sludge management
DESCRIPTION
Sludge Management LectureTRANSCRIPT
-
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