Grade 2 - Solids Handling and DisposalREVISED

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