Page - 87 - in Clean Water Using Solar and Wind - Outside the Power Grid

Used water treatment 87 the oxygen in the water. Around half of the organic matter is used for the growth of the microorganisms, in other words to increase their body mass. The other half is converted into carbon dioxide. Aeration to provide oxygen to microorganisms requires electric energy for a compressor that provides compressed air. This is normally the major part of energy demand for conventional treatment. There are many different treatment schemes using aerobic biological processes. The air is dissolved in the water, supplying the organisms with the necessary amount of oxygen for their metabolism. The oxygen supply has to be sufficiently high to satisfy the microorganisms. However, an excess of aeration will not favour the biological activity; it will only waste aeration energy. So, aeration should find a balance between the biological requirement of oxygen and the need to save energy. The energy requirement for aeration motivates to replace aerobic (using air) treatment with anaerobic treatment (requiring absence of air, see below). The key motivation to use aerobic treatment is the rate of the biological reaction. In an aerobic treatment the speed is an order of magnitude higher. As a consequence, the plant volume can be smaller, but it implies an energy cost. Therefore, when space is available there is a rationale for replacing aerobic treatment with anaerobic treatment. Even more important: in anaerobic treatment the energy in the organic matter will be converted to biogas that becomes an important energy source. In conventional systems the influent water contains not only organic matter but also nitrogen that principally arrives at the plant as ammonium NH4 + (60–80%). Most nitrogen removal plants will transform the ammonium into free nitrogen that will escape via the water surface. The removal of nitrogen is a slower process than the removal of organic carbon and takes place in two principal stages, nitrification and denitrification. In nitrification ammonium is transformed into nitrate NO3 − (an oxidation process) and in the denitrification (a reduction process, where no oxygen is allowed) nitrate is reduced to nitrogen gas N2. The concentration of dissolved oxygen (DO) governs carbon removal, nitrification and denitrification. In carbon removal and nitrification, the process rate will increase with the oxygen concentration. However, there is a limit to the process rate, and higher DO concentrations will not help the biology but only waste energy for the compressors that aerate the biological reactor. With too little DO microorganisms will Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user