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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

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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