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This study aims to develop a laboratory protocol necessary to measure the network strength of water and wastewater residuals using torque rheometry. Previous research has shown that the area under a torque-time rheogram (termed totalized torque) represents the total energy dissipation rate in a suspension, and can be used to estimate the network strength of wastewater residuals (Örmeci and Abu-Orf, 2004). In this study, several aspects of rheogram generation were investigated in order to develop a protocol for network strength measurement. These include reproducibility of the generated rheograms and the impact of impeller speed on measured torque and totalized torque values. Several residuals from different treatment facilities including alum, waste activated, aerobically and anaerobically digested residuals were used in developing and testing the protocol. The reproducibility of all rheological measurements was good which was attributed to the large sample volumes required for measurements with the torque rheometer and using the whole sample volume after conditioning. The optimum impeller speed was concluded to be the lowest impeller speed that produced a significant signal output. Higher impeller speeds decreased the sensitivity of the impeller, and decreased the accuracy and precision of the measurements due to the increased turbulence in the beaker. Among the residuals tested, activated sludge seemed to have the highest network strength. As expected, increasing the polymer dose increased the network strength of samples. In addition, the mixing conditions (G-t) used during the conditioning of samples were found to influence the final strength of residuals. Preliminary results from conditioning experiments indicated that a decrease or “dip” was observed in the rheograms around the optimum polymer dose. The dip was also observed in totalized torque and network strength values, which are proportional to the measured torque signal. This relationship can potentially provide a useful tool for the optimization and automation of sludge dewatering.
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Document Type: Research Article

Publication date: 2004-01-01

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