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CHP plants are more efficient when located near the source of demand for heat and power (i.e. a city, a village, an industrial complex). If heat has to be transported far from its production site, a significant percentage gets lost and process efficiency drops considerably. CHP plants are thus well suited as decentralized forms of energy supply. On the other hand, CHP plants require higher initial capital investment than a conventional power plant. The payback time is usually quite long, although they are more economical in the long term due to the energy savings. As with combined desalination and power plants, another disadvantage of CHP plants is the seasonal variation that affects their performance. Meeting two demands (heat and power) adds an extra layer of complexity to plant operations. During summer, it can become challenging to deal with the extra heat. 5.2.4 Sewage water energy recovery In many cities around the world and especially in developing countries, faecal sludge management is one of the most significant health problems. Anaerobic digestion is an option that could be implemented in many wastewater treatment plants to (a) reduce sludge volume and disposal costs; (b) produce a source of green energy (biogas); (c) use organic material as a fertilizer; and (d) eliminate pathogens (see Chapters 17 and 24 [Volume 2] for the case studies ‘Green energy generation in Vienna, Austria’ and ‘Green energy production from municipal sewage sludge in Japan’, respectively). The biogas produced in wastewater treatment plants (Figure 5.4) comprises primarily methane (CH4) and carbon dioxide (CO2) and is produced by anaerobic digestion or fermentation of biodegradable materials such as sewage, manure, human municipal solid waste and green waste. Biogas can be sold as gas for heat and cooking, as vehicle fuel or as fuel for a power plant, or can be burnt on-site to produce electricity and heat for the treatment plant (Box 16.3). The remaining sludge can be sold as fertilizer for agriculture purposes, which makes this practice economically viable in the long term depending on the price of gas and fertilizer. There are several environmental benefits: biogas can replace fossil fuels (e.g. natural gas, coal for cooking), thereby reducing GHG emissions; and given the reduction in sludge volume after digestion, landfill lifespan can be extended. There are several examples of this integrated water and energy solution throughout the world. The size of wastewater treatment plants with anaerobic digesters varies considerably, with important economies of scale in terms of both cost and energy consumption. La Farfana wastewater treatment plant treats urban water from 50% of the population of Santiago, Chile (population equivalent of 3.7 million people), and produces around 24 million m3 per year in biogas (Degrémont, n.d.). This biogas is sold to the gas utility company (Metrogas) and directly replaces natural gas being used in households, benefiting around 100,000 people in the metropolitan area (Aguas Andinas, n.d.). There are also examples of smaller scale decentralized biodigester projects; for example, in India (Müller, 2007), where the biogas is used for cooking. Biogas generated can also be burnt on-site in a CHP plant to generate both heat and electricity in a highly efficient process (discussed earlier). The heat produced by the CHP plant is used in the digester to dry the sludge and for space heating the plant facilities, and the power is used in the plant or sold to the grid. All or most of the plant power needs can be met by electricity generated at the plant, and thermal requirements of the biodigesters can be met by heat generated at the plant, which reduces costs by displacing fuel purchasing. Depending on local electricity prices, the CHP plant can produce electricity below retail cost, which can create a compelling case for private investors. Having its own decentralized power source also enhances plant reliability, which is important 5.4 Biogas production in a wastewater treatment plant Note: For more detail on the numbered stages, see http://news.bbc. co.uk/2/hi/uk_news/magazine/8501236.stm (Accessed Oct 2013) Source: Shah (2010). © BBC News. 52 CHAPTER 5 THEMATIC FOCUS FIGUREPDF Image | Water and Energy
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