Volume reduction and biological stabilization of sludge in small sewage plants by solar drying

High investments, high dumping costs and high electrical energy consumption turn waste water treatment in small sewage plants an expensive process. Therefore, the University of Hohenheim and the company Thermo-System have developed a fully automatic low temperature solar drying plant allowing further stabilization and volume reduction of sewage sludge by continuous mixing and aerating. To test the potential of the technology, the drying process was investigated in a commercially operated municipal sewage treatment plant with aerobic sludge digestion. During experiments, the changes of the dry solids concentration and the volatile solids, the climatic conditions and the energy consumption were constantly measured and documented. It was found, that the sludge could be dried from a dry solids concentration of 3 to 93% w/w in 64 (flocculated) or 83 days (non-flocculated). At the same time, the volatile solids content was reduced from 65 to 34% w/w, respectively. The electrical energy consumption was only 22 to 28kWh per ton of evaporated water, compared to 70 to 110kWh required for conventional drying processes, The thermal energy for evaporation was fully covered by solar energy.