The effect of rheological properties on sludge dewatering in belt filter press

Abstract

Polymers used as flocculants in the secondary sludge dewatering process are one of the most expensive inputs in these plants. The disadvantage is that these polymers cannot be recycled. Currently, controlling of polymer dosing rate is done by trial and error method. It has been shown that huge savings can be made by optimising the polymer dosing using rheological properties. It is not an easy task to optimise this process because of changing sludge characteristics on a daily, seasonal and annual basis. To try and optimise polymer dosing and polymer concentration, the variation in rheological properties needs to be understood first. The correlation between the process parameters and the rheological properties needs to be determined. There is currently no database of rheological properties of secondary wastewater sludge feeding belt filter presses available. To address these issues, a 12 week assessment of the rheological properties of the sludge feed to the belt filter press before and after conditioning in four wastewater treatment plants in Cape Town was conducted. The rheological properties were determined using an MCR-51 rheometer with parallel plate geometry under controlled temperature. After concluding the assessment, a 3-level Box-Behnken factorial trial was conducted at Plant K wastewater treatment plant to statistically analyse the correlation and/or interactions between the process parameters (sludge feed flow rate, polymer dosing concentration, polymer dosing rate and belt press speed) and the rheological properties of the sludge to optimise the plant performance.