Gray water recycle: Effect of pretreatment technologies on low pressure reverse osmosis treatment

Gray water can be a valuable source of water when properly treated to reduce the risks associated with chemical and microbial contamination to acceptable levels for the intended reuse application. In this study, the treatment of gray water using low pressure reverse osmosis (RO) filtration after pre-filtration and aerobic biological filtration pretreatment was explored. Initially, nine commercial RO membranes were compared with dead end filtration tests for water flux and salt rejection at low pressures of operation (100-400 psi). Based on the dead end filtration results, one commercially-available membrane was subjected to further evaluation using a cross flow flat sheet test unit and gray water that had undergone various levels of pretreatment. The effects of gray water pretreatment with cartridge filtration, microfiltration, and aerobic biofiltration were compared in terms of membrane flux, chemical oxygen demand reduction, and salt rejection. Results from these RO filtration tests suggest that cartridge filtration in combination with RO treatment can achieve adequate reduction of gray water chemical oxygen demand (COD), but at the cost of progressively decreasing water flux through the RO membrane. Flux decline during cartridge filtration is mainly attributed to the presence of colloids >0.2 μm in size. In comparison, microfiltration maintains the water permeation capacity of the membrane, but permeate COD levels are higher. Biofiltered/microfiltered gray water shows RO permeate flux similar to microfiltered gray water and achieves permeate COD levels similar to cartridge filtered gray water.