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Urban Water Systems12 Sludge treatment© PK, page 1 12Sludge Treatment 12.1 Overview 12.2 Thickening 12.3 Biological sludge stabilisation 12.4 Volume reduction 12.5 Sludge disposal Technische Universität Dresden Department of Hydro Science, Institute for Urban Water Management Peter Krebs Urban Water Systems
12 Sludge treatment© PK, page Overview 12 Sludge treatment
Urban Water Systems12 Sludge treatment© PK, page 3 Composition of sludge All non-degraded compounds extracted from wastewater are found in the sludge Micro-organisms Viruses, pathogens, germs in general Organic particles, heavily bio-degradable Organic compounds, inert, adsorpted to sludge flocs Heavy metals Micro-pollutants, pharmaceuticals, endocrine disrupters Predominantly water
Urban Water Systems12 Sludge treatment© PK, page 4 Goals of sludge treatment Volume reduction Elimination of pathogenic germs Stabilisation of organic substances Recycling of substances Thickening Dewatering If used in agriculture as fertiliser or compost Gas production Reduction of dry content Improvement of dewatering Reduction of odour Nutrients, fertiliser Humus Biogas
Urban Water Systems12 Sludge treatment© PK, page 5 Overview Thickening Hygienisation Stabilisation Dewatering Drying Incineration Process water Biogas Energy Agriculture Disposal site Atmosphere Wastewater treatment Primary, secondary, tertiary sludge Construction industry Gujer (1999)
Urban Water Systems12 Sludge treatment© PK, page 6 Sludge Treatment Alternatives Eckenfelder & Santhanam (1981)
Urban Water Systems12 Sludge treatment© PK, page Thickening 12 Sludge treatment
Urban Water Systems12 Sludge treatment© PK, page 8 Thickening by Gravity Gravitative separation, similar to settling tank Supernatant is introduced to primary clarifier or – if floatables and grease contents are high – to grid chamber Additional mechanic stirring to enhance flocculation and extraction of water and gas Thickened sludge is withdrawn from hopper and introduced to sludge treatment For an efficient thickening process the development of gas bubbles must be prevented
Urban Water Systems12 Sludge treatment© PK, page 9 Gravity Thickener Thickened sludge Picket fence Scum scimmer Inflow Sludge liquor
Urban Water Systems12 Sludge treatment© PK, page 10 Dimensioning of gravity thickeners surface Solids overflow rate Typical values for solids overflow rate q TSS,Th and concentration of thickened sludge X Th q TSS,Th X Th Primary sludge Primary and secondary sludge 80 – q TSS,Th Specific solids overflow rate (kg TSS / (m 2 d)) Q WAS Inflow to thickener (m 3 /d) X Th,in Solids concentration in thickeners inlet (kg TSS / m 3 ) A Th Surface of thickener (m 3 ) Secondary sludge
Urban Water Systems12 Sludge treatment© PK, page 11 Thickening by Flotation Pre treatment: mostly chemical flocculation Air bubbles attach to solid particles lower specific gravity than water Slude is placed in contact with air-saturated water (full flow or recycle pressurization) Floating Sludge bubble composite is collected at the surface Water is recovered under a scum baffle and removed
Urban Water Systems12 Sludge treatment© PK, page 12 Thickening by Flotation
Urban Water Systems12 Sludge treatment© PK, page 13 Flotation unit
Urban Water Systems12 Sludge treatment© PK, page Biological sludge stabilisation 12 Sludge treatment
Urban Water Systems12 Sludge treatment© PK, page 15 Anaerobic mesophilic sludge stabilisation Content of digester is mixed Sludge and water obtain a similar residence time Storage unit Not heated little biological activity Heated to 33 – 37°C process rates are higher Digester Not mixed separation of sludge and process water, which is directed to WWTP Further thickening Control of loading to WWTP, app. 10% of N-loading
