The Effects of Pre-Treated Membrane Backwash Water on the Quality of Finished Water from a Membrane System

Abstract

Recovering membrane backwash water (MBW) can improve productivity and reduce the costs of membrane treatment systems. To verify a recovery scheme in which MBW is pre-treated and recycled into an equalization tank, two lab-scale flocculation-microfiltration (MF) membrane systems were constructed. One membrane system used water from the Luan River (the Luan River system), and the other used a blend of pre-treated membrane backwash water plus water from the Luan River in a 1:9 ratio (the mixed water system). The operating parameters and coagulant dosages were the same for both systems. Ferric chloride was used as a coagulant in the pre-treatment of membrane backwash water, and powdered activated carbon (PAC) was added to improve water quality.

Organic compounds, as represented by UV₂₅₄ measurements, in the membrane backwash water were predominantly low molecular weight species (MW < 1 kDa), whereas the DOC fraction consisted primarily of compounds with an MW greater than 30 kDa and substances with an MW less than 1 kDa. These two fractions of organics were effectively reduced by flocculation and adsorption. No differences in the quality of treated water from the two membrane systems were observed. The results indicated that the scheme of recovering membrane backwash water was feasible.

Keywords:

• flocculation
• membrane backwash water
• microfiltration
• natural organic matter
• PAC adsorption