Effect of Na+ and Ca2+ on the aggregation properties of sieved anaerobic granular sludge

Interactions between fine particles (0¿50 ¿m) of anaerobic granular sludge were investigated by studying the effect of Na+ or Ca2+ supply on the evolution of the zeta potential and viscosity of granular sludge suspensions. In the Na+ or Ca2+ concentration range investigated, the Na+ addition affected only slightly the evolution of the viscosity of fine particles of an anaerobic granular suspension. In contrast, Ca2+ addition strongly modified the evolution of the viscosity of the studied anaerobic granular sludge suspension. The use of rheology combined with zeta potential measurements clearly described the physico-chemical interactions between small anaerobic granules and especially the importance of aggregation phenomena that occur between fine anaerobic particles under Ca2+ addition. The positive effect of Ca2+ on the aggregation of non-fed fine anaerobic granular sludge is likely to induce the formation of bigger particles and thus, prevent their wash-out from anaerobic bioreactors.