High dry solids digestion

Digestion of sewage sludge normally takes place at pH values between 7,0 and 7,5. Control methods of the process stability are based on practical experience such as the ratio of volatile fatty acids (VFA) and total alkalinity (TA). Thermal sludge disintegration for an enhanced digestion process allows a digester feed with higher dry solids concentrations. This is due to an opening of cell mass and lower viscosity of the treated sludge. At the same time the degradation of cell proteins is increased, resulting in higher ammonia concentrations and subsequently higher TA concentrations and pH values. At HIAS, Norway, thermal disintegrated sludge has been digested for several years with a volatile solids reduction by over 55%. Both pH values and VFA concentrations varied over time. Based on these measurements a recommendation is given for a changed stability control of the digestion process at pH values above 7,5.

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