Sludge Treatment

• Definition

  • Suspension with strong water-binding properties (hydrophilic) of organic and inorganic origin

  • Sludge is formed during the liquid-solid-seperation by sedimentation, flitration, flotation or micro-screening

• High-nutrient content (e.g. N, P, K, Mg, Ca), High pollutant content (e.g. heavy metals, PAH, AOX, pathogens)

• Main components

  • Water : 99%

  • Solids : 1 %

    • Organic: 70-80%

    • Inorganic: 20-30%

    • Solids in raw water like food leftovers, faeces etc.

    • Biomass (excess sludge) - microorganisms

    • Chemicals - salts from precipitation like metal hydroxide or metal phosphate

• General objective: Prepare sludge for intended disposal

• The type of disposal significantly determines the sludge treatment

• Two ways of disposal:

  • Material recycling: Agricultural use, re-cultivation

  • Material dumping: Incineration and disposal of the ash

• Stabilization of sludge

  • Break down active biomass content (high-molecular, organic substances in low-molecular, inorganic substances)

  • Reduction of odor-forming substances

  • Reduction of dry mass of sludge

• Thickening/ Dewatering / Drying

  • Reduction of volume of sludge

  • Reduction of disposal costs

• Disinfection (e.g. 30 min at 70°C)

• Conditioning: Optimization of dewatering properties

• Biogas production

  • renewable energy production (heat, electricity)

  • Reduction of energy consumption

• A) Free water: water in pores and gaps, water can flow -> gravity forces

• B) Bound to solids in suspension and colloids: surface water -> mechanical forces, change electrical load (coagulation)

• C) Capillary bounded water -> very strong mechanical forces

• D) Water bounded cells -> Destruction of cell membrane (bio degradation, freezing, evaporation)

• -> Increasind effort to remove water (A-D)

• Before Thickening

• After Thickening

• Thickening/ Dewatering / Drying

• Static Sludge Thickening

  • (Sedimentation) tanke with rake device

    • Demolition of water lenses in settled sludge

    • Sludge removal via central sludge outlet

    • Sizing according sludge surface loading

• Mechanical Sludge Thickening

  • Screen drum

  • Screw thickener

  • Belt thickener

  • Disc thickener

  • Thickener Pump

  • Centrifuge/ decanter

Belt thickener:

• Biological sludge stabilization

  • Aerobic stabilization (required sludge age > 25 d)

  • Anaerobic stabilization (sludge digestion)

• Chemical sludge stabilization

  • Addition of lime (increase of pH)

  • Pseudo-stabilization

• Thermal sludge stabilization

  • Drying (pseudo-stabilization)

• Anaerobic sludge stabilization

  • Sludge digestion tank

    • anaerobi mesophilic stabilization (35-37°C)

    • Sludge retention time: 10-30 days, sizing approx. 20 days

• Typical design of digesters

• Circulation of sludge by stirring, pumping or gas injection

• Heating of sludge by internal exchanger, external heat exchanger or introduction of steam in sludge line

• Biogas production

  • specific biogas production approx. 15-25 l/(PE*d)

  • Composition of biogas

    • Approx 65% CH4

    • Approx 35% CO2

    • N2, H2, H2S, NH3

• Chemical sludge conditioning

  • Addition of chemicals which improve dewatering properties by reducing electric repulsion of sludge particles

  • Type and amount of coagulation aid depends on thickening/ dewatering process

  • Coagulation aids

    • Iron- or aluminium salts

    • Lime

    • Polyelectrolyts

• Thermal sludge conditioning

  • (pressurised) heating, approx. 200°C

• Drying by contact

• Drying by convection

• Crying by radiation

• Radiation dryer

  • Solar dryer (sludge drying bed)

  • Solar thermal dryer

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• Thermal treatment

  • Mono-incineration

  • Co-Incineration (power plants, cement works)

• Agricultural use

• Landscaping measures - re-cultivation of mining sites

• Other material use

  • composting

• Advantages

  • Sewage sluge is thoroughly tested for pollutants, limits for heavy metals and organic pollutants are provided by law

  • High content of organic matter (favorable for the formation of humus)

  • Replenishment of necessary nutrients

  • Favorable phosphorus fertilizer, no import dependency

• Disadvantages

  • In addition to the controlled substances, sewage sludge may contain a number of unknown or unregulated pollutants (e.g. nanomaterials, microplastics, …)

  • Possible risk of entry and accumulation of pollutants in soil

  • Risk of over-fertilization and nutrient leaching

  • direct plant availability of the phosphate depends esentially on the from in which the precipitation took place