Ferric sulfate sludges demonstrated superior dewaterability over sludges formed by coagulation with aluminum. Tighter regulations governing stream discharge of coagulation sludge from the primary sedimentation process prompted the University of Iowa Water Treatment Plant to consider sludge‐handling options. This pilot study was undertaken in order to investigate differences in the mechanical dewatering of sludges formed by using either alum or ferric sulfate coagulants. The approach used various volumes of wasted lime sludge produced from the plant's softening process as a filtering aid. After the short pilot study, dewatering data from a bench‐scale plate‐and‐frame filter press indicated that ferric sludge was easier to dewater than sludges formed by coagulation with alum. The addition of lime‐softener sludge increased the dewaterability of all sludges tested, primarily by increasing the sludge feed solids concentration, which may have also decreased blinding.

This article reports on the results obtained in full‐scale parallel operation at the University of Iowa of two solids contact units at various concentrations of solids. This unit was designed for light turbidity and algae removal, lime‐soda softening, and industrial waste treatment. The article lists the operational variables used to control the functions of each unit and obtain the desired effluent quality and quantity. Details of the experimental procedure and results are discussed.