Page - 89 - in Clean Water Using Solar and Wind - Outside the Power Grid

Used water treatment 89 In the anaerobic process microorganisms assist the process of organic material conversion that produces the biogas. The processes involved in fermentation are exceedingly complex and are not completely understood. There is an impressive amount of activity going on in anaerobic process research to increase knowledge of the microbiology and the internal mechanisms taking place. A variety of factors affects the rate of digestion and biogas production. The most important is temperature. Anaerobic bacteria communities can endure temperatures from below freezing to more than 60°C and different species of bacteria thrive in different temperature ranges. At temperatures in the range 35–40°C there are mesophilic bacteria. Some of the bacteria, thermophiles, can survive at temperatures around 50–65°C. Mesophile bacteria are generally more tolerant than thermophiles of changes in environmental conditions. Therefore, mesophilic digestion systems are considered to be more stable than thermophilic ones. However, at increased temperatures the reaction rates are faster and consequently the gas yield is higher. The hydraulic retention time is also shorter. Therefore, smaller reactor volumes are required under thermophilic conditions. At lower temperatures, from 35°C to 0°C, the bacterial activity, and thus biogas production, falls off gradually. Rapid changes of the reactor temperature will upset the bacterial activity; therefore, the digester must be kept at a consistent temperature. The digesters referred to here are in subtropical or tropical regions, so they seldom need extra heating to enable their operation. Anaerobic digestion (AD) has more advantages than the energy generation via biogas: it has a high capacity to treat slowly degradable substrates at high concentrations and it can efficiently reduce pathogens. 7.2.4 Membrane separation Membrane separation has been described in Chapters 5.2 and 5.3. With the whole spectrum of membrane processes, it is possible to remove micron-sized particles like microorganisms and suspended solids. A partial or full disinfection can also be achieved. As mentioned in Chapter 5, the membrane technology is compact and provides consistent quality over a range of pollutant loading rates. It depends on energy supply. The main challenge is to maintain a consistent throughput Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user