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12 Sludge Treatment

Technische Universität Dresden. Peter Krebs. Department of Hydro Science, Institute for Urban Water Management. Urban Water Systems. 12 Sludge Treatment. 12.1 Overview 12.2 Thickening 12.3 Biological sludge stabilisation 12.4 Volume reduction 12.5 Sludge disposal.

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12 Sludge Treatment

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  1. Technische Universität Dresden Peter Krebs Department of Hydro Science, Institute for Urban Water Management Urban Water Systems 12 Sludge Treatment 12.1 Overview 12.2 Thickening 12.3 Biological sludge stabilisation 12.4 Volume reduction 12.5 Sludge disposal 12 Sludge treatment

  2. 12 Sludge treatment 12.1 Overview 12 Sludge treatment

  3. Composition of sludge • Predominantly water • 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  All non-degraded compounds extracted from wastewater are found in the sludge 12 Sludge treatment

  4. Goals of sludge treatment Volume reduction • Thickening • Dewatering • If used in agriculture as fertiliser or compost Elimination of pathogenic germs • Gas production • Reduction of dry content • Improvement of dewatering • Reduction of odour Stabilisation of organic substances Recycling of substances • Nutrients, fertiliser • Humus • Biogas 12 Sludge treatment

  5. Overview Wastewater treatment Primary, secondary, tertiary sludge Thickening Energy Process water Hygienisation Stabilisation Biogas Thickening Agriculture Dewatering Disposal site Drying Construction industry Incineration Atmosphere Gujer (1999) 12 Sludge treatment

  6. Sludge Treatment Alternatives Eckenfelder & Santhanam (1981) 12 Sludge treatment

  7. 12 Sludge treatment 12.2 Thickening 12 Sludge treatment

  8. Thickening by Gravity Gravitative separation, similar to settling tank Additional mechanic stirring to enhance flocculation and extraction of water and gas Supernatant is introduced to primary clarifier or – if floatables and grease contents are high – to grid chamber Thickened sludge is withdrawn from hopper and introduced to sludge treatment For an efficient thickening process the development of gas bubbles must be prevented 12 Sludge treatment

  9. Gravity Thickener Inflow Scum scimmer Sludge liquor Picket fence Thickened sludge 12 Sludge treatment

  10. Dimensioning of gravity thickeners surface Solids overflow rate qTSS,Th Specific solids overflow rate (kg TSS / (m2 d)) QWAS Inflow to thickener (m3/d) XTh,in Solids concentration in thickeners inlet (kg TSS / m3) ATh Surface of thickener (m3) Typical values for solids overflow rate qTSS,Thand concentration of thickened sludge XTh qTSS,Th XTh Primary sludge 80 – 120 80 - 150 Primary and secondary sludge 50 - 70 50 - 100 Secondary sludge 25 - 30 20 - 35 12 Sludge treatment

  11. Thickening by Flotation Pre treatment: mostly chemical flocculation Slude is placed in contact with air-saturated water (full flow or recycle pressurization) Air bubbles attach to solid particles  lower specific gravity than water Floating Sludge bubble composite is collected at the surface Water is recovered under a scum baffle and removed 12 Sludge treatment

  12. Thickening by Flotation 12 Sludge treatment

  13. Flotation unit 12 Sludge treatment

  14. 12 Sludge treatment 12.3 Biological sludge stabilisation 12 Sludge treatment

  15. Anaerobic mesophilic sludge stabilisation Digester Heated to 33 – 37°C  process rates are higher Content of digester is mixed  Sludge and water obtain a similar residence time Storage unit Not heated  little biological activity Not mixed  separation of sludge and process water, which is directed to WWTP  Control of loading to WWTP, app. 10% of N-loading Further thickening 12 Sludge treatment

  16. Processes in digester Anaerobic degradation Degradation of organic substances of app. 50% Biogas production: 63% CH4 (Methane) 35% CO2 2% other gases (N2, H2, H2S)  electricity and heating Organic nitrogen is converged to NH4+  N-loading of WWTP 12 Sludge treatment

  17. Characteristic values of digester Mean residence time of sludge Small units, badly mixed < 30 d Medium size units with mixing 20 d Large plants with mixing 12 – 16 d Biogas production related to degradation of organic substances 0.9 m3 / kg VSSdegr. Degradation of organic substances 40 – 55% 12 Sludge treatment

  18. Simultaneous aerobic sludge stabilisation • No primary clarifier  no primary sludge • High sludge age SRT, app. 25 d • Activated sludge tank is larger than that combined with an anaerobic sludge stabilisation • No biogas production • Possibly combined with storage or thickener unit • Stable and simple operation 12 Sludge treatment

  19. 12 Sludge treatment 12.4 Volume reduction 12 Sludge treatment

  20. Volume reduction Water content in stabilised sludge > 95% !  Reduction of water content and volume Sludge volume With water content   non-linear relation! 12 Sludge treatment

  21. Volume reduction 12 Sludge treatment

  22. Dewatering Conditioning with flocculation agents (poly-electrolytes) for efficient dewatering Unit Operation Method W DS Decanter Continuous Centrifuge > 0.7 < 0.3 Chamber filter press (large plants) Batch-wise Hydraulic pressure through plates in water-tight chambers > 0.6 ≤ 0.4 Belt filter press (small plants) continuous Pressed between two filter belts around staggered rollers > 0.7 ≤ 0.3 12 Sludge treatment

  23. Drying bed • Thin sludge layer (< 20 cm) • Sand layer as drainage and filter layer • Sludge is first dewatered by drainage then air-dried through evaporation • Applicable for small plants Dimensioning  W  0.55 (Imhoff, 1990) Plant type Specific surface Only mechanical treatment 13 PE/m2 Trickling filter 6 PE/m2 Activated sludge plant 4 PE/m2 12 Sludge treatment

  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 12 Sludge treatment

  25. 12 Sludge treatment 12.5 Sludge disposal 12 Sludge treatment

  26. Use in agriculture  Recycling of nutrients, from stabilised sludge Sludge treatment Fertiliser* Liquid sludge P- and N-fertiliser Dewatered sludge P-fertiliser, N as storage product Dried sludge P-fertiliser * Limit re. over-fertilisation Problems • Acceptance • Heavy metals • Micro-pollutants, pharmaceuticals, endocrine disruptors 12 Sludge treatment

  27. Composting  Aerobic biological degradation of organic substances Prerequisites Stabilisation 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! 12 Sludge treatment

  28. Incineration Use of energy content, but not of nutrients Mono incineration (sludge exclusively) • 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! Co- incineration • In coal power station • In solid waste incinerators • In cement production, ash is bounded to cement 12 Sludge treatment

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