Solar sewage sludge drying has become a generally accepted ecological and economical technology. To make the implementation and operation of a solar drying plant a sustainable success, all relevant aspects need to be fulfilled: economic and planning aspects but as well acceptance on the part of the operator and the public.
One of the plants that offer both, a perfect design concept and acceptance, is installed on WWTP Penzing Weil near the Upper Bavarian lake Ammersee located southwest of Munich. About 8,500 residents are connected to this aerobic stabilised sewage treatment plant. In 2008, when WWTP Penzing Weil was requested to measure PFT, high concentrations of perfluorinated tensides were found and disposal costs rose from 30,000 € to about 100,000 €. Investing into sludge treatment with the aim to reduce annual disposal costs therefore became financially rewarding. Consequently, a sludge treatment plant including sludge dewatering and drying was started to be built at the beginning of 2010.
After its completion and a test phase the new plant was officially put into operating in July 2010. The process of sludge treatment starts with a HUBER ROTAMAT® Screw Press RoS 3Q 440. A screw conveyor automatically transports the compacted sludge into the greenhouse and drops sludge onto the greenhouse floor. The sludge turning/transporting assembly SRT 9 both distributes and transports the sludge inside the greenhouse. At the end of the greenhouse the dried granular sludge is dropped into a storage bunker from where it is loaded on trucks by a wheel loader 5 to 6 times a year.
Dryer efficiency variations between summer and winter are compensated by two buffer tanks for thin sludge with 700 m³ capacity each. Depending on the weather conditions, the sewage works operator feeds more or less sludge to the dryer. Either directly the thin sludge settled in the secondary clarifier or the prethickened sludge from the tanks can be dewatered in the HUBER ROTAMAT® Screw Press RoS 3Q. The plant operation program provides for the possibility to select at the push of a button whether to feed to the screw press excess sludge with approximately 1% dry substance or statically thickened sludge with approximately 3.5% dry substance.
Where no storage capacity is available or the available space is scarce, the solar dryer efficiency can be increased by adding exhaust heat or ‘ecological heat’. ‘Ecological heat’ in this case means the heat generated ecologically from clarified wastewater by means of a heat pump. The HUBER heat exchanger RoWin guarantees reliable operation with low maintenance requirements. This type of heat exchanger provides automatic removal of algae and other pollutants that may settle on the exchanger surfaces.
The greenhouse length and thus drying surface required depends on the requested drying efficiency. The required power depends on the dewatering results the ROTAMAT® Screw Press RoS 3Q is designed to achieve. Against this background, the greenhouse hall on WWTP Penzing was designed a couple of metres longer than actually necessary. The plant operators use the resulting increased dryer capacity to reduce the polymer consumption of the dewatering system and in this way save even more costs.
Automatic sludge feeding into the greenhouse eliminates the need for material transfer points and wheel loaders, and saves operators work and time. They benefit from an excellent sludge handling concept that minimises odours. Direct feeding and immediate turning of the sludge in the dryer instantly brings the sludge into a safe, virtually aerobic condition that ensures minimum odour development. No odour at all is noticed on WWTP Penzing Weil.
For efficient and odour-free drying, climate technology and mechanical equipment components must be selected and designed to perfectly suit each other. The HUBER climate control system incorporates experiences from operation, measurements in the greenhouse and theoretical thermodynamic principles. The greenhouse is equipped with climate sensors, ventilators and ventilation flaps. The electrical control system ensures an ideal drying climate is constantly maintained inside the greenhouse hall by monitoring the water absorption capacity of the air inside and outside the hall and by limiting condensate. The run time of the ventilators that directly aerate the sludge bed is controlled according to the work cycles of the sludge turning machine (and the freshly turned sludge bed zones) and the drying capacity of the air. The air is routed differently as required in the different sludge drying areas. Of course, the control system always gives priority to the safety of mechanical components and protection of the building against storm.
The mechanical equipment components for sludge turning and transport are at the heart of the solar dryer. The sludge turning assembly consists of a rotating double shovel that draws itself through the greenhouse along chains. While the shovel rotates and at the same time travels forwards, the sludge is aerated, turned around, granulated and transported. When the sludge turning/transporting assembly arrives at the end of the hall, each individual sludge granulate grain has been aerated. This cycle normally is performed every hour. In the feeding phase, turning of sludge in the wet zones in the front section and its aeration over the full greenhouse width takes place even every 20 minutes. As the complete sludge bed is restacked, the quasi-aerobic condition of the sludge is reliably maintained and optimal preconditions for drying are achieved.