The Problems With Biofouling

Water reverse osmosis filter system 01.21.2015

Written by: Selective Micro Technologies

Reverse Osmosis systems and other membrane water filtration systems can be very effective in the water purification process. Reverse Osmosis is widely known for its use in desalination or removing salt dissolved in water, as well as its use in creating process water for dialysis machines. According to Anne Helmenstine, Ph.D, these machines work by applying pressure to force a concentrated water solution, or water that has bacteria, salt, or other foreign bodies dissolved in it, through a membrane.

The semi-permeable membrane contained in these types of systems works by trapping and filtering any ions (positively or negatively charged particles such as Na+) as well as large molecules (bacteria, urea, glucose) to pass through it. As you can imagine, those materials build up on the membrane and that can cause a lot of problems. For more information, please visit this great article on reverse osmosis:

What is Biofouling?

Biofouling, a term discussed by Thang Nguyen, Felicity A. Roddick, and Linhua Fan in their paper titled “Biofouling of Water Treatment Membranes: A Review of the Underlying Causes, Monitoring Techniques and Control Measures”, is when a membrane becomes overrun with organic debris and microorganisms. As these microorganisms attach themselves to the membrane, they are able to feed on the organic materials that are also stopped by the membrane. As this happens, the microorganisms are able to multiply. When this occurs, a wide variety of problems may present themselves, such as:

  • Decreased flow due to a buildup of bio-slime on the membrane
  • The need for increased feed pressure in order to keep the same production of process water
  • Biodegradation of the membrane due to acidic by-products of the organisms

(Nguyen et al. 2012)

How to Prevent These Problems

One of the ways to control these issues is the injection of biocides. There are many factors that affect the efficacy of these biocides such as number of microbes on the membrane, type microbes present, pH of the water, and concentration of the disinfectant used. Due to the nature of many disinfectants, the described factors will affect efficacy to a very noticeable extent.

For instance, some disinfectants’ effectiveness are reduced at certain pH balances. When considering chlorine, as the pH balance increases, the power of chlorine itself decreases. Another disadvantage of many biocides is the majority offer low selectivity, meaning a higher oxidation potential which further translates into less material compatibility and higher material corrosion. As such biocides come into contact with bio-slime, each will react with the polysaccharide layer that protects the microbes underneath of it.

Water purification field trial

 

Although these disinfectants have a higher oxidation potential, for example hydrogen peroxide, ozone or peracetic acid, they also have a lower oxidation capacity. In other words, each molecule responsible for the disinfection can accept fewer electrons, or less power to kill. Because of this, it may destroy some of the protective layer, yet leave the underlying microbes responsible for the film.

Chlorine dioxide is able to address the issues discussed above. Chlorine dioxide, or ClO₂, is effective at a wide pH range (4-10). Unlike other disinfectants, ClO₂ has a lower oxidation potential and thus is much more selective, also exhibiting a higher oxidation capacity. What this means is that it will not react solely with the aforementioned protective layer and instead choose to also react with the microbes underneath. Another advantage of ClO₂ is that it is a gas that does not carry a charge. This allows it to pass through small cracks and openings in the bio-slime. If you would like to see an actual protocol used to disinfect an RO system with ClO₂, please click here and read our available download.

Below is an image of bio-slime growth on filter media. Credit to Pure Fuel Technologies.

Biofilm on mesh filter