Investigation of bacterial predators for the prevention of membrane biofouling

Abstract

A flux decrease because of membrane biofouling is a crucial problem restricting membrane applications in conventional water treatment. Several investigations have been investigated to alleviate the biofouling problems; improving feed water qualities through coagulation, chlorine treatment, etc. Even though these methods improve the membrane performance, they are not sustainable due to the dosed chemicals. This is because the chemicals lead to the production of harmful disinfection by-products such as trihalomethanes, aldehydes, etc. In this study, the bacterial predators, BALOs (Bdellovibrio-and-like-organisms), were investigated, as an alternative treatment to alleviate biofouling and its consequent performance decrease. Dead-end microfiltration (MF) tests were conducted on Escherichia coli (E.coli) and BALOs co-culture feed solutions. Predation of E.coli was represented by the multiplicity of infection (MOI), which is explained as the proportion of predator to prey cell. The tested conditions of predation were both high MOI (high predator, HP) and low MOI (low predator, LP), and the total number of viable E.coli prey and predators were counted over 48 hr. The membrane performance of cultures such as NP (no predation), LP and HP was evaluated using a resistance-in-series model. In the performance experiments with a microbial solution containing predator bacterium, its total resistance became lower than the control (NP culture) over 48 hr. However, the LP culture showed an increase of irreversible fouling of the membranes. This was most likely due to prey cell debris produced by predation. In contrast, previous investigations of other research groups showed that coagulation using alum can mitigate membrane biofouling. Additionally, several studies found that lysing the microorganisms utilizing ultrasonication can enhance the membrane performance with alum coagulation. It was hypothesized that the predation impact may be comparable with that of the ultrasonication. This is because both predation and ultrasonication have an effect on the lysis of the bacteria leading to membrane fouling. Thus, it was predicted that a combined pre-treatment of bacterial predation and alum coagulation could improve the membrane performance. From this hypothesis, the goal of another investigation was to evaluate a combined pre-treatment using both bacterial predation and alum coagulation in order to reduce membrane biofouling, specifically, the irreversible fouling from the LP culture. Dead-end microfiltration (MF) tests were conducted on co-culture feed solutions using Escherichia coli and B. bacteriovorus after coagulation with diverse concentrations of alum. The results represented that when 10 ppm of alum was utilized, the membrane fouling got worse for both NP and LP cultures, as compared to no alum addition, because the irreversible resistance of the membrane was a lot higher. Conversely, using alum at 100 ppm reduced the total resistance similarly in both NP and LP cultures noticeably. In addition, for using 100ppm of alum, the LP culture led to both a lower total and irreversible resistance compared to the NP culture. This was because the LP culture with alum coagulation was well aggregated. These results indicate that combined treatments of both B. bacteriovorus predation and a suitable concentration of alum can be an effective pretreatment method for improving membrane performance. Powdered activated carbon (PAC) was used as alum coagulation, for the NP and LP cultures in order to decrease membrane biofouling caused by prey cell debris of the LP culture. Dead-end microfiltration (MF) tests were conducted on both NP and LP cultures, after treatment with various concentrations of PAC. The results showed that when 10 ppm or 100ppm of PAC were added, the performance of the membrane was better for both cultures as compared to no PAC addition. This was because using PAC could have effects on the reduction of reversible resistance. This finding concurs with another previous study that the application of PAC was related to decrease of reversible resistance. Furthermore, for the LP culture, 100ppm of PAC led to a decrease of irreversible resistance compared to 0ppm of PAC. In addition, when 10 ppm or 100ppm of PAC were added, the LP culture caused less total resistance of the membrane compared to the NP culture. These results also show that combined pre-treatments of bacterial predation and PAC treatment can be an effective method for enhancing membrane performance. In conclusion, this study showed that using bacterial predators at a suitably high concentration was useful at mitigating microbial fouling of the membrane. Also, even though using bacterial predators at a low concentration led to an increase of irreversible resistance of the membrane, either a proper alum concentration or PAC treatment, in combination with bacterial predation, can be a beneficial pretreatment method for reducing membrane biofouling.