-short regions. Often uses advanced or additional treatment processes, sometimes referred to as “reclamation” Biological treatment in “polishing” ponds and constructed wetlands Physical-chemical treatment processes as used for drinking water: –Coagulation-flocculation and sedimentation –Filtration: granular medium filters; membrane filters –Granular Activated Carbon adsorption –Disinfection
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