WendeWolf - Solar Drying of Sewage Sludge

In southern areas open air drying beds or sludge lagoons are widely used but have, over the years, shown some well known serious problems such as:

• Bad odours    
• Polluting the ground water table    
• General hygiene for the people handling the sludge    
• Breeding ground for insects and germs    
• Difficulties in emptying or refurbishing when filled up    
• Difficult to use it as a fertilizer in agriculture

A mechanical dewatering system is additionally required before using the WendeWolf®, but these problems mentioned above are solved.

The solar radiation warms the sludge`s surface. The rise in the temperature forces the water molecules out into the surrounding air. The moist air transports the water and has to be evacuated. However, while the surface dries, the lower parts remain moist, and have to be turned.
This is achieved by WendeWolf®, a turning and conveying machine mounted on walls, yielding a dry granulate of 10 mm as an average size. As the radial velocity is higher than the advancing speed, each time the drum turned the sludge is automatically moved from one end of the drying bed to the other.
The drying bed is fed and emptied with appropriate equipment such as conveyor belts or shovel loaders but the transport through the bed is entirely automatic!

The system consists of:

• A waterproof horizontal surface, either concrete or asphalt, similar to a road    
• Two parallel walls 12.00 m apart, 0.85 m high and up to 120 m in length, equal to a drying bed of 1440 m²    
• Electrical power supply 400 V, 12 kW    
• Road access at both ends    
• The WendeWolf® machine, mounted ready for use, is transported in a container to the site. Weighing 4000 kg, with a gauge 11.40 m, it is fully equipped with sensors and control units. The central part is a drum on which different combs and paddles are fixed which cut the sludge`s surface and aerate the lower parts    
• Operating panel with a touch screen    
• A greenhouse with ventilators (2-3 W/m² drying area) at sites with regular rainfall

The WendeWolf® moves and aerates up to 250 m³/h of sludge! The automatic programme ensures a continuous flow of the freshly pressed sludge through the drying bed.The required energy consists of electricity only, between 10 kWh to 30 kWh per ton of water evaporated. Industrial dryers require between 800 and 1000 kWh in thermal energy and electricity. Therefore the running cost of WendeWolf® is extremely low!However the system requires space to be built. As WendeWolf® is successfully used in the European climate, with an annual evaporation of 800 - 900 kg/m²year, we anticipate for example the double in the Middle East.

In Europe the reason for drying the sludge is the high cost of the final treatment, so each ton of water extracted from the sludge lowers the annual cost for the water plants.Solar drying requires space and initial funding, but the running costs are extremely low.WendeWolf® yields an end product which has the same high quality as industrially dried material but at a lower running cost.

WendeWolf® installed in 6 countries in Europe and in Australia.

Physical Background

Any drying process is based on the fact that water molecules change from liquid into gas, called vapour. This requires energy. Our option is freely available solar energy.The driving force of drying is the difference between the partial vapour pressure inside the sludge and the ambient air. In order to avoid an equilibrium between the vapour pressure inside and outside the sludge, the air has to be evacuated. This is naturally helped by the fact that water vapour is lighter than dry air.The warmer the air is the more water vapour can be transported. However the partial vapour pressure in the air rises with the amount of water dissolved in the air. Except for very damp days, the drying process still occurs even in low sunshine. This is the same process that allows a wet road to dry at night.

Open Air Drying Beds - Natural Ventilation

They are naturally ventilated by the wind. The higher the wind speed blowing over the sludge surface, the faster the drying speed. However at sites with regular rainfall, an open air drying bed is not as satisfactory.For this reason in Middle Europe open air drying beds are no longer in use. Solar drying is done in greenhouses which require controlled ventilation systems

Greenhouse Drying - Controlled Ventilation

An efficient ventilation system has been developed and monitored in the different plants and sites where WendeWolf® turning machines are installed.Our ventilation system has three components:

Roof Ventilation Flaps    

Motor driven roof ventilation flaps are installed over the entire length of the drying hall. These are similar to those used in many greenhouses for ventilation.   

Axial Fans

Fans are placed inside the hall in such a way that air turbulence is created above the entire surface of the drying bed, destroying the moist boundary layer above the sludge surface. This artificial wind is important for the drying process as it avoids any stratification of temperature or humidity.   

Aperture Slits   

An aperture slit is positioned between the greenhouse and the walls on which the WendeWolf® is travelling. Whenever the roof flaps are opened, a natural draft occurs and fresh, dry air enters the greenhouse

Control Strategy - Solar Energy Only

The temperature and the relative humidity is monitored outside and inside each hall. The PLC (Process Logical Control) calculates the temperature difference and the absolute water content inside and outside for each hall separately. The respective thresholds can be altered.

• Axial fans are switched on when the temperature inside is five degrees higher than outside    
• Roof flaps are opened when the calculated value of the absolute humidity inside exceeds a threshold of 1.5 g/kg Air

Control Strategy with Auxiliary Heating

When auxiliary heat is available, a heating system may be installed in the hall. Different systems are possible and have been used in different sites:

• Floor Heating    
• Hot Air Blowers    
• Infrared Radiators

The axial fans are controlled by the temperature difference inside/outside blowing the warm air downwards and breaking up the moist layer of air at the sludge surface. The hall must have doors and the roof flaps are able to be closed to limit thermal energy loss. The flaps are only opened when the absolute water content of the air inside has reached a certain ratio to the outside ambient air. As warm and moist air rises, the exchange is rapidly achieved and the flaps close again.