Transcript CEE 370: Lecture #8

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
Specific gravity of solids
Wf
Ws
Wv


S s  w  w S f Sv  w
Specific gravity of solids
1 Ws Wwater


S s S s S water
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 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 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
Aerobic Digestor
12-20 days of aeration
~50% reduction in solids
Digester
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
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
Acidogenesis
Alcohols, hydrogen, CO2,formate, acetate
Methanogenesis
Methane and Carbon Dioxide
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
Constituent
Problem with buildup of
heavy metals in soil
receiving sludge
Chromium
Range,
mg/dry
kg
1099,000
Typical,
mg/dry
kg
500
Copper
8417,000
800
Nickel
2-5300
80
Zinc
10149,000
1700
Cadmium
1-3410
10
PCBs
1.5-9.3
3.8
Lindane
Chlordane
Hexachlorobenzene
0.8
0.6-19
4.8
0.6
Ultimate Sludge Disposal