In biological treatment steps, the organic load in the wastewater is used by activated sludge as a food source. This purifies the wastewater and produces sludge. To keep the amount of sludge in the bioreactors constant, sludge must be removed regularly. The removed sludge consists of approximately 99% water. To minimise the volume and weight of this waste stream, dewatering takes place before disposal. The water removed during dewatering can be reintroduced into the plant. To optimise dewatering, flocculant is added to the wet sludge. Depending on the process, intermediate storage in a sludge tank is necessary. To prevent digestion processes and the associated unpleasant odours, a submersible aerator is usually installed in the sludge tank. The most common sludge dewatering processes include:
Belt filter press
In this method, the sludge to be dewatered is passed between two sieve belts through several rollers and pressed. The advantages of belt filtration include the fact that no sludge buffer is required due to continuous operation. It is also possible to dewater large quantities. In addition, a dry content of 20-35% can be achieved in the filter cake.
In a screw press, the sludge is compacted and dewatered step by step by a rotating screw with changing diameter or spacing of the screw helix. Due to its compact design, the screw press is particularly advantageous where space is limited. An achieved dry content of 18-28% in the filter cake is sufficient for the majority of applications. However, other technologies are more efficient.
In this method, the sludge water is fed under high pressure into a closed system of several filter plates. Dry contents of 20-25% can be achieved with this method. In addition, the flocculant requirement is lower compared to other technologies. A disadvantage of chamber filtration is the discontinuous operation, which makes a sludge storage tank necessary.