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Sludge Disposal Methods
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Digestion of Sludge Aerobic
Anaerobic
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Aerobic Digestion Biological sludge treatment process
Employs aeration to stabilize waste sludge
Reduces total volume of sludge by aerobicbacteria that breakdown volatile solids
Creates condition that forces the
microorganisms to use their own cellularmass as a food source
Process requires a source of aeration
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Aerobic Digestion Operates either on a batch or continuous
basis
Supernatant may be drawn off to allowadditional sludge to be added
Solids concentration entering the digester
may run 0.5 to 1.0 percent
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Aerobic Digestion Control Factors Dissolved Oxygen
0.5 to 2.0 mg/L
Temperature > 50 degrees F
Solids Retention Time > SRT the > volatile solids reduction )
pH carbon dioxide produced from digestion reduces the pH
best if kept at 7.0
Organic Loading (> OL = >DO levels needed )
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Anaerobic Digestion Absence of Oxygen
Most economical option if the process produce
energy by using methane Stabilized sludge is relatively inert
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Anaerobic Digestion 3 Stages 1st - Extracellular enzymes ( outside cell)
breaks down solid complex organics
2nd - Acid formers ( acid production and acidregression stages ) microorganisms convert
products of first stage to organic acids &
carbon dioxide & hydrogen
3rd - Methane forming bacteria converts
hydrogen and carbon dioxide to methane
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Anaerobic Digestion - Methane Formers Controls the process
Sensitive to environmental factors ( pH ,
temperature , toxins ) Reproduce very slowly
Easy to kill and difficult to grow
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Anaerobic Digestion Uniformity and consistency in operations
Sudden changes can inhibit performance
Raw sludge is blended with recirculateddigester sludge
Two criteria that determines capacity , is
hydraulic detention time & volatile solidsloading rate
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Anaerobic Digestion Hydraulic detention time
15 to 20 days
Organic loading
0.1 to 0.4 lbs. VS/cu.ft./day
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Anaerobic Digestion Types Fixed Covers - number of different
constructions
Floating Covers - Floats directly on the liquidsurface
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Anaerobic Digestion Heating
Systems Maintaining a constant temperature for
methane-formers is one of the most important
control functions Heat is required to maintain digester
temperature and to heat incoming sludge
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Anaerobic Digestion - Temperature
Ranges Psychrophilic
50 to 68 degrees F
50 to 180 days
Generally not very effective Mesophilic
85 to 100 degrees F
25 to 30 days
Most popular Thermophilic
120 to 135 degrees F
5 to 12 days
Mostly used for class A sludge
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Anaerobic Digester - Foaming Fine bubbles of gas trapped in a semi-liquid
matrix
Commonly forms during start-up and whenthe digester is under stress
Start-up ( carbon dioxide to methane ratio is not
equal )
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Sludge Processing Concentration and stabilization of sludge prior to final
disposal
Methods thickening stabilization
conditioning
dewatering
Purpose is to reduce potential problems from pathogens
quality of undesirable organic material
potential for odor problems
reduce vector attraction
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Sludge Processing Thickening
increases solids content by removing excess
water promotes efficient sludge digestion
Methods
gravity
solids content of 1- 3 %
dissolved air floation
solids content 2 - 8 %
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Sludge Processing Stabilization - reduces organic matter in sludge to a
more stable form
Methods
thermal
applying heat to drive away water
lime stabilization
applying lime until pH of 12 or higher for 2 hrs
aerobic & anaerobic digestion Composting
microorganisms break down organic matter to a humus-
like material
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Sludge Processing Composting
windrow
mechanical
Suitable environment must be established &
maintained for bacteria to survive
Blending
aeration temperature
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Sludge ProcessingFactors affecting composting
sludge type
initial moisture content uniformity of mixture
frequency of aeration or turning
climatic conditions
desired moisture content of final product
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Sludge Processing Dewatering - removes liquids
Drying beds
solids concentration up to 40% used for well digested, stabilized residuals
Centrifuges
solids concentration 20 to 40 %
rotates liquids at high speeds & spins the water away
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Sludge Processing Vacuum Filters
solids concentration 15 to 30 %
rotating filter media passes through vat of residuals,
residuals coat the filter and water is extracted by vacuum,
polymers may be used
Pressure filters
solids concentration up to 50%
series of verticle plates,belts,or frames, water squeezed
out under pressure, solids are retained between the belts
or plates
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Sludge Reuse or Disposal OptionsIncineration
burning sludge at high temperatures in
furnace volume is reduced
ash is stabilized
requires air permit & pollutant removal high operating costs
complex
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Sludge Reuse or Disposal OptionsLandfill
disposal of sludge in a lined landfill
leachate collection required odor potential
local regulatory requirements
inexpensive
trucking costs
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Sludge Reuse or Disposal OptionsSurface disposal
trench, waste pile, lagoon
biosolids remain longer than 2 years monofills
surface impoundment
dedicated disposal sites
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Requirements for Sludge Surface
Disposal Systems without liners
have pollutant limits for arsenic,chromium, andnickel
must have leachate collection system and monitorleachate
systems with covers have methane limits
cannot be in the 100yr. floodplain, wetland, orarea with geologic faults
requires an NPDES stormwater permit
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Requirements for Sludge Surface
Disposal Special restrictions on crops and grazing
public access restricted for 3 years after
closure groundwater monitoring
pathogen and vector reduction requirements
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Sludge Reuse or Disposal OptionsLand Application
application of sludge to condition the soil or fertilize
crops / other vegetation
sprayed or spread on soil surface
tilled into soil or injected directly below the surface
must be applied at agronomic rates, meet pollutant
limits, and pathogen & vector attraction reductionrequirements
requires 30 days or more of storage
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Sludge Reuse or Disposal Options sites must be evaluated for soils, depth to
SHWT,depth to bedrock,buffer requirements
application rate limited by type of crop to be grown
application events must be timed to coordinate withfarmers schedules
records must be maintained & submitted to DWQyearly
requires certified operator requires large amounts of land
odor potential
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Sludge Reuse or Disposal Options Distributing & Marketing : composted sludge
given away or sold in bulk
must achieve class A sludge end use is not regulated
bag or bulk compost
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Sludge Reuse or Disposal OptionsPathogen & Vector Attraction Reduction
two levels of pathogen reduction are
recognized by both EPA and DWQ class B or PSRP
class A or PFRP
vector attraction reduction
reducing the attractiveness of sludge to
flies,mosquitoes,fleas,rodents, and birds