Comparison of pilot scale performances between membrane bioreactor and hybrid conventional wastewater treatment systems

Abstract

A number of municipal wastewater treatment systems were developed to improve effluent water quality, however economic aspects should be considered. In this study, one of the membrane bioreactor (MBR) systems that have recently attracted much attention was compared to combined biological and chemical processes (CBCP), focusing on effluent water qualities and economic assessment. A HANT ®, modified MBR system, was composed of anoxic/anaerobic/oxic/anoxic stages with hollow fiber membrane, while the CBCP was consisted of anoxic and oxic steps with addition of clinoptilolite powder followed by rapid coagulation process. Both treatment systems showed enhancing efficacy in removing organic compounds and suspended solids. Due to membrane separation in the HANT ®, solids retention times (SRT) was increased up to 50 days. Such long SRT control along with high biomass concentration could enable to achieve nearly complete nitrification, where nitrogen removal was stably attained, as high as at average of 71.7%. However, effluent total phosphorus (TP) concentration in the HANT ® hardly met the requirement limiting eutrophication, which moreover presented instable effluent TP concentrations since biological phosphorus removal immediately reflected from temporarily varying influent COD/P ratio. In contrast, the CBCP presented more improvements on COD and TP removals as compared to the HANT ®, because it well functionally employed long SRT control in biological process in conjunction with post-coagulation process in the CBCP. Especially, effluent TP concentration was averaged at 0.04 mg/l in the CBCP, extremely lower than that in the HANT ®. It indicated that additional measure in the system, simply manipulating coagulants dosing, could lower its effluent concentration as low as 0.02 mg/l to limit eutrophication. From economic assessment, the CBCP was superior to the HANT ®. The CBCP showed capital investment of 204 $ m −3 lower than that of the HANT ®, and operating cost was estimated at 0.048 $ m −3 less than that of the HANT ®. The major factor largely attributing to less economical for the MBR was originated from both membrane price and its life expectancy. In present, the MBR system was investigated to be less economical, whose effluent water quality furthermore could not be greatly improved compared to that of conventional biological and chemical process. However, in near future, MBR systems would have economical competitiveness, which could facilitate it with supreme technology for wastewater treatment, considering the trend in reducing membrane price and enhancing membrane service life.