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Domestic Waste Water Treatment

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Presentation on theme: "Domestic Waste Water Treatment"— Presentation transcript:

1 Domestic Waste Water Treatment

2 Composition of domestic sewage
Human feces and urine g of feces and 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)

3 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

4 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

5 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

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

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

8 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

9 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

10 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

11 Trickling Filter Porous media Microbial biofilm Organic matter
CO2 + H2O

12 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

13 Rotating Biological Contactors

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

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

16 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

17 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

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