Secondary sedimentation is one of the most commonly used unit operations in wastewater treatment plants. It is customarily designed to achieve solids separation from biologically treated effluent through clarification of biological solids and thickening of sludge. As treatment plants receive increasingly high wastewater flow, conventional sedimentation tanks suffer from overloading problems which result in poor performance. Inclined plate settlers, a form of tube settlers, may have good potential in upgrading sedimentation tanks. This study was conducted to examine the possibility of applying inclined plate settlers in secondary sedimentation in order to upgrade conventional rectangular sedimentation tanks and improve their performance. Experiments were carried out at Al-Awir sewage treatment plant in Dubai using a pilot-scale inclined plate settler which received a mixed liquor stream from the high-rate activated sludge aeration tank. The results showed that inclined plate settlers perform slightly better than conventional type settlers during normal operation of plants, but during peak flows the inclined plate settlers showed much better performance than conventional settlers. The inclined plate settlers are less affected by overloading in comparison to conventional settlers. The solids removal efficiency increased as the hydraulic residence time was increased or as the surface loading rate was decreased. Application of these plates will not cause any interruption of daily operation of treatment plants and could be achieved at minimal cost when compared with other methods such as addition of chemicals, application of deep tanks, … etc. The study revealed that SS is a better parameter than TS, BOD, COD to evaluate the performance of sedimentation tanks. A statistical model was formulated to describe tank performance and design parameters were obtained based on the experimental results.

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