A Coagulant Survey for Chemically Enhanced Primary Treatment of Synthetic CSOs

Combined sewer overflow (CSO) wastewaters tend to be highly variable in composition, presenting a challenge to effective treatment. The robustness of various coagulants and flocculants was studied in terms of their ability to bring about efficient contaminant removal under variable conditions. To accomplish this, the effects of total suspended solids (TSS) and dissolved organic carbon (DOC) concentrations on chemically enhanced settling of synthetic CSO wastewater were evaluated using various inorganic salt and organic polyelectrolyte products. The evaluated products varied in their responses to changes in solids and organics concentrations, particularly in terms of sensitivity to DOC concentration and ability to bring about its removal. The required dosages of the coagulants (both inorganic and organic) were quite sensitive to DOC concentration, but good to excellent TSS and DOC removals were obtained. In comparison, the polymer flocculants displayed good TSS removal but poor reduction of DOC under all conditions, in spite of sensitivity to initial DOC concentration. The coagulants and flocculants studied also varied in tendency to overdosing and effect on pH. While no coagulant or flocculant is universally applicable, understanding the trends in treatment efficiency can help in choosing the type of product that is most suitable for treatment under given wastewater quality conditions.