Iron amended gravity-driven membrane (IGDM) system for heavy-metal-containing groundwater treatment

Gravity-driven membrane (GDM) system is a low-maintenance ultrafiltration system for decentralized drinking water treatment. GDM shows a high removal of pathogens, but a low removal of soluble heavy metal ions. In this study, to overcome this limitation, iron nails were introduced into a GDM system (IGDM) for the purification of heavy-metal-containing groundwater. The corrosion of iron nails continuously occurred in the influent, and rust particles were produced to remove heavy metal ions. The results suggested that the removal of As(V), Pb(II), and Cd(II) were all higher than 90% in the IGDM system; these removal efficiencies could be maintained because of the abundance of iron nails and the continuous formation of fresh rust. During the filtration, rust particles were partly deposited on the membrane surface, and the thickness of the cake layer in the IGDM correspondingly increased by 143% compared with that in the control GDM. The deposited rust had an inhibitory effect on the microorganisms in the cake layer, and the content of adenosine triphosphate (ATP) was reduced by 75% accordingly. However, the rust particles could adsorb organic membrane foulants, thereby alleviating organic fouling. Moreover, the bird's-nest-like structure of the rust particles facilitated the formation of a porous cake layer, and no synergistic fouling effect of rust particles and organic foulants was observed. These positive effects offset the inhibitory effect of rusts on microorganisms and the thickness increase of the cake layer. Consequently, a stable flux of 3.5 L m(2) h(-1 ) could still be achieved during the 105-day operation in IGDM system, although the value was 30% lower than that in the control GDM. All these results indicated that IGDM could be an economical and low-maintenance method for heavy metal removal in undeveloped areas. In addition, a house-hold scale (similar to 50 L d(-1)) IGDM was successfully operated, demonstrating the practical potential of this technology.