Study of membrane attached biofilm performance with nitrate as electron acceptor

This paper presents a study that aims to overcome oxygen limitations in membrane-attached biofilms in the Extractive Membrane Bioreactor (EMB), by using nitrate instead of oxygen as an electron acceptor. In the EMB target organic compounds are extracted through a dense membrane into a bioreactor, where they are biodegraded. However, due to bacterial attachment, a membrane-attached biofilm forms on the biomedium side of the membrane. This is problematic as an inverse relationship between the organic flux and biofilm growth is commonly observed. In the aerobic EMB, this is thought to be a consequence of the low solubility of oxygen in water — oxygen can therefore only penetrate thin biofilms. Thus this problem might be overcome by using nitrate, which is very soluble in water. It is hypothesized in this anoxic EMB that nitrate can fully penetrate the biofilm and therefore overcome the inactive zones of bacteria. To test this hypothesis, four experiments have been performed in an EMB with toluene as a model pollutant. The effect of excess and low nitrate concentrations on biofilm formation and toluene flux has been investigated. At low nitrate concentrations, a decrease in pollutant flux concomitant with biofilm growth was observed. Conversely, no biofilm was formed and the flux remained high using excess nitrate. Therefore, to investigate the effect of excess nitrate with biofilm formation, a ferric nitrate flocculent was used to force the formation of a biofilm. However, when the biofilm was formed, the toluene flux across the membrane decreased, similar to the experiment with low nitrate concentrations. This indicates that the solubility of the electron acceptor is not the sole factor controlling the decrease in organic flux with biofilm growth, as previously thought in the aerobic EMB.

This study has investigated the feasibility of the anoxic EMB and the influence of nitrate concentrations. These experiments showed: • A biofilm is only formed naturally under low nitrate concentrations. • A low nitrate concentration results in a toluene flux decrease concomitant with biofilm growth. • In the presence of excess nitrate concentrations, no biofilm is formed. The toluene flux therefore remains constant over time. • Using excess nitrate and forcing a biofilm to form using a ferric nitrate flocculent, the toluene flux across the biofilm decreases concomitant with biofilm growth. This therefore shows that contrary to our hypothesis; the solubility of the electron acceptor is not the primary factor controlling the decrease in organic flux with biofilm growth in the EMB.