(2) Sludge Treatment (Anaerobic Stabilization to the end)

(Anaerobic Stabilization to the end pg. 13-20)

anaerobic digester

Most important factors effecting the performance of ___

• SRT

• HRT

• Temp

• pH

• toxic materials

Standard (Low) Rate Process

what type of anaerobic digestion process

• does not employ sludge mixing/ heating

• SRT - 30-60 days

high rate process

what type of anaerobic digestion process

• w/ mixing and heating

• SRT - 15-20 days

• 2 digesters operating in series separate the functions of fermentation and solid-liq separation and residual gas extraction

Primary Digester

Kind of Anaerobic Digester

• to increase the reaction rate, reactor is;

  • covered

  • heated

  • mixed

• 35C sludge

Secondary Digester

Kind of Anaerobic Digester

• not mixed/ heated

• used

  • storage of gas

  • concentrating of sludge by settling

• gas is high enough in CH4 to be used as a fuel and is usually used to heat the primary digester

Anaerobic Digester Gas Production

• 810 to 1120L of digester gas/ kg volatile solids

• 65-69% methane

• 31-35% CO2

  • if more than 35% CO2 there is something w the digestion

Egg-shaped digester

Digester

• first installed in Germany 1950s

American Digester

shallow cylindrical vessel w moderate floor and roof slopes

Conventional German Digester

deep cylindrical tank w steeply sloped top and bottom cones

aerobic digestion

• used to sltabilize;

  • primary sludge

  • secondary sludge

  • combination

• process converts organic sludge to CO2, NH3, H2O

bacteria

lack of organic matter leads to the death of __

CO2

organic matter is consumed by bacteria and converts it into __

Conventional

Type of Aerobic Digestion

• concentration VSS influent - 3% max for RT of 15-20 days

• by batch (mostly), semi batch, continuous basis

• Activated sludge - SRT 15-20 days

• Primary & Activated Sludge - SRT 20-25 days

Conventional batch basis

The way of Conventional Aerobic Digester introduces the sludge

• digester is filler w raw sludge and aerated for 2-3 weeks then stopped

Conventional semi batch basis

The way of Conventional Aerobic Digester introduces the sludge

• raw sludge is added every couple of days

• supernatant is decanted periodically

• settled solids are held long time before removing

Conventional Aeration period

• 200 to 300C-days

• multiplying digester’s temp in C by the sludge age

AutoThermal Thermophilic Aerobic Digestion (ATAD)

Type of Aerobic Digestion

• variation of both conventional and high purity oxygen aerobic digestion

• reqs

  • insulated reactors 120-140F

  • aerators

  • foam controllers

  • sludge

• 70% removal rate of the biodegradable organics at 3-4 days

Biofilter

ATAD Challenge - odor control

High Purity Oxygen Aerobic Digestion

Type of Aerobic Digestion

• used in lieu of air

• applicable in cold weather

• costing disadvantage

cryophilic aerobic digestion

Type of Aerobic Digestion

• ow temp - less than 20C

• sludge age increase = operating temp increase

Composting

• biological degradable to a stable end product

• microorganisms involved are

  • bacteria - meso and thermophilic

  • actinomycetes

  • fungi

• 20-30% of volatile solids converted to CO2

• Water at temp in the pasteurization of 50-70C

• optimum moisture content is 50-60%

40C at least 5 days

Composting PSRP Req

• min temp & days

55C at least 3 days

Composting PFRP Req

• min temp & days

Agitated Composting

Principal Method of Composting examples are;

• window composting

• in vessel composting

Static Composting

Principal Method of Composting examples are;

• static aerated pile composting

Sludge conditioning

• to improve its dewatering characteristics

• 2 methods

  • adding chemicals

  • heat treatment

Sludge Dewatering

physical unit operation used to reduce moisture content of sludge and biosolids by subjecting it to vacuum, pressure, drying

Cake

the dewatered solids coming out from a dewatering device

Belt Filter Press

Device used in sludge dewatering

• continuously feed sludge dewatering device

• involve

  • chemical conditioning

  • gravity drainage

  • mechanically applied pressure

Pressure Filter Press

Device used in sludge dewatering

• req lower polymer input due to higher pressure and finer cloths

• sludge is pumped into the press at pressured up to 225 psi for less than 2hrs

vacuum filters

Device used in sludge dewatering

• used for almost 7 decades

• drum is partly submerged 20-40% in a vat containing sludge

• small plants

  • 35h operation / week

• larger plants

  • 16-20hrs operation/ day

• optimum solids content for filtration is about 6-8%

• chemical conditioners

  • ferric chloride

  • lime

  • polymers

Centrifuge

Device used in sludge dewatering

• used in solid bowl decanter

• w/o chemical conditioner - 50-80% solids capture

• w/ chemical conditioner - 80-95% solids capture

• cake - 15-30% dry solids

Sludge Drying Bed

Device used in sludge dewatering

• remove moisture by natural evaporation, gravity, most widely used in municipal in USA

Lagoon

Device/Location used in sludge dewatering

• substitute for drying beds

• not suitable for ;

  • untreated sludge

  • limed sludge

  • high strength supernatant

• sludge drying - 18 months

• clean and rested 6 months

Sand Drying Bed

Type of Sludge Drying Bed

• most cost effective

• seepage and evaporation mechanism

• sand content - 10-23cm

• gravel layer - 20-50 cm

• 15cm deep dewatered sludge

Paved Drying Bed

Type of Sludge Drying Bed

• consist of concrete and asphalt

• slope 1.5%

artificial media drying bed

Type of Sludge Drying Bed

• use of stainless-steel wedge wire

vacuum- assisted drying beds

Type of Sludge Drying Bed

• consists of reinforced concrete ground slab, layer of supporting aggregate, rigid porous media plate on top