A three year field and investigative study carried out on 33 individual sanitation systems showed that the frequency of septic tank emptying could be at least 5 years, corresponding to a sludge accumulation rate of 0.21/user.day. This value should be included in the calculation of septic tank sizing. Other data showed within a septic tank the methanogenesis is only effective after 2 years of operation. Thereby a septic tank should not be desludged before this period of time. At last, among all the physico-chemical parameters monitored to study the sludge quality, the best indicators of system failures were both the solids (TS and VS) and soluble matter (COD and some VFA). Then, biochemical study of septic tank sludge showed that the main hydrolytic enzymes (cellulase, phosphatase, protease, lipase and urease) are present in septic tank sludges and linked to the insoluble particles. Their solubilization with different solvents (buffers, surfactants…) showed each enzyme was linked in a different way.

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