Domestic Waste Water Treatment

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)

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

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

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

Primary Clarifiers Separate liquids from solids Skimmer removes grease at the surface and sends it to anaerobic digester

Secondary treatment: Aerobic Activated sludge process Trickling filters Rotating discs Rotating drums Oxidation Ponds

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

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

Trickling Filters Not a true filtering or sieving process Material only provides surface on which bacteria to grow Can use plastic media lighter - can get deeper beds (up to 12 m) reduced space requirement larger surface area for growth greater void ratios (better air flow) less prone to plugging by accumulating slime

Trickling Filter Porous media Microbial biofilm Organic matter CO2 + H2O

Rotating Biological Contactors Called RBCs Consists of series of closely spaced discs mounted on a horizontal shaft and rotated while ~40% of each disc is submerged in wastewater Discs: light-weight plastic Slime is 1-3 mm in thickness on disc

Rotating Biological Contactors

Aerobic ponds Shallow ponds (<1 m deep) Light penetrates to bottom Active algal photosynthesis Organic matter con- verted to CO2, NO3-, HSO4-, HPO42-, etc.

Secondary treatment: Anaerobic Anaerobic digestion Anaerobic Up flow filters (AUF) Anaerobic filters

Anaerobic Sludge Digester CH4 Energy to run plant 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

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 Disinfection tank Discharge to environment filters out protozoans & pathogenic bacteria Sludge digestor