AWWA WQTC64166

Conventional treatment plants are often limited by the effectiveness of their coagulant. The Philadelphia Water Department (PWD) relies upon ferric chloride as the coagulant in its three drinking water treatment plants. This is the case for a number of reasons that include: ferric chloride generally yields good turbidity and particle removal; superior organics removal; and, is economically sensible. However, there are also undesirable characteristics of ferric chloride. These include marginal performance in cold water, nuisance metals content (e.g. Mn) and product impurities that have clogged tanks and chemical feed lines. These concerns, along with others have prompted PWD to revisit exploration into alternative coagulants. This research was done at the pilot-scale. Coagulant screening tests (jar tests) performed by two vendors resulted in recommendation of four chemicals for pilot scale evaluation: ferric sulfate (one from each of the two manufacturers); polyaluminum chloride (PACl); and, an alum-polymer blend. Each of these coagulants was tested in a conventional treatment pilot plant in side-by-side comparison to ferric chloride. This research was conducted over six seasons. Parameters used to develop the analysis were turbidity removal, organics removal, filter production, residual metals concentration, solids generation, DBP production, and a cost index. Results indicated that in general, performance of both ferric sulfate coagulants was comparable to ferric chloride in terms of water quality. However, cost of the ferric sulfates was considerably higher than the ferric chloride in use. The alum-polymer blend worked well only inside a narrow optimal dose and pH range. This range was difficult to discern and performance outside of the optimal dose and pH range was unacceptable. The polyaluminum chloride showed significant advantages over ferric chloride with respect to settled water turbidity, low addition of nuisance metals and less production of treatment residuals. Drawbacks included cost, lower organics removal and therefore higher DBP production. The results of this analysis led to further testing. Includes 4 references, tables, figures.