Chlorination and coagulation as pretreatments for greywater desalination

On-site greywater reclamation is thought to enhance water usage efficiency and decrease urban water demand. Direct membrane filtration is an attractive approach for treatment of greywater in residential areas due to its small footprint and high reliability, as product quality is insensitive to fluctuations in inflow quality. The research evaluated coagulation or chlorination as possible pretreatments for ultrafiltration followed by RO desalination. The system was fed with “light” greywater (avg. TOC, TN and P — 24.9, 4.6 and 0.7 mg L −1; turbidity — 34 NTU). Direct filtration, without pretreatment, resulted in sharp decrease of the UF permeability, due to organic fouling and biofouling. The RO membrane was suspected for phosphate salt scaling. To overcome these drawbacks, coagulation and chlorination were evaluated as pretreatment options prior to UF. Chlorine demand of the greywater was 10–20 mg L −1. Applying this dose and controlling residual chlorine in the UF feed, lowered the fouling rate by 33%. Mass balances calculations indicated that chlorination inhibited microbial activity in the UF system. The partial oxidation of the organic matter, achieved by the chlorination pretreatment, may have changed the properties of the organic matter and thus its reactivity with the UF and RO membranes. Next, ferric chloride was tested as a coagulant. Dosing 50 mg L −1 ferric chloride reduced the UF flux decline rate by 43%, mainly due to a 38% decrease of the organic load. After coagulation, larger particles with narrower size distribution were observed in the feed (average size ~0.5 μm). This upward shift resulted in a more porous filtration cake buildup on the UF membrane, a finding which supports the enhanced performance. Although a positive synergistic effect could be anticipated, the combined coagulation-chlorination process increased fouling rate in the RO membrane probably due to the increase in the concentration polarization phenomenon, as a result of a rise in the concentrations of counter ions associated with the pretreatment reactants and possible surplus of the latter. The results indicate that greywater pretreatment is a prerequisite to hamper UF membrane biofouling. Coagulation was found to be superior to chlorination for the UF. Effluents produced by each of the membranal steps were of excellent quality that can be used for various purposes, being low in organic content, suspended matter and colloids (UF), or desalinated (RO).