12 Sludge Treatment 12.1 Overview 12.2 Thickening 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 Urban Water Systems 12 Sludge treatment

12.1 Overview 12 Sludge treatment Urban Water Systems

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 Urban Water Systems 12 Sludge treatment

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

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

Sludge Treatment Alternatives Eckenfelder & Santhanam (1981) Urban Water Systems 12 Sludge treatment

12.2 Thickening 12 Sludge treatment Urban Water Systems

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introduced to primary clarifier or – if floatables and grease contents are high – to grid chamber Thickened sludge is withdrawn from hopper and introduced to sludge treatment For an efficient thickening process the development of gas bubbles must be prevented Urban Water Systems 12 Sludge treatment

Gravity Thickener Inflow Scum scimmer Sludge liquor Picket fence Thickened sludge Urban Water Systems 12 Sludge treatment

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,Th and 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 Urban Water Systems 12 Sludge treatment

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 Urban Water Systems 12 Sludge treatment

Thickening by Flotation Urban Water Systems 12 Sludge treatment

Flotation unit Urban Water Systems 12 Sludge treatment

12.3 Biological sludge stabilisation 12 Sludge treatment 12.3 Biological sludge stabilisation Urban Water Systems 12 Sludge treatment

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 Urban Water Systems 12 Sludge treatment

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 Urban Water Systems 12 Sludge treatment

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% Urban Water Systems 12 Sludge treatment

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 Urban Water Systems 12 Sludge treatment

12.4 Volume reduction 12 Sludge treatment Urban Water Systems

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

Volume reduction Urban Water Systems 12 Sludge treatment

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 Urban Water Systems 12 Sludge treatment

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 Urban Water Systems 12 Sludge treatment

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 Systems 12 Sludge treatment

12.5 Sludge disposal 12 Sludge treatment Urban Water Systems

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 Urban Water Systems 12 Sludge treatment

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! Urban Water Systems 12 Sludge treatment

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 Urban Water Systems 12 Sludge treatment