Development of an integrated membrane process for water reclamation

Abstract

An integrated membrane process (IMP) comprising a membrane bioreactor (MBR) and a reverse osmosis (RO) process was developed for water reclamation. Wastewater was treated by the MBR operated at a hydraulic retention time (HRT) of 5.5 hours and average sludge retention times (SRTs) of 20 and 35 days. The IMP had an overall water recovery efficiency of 80%. A unique feature of this IMP was the recycling of a fraction of the RO concentrate back to the MBR. Concentrate recycling had an inhibitory effect on the microorganisms and resulted in a decrease in specific oxygen uptake rate (sOUR) and a decrease in specific MBR TOC and TCOD mass removal rates. The inhibition was, however, not severe and the microorganisms remained viable. A portion of the hard-to-degrade organic constituents in the concentrate recycle stream could be degraded by an acclimated biomass. Thus, concentrate recycling could support a higher MLVSS concentration and this brought about higher MBR TOC and TCOD mass removal rates. In terms of sOUR, the effects of increasing average SRT from 20 days to 35 days were found to be dependent on F/M ratios. Critical F/M ratios at 0.040 g TOC/g VSS.day and 0.20 g TCOD/g VSS.day were identified. Below the critical ratios, the sOURs were observed to be higher at 35 days SRT, compared to those at 20 days SRT, and vice versa for ratios above the critical values. Higher TOC and TCOD mass removal rates can also be achieved at 35 days SRT. Concentrate recycling and the prolonging of SRT both worsened the MBR effluent quality. However, this drawback could be mitigated somewhat by operating the MBR at an F/M ratio below 0.030 g TOC/g VSS.day, yielding an effluent quality comparable to that achievable without concentrate recycling. Operating the IMP at 35 days SRT led to higher IMP TOC mass removal efficiencies, as compared to that achievable at 20 days SRT. Improvements in mean permeate and concentrate qualities were also observed when SRT was increased from 20 days to 35 days. Based on the results from this study, it is recommended that the IMP be operated at a SRT of 35 days, and at F/M ratios below the critical values.