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Utilities with biological phosphorus removal in combination with anaerobic digestion have been reporting a deterioration in dewatering properties. The purpose of this research was to evaluate if the deterioration in dewaterability could be described by the divalent cation bridging theory (DCBT) and the deleterious impact of phosphorus and potassium release in the digester. A survey of laboratory and full-scale digestion systems receiving waste activated sludge with and without bio-P removal was performed. The samples were dewatered in the laboratory using the same protocol and the soluble cation concentrations were measured along with the soluble phosphorus. The results showed that the cake solids obtained after dewatering was directly related to the monovalent to divalent (M/D) cation ratio, as would be expected from the DCBT. In particular, as the M/D ratio increased, cake solids decreased. The phosphorus concentration was directly correlated to the M/D ratio, and suggested the phosphate was influencing the cation chemistry -as phosphorus concentration increased, the dissolved Ca 2+ and Mg 2+ concentrations decreased -likely through complexation and precipitation reactions. Addition of the WASSTRIP process to release the phosphorus prior to digestion resulted in an improvement in the dewatering properties. The results fit well overall with the DCBT, and show that bio-P removal systems may experience a deterioration in dewaterability after anaerobic digestion.

A research study was performed to evaluate the rapid rise/volume expansion concept proposed by Chapman and Krugel (2011). The concept relates the interplay of mixing, gas production and rheology of the solids and how they will impact volume expansion due to gas holdup. Gas holdup is defined as the production of gas bubbles that do not rise out of solution and instead contribute to the expansion of the solids volume. This research demonstrated that loss of mixing can increase the digester volume by 20% or more, and this was accompanied by a decrease in gas production rates measured by respirometer. The decrease in gas production was a good prediction of the subsequent gas holdup expansion. Volume expansion also occurred when mixing was decreased and not just halted completely. The results suggested a critical mixing or shear rate exists, below which the viscosity of the suspension increases rapidly, resulting in gas holdup and volume expansion. Additional research is needed to further evaluate the many variables that will impact volume expansion due to gas holdup and foaming.

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