43. Solar Sludge Drying

The use of solar sludge drying is a further development of the natural drying process and is aimed at reducing sludge volume and disposal costs. Drying is achieved through the evaporation of the water contained in the sewage sludge. The evaporation rate is increased by the greenhouse effect, as solar radiation is converted into thermal radiation for heating the air in the drying hall. In order to enhance this process, the water-saturated air has to be evacuated through targeted ventilation using circulating air fans and/or the targeted control of the ventilation flaps. High drying values can be achieved even during low solar radiation periods by the employment of additional thermal energy and thus constant sludge quality can be guaranteed throughout the year. Solar driers are characterized by low energy demand of 20–30 kWh/t and are therefore very environment-friendly and have a small CO₂ footprint. An optimum drying degree is reached with 70%, however drying degrees of up to 85% are possible. Drying takes place in transparent, semi-closed structures, which support heating and air circulation. Apart from the greenhouse, the main parts of the solar drier are an air exchange system and a sludge turning and mixing device. The most important process parameters are the sludge retention time and the drying capacity in terms of the tones of water eliminated per area and year. At present, there are several manufacturers in the market. Their process set-ups are more or less all identical, but the sludge turning, mixing and distributing devices, which constitute the vital part of the solar drier, are generally patented and therefore differ.