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RECOVERY OF RESOURCES FROM WATER 第二部分-能源 Energy 1. Types of Energy Energy plays a big role in the resource recovery sector, both since electricity is needed for the treatment process and since energy in different forms can be recovered in the treatment process. The majority of recovered energy from treatment plants is used on-site, producing both electricity and heat needed for the ongoing processes. Energy contained in used water sometimes even exceeds the energy that is necessary for treatment. The onus in energy recovery is currently moving from energy efficiency to energy neutrality and to having a production that exceeds consumption. Used water contains energy in different major forms: potential energy, chemically bound energy and thermal energy. Energy from used water is to a large extent stored as thermal energy. The kinetic energy contentof used water is negligibly small and depends mainly on flow rates. There are two major routes to recover energy from used water. One is to on-site turn sludge into biogas through anaerobic digestion (AD), thus recovering electricity and using the heat to heat up the reactor. The other major route is to concentrate the sludge and transport the digested sludge product to central incineration. A positive correlation has been seen between the size of treatment worksand the energy recovery potential. 2. Biosolids and Biogas The chemical energy embedded in biosolids is theoretically enough to cover the energy necessary for treatment. AD is the most typical method in which biosolids can be converted into energy. The process involves the readily biodegradable portion of the volatile solids in sludge which is transformed into biogas by microorganisms in the absence of oxygen. Apart from used water, sources for biogas include landfills, livestock operations and food wastes. The end product of biogas consists predominantly of methane (60-65%) and carbon dioxide (30- 40%) as well as small concentrations of nitrogen, hydrogen sulphide and other constituents. Biogas is mainly used to produce electricity and heat. The methane portion of biogas is a valuable fuel and can with conditioning be used in place of natural gas. Currently around 1% of the biogas beneficially used is upgraded to the same quality as natural gas for transmission to the natural gas system and can be used to provide heat and power. Pipeline injection is one method of use, which is also known as biomethane or ‘green gas’. In order to resemble the qualities of natural gas, biogas needs to be enriched in methane and carbon dioxide needs to be removed. The biogas should be further cleaned from sediment, water and foam before it is compressed for injection. This approach is being currently used in many countries and companies. Biogas can further be upgraded to compressed natural gas (CNG) or liquid natural gas (LNG) to be used in vehicles capable of using such fuels. Biogas production through AD typically only converts the readily biodegradable portion of the solids. Ways to enhance the degraded fraction include processes such as pre-treatment and co-digestion. Pre-treatment breaks open the bacterial cells in the activated sludge, thus releasing the cell contents and making them available to the anaerobic bacteria for conversion to biogas. Technologies for pre-treatment include thermal hydrolysis, sonication, mechanical disintegration and electrical pulse treatment. For co-digestion, readily biodegradable feedstock is added to the digester and thus these are co-digested with the biosolids, thereby increasing the biogas production. Fats, oils and grease (FOG) can for example be co-digested. Another way to recover energy is through incineration of biosolids in fluidized bed or multiple-hearth furnaces. 3. Microbial Fuel Cells Microbial fuel cells (MFC) are an alternative for AD, which directly delivers electricity. This is a system which generates bioelectricity from biomass using bacteria. Through oxidation of organic matter by microorganisms, electrons are produced which are used to create power. Common MFC systems consist of an anode and a cathode chamber separated by a membrane (see Figure 1). The bacteria grow in the anode chamber while electrons react with the catholyte in the cathode chamber. In the system, used water is treated at the same time as energyis produced through conversion of chemical energy into electrical energy. Ammoniacan further be recovered through this process. Figure1 Microbial Fuel Cell 4.Heat Recovery Heat recovered from used water treatment plants can either be used for district heating, sludge drying or thermophilic heating in a sludge digestion process. The financial feasibility of the different heating options typically depends on the current shadow price of carbon. One study comparing different plants recovering energy in the UK showed that districtheating applications have the greatest carbon reduction potential and thus the greatest energy savings. Thermal energy from used water can be concentrated by heat pumps, to be used on-site in the process or off-site for district heating. It has been reported that over 500 used waterheat pumps are in operation worldwide, with thermal capacities ranging from 10kW to 20 MW. The heat pump systems can be used when used water treatment plants are located near a residential area and these typically use existing underground sewage piping as their heat source. Thermal energy recovery from used water has successfully been tested and implemented in countries such as Canada, China, Finland, Switzerland and the United States of America. 5. New Trends Combining energy recovery from organic constituents and use of waste heat provides opportunities for highly integrated resource recovery strategies that represent a role model for the future. In IWA’s survey one researcher noted that recovery of carbon dioxide to valuable products such as methane and carboxylic acids are new trends along with recovery of carbon wastes with gastification. Co-digestion and mainstream deammonification are other areas which are anticipated to increase in scope in the upcoming years. One researcher in the survey predicted that nitrous oxide is an upcoming trend within the field of resource recovery. This process has been documented in some areas already. For example, the Stanford Nitrogen Group has developed aprocess called the Coupled Aerobic-anoxic Nitrous Decomposition Operation (CANDO) in which ammonia is converted to nitrous oxide before combusting fuel with nitrous oxide to recover energy. Another noticed trend is to rethink how large surface areas on used water treatment plants have traditionally been used through installing on-site wind and solar power. This is currently being carried out in several locations in the USA used water treatment plant in California, for example, uses solar integration to provide 80% of the facility needs. Although renewable energy can be integrated in treatment plants in many different ways, few such studies investigating this have been carried out. An area that further needs investigation is looking at the effects of discharging cooled effluent into aquatic ecosystems as these are currently relatively unknown. 未完待续 Authors: Katrin Eitrem Holmgren, Hong Li, Willy Verstraete and Peter Cornel 本文节选自IWA Resource Recovery Cluster的最新汇编报告 State of the Art Compendium Report on Resource Recovery from Water。更多内容，请访问IWA网站: http://www./cluster/resource-recovery-from-water-cluster 国际水协会（IWA）官网 www. |
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