Gaining deeper insights into the bioflocculation process occurring in a high loaded membrane bioreactor used for the treatment of synthetic greywater

Abstract

In the present study, a high loaded membrane bioreactor (HL-MBR) operated at a hydraulic retention time (HRT) of 1.5 h, and three different sludge retention times (SRTs) in the range of 0.5–2 days, was used for the treatment of synthetic greywater. The chemical oxygen demand (COD) removal efficiency of the system was in the range 87–89% at all SRTs. Bioflocculation efficiency (defined as the percentage of suspended COD in the concentrate stream), COD bio-oxidation, total extracellular polymeric substances (EPS), tightly bound (TB) EPS and the ratio of EPS protein (EPSp) to carbohydrate (EPSc) increased when SRT was increased from 0.5 to 2 days. Sludge supernatant soluble microbial products (SMP) increased with increase in SRT from 0.5 to 2 days, while the effluent SMP was negligible. Particle size distribution analyses revealed a bimodal distribution at an SRT of 0.5 days, and normal distributions at other SRTs. Furthermore, depending on the value of the F/M ratio, different SRTs in the range of 0.5–2 days had either positive or negative effects on the mean particle size. Linear correlation analyses were performed using the data obtained during both transient and steady-state operations of the HL-MBR system. TB-EPS and EPSp showed strong correlations with the biofloccultaion efficiency, whereas loosely bound (LB) EPS correlated with soluble COD removal. TB-EPS and EPSc had negative correlations with the energy recovery potential of the system. The trend of change of parameters affecting membrane fouling intensity with SRT suggested that, in the range studied, the lowest rate of membrane fouling would be expected at SRT of 0.5 days.