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Chapter 4  How Biogas works

The making of biogas (methane and carbon dioxide) from food materials is an anaerobic microbiological process, involving a symbiotic population of microbes (bacteria and archaea). The process requires the right conditions, such as pH, temperature, retention time, the chemical and physical nature of the feed, and moisture content. There are several ways in which an anaerobic digester can be run, such as in batch, continuous or semi-continuous modes. Different approaches to the process include using stirred tanks (CSTR), plug flow, upflow sludge blanket (UASB), anaerobic filters and baffled reactors. There are also different ways to pre-process the feed including chopping and pre-digestion using flooded pre-digesters and leach beds. The qualtiy of the feed material can be monitored using different parameters, such as TS (Total Solids), VS (Volatile Solids), COD (Chemical Oxygen Demand), BOD (Biological Oxygen Demand), batch or semi-continuous tests. The process can be mathematically modelled.


Chapter 5  Biogas Effluent as Compost

The value of anaerobic digestion to produce compost has received little interest until recent years. Biogas effluent slurry, or bio-slurry, is increasingly seen as having value, not only as a fertilizer, but also in reducing crop disease and acting as a soil conditioner. Its use has lasting effects on the soil. The ways in which it is processed affect the value of the product: composting and vermi-composting increase its usefulness. The benefits of bio-slurry are shown by the results from many practical examples.


Chapter 6  Main Domestic Biogas Plant Designs

From the wide range of biogas digester designs, only a few meet the criteria for use in the programmes in Asia which have been made in large numbers. The floating drum system was developed in India, but has largely been replaced by fixed dome systems. Fixed dome systems were developed in China and the two main approaches are to use a dome made from brick or concrete. There are several aspects that are common to both fixed dome designs: such as how the dome is sealed, how reservoir pits are made, the covers over reservoir pits and inlet pits. Various attempts have been made to adapt such plants for climates that have lower ambient temperatures.


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