Abstract
Pure culture studies have shown that biofilm dispersal can be triggered if the nutrient supply is discontinued by stopping the flow. Stimulating biofilm dispersal in this manner would provide a sustainable manner to control unwanted biofilm growth in industrial settings, for instance on synthetic membranes used to purify water. The response of multispecies biofilms to nutrient limitation has not been thoroughly studied. To assess biomass dispersal during nutrient limitation it is common practise to flush the biofilm after a stop-period. Hence, flow-stop-induced biomass removal could occur as a response to nutrient limitation followed by mechanical removal due to biofilm flushing (e.g. biofilm detachment). Here, we investigated the feasibility to reduce membrane biofouling by stopping the flow and flushing the membrane. Using a membrane fouling simulator, biomass removal from synthetic membranes after different stop-periods was determined, as well as biomass removal at different cross flow velocities. Biomass removal from membrane surfaces depended on the nutrient limiting period and on the flow velocity during the biofilm flush. When flushed at a low flow velocity (0.1 m.s−1), the duration of the stop-period had a large effect on the biomass removal rate, but when the flow velocity was increased to 0.2 m.s−1, the length of the stop period became less considerable. The flow velocity during membrane flushing has an effect on the bacterial community that colonized the membranes afterwards. Repetition of the stop-period and biofilm flushing after three repetitive biofouling cycles led to a stable bacterial community. The increase in bacterial community stability coincided with a decrease in cleaning effectivity to restore membrane performance. This shows that membrane cleaning comes at the costs of a more stable bacterial community that is increasingly difficult to remove.
Highlights
Membrane filtration is the leading technology for seawater purification (Gude 2016)
It is common practise to assess the effectivity of biofilm dispersal stimulants by determining the amount of released biomass after the liberated components have been removed by a flush
An feed channel pressure drop (FCP) increase of 125 mbar was used as criterion to start the stop-period, but due to the logarithmic biomass increase, this criterion was frequently exceeded (Figure S2, S3 and S4)
Summary
Membrane filtration is the leading technology for seawater purification (Gude 2016). One disadvantage is the inevitable accumulation of material at the membrane surface, causing membrane fouling. Biofouling is the fouling type that most frequently diminishes membrane performance and leads to operational problems, such as an increase in pressure drop, decrease in normalized flux and altered membrane selectivity (Nguyen et al 2012). Microfiltration (MF) and ultrafiltration (UF) membranes are cleaned physically via forward flush, backflush, pneumatic cleaning by gases or by applying ultrasound or electrical fields (Regula et al 2014). Despite these alternatives, cleaning in place is the only option for MF and UF membranes to remove recalcitrant fouling components such as proteins (Maskooki et al 2010)
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