Query: We found 68 stations located  at wastewater treatment plants, where the process of waste digestion  causes temperatures to be higher than in surrounding areas.

Relevant passages:
Sewage sludge <=SEP=> Sewage sludge refers to the residual , semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater . The term septage is also referring to sludge from simple wastewater treatment but is connected to simple on-site sanitation systems , such as septic tanks .   When fresh sewage or wastewater enters a primary settling tank , approximately 50 % of the suspended solid matter will settle out in an hour and a half . This collection of solids is known as raw sludge or primary solids and is said to be `` fresh '' before anaerobic processes become active . The sludge will become putrescent in a short time once anaerobic bacteria take over , and must be removed from the sedimentation tank before this happens .   This is accomplished in one of two ways . In an Imhoff tank , fresh sludge is passed through a slot to the lower story or digestion chamber where it is decomposed by anaerobic bacteria , resulting in liquefaction and reduced volume of the sludge . After digesting for an extended period , the result is called `` digested '' sludge and may be disposed of by drying and then landfilling . More commonly with domestic sewage , the fresh sludge is continuously extracted from the tank mechanically and passed to separate sludge digestion tanks that operate at higher temperatures than the lower story of the Imhoff tank and , as a result , digest much more rapidly and efficiently .   Biosolids is a term often used in conjunction with reuse of sewage sludge after sewage sludge treatment . Biosolids can be defined as organic wastewater solids that can be reused after stabilization processes such as anaerobic digestion and composting . Opponents of sewage sludge reuse reject this term as a public relations term .

Sewage sludge treatment <=SEP=> Sewage sludge treatment describes the processes used to manage and dispose of sewage sludge produced during sewage treatment . Sludge is mostly water with lesser amounts of solid material removed from liquid sewage . Primary sludge includes settleable solids removed during primary treatment in primary clarifiers . Secondary sludge separated in secondary clarifiers includes treated sewage sludge from secondary treatment bioreactors .   Sludge treatment is focused on reducing sludge weight and volume to reduce 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 .   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 .

Anaerobic digestion <=SEP=> Anaerobic digestion is a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen . The process is used for industrial or domestic purposes to manage waste or to produce fuels . Much of the fermentation used industrially to produce food and drink products , as well as home fermentation , uses anaerobic digestion .   Anaerobic digestion occurs naturally in some soils and in lake and oceanic basin sediments , where it is usually referred to as `` anaerobic activity '' . This is the source of marsh gas methane as discovered by Volta in 1776 .   The digestion process begins with bacterial hydrolysis of the input materials . Insoluble organic polymers , such as carbohydrates , are broken down to soluble derivatives that become available for other bacteria . Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide , hydrogen , ammonia , and organic acids . These bacteria convert these resulting organic acids into acetic acid , along with additional ammonia , hydrogen , and carbon dioxide . Finally , methanogens convert these products to methane and carbon dioxide . The methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments .   Anaerobic digestion is used as part of the process to treat biodegradable waste and sewage sludge . As part of an integrated waste management system , anaerobic digestion reduces the emission of landfill gas into the atmosphere . Anaerobic digesters can also be fed with purpose-grown energy crops , such as maize .   Anaerobic digestion is widely used as a source of renewable energy . The process produces a biogas , consisting of methane , carbon dioxide and traces of other ` contaminant ' gases . This biogas can be used directly as fuel , in combined heat and power gas engines or upgraded to natural gas-quality biomethane . The nutrient-rich digestate also produced can be used as fertilizer .   With the re-use of waste as a resource and new technological approaches that have lowered capital costs , anaerobic digestion has in recent years received increased attention among governments in a number of countries , among these the United Kingdom ( 2011 ) , Germany and Denmark ( 2011 ) .