Due to the new regulations and legislation for sewage sludge, disposal costs have doubled for many sewage treatment plants or increased to far more than 100 € per ton original substance. The reason for the cost increase is that the standard solution for meeting the requirements of the new legislation is sludge incineration.
Incineration capacities are however unequally distributed and exploited to a large extent, and transport ways have become longer on many sites. Disposal costs can be reduced if the sludge volume is reduced through drying.
The Bavarian municipality Freystadt decided to build its own decentralized solution: a complete sludge treatment plant that processes wet thin sludge into dry granulate installed at the beginning of last year has since been operated fully automatically there. “Of course, we had to get familiar with the new technology in the beginning, but after a while we knew how everything works. Now, it takes us not more than one and a half hour per day to handle the complete sludge treatment on site”, said plant manager Johann Weihrich.
The sludge treatment plant consists of a screw press and a solar sewage sludge drying system. Since years, solar sewage sludge drying has been a recognized technology for producing dry grains from a partly sticky sludge cake, reducing the sludge volume to approximately a quarter at the same time. Solar sludge drying represents an economically efficient method if the disposal price is approx. 80 € per ton or higher – and there are hardly any operating costs. The low operating costs include low energy consumption and a very good climate balance.
On STP Freystadt, the installation of the solar sewage sludge drying plant resulted in a number of advantages: before the plant was built, contract dewatering companies emptied the sludge storage ponds at regular intervals. Emptying the ponds meant considerable efforts and operating expense: stirring the 8,000 m³ large sludge ponds for homogenisation, the load on the treatment stages due to the filtrate water, the effort necessary to regularly set up the mobile machines, and sludge removal and transport.
Stationary dewatering eliminates peak loads caused by the filtrate water, ensures that the dry granulate is pourable and not sticky so that sludge transport is clean and easy. Moreover, sludge volumes are significantly reduced: only 180 t/a have to be removed from the plant.