Control of calcium buildup in GAC: Effect of iron coagulation

Abstract

In response to proposed EPA regulations governing disinfection by-products the water treatment industry may need to remove natural organic matter (NOM) more extensively prior to chlorination. Unit processes such as granular activated carbon (GAC) adsorption and enhanced coagulation can be used to achieve conformance with these proposed regulations; and the water treatment industry actively seeks means to reduce the cost of these unit processes. One of the cost-saving steps in GAC use pertains to its reactivation; however the buildup of calcium within GAC pores can deleteriously alter the micropores of the GAC during thermal reactivation and affect its performance. Thus, it is proposed that by reducing the buildup of calcium within GAC during water treatment service, a utility could improve its reactivation effectiveness. The purpose of this paper is to explore how iron coagulation and competitive complexation with NOM can limit calcium accumulation within GAC, and therefore contribute to favorable reactivation. Research conducted by the authors has shown that iron addition and coagulation prior to GAC treatment is capable of the following favorable effects: (1) promoting complexation between Fe and NOM, preventing Ca-NOM complexes from forming, and thus limiting calcium accumulation in complexed form onto GAC; and (2) removing substantial fractions of NOM ahead of the GAC beds such that subsequent GAC bed life increases considerably. By minimizing calcium accumulation, less destruction of micropores occurs during reactivation. Furthermore, reactivation becomes less frequent when bed life is increased. Both high (20 mg L −1) and moderate (5 mg L −1) iron (as Fe) dosages were found to be effective at preventing calcium accumulation. Also, tests determined that a pH 6.0 or lower was better than a pH of 7.0 or 8.0 when attempting to achieve calcium exclusion.