The sludge can be placed directly after dewatering without intermediate storage.


Full-surface turning enables optimum evaporation and consistently avoids the development of anaerobic zones.


The sludge is automatically transported through the drying hall.


The dried granules can be accumulated in the drying hall up to a height of 80cm and thus temporarily stored until removal.


At the end of the hall, the dried sewage sludge is discharged e.g. into a storage bunker or a conveyor.

Contact us for more information or a price quote!



The WendeWolf, a turning and conveying machine, is the heart of the solar dryer. The machine essentially consists of a moving steel bridge, a height-adjustable turning drum and controls for the machines and the hall climate.

The drum is driven and rotated by frequency-controlled drive motors. The drive motors allow a rapid traverse speed of 8 m/min and a working speed of between 0.8 and 2.0 m/min. The turning drum rotates at a speed of approx. 2.5 m/s during turning. The operating speed is reduced under heavy load.

The serrated metal combs of the drum turn the sludge to be dried over the entire width of the bed at an adjustable depth and transport the sludge from the feed opening through the hall to the clearing opening. The sludge can be turned and moved in both directions, there is no preferred direction. The drum movement also makes it possible to remove larger heaps that occur when tipping the filter cake from skips or with wheel loaders.

Furthermore, the turning machine distributes the sludge in the hall and the dried granulate can be piled up to a height of 80 cm at the end of the hall. The drying area itself does not need to be entered or driven on.
The height of the drum is adjusted using a spindle drive.
The roller stroke is over 750 mm.

Example: WendeWolf SW-12 Values
Diameter of the turning drum 1,0 m
Length 4,3 m
Total width 11,7 m
Weight 4100 kg
Clearance height 2,6 m
Lane distance 11,3 m
Protection class of the motors IP66
Total connected load 15 kW
Tension 400 V
Frequency 50 Hz
Drum speed 30-60 rpm
Fast gear 8.0 m/min
Operation 0.8-2.0 m/min
Bild1 - Wendewolf


Electrical engineering

The connected load of the turning machine is approx. 14 kW, the connected loads for the circulating air fans (whirlers) are approx. 0.6 – 0.7 kW per 100 m² drying area.
A separate Internet connection must be provided on site for remote monitoring of the system and for transferring software updates.

Drying hall (greenhouse)

These are commercially available greenhouse constructions that are slightly adapted to the requirements of a drying hall.
The basic construction is sufficiently protected against corrosion with hot-dip galvanized steel.
Various materials can be used for the roof and wall cladding, whereby the materials used should have a transparency of 85% or more.

The following materials can be used for the covering:

Greenhouse films
Polyethylene films (UV-5) as single films, inflatable double films or air bubble films commonly used in greenhouse construction
ETFE (f-clean) as single film or inflatable double film

Roof panels
Polycarbonate or PVC sheets in single or double bar design.

Glass – single or double glazing
The choice of roofing and the dimensioning of the supporting structure, the roofing and the side walls must take into account the local conditions (wind load / snow load).
It has been shown that although glass has excellent transparency, the supporting structure must be more complex than with plastic roofing.

Weight comparison covering Values
4 mm glass approx. 10.0 kg/m²
Double-skin sheets approx. 1.5 kg/m²
inflatable double foil approx. 0.7 kg/m²

Furthermore, good thermal insulation of the roofing is an advantage; poorly insulated roofs fog up more easily. This leads to a lower radiation input and thus to less evaporation.

The WendeWolf® requires a free clearance height of approx. 2.2 meters above the ground in the area of the roadway, the side walls must be designed accordingly high.
A height of approx. 2.5 meters should be selected as a clear passage through the hall.


Civil engineering and side walls

An asphalted surface as used for road construction is suitable for the hall floor; a reinforced concrete slab can be dispensed with. The side boundary walls, which also serve as a roadway for the WendeWolf®, can be constructed using precast concrete elements or poured on site using traditional concrete construction methods.
The plinth height incl. The running rail should be approx. 0.85 m, the support surface for the machine must be 0.20 meters. The clear width between the boundary walls is 11.3 m, but the machine technology can also be adapted to other widths.
The local requirements and regulations must be observed.

