IST-Anlagenbau GmbH

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Frequently Asked Questions
Question: For what size of water works is the WendeWolf® especially suitable?
The dryers in use in 2010, are installed in plants serving between 5,000 and 500,000 inhabitants equivalent, having an area from 500 m² to 7,200 m². The annual volume treated varies in these plants between 400 and 25,000 t/year. The system is modular. The largest drying hall for one machine is 12 x 120 m or 1,440 m². If the dimensioning requires a larger surface, a second hall can be built beside it.
There is no technical reason to limit the number of drying halls.

Question: Which are the particularities of the WendeWolf®-Dryer?
The sludge is dumped at one end. Then the machine turns and transports the sludge with a paddle drum through the drying hall. Each time the WendeWolf® turns and aerates the sludge, it moves the particles behind the slowly advancing drum. Once the machine is at the entrance, the drum is lifted and the machine runs back to the starting point.  The main drying hall is neither entered by the staff nor by a shovel loader. In summer the turning cycle is higher, up to 10 times per day, than in winter when the sludge is moved only to get additional area for the newly arriving material. Due to frequent turning the sludge is transformed  into a granulate of typically 1 - 20 mm diameter, easy to handle and free of any weeds.

Question: What kind of sludges can be treated?
The sludge must not be liquid. WendeWolf® will treat sludge with a dry residue from 15% or higher. This requires in general a pre-treatment with a press or decanter. We are able to process sludge even if it turns into a paste. For this combs and knives have been developed which are protected by a patent.

Question: What about bad smells?
The initial smell of the plant’s sludge disappears rapidly. The sludge is turned and aerated on the entire width of the floor. The aerobic bacterias change the smell into one similar to compost. As long as we get a non sticking sludge, we guarantee that no additional bad smells are forming during the drying process. If the sludge turns into a paste and becomes anaerobic, bad smells may develop. In such a case it is advisable to recycle parts of the dry granulate from the previous drying period. In the plants operating in Europe, the oldest since 1994, we had no problems with bad odours from the WendeWolf®.

Question: How long does the drying take?
The specific evaporation per m² ground area varies with the meteorological conditions. The speed of evaporation is dependant on the amount of solar radiation reaching the sludge surface and the dryness of the ambient air. The drying time is a function of the thickness of the layer, e.g. the water per square meter to be evaporated, and the climatic conditions. In summer the required time to dry a 10 cm thick layer to a dry residue of 90%, may be less than one week. During the winter it may take several months.

Question: How dry does the end product become?
The final % of dryness achieved, is a product of time. In summer we achieve regularely about 90%.  Even in winter water evaporates, however the specific water output per ground area in winter is much smaller than in summer. The rate of evaporation slows when the dried residue reaches 70%.

Question: How much area is required?
The active drying area is a function of the initial humidity of the cake, the desired final dry residue and the stacking volume between the possible emptying periods. A modelling programme, using the local meteorological data, establishes the dryer’s surface. Including the necessary area for loading and unloading, the installation in Middle Europe required about 1.5 m² per ton of annual sludge output. In warmer climates the required area drops below 1 m² per ton.

Question: Can one speed up the drying process?
If the sludge is treated in digesting towers, the resulting gas is normally used to drive a stationary motor to produce electricity. The motor’s thermal  energy can be used to reduce the drying time.
Different systems are used in different sites. All are available on the marked and are commonly used to heat large buildings such as conference halls, sporting facilities or airports:

-  Floor Heating, running with a low temperature of  30-35 °C, using auxiliary heat from gas motors. This requires a concrete floor in which plastic tubes are embedded.
(Implemented in: Bilten ARA Glarnerland, Switzerland; Murnau, Germany; Ilawa and Myszkow, Poland)

-  Hot Air Blowers, operating at 60/40 °C.
(Implemented in: Miltenberg + Weil am Rhein + Riedlingen, Germany; Sargans, Switzerland; Veszprém, Hungaria; Rzeszów, Poland)

-  Infrared Radiators using surplus gas from the digesters whenever available.
(Implemented in: Vils, Austria)