The effect of divalent cation complexation on anaerobically digested enhanced biological phosphorus removal sludge dewatering performance

The effect of mono- and divalent cation concentrations on digested sludge dewatering characteristics was investigated in a full-scale study at the Harrisonburg-Rockingham Regional Sewer Authority, VA (HRRSA) and at an additional POTW (POTW B). At HRRSA, anaerobic digesters were fed with primary sludge, enhanced biological phosphorus removal (EBPR) WAS, and trucked-in DAF solids from a poultry processing facility, and digested sludge was dewatered by belt filter press. The POTW B digesters received EBPR WAS and primary sludge, and dewatering was by centrifuge. Visual MINTEQ 3.1 (Gustafsson, 2014, Visual MINTEQ 3.1. User's manual) was used to determine cation speciation and struvite, tricalcium phosphate, vivianite, and ferrous sulfide formation at a range of digestion conditions. Significant Mg and Ca complexation was predicted at all conditions evaluated, and the majority of the complexes were monovalent cations. Complexation reduced the divalent cation concentration on the order of 40%. The HRRSA and POTW B data both showed that dewatered cake total solids (TS) increased linearly with uncomplexed divalent cation concentration and that there was an increase of 1.0%-1.1% TS for every additional 1.0 meq/L of uncomplexed divalent cations. Because POTW B used centrifuge dewatering, this relationship could be independent of dewatering technique. Further, the relationship between uncomplexed divalent cation concentration and dewaterability suggests that the selectivity of exocellular polymeric substances cation exchange sites is much greater for divalent cations so that monovalent cations only bind to them when their concentrations are much higher than those of the uncomplexed divalent cations. Thus, monovalent cation concentration may not be important in anaerobic digestion or other high alkalinity environments. PRACTITIONER POINTS: It is important to account for complexation in high alkalinity environments such as anaerobic digesters; The findings of this study suggest that dewatering characteristics of anaerobically digested enhanced biological phosphorus removal (EBPR) waste sludge can be significantly improved by preserving cation-mediated exocellular polymeric substances bonding in the solids stream by reducing struvite formation; Data from two full-scale POTW's indicated that dewatered cake TS was increased by 1.0%-1.1% for every 1.0 meq/L of dissolved uncomplexed divalent cations in the anaerobic digestate bulk solution; This relationship existed for dewatering both by a belt filter press and by a centrifuge suggesting that this relationship is independent of dewatering technology; Dissolved monovalent cation concentrations may not be important in anaerobic digestion or other high alkalinity environments unless their concentrations are much greater than those of the dissolved divalent cations.