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A further reduction of the sludge amount is almost always necessary after the thickening. The liquid sludge has to be dewatered (removal of water from the sludge) and has to conform to a dry and granular form. Dewatering can be done naturally (dry beds, solar drying), however this is only possible during a long period of time. Faster and smaller, but also more cost intensive, are machine processes such as pressing (filter press and belt filter), and centrifugation (centrifuge). For a good dewatering, size and firmness of the sludge agglomerates are important, so that these remain porous during the compression.
Flocculants are often used to achieve as high as possible dry matter in the machine dewatering and must be specifically selected for the sludge.
Lime is commonly used on press filter, belt filter and centrifuge dewatering. LHOIST has recently developed a new approach to improve dewatering efficiency, which is adding a mix of lime + flocculant to the sludge. This new approach involves Neutralac® reagents. Together, Neutralac® reagents and the polymer improve the sludge dry matter increase, and produce a limed sludge with a good structure and better storage properties.