Frequently asked questions

The WendeWolf® process is designed for medium-sized wastewater treatment plants. The projects we have realized range from 5,000 to 500,000 population equivalents with a drying area of 500 to 12,500m². Sludge volumes per year at these plants are approximately 400 – 32,850 tons (mechanically dewatered). The system is modular, the size of a single hall is 12 x 120 meters, i.e. 1’440m².

Additional halls may be erected if required by the quantities of sludge produced.

‍We turn and transport the sludge through the drying hall with a rotating pusher-turner. The mechanically dewatered sludge can be regularly delivered to one face and collected from the opposite face when the degree of dryness is reached. The hall also serves as a storage area for the dry material and does not need to be entered by personnel or accessed by a wheel loader.
We need machine dewatered sludge. The easiest to treat are sludges with DS content higher than 20%, but we also have plants in which we successfully dry sludges with DS content around 15%. With the WendeWolf® process, slurries can also be processed in the so-called glue phase. Patented blades and knives have been developed for this purpose.
‍We have to accept the odor emissions of the delivered sludge as a given. Because we turn and aerate the entire surface, this odor dissipates very quickly, and the aerobic bacteria make the sludge smell similar to earthy compost. As long as the above-mentioned boundary conditions are adhered to by the wastewater treatment plant operator, we guarantee that no additional odor pollution will occur during the drying period. This also applies to a storage period of several months.
Since only a limited amount of water can evaporate per m² of floor area, the residence time is a function of the fill height and the weather. In summer, for example, drying can be completed in a week at 10 cm bulk height. In winter, only partial drying is achieved. With the help of a detailed simulation program and the local weather data, we calculate the monthly water discharge and the drying rates to be achieved then.
‍Theachievable degree of dryness is exclusively a function of time. We achieve drying degrees around 90% with sufficiently long residence time in summer. Water is also evaporated in the drying halls in winter, but the specific water withdrawal per m² of floor area is understandably smaller then than in summer.

The evaporation rate decreases as soon as a degree of dryness of about 65% DS is reached.  If invoicing is based on weight only, drying above about 65% DS is usually no longer worthwhile. Example: Feeding the filter cake into the dryer with 25% DS. Each ton contains 750 kg of water and 250 kg of solids. If dried to 65%, the remaining quantity is 250/0.65 = 384 kg, of which 250 kg is solids and 134 kg is water. Of the original water content of 750 kg, 616 kg or 82% have therefore already evaporated. If it is desired to dry to 85%, the remaining quantity is still 250/0.85 = 294 kg, of which 44 kg is water.

Larger wastewater treatment plants, such as ARA Glarnerland, Veszprém and Rzeszów, therefore operate their drying plant in uniform continuous operation, but have concluded a purchase agreement with the disposal company that allows different degrees of drying.
‍With the necessary access and exit routes, approx. 1.5 m² per ton of delivered sludge should be provided. We dimension on the one hand according to the delivered DS content and the desired degree of dryness and on the other hand according to the stack volume between the possible application times.
We have three different auxiliary heating systems in use in our plants: Bottom heating in the WWTP Glarnerland, KA Murnau, KA Ilawa and Myszkow, air heating in the WWTPs Sargans, Veszprém, Rzeszów, Miltenberg, Weil am Rhein and Riedlingen, infrared radiators (dark radiators) in the WWTP Vils. The heat sources are the waste heat from the gas engines and a heat recovery from a heat pump system. The infrared heaters use excess gas from the digestion tower, which would otherwise have to be flared.
‍The investment costs are divided into civil engineering work, hall construction and machine technology in roughly equal parts. Per annual ton of filter cake, investment costs vary between 400 and 500 €/t.
Operating costs are extremely low thanks to the extremely low electrical power consumption, about 10-30 kWh per ton of water extracted. The simple operation and automatic process flow require a low level of personnel. We calculate 0.5 h per hall and working day, this includes the usual maintenance work.
The financing costs, which can vary depending on the subsidies, interest rates of bank loans.
If one bases the cost calculation on the VDI guideline 2067, then one receives for a pure solar dryer with 1,200 m² surface area, corresponding to about  per ton of water extraction approx. 15, – € for the operating costs and approx.  50, – € for the annuities. If the disposal costs are above 65,- €/t a solar sludge drying is worthwhile.
These costs can be reduced by possible state subsidies and/or the utilization of waste heat.
‍Approval is granted under water law as a process for sludge dewatering and is also included in any subsidy (e.g. in Baden-Württemberg).