Integrated MBR and low cost adsorbent system for the treatment of municipal wastewater and metals removal.

Membrane Biological Reactors (MBR) are largely used for municipal wastewater treatment, because they combine biological treatment with membrane filtration. Nevertheless, the presence of metals in municipal wastewater affects significantly MBR performances. Even if wastewaters show a low concentration of metals, they can still be extremely polluting, because of their effects on human health and on aquatic life, consequently their removal is still important. MBR processes provide good performances in some metals removal but they show a lower efficiency of arsenic, mercury and zinc removal. Furthermore, metal presence in the MBR in-fluent could increase membrane fouling and induce changes of the Extracellular Polymeric Substance (EPS), flocs size and sludge features. Heavy metal toxicity could be responsible of the inhibition of microbial growth and causes a reduction in nitrification and de-nitrification reaction rate. In order to improve MBR metal removal capacity, a possible solution is the use of a MBR combined with a low-cost adsorbent, particularly eggshell and eggshell membrane (ESM), which porosity and surface properties make them good adsorbents, able to remove heavy metals, phenolic compounds, dyes and pesticides from wastewater. In this paper an experimental study has been carried out in order to find the system configuration and the optimal operative conditions that could be adopted to reach the complete metals removal and wastewater purification with an integrated systems consisting of MBR, adsorption and sedimentation units. A preliminary analysis has been carried out in order to verify the compatibility between the two processes of adsorption and membrane separation and to find the sorbent concentration and size that don’t affect membrane fouling. Then eggshell removal capacity has been checked with aqueous solution containing the metals as water pollutants, at different operative conditions. Synthetic wastewater, containing the metal species, was treated preliminary in three process alternatives, then in the optimal involving a pretreatment unit (consisting of adsorption and sedimentation units), followed by the MBR placed downstream of the pretreatment. Membrane fouling, BOD and COD reduction as well as metals concentration have been evaluated. Results demonstrate that the integrated system permits metals removal and high quality of purified water.