Infrastructure Engineering - Theses
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ItemDewatering of sewage sludge( 1996)Sewage sludges are biological sludges which exhibit wide variations in their physical, chemical and biological properties. The large and increasing amount of sludge produced has caused significant disposal problems. Apart from the steadily growing sludge volumes the cost per tonne of dumped waste also shows a continuous upward trend. Therefore, this research aimed at investigating the application of one of the most common dewatering technique which was vacuum filtration to dewater sewage sludge. The effects of impermeable membrane, vacuum level, conditioning, fibre, solids loading and cost of various dewatering techniques were investigated. The performance of the sludge was determined by capillary suction time (CST), torque (viscosity) and filtration rate. The CST and torque were determined using the CST apparatus and viscometer, respectively. The filtration was carried out using laboratory scale vacuum filters. The solids content of alum, South-Eastern and Mornington sludges were found to be 1.4%, 2.5% and 2%, respectively. The alum had the lowest fibre content (approximately none) while the Mornington had the highest fibre content (9.9%). Alum sludge had a much lower CST when compared to sewage sludges. Cationic polyelectrolytes reduced the CST of sewage sludges dramatically. The performance of most Zetag polyelectrolytes was good and consistent while CP408HMW had the poorest performance. For lllost polyelectrolytes, the pattern showed a rapid decrease in the first instance leading to a very small decrease (almost constant), followed by another rapid decrease leading to a constant again. In all cases, yield torque was found to increase with polyelectrolyte dose. Both the CST and yield torque decreased with stirring time to a minimum and then increased again with further stirring. The change in torque was found to change dramatically with the change in sludge volume. When the impermeable membrane was introduced, it was found that it was not feasible to carry out the experiment in the absence of polyelectrolyte. When polyelectrolyte was used, the filtration rates and final percentage solids (20% to 25%) were found to improve dramatically. The impermeable membrane itself did not have a profound effect on the filtration performance. Filtration rate was found to increase with increasing vacuum level. When different solids loadings were tested, it was found that the performance of the filtration improved with increasing solids loadings. Both the CST and filtration results showed that conditioned sludge performed better after aging. However, unconditioned sludge gave an entirely opposite result in which the sludge had poorer filterability with increasing age. Fibres were found not to have a profound effect on filtration rates. However, the fibres were found to be able to reduce or even eliminate cracking of the cake. From the economic analysis performed, it can be concluded that filter press had the highest total cost followed by centrifuge, sand drying beds and vacuum assisted drying beds. The cheapest alternative was the vacuum assisted drying bed. Vacuum assisted drying bed was the most feasible option if the plant life was short (<20 years).