Comparison with conventional activated sludge (CAS)
Trace organic removal characteristics of MBR are different from those of CAS. It is mainly due to its longer SRT and lower intrinsic excess sludge production than CAS. (Note: apparent excess sludge production of MBR is comparable to that of CAS as discussed here) While longer SRT can promote biodegradation by allowing longer time to microorganisms to adapt with trace organics, lower sludge production can partially hamper the removal by adsorption. As a result, depending on which removal mechanism a trace organic compound mainly relies on, either MBR or CAS can show better removal efficiency.
Fig.1 shows the difference of removal efficiencies between either MBR and CAS or MBR and CAS-UF. For relatively hydrophilic compounds, e.g. TMP and SMX, of which Kow is <1.0, MBR showed much better removal efficiencies. Since these compounds do not adsorb on biomass as much as hydrophobic compounds, they rely more on biodegradation to be removed. Thus, MBR can provide better degradation environment by providing longer contact for microorganisms to adapt with the compounds. On the contrary, hydrophobic compounds, e.g. CLA, ROX, and ERY, were removed better by CAS or CAS-UF in general. This is likely because these hydrophobic compounds rely more on adsorption mechanisms to be removed. The higher sludge production by CAS certainly contributes positively to the removal of such compounds.
Fig. 1. Removal differences of Trimethoprim (TMP), sulfamethoxazole (SMX), Clarithromycin (CLA), roxithromycin (ROX), and erythromycin (ERY) by MBR and CAS (Sahar, 2011)
© Seong Hoon Yoon