The use of membrane bioreactors in high rate activated sludge processes: How and why sludge retention time affects membrane fouling

In the current study, the performance and membrane fouling characteristics of a high loaded membrane bioreactor (HL-MBR) treating municipal wastewater at sludge retention times (SRTs) of 0.5, 1 and 2 days, was investigated. Results demonstrated that membrane fouling rate rose with increase in SRT. The cake layer, which according to Fourier transform infrared analysis (FTIR), was mostly composed of protein and carbohydrate, was found to be the principle mechanism responsible for the membrane fouling. With increase in SRT, there was a higher increase in soluble microbial products (SMP) and higher drop in particle size in the cake layer compared to the mixed liquor. On the other hand, extracellular polymeric substances (EPS) did not change in the cake layer whereas it increased in the mixed liquor. SMP and its components, protein to carbohydrate ratio in the EPS, relative hydrophobicity of SMP and bacterial floc as well as submicron particles contributed to the increase in the membrane fouling rate and cake resistance with increasing SRT. Multiple regression analysis results showed that SMP was the principal factor that resulted in rise in the membrane fouling rate with increasing SRT. Finally, in terms of membrane fouling and system performance, the lowest SRT studied yielded the best performance for the HL-MBR.