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SLUDGE

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  1. SLUDGE • Screenings • Grit • Scum • Solids • Biosolids • Substances responsible for offensive character of wastewater • Highly organic in nature • Pathogenic • High water content Reduce water content, organic content and render solids suitable for reuse or final disposal

  2. Sludge Management and Disposal • Thickening, Conditioning • gravity, flotation • Dewatering, Drying • Vacuum filtration, centrifugation, pressure filtr. • Digestion, Composting, Stabilization • aerobic, anaerobic, alkaline treatment • Disposal • land application, burial, incineration

  3. REGULATIONS • 40 CFR 503 • Land application of sludge (Class A and Class B) • Surface disposal • Patogen and vector reduction • incineration

  4. Volume Mass Relationships Specific gravity of solids Specific gravity of solids

  5. THICKENING • Increase the solids content of sludge by removing a portion of the liquid fraction • Activated sludge 0.8% to 4% results in fivefold decrease in sludge volume • Settling, flotation, centrifugation, gravity belt, rotary drum

  6. Sludge Dewatering • Sludge drying beds • historically the most common • sand bed, 15-30 days, evaporation & seepage • Vacuum Filtration • cylindrical rotating drum covered with fabric • submerged with applied vacuum • Continuous belt filter presses (follows) • Plate pressure filters • vertical plates mounted on a frame

  7. Belt Filter Press: Description In the belt press process, chemical conditioned sludge is resting on a gravity drainage section so that it can be thicken. Water is able to fall from the sludge by the force of gravity. Now pressure is being applied in a low pressure section, where the sludge is squeezed between opposing porous cloth belts. Next it will travel through a high pressure section, where the sludge is subjected to a shear force as the belts pass through a series of rollers. This shearing force and squeezing process reduces additional quantities of water from the sludge. Finally dewatered sludge cake is removed from the belts by scraper blades.

  8. Belt Filter Press (Komline-Sanderson)

  9. Filter Press

  10. Sludge Volume Reduction • A. Start with 1 liter of 1% by weight (i.e., 10 g/L) sludge. • Mass of sludge = (1 liter)(1000 g/L) = 1000 g sludge • Mass of solids = (1 liter)(10 g/L) = 10 g dry sludge solids • Mass of water = 1000 g - 10 g = 990 g H2O • B. Gravity Thicken to 4% dry solids (i.e., 40 g/L). • Mass of sludge = (10 g)/(0.04) = 250 g sludge • Mass of solids = unchanged = 10 g dry sludge solids • Volume Removed = (1000 mL - 250 mL)/1000 mL = 75% • Mass of water = 250 g - 10 g =240 g H2O • C. Vacuum Filter to 30% dry solids (i.e., 300 g/L). • Mass of sludge = (10 g)/(0.30) = 33.3 g sludge • Mass of solids = unchanged = 10 g dry sludge solids • Volume Removed = (1000 mL - 33.3 mL)/1000 mL = 96.7% • Mass of water = 33.3 g - 10 g = 23.3 g H2O

  11. Aerobic Digestor 12-20 days of aeration ~50% reduction in solids

  12. Digester

  13. Anaerobic Digestion • Sludge held without aeration for 10-90 days • Process can be accelerated by heating to 35-40oC • These are called High Rate Digestors (10-20 days) • Advantages • low solids production • useable methane gas produced • Disadvantages • high capital costs • susceptibility to shocks and overloads

  14. Conventional standard rate single-stage High rate Completely mixed single-stage Two-stage Process

  15. Process Microbiology Lipids, Proteins, Nucleic Acids, Polysaccharides Hydrolysis Acidogenesis Fatty Acids, Amino Acids, Monosaccharides etc Alcohols, hydrogen, CO2,formate, acetate Methanogenesis Methane and Carbon Dioxide

  16. Process Microbiology Methanogens or Methane Formers 4H2 + CO2 = CH4 + 2H2O 4HCOOH =CH4 + 2H2O + 3CO2 CH3COOH = CH4 + CO2 CH3OH = 3CH4 + CO2 +2H2O 4(CH3)3N + H2O = 9CH4 +3 CO2 + 6H2O+ 4NH3 pH 6.6-7.6, alkalinity should be present; slow growth rates Y =0.06

  17. Digester Design • Mean Cell Residence Time • Volumetric Loading Factor • Observed Volume Reduction • Loading Factors Based on Populations

  18. Toxics in Municipal Sludge Problem with buildup of heavy metals in soil receiving sludge

  19. Ultimate Sludge Disposal