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24 green energy production from municipal sewage sludge in Japan A 15 m high tsunami, triggered by the magnitude nine Great East Japan Earthquake on 11 March 2011, caused a nuclear accident by damaging the cooling systems of three reactors in the Fukushima Daiichi Nuclear Power Plant. This event required more than 100,000 people to be evacuated from their homes (WNA, 2013). In the wake of this major incident, the promotion of green energy became one of Japan’s national goals. In this process, municipal sewage sludge was identified as an untapped resource with great potential. The Japanese government was prompted to implement policies to support the development of suitable technologies to benefit from the sludge to the greatest possible extent. This case study features three leading projects where green energy is produced from municipal sewage sludge: gasification in the Tokyo Metropolitan Area, biochar production in Hiroshima City and the use of biogas as vehicle fuel and city gas in Kobe City. 24.1 Gasification in Tokyo In 2006, the Tokyo Metropolitan Government implemented a programme to reduce greenhouse gas emissions in the area by 25% by 2020 (the year 2000 was taken as the baseline). Wastewater service activities, generating 40% of the emissions, were identified as the main challenge. The gasification of sewage sludge was proposed as an effective solution. This process converts the organic materials in wastewater into a gas mixture, which is then used as fuel for drying sludge and generating power. The high temperature in the heat recovery furnace helps to minimize the emission of nitrous oxide, which is an even more potent greenhouse agent than carbon dioxide. With the exception of a period following the Great East Japan Earthquake, the project has remained operational and met the targets set for the generation of power. However, the energy output of the dewatered sludge has been less than expected, which means that natural gas has had to be added to the fuel mix to achieve stable power generation. This aspect will require further enhancement to reduce the cost of the operation as well as to improve its environmental benefits. 24.2 Biochar production in Hiroshima Hiroshima City had started considering biochar production as early as 2007, some four years before the 2011 earthquake. The amount of sewage sludge being generated at that time was 58,000 tonnes per year. Of this sludge, 31,000 tonnes was recycled as either compost or cement. The remainder was incinerated and used for land reclamation. However, it was subsequently considered that reclamation was not an environmentally conscious practice, and site capacity was limited. In addition, the incinerators had reached the end of their designed life and it would have been too costly to renew them. Most importantly, the unsecure disposal of sewage sludge remained a risk in view of the fluctuating need for compost and cement. Under these circumstances, Hiroshima City contracted private companies in a joint venture on a design–build–operate basis. The design and construction of the facility lasted three years and ended in March 2012. The operational period will run for 20 years – up to 2032. The contractor handles 28,000 tonnes of dewatered sludge per year in a plant that has a daily capacity of 100 tonnes. The project uses low-temperature (250°C to 350°C) carbonization technology, which allows the production of biochar that has a high calorie, low risk of self-ignition and a low odour level. In the first six months of operation (April to September 2012), 14,000 tonnes of dewatered sludge was processed and 2,300 tonnes of biochar was produced. From an environmental perspective, the use of biogas (which is generated as part of the sludge dewatering process) as a fuel to produce biochar reduced the annual greenhouse gas emissions (carbon dioxide) of the city’s wastewater services by 12%. Furthermore, using biochar at coal-fired power plants is expected to reduce yearly greenhouse gas emission by another 9%. 24.3 Biogas as vehicle fuel and city gas in Kobe Six wastewater treatment plants in Kobe treat approximately 500,000 m3 sewage per day, generating 37,000 m3 biogas. Because of its poor quality, this combustible gas was mainly used on site to heat digester 158 CHapTer 24 CASE STUDIES CHAPTERPDF Image | Water and Energy
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