Examining Coagulation and Precoating for Seawater UF/RO System Pretreatment

Abstract

Ultrafiltration (UF) is gradually becoming the technology of choice for seawater reverse osmosis (RO) pretreatment because of its ability to provide consistent, high-quality RO feed and its small footprint, among other advantages. However, because of the complexity and cost of coagulation sludge handling and disposal, coagulation as a UF pretreatment is a concern for most plant operators. Although efforts are being made to reduce or eliminate coagulation prior to UF, potential benefits may be overlooked. This study focused on the effect of coagulation conditions and modes of application on UF performance as it relates to hydraulic operation and product water quality. Coagulation was performed in two modes—inline coagulation and precoating. Filtration was performed in unstirred cells through membranes in dead-end constant pressure. UF hydraulic performance was evaluated, and UF permeate quality was characterized in terms of residual phosphate. Although precoating improved hydraulic performance, results showed that permeate quality was better when inline coagulation was applied. Furthermore, hydraulic performance for inline coagulation improved considerably when flocculation time was increased. Maximum phosphate removal was achieved in inline coagulation when pH was 6 and flocculation time was 45 min.