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Conventional treatment plants are often limited by the effectiveness of their coagulant. The PhiladelphiaWater Department (PWD) relies upon ferric chloride as the coagulant in its three drinking watertreatment plants. This is the case for a number of reasons that include: ferric chloride generally yields good turbidityand particle removal; superior organics removal; and, is economically sensible. However, there are alsoundesirable 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 feedlines. These concerns, along with others have prompted PWD to revisit exploration into alternativecoagulants. This research was done at the pilot-scale.Coagulant screening tests (jar tests) performed by two vendors resulted in recommendation of fourchemicals 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 aconventional treatment pilot plant in side-by-side comparison to ferric chloride. This research wasconducted over six seasons. Parameters used to develop the analysis were turbidity removal, organicsremoval, filter production, residual metals concentration, solids generation, DBP production, and a costindex.Results indicated that in general, performance of both ferric sulfate coagulants was comparable to ferricchloride in terms of water quality. However, cost of the ferric sulfates was considerably higher than theferric chloride in use. The alum-polymer blend worked well only inside a narrow optimal dose and pHrange. This range was difficult to discern and performance outside of the optimal dose and pH rangewas unacceptable. The polyaluminum chloride showed significant advantages over ferric chloride withrespect to settled water turbidity, low addition of nuisance metals and less production of treatmentresiduals. Drawbacks included cost, lower organics removal and therefore higher DBP production. Theresults of this analysis led to further testing. Includes 4 references, tables, figures.

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Edition: Vol. - No. Published: 11/01/2006 Number of Pages: 18File Size: 1 file , 460 KB