Thermosonication –another way to tackle biofouling.
A review of: Al-juboon, R.A., Yusaf, T., Aravinthan, V., (2012), Investigating the efficiency of thermosonication for controlling biofouling in batch membrane systems, Desalination, 286:349-367
Thermosonification is the combination of ultrasound and heat and can applied in the destruction of food-borne pathogens and to help preserve foods.This study aims to establish the effects of thermosonication as a pre-treatment technique to reduce biofouling in Reverse Osmosis (RO) systems. RO technology is used for water treatment, purifying water from seas and wastewater facilities to a potable water quality. However, the use of RO membrane is hindered by biofouling problems. Microorganisms are thought to be mainly responsible, developing biofilms on the membranes and whose metabolic activity causes degradation of the membrane and detrimental effects to the system, inevitably causing a rise in energy requirements and operation costs. It is therefore vital to establish techniques to control the adhesion of the microbes, and specifically an environmentally friendly method such as thermosonication, as current techniques involving disinfection with chemical antimicrobial agents have limitations with negative effects on the ecosystem and the membranes.
In order to assess the efficiency of thermosonication, biofilm formation on treated and untreated membranes was analysed, including numerating living and dead cells using staining and epiflourescence. E.coli was the sample microorganism used, due to its known ability to adhere and develop biofilms on RO membrane. The E.coli suspensions were placed in different mediums; distilled water and MacConkey nutrient broth, with one set treated with thermosonication and another left untreated, as a control.
They reported that overall the number of injured E.coli cells increased as treatment increased, with largest effects observed after 4minutes of treatment. Released protein from the cells also increased, which is an indicator of cell damage due to sonication treatment. Comparing results of treated E.coli suspension with distilled water compared to MacConkey broth, they found higher concentrations of viable cells in the treated broth suspension. They attributed this to two reasons; the formation of large bubbles in the broth due to the reaction of free radicals and some of the broth components, where the large bubbles can hinder the transmission of ultrasonic energy into the suspension. Similarly, the sonoprotective effects of lactose (one of MacCONKEY broth components) could strengthen the resistance of E.coli against ultrasound treatment, allowing for more successful growth.
They also found that after peak concentration was reached, after 36 hours of the RO membrane operating, the reproduction rate of E.coli in the treated suspension was significantly lower (Peak conc: untreated – 11.368log10, treated – 6.5log10). The colonies formed in the treated suspension were also noticeably smaller and could not develop a biofilm with coverage area as large as that of the untreated cells. They explain that these differences were due to increased numbers of injured cells. This prevents them from producing EPs, an important tool in the adhesion of cells and therefore biofilm formation was hindered. The paper also looks at the permeate fluxes of the membrane with the suspensions, finding that using pure water the permeate flux remained almost constant, but decreased with the other suspensions due to the fouling produced, especially for the untreated suspension, where it remained considerably low compared to that of the treated suspension.
Overall, the study shows that thermosonication treatments, specifically for 4minutes, can reduce biofouling and recover the permeate flux of RO membranes and should be introduced as a pre-treatment method. Moreover, this efficiency was heightened by the use of distilled water, as opposed to nutrient broth as medium, suggesting that considerable attention should be given to the type of medium used when applying thermosonication.
4 comments:
This is an interesting approach to managing biofilm development. It is clear that this environmentally friendly method is extremely appropriate to maintain these membranes. The use of these RO systems in roles linked to direct human consumption makes it is clear that more traditional methods like applying toxic antifouling coats is not a viable options as it would cause significant health and safety issues.
The manner in which MacCONKEY broth can inhibit the full potential of thermosonification is very interesting. Distilled water seems like the logical medium choice, did they explain why they used the MacCONKEY broth? I imagine it is to replicate environmental conditions where these systems are commonly located.
Hi Dave,
I agree, knowing how important this technology is and how dependant we are on its efficiency means we should be focusing on ways to make its maintenance safer and environmentally friendly, especially considering the large scale that it is used.
In regards to your question, well they actually used another medium also, Tryptone Soya Agar, TSA (however, they did not include many results with this medium, with little focus on it in the discussion as well, so I did not include it in my review). Basically TSA is a non-selective medium which provides the microorganisms with a nutritious environment, so that the injured cells can grow and multiply, essentially a bad medium to use if you're trying to eliminate them! MacConkey Agar on the other hand is a selective medium. This is considered a harsher environment for the injured microorganisms, however it provides the E.coli with sufficient nutrients to develop biofilm and grow. So I'm assuming they used this to ensure that biofilm production was possible in the experiment, eliminating some variables regarding other reasons why biofilm production decreased. I also imagine that your reasoning is correct and that this replicates the natural environment, especially compared to the distilled water! This medium is also a good indicator for growth, causing a distinct colour change, so I guess that’s also handy!
Thanks for getting back to me Jelena.
It is interesting that they would even consider using the TSA medium in this investigation. Possibly they used this medium to replicate eutropic waters in order to determine how the thermosonification treatment would fair in some of the most problematic envrionments.
Yes that seems the likely reason. To establish a successful treatment it needs to be effective in all environments, even those that would favour bacterial growth, such as eutropic waters. The next step would be to look at how to heighten the effects of thermosonication in nutrient rich environments so it can be applicable to all areas and RO membranes.
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