DRYNESS LIMIT: A USEFUL PARAMETER TO ASSESS SLUDGE DEWATERING

International audienceIt is usually recognized that sludge management is a major problem in water and wastewater treatment, as it can account for up to 50% of total operating costs. The effectiveness and cost of sludge treatment and disposal operations are strongly affected by its volume. Hence, the water content (or solids concentration) of the sludge is a key parameter which can impact the different steps of the whole treatment. Consequently, thickening and dewatering are key steps in the total sludge processing train. Sludge is classically dewatered by centrifuge, filter-press, belt filter or screw filters. In every case, the process efficiency is strongly affected by sludge properties. That is why users and suppliers need a parameter to assess sludge dewaterability and a standard method to measure it. Suppliers might use this standard data to guarantee dewatering performances of their equipment. The dryness limit is a well-known parameter used by French actors of sludge treatment. It is classically measured by a filtration-compression test until filtrate flowing stops. However, for highly compressible sludges, this condition is difficult to reach. That is why a simple mathematical model fitting compression data was used to extrapolate data for dryness limit assessment. The objective of the research is to study the role of every operating condition listed in FD-T 97001-1 relative to sludge dryness in order to recommend values for analysts. The influence of test time, pressure, cake thickness, filter cloth, flocculation conditions, pre-thickening and storage time on the dryness limit was strongly discussed for different kind of sludge.The results enable to establish a proper procedure to measure the dryness limit. This limit is always higher than the dryness reached with industrial devices. The deviation depends mainly on the sludge nature and the type of dewatering equipment used