Frequently asked questions

The WendeWolf® process is designed for medium-sized wastewater treatment plants. The projects we have realized are between 5,000 and 500,000 population equivalents with a drying area of 500 to 12,500m². The sludge quantities per year in these plants are approx. 400 – 32,850 tons (mechanically dewatered). The system has a modular structure, the size of a single hall is 12 x 120 meters, i.e. 1,440 m².

Additional halls can be erected if this becomes necessary due to 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 end and collected from the opposite end when it has reached the required degree of dryness. The hall also serves as a storage area for the dry goods and does not need to be entered by staff or accessed with a wheel loader.
We need mechanically dewatered sludge. Sludge with a dry matter content of more than 20% is the easiest to treat, but we also have plants in which we successfully dry sludge with a dry matter content of around 15%. The WendeWolf® process can also be used to process sludge in the so-called glue phase. Patented blades and knives have been developed for this purpose.
We have to accept the odor emissions from the delivered sludge as a given. As we turn and aerate the entire surface, this odor disappears very quickly and the aerobic bacteria make the sludge smell similar to earthy compost. As long as the above-mentioned conditions are met by the wastewater treatment plant operator, we guarantee that no additional odor nuisance will occur during the drying period. This also applies to a storage period of several months.
As only a limited amount of water can evaporate per m² of surface area, the retention time is a function of the fill level and the weather. In summer, for example, drying can be completed in a week with a fill height of 10 cm. Only partial drying is achieved in winter. Using a detailed simulation program and local weather data, we calculate the monthly water discharge and the degree of drying to be achieved.
‍Theachievable degree of drying is solely a function of time. We achieve drying rates of around 90% in the summer, provided the residence time is long enough. Water also evaporates in the drying halls in winter, but the specific water removal per m² of floor space is understandably lower than in summer.

The evaporation rate decreases as soon as a degree of dryness of around 65% dry matter is reached.  If you only charge by weight, drying above around 65% dry matter is generally no longer worthwhile. Example: Feeding the filter cake into the dryer with 25% dry matter. 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. This means that 616 kg or 82% of the original water content of 750 kg has already evaporated. If it is desired to dry to 85%, the remaining quantity is 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 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. On the one hand, we dimension according to the delivered dry matter content and the desired degree of drying and, on the other hand, according to the stack volume between the possible discharge times.
We have three different auxiliary heating systems in use in our plants: Underfloor heating in the Glarnerland, Murnau, Ilawa and Myszkow wastewater treatment plants, air heating in the Sargans, Veszprém, Rzeszów, Miltenberg, Weil am Rhein and Riedlingen wastewater treatment plants, infrared radiators (dark radiators) in the Vils wastewater treatment plant. The waste heat from the gas engines and additional heat from a heat pump system are used as heat sources. The infrared heaters use excess gas from the digestion tower, which would otherwise have to be flared off.

The investment costs are divided roughly equally between civil engineering work, hall construction and machine technology. The investment costs per annual tonne of filter cake vary between 400 and 500 €/t.
The operating costs are extremely low thanks to the extremely low electrical power consumption, around 10-30 kWh per ton of water extracted. The simple operation and automatic process sequence require little personnel input. We calculate 0.5 hours per hall and working day, including the usual maintenance work.
The financing costs, which can vary depending on the subsidies and interest rates of bank loans, have to be taken into account.
If the cost calculation is based on the VDI guideline 2067, the operating costs for a pure solar dryer with a floor area of 1,200 m², corresponding to approx.  per ton of water removal, are approx. 15 € for the operating costs and approx.  50 € for the annuities. If the disposal costs are above €65/t, solar sludge drying is worthwhile.
These costs can be reduced through possible state subsidies and/or the utilization of waste heat.
Approval is granted under water law as a sludge dewatering process and is also included in any subsidies (e.g. in Baden-Württemberg).