Sludge ManagementTypes of Sludge:Primary sludge

- organic solids, grit, inorganic fines, greasy, odorous slurry- includes ~ 1 % tank skimmings- solids conc ~ 4-6% - VSS ~ 60-80%

Waste activated sludge (secondary sludge)- active microbial mass- dark brown suspension- inoffensive at first, can rapidly become odorous- solids conc ~ 0.5-1.5 %- VSS ~ 70-80%

Trickling filter sludge (humus)- similar to waste activated sludge- solids conc ~ 0.5-1.5- VSS 70-80%

Sludge ManagementAnaerobically digested sludge

- dark brown thick slurry - smells like garden soil- VSS ~ 30-60 % - solids conc. ~ 3-12%

Aerobically digested sludge - dark brown- more difficult to process due to flocculent nature- solids conc ~ 1-2%- VSS ~ 35-40%

Sludge ManagementEstimation of Solids Production:Ws = Wsp + Wss

Ws = total dry solids [M/T]Wsp = raw primary solids [M/T]Wss = raw secondary solids [M/T]

Wsp = f x SS x Qf = fraction of suspended solids removed in primary settling ~ 0.4-0.6SS = suspended solids conc. in wastewater[M/L3]Q = flow rate [L3/T]

Wss = k x BOD x Qk = fraction of influent BOD that becomes excess biomass

= 0.3-0.5 (for F/M = 0.05-0.5)= lower for extended aeration AST and RBCs

Sludge Management

Aeration

ThickeningHoldingtank

Sludgedigestion dewatering

influent Primclarif

Secclarif

Return sludge

Waste sludge

disposal

effluent

Sludge ManagementSludge ThickeningGravity thickening

- most common thickening method- pickets rake sludge - primary sludge :from 4 to 8%activated sludge : from 1 to 3 %primary-secondary mixture : from 4 to 6%

Operating criterion:solids applied per unit bottom areaprimary: 100-150 kg/m2-dayprimary plus AST: 40-80AST: 20-40

Thickeners should recover 90-95 % of solidsBottom sludge blanket ~ 1 m deep

Sludge ManagementDissolved Air Flotation (DAF)

- most applicable to solids near neutral buoyancy- air is dissolved into wastewater under high pressure- thickened solids are 3-6 % solids- not recommende for primary sludge or trickling filter humus- typical loading: 240 kg/ m2-day

Sludge ManagementCentrifugal Thickening

- sludge is centrifuged to concentrate solids- with polymer addition- thickened solids ~ 5 8 %

Gravity belt thickening- sludge is treated with polymer flocculants, then conveyed on continuous porous belt- typical prformancePrimary sludge from 2-5 to 8-12Secondary sludge from 0.4-1.5 to 4-650-50 primary-sec from 1-2.5 to 6-8

Centrifuge

Belt Thickener

Belt Thickener

Sludge TreatmentSludge Stabilization

- process of treating solids to make them stable- reduces odor and pathogens

Processes:anaerobic digestionaerobic digestioncompostingalkaline stabilizationincineration

Sludge TreatmentAnaerobic digestionOverall process consists of four steps:

1. Hydrolysis2. Acidogenesis (fermentation)3. Acetogenesis4. Methanogenesis

Overall process stabilizes about 40-65 % of VSS

Chemistry of Anaerobic Digestion

Chemistry of Anaerobic Digestion

Organic matter + combined oxygen + (anaerobes) new cells + CH4 + CO2 + other end products + energy for cells

Organic matter: carbohydrates, proteins, fats, oilsCombined oxygen: CO3 2- , SO4 2- , NO3 - , PO4 3-

Other end products: H2S, H2, N2

Microbiology of Anaerobic Digestion

Micro-organisms: methanogens, acidogens, acetogens

pH: methanogens are inhibited at pH < 6.5acidogens and acetogens at pH < 5

Parameters to monitor pH and volatile acid concentrations (reported as acetic acid)

Anaerobic DigestionOperating Conditions:

Usual rangeOptimal

50-250 mg/L as acetic acid

Volatile acids

1000-5000 mg/L as CaCO3

Alk

6.7 7.47 7.2pH

29-35oC35 oCTemperature

Anaerobic DigestionGas Composition : 55 75 % methane

25 45 % CO2Solids reduction: 50 75 % VSS

35 50 % TSSGas production: 0.75-1.1 m3per kg VSS stabilizedInhibitory compounds:

high ion concentrationssome metalsammonia and sulfide gas

Digester DesignLow rate digesters

- cylindrical tank with roof- no mixing or heating- low loading rates, large tank sizes- detention time: 30 60 days (SRT)- used only for small plants

Low Rate Digester

Low Rate Digester

Low Rate Digester

Digester Design

High Rate Digesters- supplemental mixing and heating- SRT : 15- 20 days- different mixing systems- one variation is egg-shaped digester

High Rate Digester

High Rate Digester

Digester DesignMinimum solids retention time(FMT w/o recycle)1/ min = [YKS in / (Ks + S in)] KeWhere: min = minimum retention time [T]

Y = yield coeff [M VSS/ M COD]= 0.04 g VSS/ g COD

K = max specific substrate use rate [M COD/M VSS-T]

= 6.67 (1.035 T-35) g COD/ g VSS-dT = temperature (oC)Ks = half saturation constant

= 1.8 (1.112 T-35) g COD/ LSin = conc of biodegradable substrate [M COD/ L3]Ke = endogenous decay coeff [T-1]

= 0.03 (1.035 T-35) day-1

Digester DesignTypical Values min = 4 days at 35-40oC

= 11 days at 18 oCDesign is 2.5 times minLoading rates: 1.9 2.5 kg VSS/m3-d

3.2 kg VSS/m3-d is maximum to avoid accumulation of toxics, washout of methanogens

Gas production: 0.8 to 1.1 m3/kg VSS stabilizedVSS destruction : Vd = 13.7 ln + 18.9

where Vd is in % and is in days

Sludge Management

Chemistry Sludge CH4 + CO2 + H2O + stable

solids Stable solids are removed, sent to final

disposal Leftover water returned to treatment plant

Sludge Management

Methane Use Methane (CH4) can be burned as fuel Heat digester Run electric generator Flare

Sludge Management

DisinfectionObjective: Kill pathogens that survive

stabilization process Physical processes

Heat Chemical processes

Chlorination Lime

Sludge Management

DewateringObjective: Remove water from stabilized

sludge Processes:

Vacuum filter Centrifuge Drying beds

Drying Bed

Pressure Filter

Sludge ManagementFinal Disposal

Objective: Dispose of sludge in environmentally sound way

Processes Landfill Land application Composting

Sludge ManagementLandfill Place sludge in a typical solid waste landfill No beneficial use with this method

Land Application Sludge is injected or tilled into soil Done on either non-agricultural or agricultural land

Composting Sludge solids are composted with other solids Compost is sold or given away