Study On The Effect Of Heavy Metal Wastewater On A/O-MBR Process

Toxic metals are almost ubiquitous in wastewater, arising from a diverse range ofsources. Despite industrial activities have always been significant sources of heavy metalsin the past, more dilute diffuse sources such as traffic related emissions, effluents fromsmall businesses, domestic effluents and several other chemical treatment processes havebecome important sources for heavy metal emission. Since the wide-spread application ofMBR, the effect of heavy metals on MBRs is an inevitable and valuable issue to discuss. Aprimary treated municipal wastewater added with Zn(Ⅱ), Cu(Ⅱ), Pb(Ⅱ), and Cd(Ⅱ) wasused for this study.The changes of performance of MBR fed with municipal wastewater containing0.25-2.56mg/l (low concentrations) and3.7-32.3mg/l (high concentrations) of zinc,copper, lead, cadmium and iron were studied in this paper. The effect of heavy metals onA/O-MBR has been studied in the following aspects:1) COD removal;2) Nitrificationand denitrification;3) Sludge activities;4) EPS and fouling;5) Activated sludge PSD andfouling;6) Variation of TMP and Appearance of fouled membraneThe experimental results can be concluded in the following aspects:(1) COD removal in the1st run was above95%and the permeate COD was very low(10-23mg/l). The2ndand3rdruns both had a lower, but pleased COD removal with anaverage of93%and87%respectively.(2) During the2ndrun and3rdrun biomass inhibition was on average15%and41%respectively. The inhibition of autotrophic biomass was found to be on average28%and62%for the2ndand3rdruns respectively compared to the control conditions.(3) The nitrification rate decreased27%and46%whereas denitrification ratedecreased20%and34%when treating low and high concentrations of heavy metals,indicating that heavy metals had a greater impact on nitrification than on denitrification.(4) The protein of EPS (EPSp)/carbohydrate of EPS (EPSc) of the activated sludgedecreased from1.85to0.85. With the heavy metal fed into the system, as it was observed,the concentration of EPSc increased from27.19to33.05(mg/g SS) and from33.76to41.30(mg/g SS) in the2ndrun and3rdrun respectively.(5) Heavy metals also lead to a smaller particle size distribution (PSD). The meanparticle sizes of sludge were54μm for1strun,68μm for2ndrun, and50μm for3rdrun,respectively. It was found that the MBR sludge at2ndrun exhibited a broader PSD, andlarger mean size. For the sludge at3rdrun, the sludge PSD had the tendency to be small size.(6) SEM images of fouled membranes found solid inorganic scale deposits onmembrane. All these results suggest that the effects of heavy metals on the membranefouling could be concluded into two aspects: a) modifying sludge characteristics; b)contributing to inorganic fouling.