Abstract
An integrated membrane system, membrane bioreactor-reverse osmosis (MBR-RO), has become highly efficient in producing high-quality water for municipal wastewater reclamation. However, disposal of a highly concentrated waste stream (RO concentrate or RO retentate) generated in this combination is an important issue. This work investigated RO behaviour in an integrated pilot scale MBR-RO system for municipal wastewater reuse with the continuous recycling of RO retentate to the MBR influent. RO membrane retention and the fouling propensity were studied. RO concentrate, produced by the RO process at a fixed concentration factor (CF) of 3, was recycled continuously to the MBR, leading to water recovery of the entire process around 92%. Osmotic pressure model, saturation index method, high performance liquid chromatography equipped with size exclusion column (HPLC-SEC) and specific filtration test were used to analyse the fouling potential of the RO membrane. The results obtained showed that even though RO concentrate recycling changed remarkably, the compositions of both MBR permeate and RO concentrate, the quality of RO permeate remained almost constant in terms of organic matters, conductivity, and ionic salts. However, these high concentrations of organic or inorganic substances in RO concentrate were major factors leading to the RO membrane fouling. Before RO concentrate recycling, a decline of approximately 30% of the initial RO permeate flux was observed in the period when the CF was increasing to 3, mainly due to the osmotic pressure effect of retained ions and the deposits of organic matters at the RO membrane surface. After RO concentrate addition to the MBR, due to the continuous accumulation of ionic salts on the RO membrane surface, a gradual reduction in RO permeate flux (additional 19%) was also mainly attributed to the osmotic pressure effect of the retained ions. These observations showed that the continuous addition of RO concentrate to the MBR was successful in a combined MBR and RO process in terms of the excellent quality of RO permeate.
Highlights
At present, MBR followed by an RO process (MBR-RO) has gained worldwide acceptance as the most important technology in municipal wastewater reuse [1] [2] [3] [4]
The concentration factor (CF) is a key parameter for RO membrane fouling behaviour. These findings demonstrate the important role of the characteristics of MBR permeate in the fouling mechanisms of the RO membrane
Dissolved organic carbon (NPOC) and proteins were considered as indicators to evaluate RO performance in terms of RO retention capacity for organic matters in the membrane bioreactor-reverse osmosis (MBR-RO) system with RO concentrate recycling
Summary
MBR followed by an RO process (MBR-RO) has gained worldwide acceptance as the most important technology in municipal wastewater reuse [1] [2] [3] [4]. The constituents of MBR permeate as RO feed, including inorganic ions and organic matters, are mainly associated with how fouling develops in the RO membrane. Soluble microbial products (SMPs) present in MBR permeate are found to be important constituents leading to RO membrane fouling [6]. The presence of inorganic salts in MBR permeate leads to a reduction in RO permeate flux in MBR-RO systems for real domestic wastewater reuse [7] [8]. The concentration factor (CF) is a key parameter for RO membrane fouling behaviour. These findings demonstrate the important role of the characteristics of MBR permeate in the fouling mechanisms of the RO membrane
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