Processes to manage and dispose of sludge during sewage treatment
Sludge treatment in anaerobic digesters at a sewage treatment plant in Cottbus, Germany
Sewage sludge treatment describes the processes used to manage and dispose of sewage sludge produced during sewage treatment. Sludge treatment is focused on reducing sludge weight and volume to reduce transportation and disposal costs, and on reducing potential health risks of disposal options. Water removal is the primary means of weight and volume reduction, while pathogen destruction is frequently accomplished through heating during thermophilic digestion, composting, or incineration. The choice of a sludge treatment method depends on the volume of sludge generated, and comparison of treatment costs required for available disposal options. Air-drying and composting may be attractive to rural communities, while limited land availability may make aerobic digestion and mechanical dewatering preferable for cities, and economies of scale may encourage energy recovery alternatives in metropolitan areas.
Sludge is mostly water with some amounts of solid material removed from liquid sewage. Primary sludge includes settleable solids removed during primary treatment in primary clarifiers. Secondary sludge is sludge separated in secondary clarifiers that are used in secondary treatment bioreactors or processes using inorganic oxidizing agents. In intensive sewage treatment processes, the sludge produced needs to be removed from the liquid line on a continuous basis because the volumes of the tanks in the liquid line have insufficient volume to store sludge.[1] This is done in order to keep the treatment processes compact and in balance (production of sludge approximately equal to the removal of sludge). The sludge removed from the liquid line goes to the sludge treatment line. Aerobic processes (such as the activated sludge process) tend to produce more sludge compared with anaerobic processes. On the other hand, in extensive (natural) treatment processes, such as ponds and constructed wetlands, the produced sludge remains accumulated in the treatment units (liquid line) and is only removed after several years of operation.[2]
Sludge treatment options depend on the amount of solids generated and other site-specific conditions. Composting is most often applied to small-scale plants with aerobic digestion for mid-sized operations, and anaerobic digestion for the larger-scale operations. The sludge is sometimes passed through a so-called pre-thickener which de-waters the sludge. Types of pre-thickeners include centrifugal sludge thickeners,[3] rotary drum sludge thickeners and belt filter presses.[4] Dewatered sludge may be incinerated or transported offsite for disposal in a landfill or use as an agricultural soil amendment.[5]
Energy may be recovered from sludge through methane gas production during anaerobic digestion or through incineration of dried sludge, but energy yield is often insufficient to evaporate sludge water content or to power blowers, pumps, or centrifuges required for dewatering. Coarse primary solids and secondary sewage sludge may include toxic chemicals removed from liquid sewage by sorption onto solid particles in clarifier sludge. Reducing sludge volume may increase the concentration of some of these toxic chemicals in the sludge.[6]
^Henze, M.; van Loosdrecht, M.C.M.; Ekama, G.A.; Brdjanovic, D. (2008). Biological Wastewater Treatment: Principles, Modelling and Design. IWA Publishing. doi:10.2166/9781780401867. ISBN 978-1-78040-186-7. S2CID 108595515. (Spanish and Arabic versions are available online for free)
^Von Sperling, M. (2015). "Wastewater Characteristics, Treatment and Disposal". Water Intelligence Online. 6: 9781780402086. doi:10.2166/9781780402086. ISSN 1476-1777.
^"Centrifuge Thickening and Dewatering. Fact sheet". EPA. September 2000. EPA 832-F-00-053.
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wastewater (sewage) flows for basic sewagetreatment. Settling and anaerobic digestion processes reduce solids and organics, but the treatment efficiency...
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50540°N 0.14202°E / 51.50540; 0.14202 The Crossness SewageTreatment Works is a sewagetreatment plant located at Crossness in the London Borough of Bexley...
materials. Sewagesludgetreatment describes the processes used to manage and dispose of sewagesludge produced during sewagetreatment. Sludgetreatment is focused...
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phosphorus from sewagesludge directly, from the ash after incineration of the sewagesludge or from other products of sewagesludgetreatment. This includes...
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In treatment of sewage one process used is the activated sludge process in which air is passed through a mixture of sewage and old sludge to allow the...
soil, fresh water, and sewage, or even for several years in feces, fecal sludge (historically called night soil), and sewagesludge – a period that is much...
impacts on treatment processes.[citation needed] The activated sludge process is a type of biological wastewater treatment process for treating sewage or industrial...
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