Protocol to Measure Network Strength of Sludges and Its Implications for Dewatering

The ability to measure network strength is important in sludge conditioning and dewatering applications. Previous research has mostly focused on determining sludge floc strength indirectly through measuring other parameters such as floc size, floc density, and dewaterability of sludge. Few methods were developed for direct measurement of floc strength but these methods are not very practical for sludge applications. For the purpose of sludge characterization, it is more appropriate to determine the overall network strength of sludge rather than determining the strength of individual sludge flocs. This is because individual floc strengths will vary greatly, especially during conditioning, and therefore network strength is a more meaningful measurement compared to floc strength. This paper refers to raw or conditioned sludge as "network," and provides a protocol to measure "network strength" which can be used to evaluate the dewaterability of water and wastewater sludges. The proposed protocol uses rheology, a fundamental character of sludge, to measure network strength in terms of energy required to break up the structure of a certain volume of sludge. Two methods are presented to directly measure network strength using either torque or concentric cylinder rheometers. A mathematical derivation showed that the area under a shear stress-shear rate or a torque-time rheogram is representative of the energy dissipation within the sludge system, and the total dissipated energy is related to the network strength. This study does not intend to measure the absolute network strength, rather a comparative strength of different sludges using the same instrument and measurement conditions.