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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
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
REGULATIONS
40 CFR 503– Land application of sludge (Class A and Class
B)– Surface disposal– Patogen and vector reduction– incineration
Volume Mass Relationships
wvfw
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Wv
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water
water
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1
Specific gravity of solids
Specific gravity of solids
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
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
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 issqueezed between opposing porous cloth belts. Next it will travel through a high pressure section, where the sludge issubjected to a shear force as the belts pass through a series of rollers. This shearing force and squeezing process reducesadditional 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 ReductionA. Start with 1 liter of 1% by weight (i.e., 10 g/L) sludge.
Mass of sludge = (1 liter)(1000 g/L) = 1000 g sludgeMass of sludge = (1 liter)(1000 g/L) = 1000 g sludgeMass of solids = (1 liter)(10 g/L) = 10 g dry sludge solidsMass of solids = (1 liter)(10 g/L) = 10 g dry sludge solidsMass of water = 1000 g - 10 g = 990 g HMass of water = 1000 g - 10 g = 990 g H22OO
B. Gravity Thicken to 4% dry solids (i.e., 40 g/L).Mass of sludge = (10 g)/(0.04) = 250 g sludgeMass of sludge = (10 g)/(0.04) = 250 g sludgeMass of solids = unchanged = 10 g dry sludge solidsMass of solids = unchanged = 10 g dry sludge solidsVolume Removed = (1000 mL - 250 mL)/1000 mL = 75%Volume Removed = (1000 mL - 250 mL)/1000 mL = 75%Mass of water = 250 g - 10 g =240 g HMass of water = 250 g - 10 g =240 g H22OO
C. Vacuum Filter to 30% dry solids (i.e., 300 g/L).Mass of sludge = (10 g)/(0.30) = 33.3 g sludgeMass of sludge = (10 g)/(0.30) = 33.3 g sludgeMass of solids = unchanged = 10 g dry sludge solidsMass of solids = unchanged = 10 g dry sludge solidsVolume Removed = (1000 mL - 33.3 mL)/1000 mL = 96.7%Volume Removed = (1000 mL - 33.3 mL)/1000 mL = 96.7%Mass of water = 33.3 g - 10 g = 23.3 g HMass of water = 33.3 g - 10 g = 23.3 g H22OO
Aerobic Digestor
12-20 days of aeration~50% reduction in solids
Digester
Anaerobic Digestion
• Sludge held without aeration for 10-90 daysSludge held without aeration for 10-90 days• Process can be accelerated by heating to 35-40Process can be accelerated by heating to 35-40ooCC• 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
HydrolysisFatty Acids, Amino Acids, Monosaccharides etc
Methane and Carbon Dioxide
Acidogenesis
Methanogenesis
Alcohols, hydrogen, CO2,formate, acetate
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
Digester Design
Mean Cell Residence Time Volumetric Loading Factor Observed Volume Reduction Loading Factors Based on Populations
Toxics in Municipal Sludge
Problem with buildup of heavy metals in soil receiving sludge
Ultimate Sludge Disposal