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Environmental Aspects of Anaerobic Digestion

The application of biogas technology will also minimise the spread of disease because of the anaerobic digestion of organic wastes. The more fully the wastes are digested, the more pathogens are destroyed. The reduction of pathogens in this processes is influenced by the reduction time and temperature of the digester contents. The higher the temperature, the shorter the time require for complete digestion. It has been reported that the principal organisms killed in biogas plants are typhoid, paratyphoid, cholera and dysentery, hookworm and bilharzia. Feacehem reported that average removal was about 90 percent at a digested temperature of 35° C at twenty days retention time. Therefore, it can be concluded  that a great reduction of pathogens occurs in the biogas generation process but in the case of high concentrations of pathogens in the excreta of infected persons or animals, this reduction does not lead to a complexly pathogen-free effluent.

digester slurry

a digester slurry

Complete removal of pathogens is possible with further treatment of digester slurry. The most common treatment methods are drying, composting and composting with additional chemicals. Tapeworm and roundworm die completely  if the fermented slurry is dried in the sun. Composting is an exothermic reaction and a temperature of 60-70° C  can be obtained. At higher  temperature, most of the pathogen are reduce as all are biological entities and rapidly denature at temperature higher than 55°  C, even at a retention time of just one day. The additional of liquid urea to a compost heap as carried out by the Chinese results in a high concentration of ammonia, which disinfects the sludge.