Domestic Waste Water Treatment

1. Domestic Waste Water Treatment

2. 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)

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. 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

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. Aeration tank or Trickling filter Activated Sludge Process • 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

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

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. Energy to run plant CH4 Anaerobic Sludge Digester • 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 Disinfection tank • 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