Urban Water Systems12 Sludge treatment© PK, page 16 Processes in digester Biogas production: 63% CH 4 (Methane) 35% CO 2 2% other gases (N 2, H 2, H 2 S) electricity and heating Anaerobic degradation Organic nitrogen is converged to NH 4 + N-loading of WWTP Degradation of organic substances of app. 50%
Urban Water Systems12 Sludge treatment© PK, page 17 Characteristic values of digester Mean residence time of sludge Small units, badly mixed Medium size units with mixing Large plants with mixing < 30 d 20 d 12 – 16 d Biogas production related to degradation of organic substances 0.9 m 3 / kg VSS degr. Degradation of organic substances40 – 55%
Urban Water Systems12 Sludge treatment© PK, page 18 Activated sludge tank is larger than that combined with an anaerobic sludge stabilisation No biogas production High sludge age SRT, app. 25 d Simultaneous aerobic sludge stabilisation No primary clarifier no primary sludge Possibly combined with storage or thickener unit Stable and simple operation
Urban Water Systems12 Sludge treatment© PK, page Volume reduction 12 Sludge treatment
Urban Water Systems12 Sludge treatment© PK, page 20 Volume reduction Water content in stabilised sludge > 95% ! Reduction of water content and volume Sludge volume With water content non-linear relation!
Urban Water Systems12 Sludge treatment© PK, page 21 Volume reduction
Urban Water Systems12 Sludge treatment© PK, page 22 Dewatering Conditioning with flocculation agents (poly-electrolytes) for efficient dewatering DecanterContinuous Chamber filter press (large plants) Batch-wise Belt filter press (small plants) continuous Centrifuge Hydraulic pressure through plates in water-tight chambers UnitOperationMethod Pressed between two filter belts around staggered rollers W DS > 0.7< 0.3 > >
Urban Water Systems12 Sludge treatment© PK, page 23 Drying bed Thin sludge layer (< 20 cm) Sand layer as drainage and filter layer Sludge isfirst dewatered by drainage then air-dried through evaporation Applicable for small plants Plant typeSpecific surface Only mechanical treatment13 PE/m 2 Trickling filter6 PE/m 2 Activated sludge plant4 PE/m 2 Dimensioning W 0.55 (Imhoff, 1990)
Urban Water Systems12 Sludge treatment© PK, page 24 Drying Vaporisation of water content Partial drying W 0.3 – 0.4 Full drying W down to < 0.1 Contact drying over heated areas Drying by convection through hot air counter-current inlet app. 600°C, outlet app. 300°C (Imhoff, 1999) For large plants Disposal is critical: fire, dust explosion In granulate form as fertiliser
Urban Water Systems12 Sludge treatment© PK, page Sludge disposal 12 Sludge treatment
Urban Water Systems12 Sludge treatment© PK, page 26 Use in agriculture Recycling of nutrients, from stabilised sludge Problems Acceptance Heavy metals Micro-pollutants, pharmaceuticals, endocrine disruptors Liquid sludge Dewatered sludge Dried sludge P- and N-fertiliser P-fertiliser, N as storage product P-fertiliser Sludge treatmentFertiliser * * Limit re. over-fertilisation
Urban Water Systems12 Sludge treatment© PK, page 27 Composting Aerobic biological degradation of organic substances PrerequisitesStabilisation Dewatering Hygienisation Approach Structure means: straw, wood, saw dust, wood chips Mixture app. 1:1 Water content app. 0,65 Requirements are more demanding than for sludge use as fertiliser!
Urban Water Systems12 Sludge treatment© PK, page 28 Incineration Use of energy content, but not of nutrients Mono incineration (sludge exclusively) Co- incineration In solid waste incinerators In cement production, ash is bounded to cement Calorific value of sludge high enough no biogas use before, no stabilisation Water content not minimised (no full drying) Fluidised bed incinerator, incineration at 800 – 950°C in fluidised sand bed Expensive! In coal power station
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