Evaluation of pretreatments for a blowdown stream to feed a filtration system with discarded reverse osmosis membranes

• This work studied the reuse of discarded reverse osmosis membranes.
• Blowdown wastewater was treated to reuse the permeate as make-up water.
• Coagulation, flocculation, sand filtration and GAC were proposed as pretreatments.
• Coagulation/flocculation/sand filtration showed the best result to feed RO systems.
• The permeate quality was good; the membranes presented high retention.

In reverse osmosis demineralization processes, spiral wound modules are used and after approximately 5 years are discarded, since they lose their performance. These modules could be reused for other treatments that do not require a final permeate with so high quality, but still have limitations for feed water quality. The aim of this study is to propose a pretreatment configuration to a membrane filtration system that uses discarded membranes to treat the stream of a cooling tower for reuse as make-up water. Steps of coagulation/flocculation, sand filtration, sorption with GAC and combinations of these were proposed. Parameters as pH, electrical conductivity, turbidity, total hardness, COD, silica content and SDI were analyzed. The pretreated effluent was tested in a bench scale membrane filtration system. Permeate flux, salt retention and hydraulic permeability were evaluated to identify fouling. The best results were obtained coupling coagulation/flocculation with sand filtration. The RO discarded membranes presented high salt retention, about 97% and analysis of permeates indicates the possibility of reuse. MEV and EDS of SDI membranes showed a decrease of silica and total hardness scale, which are the main causes of fouling and the pretreated stream seems to present suitable characteristics to feed the RO filtration system.