Sludge Disposal Methods

Digestion of Sludge Aerobic Anaerobic

Aerobic Digestion 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 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 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 )

Anaerobic Digestion Absence of Oxygen Most economical option if the process produce

energy by using methane Stabilized sludge is relatively inert

Anaerobic Digestion 3 Stages 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 Controls the process Sensitive to environmental factors ( pH ,

temperature , toxins ) Reproduce very slowly Easy to kill and difficult to grow

Anaerobic Digestion 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 Hydraulic detention time

15 to 20 days

Organic loading 0.1 to 0.4 lbs. VS/cu.ft./day

Anaerobic Digestion Types Fixed Covers - number of different

constructions Floating Covers - Floats directly on the liquid

surface

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

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

Anaerobic Digester - Foaming Fine bubbles of gas trapped in a semi-liquid

matrix Commonly forms during start-up and when

the digester is under stress Start-up ( carbon dioxide to methane ratio is not

equal )

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

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 %

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

Sludge Processing Composting

windrow mechanical

Suitable environment must be established & maintained for bacteria to survive

Blending aeration temperature

Sludge Processing

Factors affecting composting sludge type initial moisture content uniformity of mixture frequency of aeration or turning climatic conditions desired moisture content of final product

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

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

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 dedicated disposal sites

Requirements for Sludge Surface Disposal Systems without liners

have pollutant limits for arsenic,chromium, and nickel

must have leachate collection system and monitor leachate

systems with covers 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 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 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 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 class B or PSRP class A or PFRP

vector attraction reduction reducing the attractiveness of sludge to

flies,mosquitoes,fleas,rodents, and birds