Water treatment plants produce enormous amounts of sludge. Due to increased environmental concern, recently, there is considerable amount of pressure on the water authorities for the safe disposal of the sludge. Currently, the most acceptable form of disposal is sanitary landfilling, which requires sludge to be dewatered to 20-40% of solids. One of the most commonly used dewatering techniques is sand drying beds. This process is popular because of its reliability, ease of use and low cost. However, one of the basic concerns with this process is the requirement of a large area of land. This could be overcome by chemical conditioning of the sludge, which can accelerate the dewatering rate substantially. At this stage, however, there are no guidelines for the design of the sand drying beds for treating chemically conditioned sludges.
This study is conducted to investigate the design and performance of sand drying beds for treating chemically conditioned water treatment plant (WTP) sludge. Experiments are conducted using laboratory scale sand drainage columns. The results indicate that the drainage rates, after adding polymer, increased by 9 times for clarifier sludge and 20-25 times for filter backwash sludge. Thus, the land area requirement for sand drainage bed can be drastically reduced through polymer addition. Optimal dose of polymer is found to be between 3 and 6 kg/t of dry solids, with dilute sludges requiring high dosages. The cake solids concentration increased from 3 to 12% as the polymer dose increased from 0 to 16 kg/t of dry solids. The variation of drainage rates and cake solids with solids/hydraulic loading rates are found to vary depending on the type of sludges and polymers. The filtrate quality is found to be independent of polymer dose and solids loading rates.