Technische Universitt Dresden Peter Krebs Department of Hydro

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 1

12 Sludge treatment 12. 1 Overview Urban Water Systems 12 Sludge treatment 2

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 3

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

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

Sludge Treatment Alternatives Eckenfelder & Santhanam (1981) Urban Water Systems 12 Sludge treatment 6

12 Sludge treatment 12. 2 Thickening Urban Water Systems 12 Sludge treatment 7

Thickening by 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introduced to primary clarifier or – if floatables and grease contents are high – to grid chamber Thickened sludge is withdrawn from hopper and introduced to sludge treatment For an efficient thickening process the development of gas bubbles must be prevented Urban Water Systems 12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 8

Gravity Thickener Inflow Scum scimmer Sludge liquor Picket fence Thickened slud Urban Water Systems 12 Sludge treatment 9

Dimensioning of gravity thickeners surface Solids overflow rate q. TSS, Th Specific solids overflow rate (kg TSS / (m 2 d)) QWAS Inflow to thickener (m 3/d) XTh, in Solids concentration in thickeners inlet (kg TSS / m 3) ATh Surface of thickener (m 3) Typical values for solids overflow rate q. TSS, Th and concentration of thickened sludge XTh Primary sludge Primary and secondary sludge Secondary sludge Urban Water Systems 12 Sludge treatment q. TSS, Th XTh 80 – 120 80 - 150 50 - 70 25 - 30 50 - 100 20 - 35 10

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 11

Thickening by Flotation Urban Water Systems 12 Sludge treatment 12

Flotation unit Urban Water Systems 12 Sludge treatment 13

12 Sludge treatment 12. 3 Biological sludge stabilisation Urban Water Systems 12 Sludge treatment 14

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 15

Processes in digester Anaerobic degradation Degradation of organic substances of app. 50% Biogas production: 63% CH 4 (Methane) 35% CO 2 2% other gases (N 2, H 2 S) electricity and heating Organic nitrogen is converged to NH 4+ N-loading of WWTP Urban Water Systems 12 Sludge treatment 16

Characteristic values of digester Mean residence time of sludge Small units, badly mixed Medium size units with mixing < 30 d 20 d Large plants with mixing 12 – 16 d Biogas production related to degradation of organic substances 0. 9 m 3 / kg VSSdegr. Degradation of organic substances 40 – 55% Urban Water Systems 12 Sludge treatment 17

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 18

12 Sludge treatment 12. 4 Volume reduction Urban Water Systems 12 Sludge treatment 19

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 20

Volume reduction Urban Water Systems 12 Sludge treatment 21

Dewatering Conditioning with flocculation agents (poly-electrolytes) for efficient dewatering W DS Centrifuge > 0. 7 < 0. 3 Batch-wise Hydraulic pressure through plates in water -tight chambers > 0. 6 ≤ 0. 4 continuous Pressed between two filter belts around staggered rollers > 0. 7 ≤ 0. 3 Unit Operation Method Decanter Continuous Chamber filter press (large plants) Belt filter press (small plants) Urban Water Systems 12 Sludge treatment 22

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/m 2 Trickling filter 6 PE/m 2 Activated sludge plant 4 PE/m 2 Urban Water Systems 12 Sludge treatment 23

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 24

12 Sludge treatment 12. 5 Sludge disposal Urban Water Systems 12 Sludge treatment 25

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

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 27

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 28