Effect of sludge ages on formaldehyde and phenol removal in plug-flow and cstr anaerobic membrane bioreactors

Abstract

The aim of this study was to investigate effects of sludge ages on removal efficiencies of formaldehyde (FA) and phenol, main ingredients of embalming fluid, using both the continuous stirred-tank reactor (CSTR) and plug-flow anaerobic membrane bioreactors. A microfiltration membrane module having pore size of 0.4 µm was installed inside each reactor for retaining sludge to control the sludge retention time (SRT) at the desired values. The CSTR was operated under 65,000 and 6,000 d of sludge age, whilst the plug-flow reactor was operated under 4 different SRTs, i.e. 6,010, 1,000 100 and 50 d. Ratio of embalming fluid to domestic wastewater was varied to at each studied SRT, corresponding to FA concentrations of 25 to 435 mg/l and phenol concentrations of 3.7 to 64 mg/l, respectively. Both the CSTR and plug-flow anaerobic membrane bioreactors could efficiently remove FA at all studied FA concentrations. On the other hand, these two reactors failed to efficiently remove phenol at the concentration higher than 38 mg/l under SRT of 65,000 and 6,010 d ( CSTR and plug-flow reactor, respectively) corresponding to the embalming fluid to wastewater ratio of 0.008:1, due to the possible toxicity of FA on phenol degrading microorganisms. However, the deterioration of phenol removal was occurred when anaerobic microorganism possibly responsible in phenol degradation had not acclimatised to FA existence. That phenol was efficiently removed at the lower SRT on CSTR and plug-flow reactor at the ratio of 0.008:1 proved the assumption that microbial acclimatization had greater effect on both CSTR and plug-flow reactor.