SLUDGE u Screenings u Grit u Scum u Solids u 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

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

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

Volume Mass Relationships Specific gravity of solids

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

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

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.

Belt Filter Press (Komline-Sanderson)

Filter Press

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 H 2 O 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 mL)/1000 mL = 75% Mass of water = 250 g - 10 g =240 g H 2 O 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 mL)/1000 mL = 96.7% Mass of water = 33.3 g - 10 g = 23.3 g H 2 O

Aerobic Digestor days of aeration ~50% reduction in solids

Digester

Anaerobic Digestion Sludge held without aeration for daysSludge held without aeration for days Process can be accelerated by heating to o CProcess can be accelerated by heating to o C These are called High Rate Digestors (10-20 days)These are called High Rate Digestors (10-20 days) AdvantagesAdvantages low solids productionlow solids production useable methane gas produceduseable methane gas produced DisadvantagesDisadvantages high capital costshigh capital costs susceptibility to shocks and overloadssusceptibility to shocks and overloads

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

Process Microbiology Lipids, Proteins, Nucleic Acids, Polysaccharides Hydrolysis Fatty Acids, Amino Acids, Monosaccharides etc Methane and Carbon Dioxide Acidogenesis Methanogenesis Alcohols, hydrogen, CO 2,formate, acetate

Process Microbiology Methanogens or Methane Formers 4H 2 + CO 2 = CH 4 + 2H 2 O 4HCOOH =CH 4 + 2H 2 O + 3CO 2 CH 3 COOH = CH 4 + CO 2 CH 3 OH = 3CH 4 + CO 2 + 2H 2 O 4(CH 3 ) 3 N + H 2 O = 9CH 4 +3 CO 2 + 6H 2 O+ 4NH 3 pH , alkalinity should be present; slow growth rates Y =0.06

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

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

Ultimate Sludge Disposal