Rheological characteristics of a mixture of primary and secondary municipal sludge

Municipal wastewater sludges are complex fluids displaying non-Newtonian characteristics. Whilst rheological properties of sludges are very important for the design and operation of wastewater treatment facilities, there is a lack of information regarding these properties, particularly for mixtures of primary and secondary sludges. The rheology of wastewater treatment sludge has been widely studied but most of the literature has concentrated on secondary or digested sludges alone. Therefore, the current study aimed to investigate the rheology of mixed primary and secondary sludge. A mixed sludge containing 40 % primary and 60 % secondary sludges, were collected from Rotorua wastewater treatment plant, Rotorua, New Zealand. The sludge was diluted to total solid contents of 4.3, 7.3 and 9.8 %, and the rheological properties were measured at different temperature. Results showed that the temperature and solid concentration are critical parameters affecting the mixed sludge rheology. It was found that the yield stress increases with an increase in the sludge solid content and decreases with increasing temperature.