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floors or collected rainwater from the roof can be used in the lower floors to minimize energy required for pumping of freshwater. The thermal energy in wastewater comes from its temperature when leaving a building – about 27°C for mixed wastewater and 38–40°C for grey water (Roest et al., 2010). The thermal energy of wastewater is particularly useful in places where a large amount of energy is required for heating water, because it can be used to preheat the water via heat exchangers or heat pumps. In Dalian, a city of 5.7 million people in north- east China (recognized as a National Model City in Environmental Protection by the Chinese government), heat is reclaimed from sewage to meet part of the heating and cooling requirements of the Xinghai Bay Business District. Authorities claim they save more than 30% in energy compared to conventional solutions (Friotherm, 2012). The chemically bound energy in wastewater results from its carbon content, which can be converted to methane under anaerobic conditions. The methane can then be used for cooking and heating, converted to electricity, or used to fuel vehicles. Based on the maximum chemical oxygen demand (COD) load per capita of 110–120 mg/L, Lazarova et al. (2012) estimates the maximum theoretical chemical energy content of wastewater to be 146 kWh per capita per year. Although the chemically bound energy content in wastewater is less than its thermal energy, it can be transported without much loss, whereas the thermal energy has to be reclaimed as close to the source as possible. Many wastewater treatment plants have been able to generate biogas from wastewater or sludge and convert it to heat or electricity. In Stockholm, for example, public buses, waste collection trucks and taxis run on biogas produced from sewage treatment plants (Osterlin, 2012) (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). In developing countries, particularly in warm climates, there is little opportunity for using thermal energy from wastewater, but generating biogas from wastewater can be very useful. This is now a widespread practice in many cities in Africa and in Asia. More than 300 households and institutions in Maseru, Lesotho, are generating biogas from wastewater and using it as a cooking fuel. Although the initial cost for decentralized wastewater treatment systems – which includes a biogas digester, an anaerobic baffle reactor and planted gravel filters – is slightly higher than septic tanks, the additional benefits of biogas allow the system to pay for itself in three years (Mantopi and Huba, 2011). When biogas is used for cooking it often replaces inefficient and potentially harmful solid biomass fuels (Chapters 3, 9). Decentralized biogas systems for wastewater treatment also reduce the cost of transporting and pumping wastewater. Another potential option for energy production is the use of dried faecal sludge (FS) as fuel. Nakato et al. (2012) analysed FS samples from three cities (Dakar, Senegal; Kampala, Uganda; Kumasi, Ghana) and found the average calorific value of the samples to be 17.2 MJ/kg dry solids (DS), which is comparable to other commonly used fuels such as rice husk (15.6 MJ/kg DS), forest residue (19.5 MJ/kg DS), coffee husk (19.8 MJ/kg DS) and sawdust (20.9 MJ/kg DS).19 The study found that the FS must be dried to ≥27% DS to enable industry to derive the net energy of 17.2 MJ/kg DS. Experience from Uganda has shown that it is possible to achieve a DS concentration of more than 30% by drying the FS in simple drying beds for two weeks, thus indicating that additional energy inputs would not be required. 7.4.4 Waterborne transit Many urban areas are located next to large water bodies, making waterborne transit another area where water and energy come together in the urban context. Several studies have shown that waterborne transit is one of the most energy efficient means of transport. Studies done in the USA indicate that inland towing barges are more than three times more energy efficient than road (trucks) and 40% more efficient than rail in transporting cargo (PIANC, 2011). A study done by India’s National Transport Policy Committee (1980) showed that the energy consumption of transporting goods by barge was 328.0 BTU per tonne-km,20 whereas for diesel trucks it was five times higher at 1587.3 BTU per tonne-km. As transportation is one of the most energy intensive sectors in the urban context, increasing the use of waterways for passenger or goods transit can lead to substantial energy savings. 19 20 1 MJ = 0.278 kWh. 1 BTU = 0.293 Wh. 68 CHAPTER 7 THEMATIC FOCUSPDF Image | Water and Energy
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