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High investments, high dumping costs and high electrical energy consumption turn waste water treatment an expensive process. Therefore, the University of Hohenheim and the company Thermo-System have developed a fully automatic low temperature solar drying plant allowing pathogen reduction,
further stabilization and volume reduction of sewage sludge by continuous mixing and aerating it in a climate controlled environment. To find out the potential of the technology, the drying process and the pathogen reduction was investigated in two commercially operated municipal sewage treatment
plants with aerobic and anaerobic sludge digestion. During the experiments, the climatic conditions, the energy consumption and the changes of the dry solids concentration, the volatile solids and the number of fecal coliforms were measured, analysed and documented. It was found, that the
volatile solids content was reduced during the drying process from 61 to 37% w/w for aerobically digested sludge and from 52 to 43 % w/w for anaerobically digested sludge. At the same time the concentration of fecal coliform was reduced from 105 to 103
or less. Adding lime at a concentration of 0,2 kg/kg in a late stage of the drying process reduced the number of pathogens below a detectable limit and increased the pH-value to over 12,5. The electrical energy consumption was only 17 to 28 kWh per ton of evaporated water, compared to
70 to 110 kWh required for conventional drying processes. The thermal energy for evaporation was fully covered by solar energy.
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