How can membrane fouling be prevented?
Membrane fouling by bacterial biofilms has remained a persistent and unmet challenge for membrane-based water purification systems worldwide.
Even if you remove 99.9% of pathogens from water there will still be enough cells remaining which will continue to grow and form biofilms.
Biofouling alone contributes to more than 45% of all membrane fouling and has been reported as a major problem in ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membrane filtration.
As an owner or operator of membranes in any industry, you will know all too well, biofouling can have several adverse effects on membrane systems such as:
- Membrane flux decline, due to the formation of a low permeability biofilm on the membrane surface.
- Increased differential pressure and feed pressure needs to maintained to ensure the same production rate due to biofilm resistance.
- Membrane biodegradation can be caused by acidic by-products which are concentrated at the membrane surface.
- Increased contaminant in the passage through the membrane results in reduced quality of the product water due to the accumulation of dissolved ions in the biofilm at the membrane.
- Increased energy consumption is needed due to higher pressure being required to overcome the biofilm resistance and the flux decline.
Biofouling can be considered as a biotic form of organic fouling, while fouling caused by organic matter derived from microbial cellular debris can be considered as an abiotic form of biofouling.
How do you control membrane biofouling?
There are several strategies for controlling membrane biofouling:
- Adding disinfectants and biocides
- Adding specific molecules to influence quorum sensing (QS) in biofilms to trigger their dispersal
- Modifying the membrane surface (or spacers) to reduce biofilm attachment and growth
Most current biofouling control techniques are either effective only initially due to the ability of the biofilms adaptation to the conditions over time. Or the process needs a repeated application to control biofouling effectively in the long run.
New methods are needed to control persistent biofouling.
Chlorine is the most widely used disinfectant in water and wastewater treatment.
Unfortunately in most cases chlorine (either as a gas or in the hypochlorite form) cannot be used for membrane treatment. Most commercially available polymeric membranes are sensitive to chlorine due to the production of a large amount of assimilable organic carbon (AOC) which stimulates more bacterial growth.
Other biocide treatments have been used in water treatment, such as:
- Non-oxidizing biocides (formaldehyde, glutaraldehyde and quaternary ammonium compounds)
However, as with most current practices, their long term use may lead to microbe resistance. This is a large drawback with using biocides in water treatment processes.
The physical cleaning/backwashing of membranes can effectively remove the non-adhesive foulants from membrane surfaces and thus reduces fouling at relatively low maintenance cost. However, this process requires high energy and adding a further step like ultrasound can again damage membranes.
What is the best technology to control membrane biofouling?
One emerging technology that is has the potential to play a big role in preventing biofouling is ESOL technology.
ESOL is a strongly oxidising but low chlorine disinfecting solution suitable for use in a range of industries.
ESOL is produced via the electrolysis of a low mineral salt solution (the electrolyte) which when a current is applied, electrochemical processes at the material electrode surface transform the electrolyte (NaCl) into an activated ‘metastable’ state, exhibiting elevated chemical reactivity and resulting in the modification of molecular ionic structures.
ESOL has an Oxidation Reduction Potential of between +1,100mV and +1,200 mV which exerts an 'oxidative stress' and prevents microbes from adapting to its mode of action. ESOL has a broad spectrum of antimicrobial activity with a rapid disinfection time, is created using only salt and water making it inexpensive to produce, is produced in-situ requiring little operator skill, and is non-toxic and biodegradable.
In a paper published in 2017 in ‘the Journal of Water Process Engineering’ ESOL was assessed in terms of water disinfection capability and also biofouling potential. UF membrane health was determined by calculating the pressure diﬀerential across the UF membrane column module and converting this to membrane permeability, the industry standard for membrane health, as shown below:
The stable permeability of the UF membranes during ﬁeld trial 1 (whereby 0.5% (v/v) ECAS was dosed pre-UF membranes) indicated that ESOL managed bioﬁlm formation on the UF membranes throughout the duration of the trial. During field trial 2 (control; no ESOL added) large ﬂuctuations in permeability within the UF membranes was observed, which the authors suggested was “indicative of less stability, and possible bioﬁlm formation”.
There is a growing body of evidence that demonstrates ESOL’s effectiveness at managing biofilms. Biofilm causing microbes are not able to form a resistance to ESOL due to its rapid mode of action and the presence of hydroxyl radical in ESOL solutions also contributes to a reduction in AOC concentration minimising further bacterial growth. The fact that it is also a low chlorine technology makes it an ideal solution to control biofouling on delicate membranes.
- E. Clayton, R. M.S. Thorn & D. M. Reynolds. (2017) Development of a novel oﬀ-grid drinking water production system integrating electrochemically activated solutions and ultraﬁltration membranes. Journal of Water Process Engineering. In Press