Optimizing Organics Removal from Surface Water in a Coagulation / Ultrafiltration System

Surface waters contain inorganic and organic particulate substances that need to be addressed and potentially reduced for drinking water production. Inorganic particulate constituents are amongst others clay, silt and minerals. Organic particulate substances can include microorganism like viruses, bacteria and protozoa. One of the major problems of using surface water as source for drinking water is the high content of natural organic matter (NOM). NOM can cause odor, can influence the taste and can increase corrosion and biofilm growth in the distribution network. Furthermore, NOM could be a source for the formation of disinfection by-products when water is disinfected. In recent years, surface water treatment with ultrafiltration membrane has become an attractive alternative to conventional clarification. Minimal or no addition of chemical agents, superior final water quality independent of feed water quality, superior removal efficiency towards microorganisms, reduced sludge generation, compact process, and easy automation are some of the advantages of ultrafiltration compared to conventional treatment. In drinking water production, coagulation as a pre-treatment process for Ultrafiltration can improve the efficiency of the filtration process, resulting in a higher flux level, lower transmembrane pressure and increased NOM removal. Coagulant dose rate is critical to proper operation: both over- as well as underdosing can result in unstable operation, with potentially insufficient organics removal, coagulant carry-over and membrane fouling as a consequence. A one-year pilot study confirmed a good correlation between dose rate and removal efficiency. The results gave the confidence to build a full-scale plant that employs the same principle: online monitoring of the feed UV absorption and desired filtrate quality dictate the coagulant dose. This paper describes the results from 2½ years operation of a full-scale hollow fiber Ultrafiltration plant. During operation continuous high removal rates of NOM > 60% (based on UV254), true Color > 75% and DOC > 55% were established.

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