Pretreatment of the municipal wastewater feed for reverse osmosis plants

Potable water can be produced at a reasonable cost if reverse osmosis (RO) technology can be applied to renovate secondary/tertiary wastewater effluent. This implementation would yield many advantages to Kuwait, namely satisfying the increasingly agricultural, industrial and domestic demands for good quality water free of viruses and bacteria, preserving the natural strategic water resources, reducing environmental pollution resulting from direct discharge of secondary/tertiary municipal effluents to the sea and meeting unexpected emergency cases of shortages in fresh water production for certain applications. Membrane fouling is the most important obstacle that has to be overcome for successful RO. Membrane fouling usually results in reduction of the permeate production rate, an increase in salt passage with time and/or membrane damage. Causes of fouling include scaling, plugging of membrane pores by suspended matter, biological fouling and degradation of the membrane itself. Membrane fouling is very much aggravated with the use of wastewater effluent as a feed for RO systems. The Kuwait institute for Scientific Research (KISR) is implementing a research project to desalinate tertiary treated wastewater using RO in Kuwait. This paper describes the process of designing a proper pretreatment process capable of producing a substantial reduction in potential membrane foulants. Results obtained from laboratory and pilot studies revealed that fast mixing, coagulation, flocculation and sedimentation using FeIII, cationic polyelectrolyte and a sanitizing agent can produce an acceptable quality of effluent feed for the RO process. This paper discusses the progress made in establishing proper pretreatment of this feed to meet the requirements of RO membrane manufacturers.