Rotary Vacuum Filter in Wastewater Application

The Rotary Vacuum Filter is made up of a large drum with tiny holes rotating in a vat containing the final treated wastewater. It is often used to dewater and extract out the waste sludge so that a clear discharge can be obtained. What is different between this compared to other unit of operations is that vacuum is continuously applied to the system pushing the liquid through a filter media so that clearer separation can be accomplished.

The sequence of the rotary vacuum filter first involves the cake formation whereby the cake layer will act as a solid medium to filter out subsequent solids. As this happen, vacuum present in the system will suck out the water from the solid cake and then diverted it to a receiver. As filter cake starts to buildup, a mechanism that usually involves a scrapper is needed to remove it and then this follows with the media washing step. Normally this can be achieved using a spray jet combined with acid wash for better cleaning results.

There are different types of vacuum filter mainly the 1) drum scraper blade type 2) belt filter and 3) coil type filter. The drum scraper blade vacuum operated filter is the most commonly used system which involves direct scrapping off the cake formation on the surface of the mesh or synthetic filter surface. Sometimes there is also an air-driven pressure agitator that is mounted inside of the drum to help to create vibration and loosen the sludge cake. Depending on the amount of solids present in the wastewater load, a system consisting of a stainless steel mesh should be good enough for general application.

Rotary Vacuum Filter Belt Type

Diagram showing a typical buildup of a rotary vacuum filter (belt type)

Another type of rotary vacuum filter is called the belt filter. The unit consists of a large roll of turning belt lined on the drum surface which acts as a filter medium. As the belt turns and dragged along a sharp bent, the sludge cake on the surface will start to loosen and then dropped off collected in a receiver. Main advantage using the belt type is that the cake removal is far more complete with washing accomplished on both sides of the belt filter compared to only one using the scraper blade drum. The coil type involves a more complex mechanism as there is a stainless steel coil spring attached to the drum that acts as a filter media. In comparison, the unit of operation for the coil type is closely similar to the belt filter, however, the only disadvantages is that high maintenance requirement is needed to constantly monitor the spring performance.

Related topic on wastewater management: Rotating Biological Contactors

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