Tackling the biofouling challenge in SWRO
Biofouling is one of the most significant challenges in seawater reverse osomosis desalination plants. Hiroko Kasama looks at the effectiveness of different pretreatment methods and examines the market prospects for this sector.
Most seawater reverse osmosis (SWRO) failures are caused by pretreatment failures, and most pretreatment failures are associated with biofouling – the formation of biofilm over membranes. Biofouling can cause a serious decline in operational efficiency and increased costs due to impaired filtration efficiency plus increased frequency of RO membrane cleaning and replacement.
Typically, a permeate flux decline of about 20-30% is the threshold level of an ‘unacceptable' operational problem which requires management. Globally, biofouling is the most troublesome result of inadequate pretreatment, and the single biggest technological challenge faced by the SWRO desalination industry.
Unlike RO itself which is widely accepted, pretreatment technology for SWRO systems remains a relatively immature technology, with its success dependent on both good engineering and expert operational management. There are an increasing number of tools available to prevent biofouling, and a growing understanding of how those tools can be effectively employed.
Biofouling mechanisms, causes and impacts
RO membrane biofouling is associated with the development of biofilms made up of layers of various bacterial species and other microbial populations. They are held together in a sticky matrix of Extracellular Polymeric Substances (EPS), largely comprised of mucopolysaccharides, longchain polymers of amino sugars.
EPS are produced by bacteria to protect them from changing environmental conditions in the water. EPS trap nutrients from dissolved organic material (DOM), other suspended debris and other bacteria, which improves the resilience and durability of the biofilm. The phenomenon is often confused with organic fouling (i.e. deposition of organic substances such as oil, proteins and humic substances).
Biofouling potential varies from one location to another, as well as from one operator to another. The potential for fouling is generally higher in large SWRO plants with open intakes (especially shallow intakes) where local water quality is inferior and/or subject to fluctuation and alagal blooms. Other risk factors include an intake water temperature of 20°C or higher, storms and heavy rainfalls. Aside from water conditions or natural occurrences such as red tides, an important factor in biofouling is operational experience within the facility. An overdose of coagulant and/or inadequate mixing, as well as continuous chlorination, can lead to a higher risk of bacterial growth. Biofouling is especially problematic when the plant cannot go off-line in order to be disinfected (this is often the case when the plant is supplying domestic water in water-scarce regions). The energy consumption of a plant is impacted significantly due to the requirement for increased pressure to overcome the barrier. Biofouling is caused by a small number of bacterial species developing under specific conditions.
While seawater quality is is the principal factor determining biofouling potential, in many cases the problem is created by plant operators with insufficient experience. The understanding of the mechanism of biofouling in SWRO plants is still incomplete, and the conditions under which different types of bacteria can attach and multiply are not fully understood.
The quality of intake seawater varies from one region to another, and it is highly dependent on the type and depth of intake system, regional climate and microbiology. A beach well intake system is the most effective way to alleviate the risk of biofouling, as seawater of good and stable quality can be obtained. Open intake, especially shallow intake, makes the plant more susceptible to changes in water quality such as in the event of algal blooms and storms, when levels of particulates and dissolved organic substances tend to rise. However, for larger plants open intakes are used despite the disadvantage of greater complexity in the pre-treatment stage, as a greater production flow can be secured. There are two main pretreatment options – conventional and membrane pretreatment.
Conventional pretreatment – which is the most common form, consisting of coagulation/flocculation followed by media filtration, two-stage dual media filtration (DMF) or dissolved air flotation – as well as membrane treatments, have proven effective in various applications.
There is no single perfect solution to eliminate biofouling. Conventional pretreatment is very complicated to operate and the filters themselves can become filled with bacteria. However, the conventional pretreatment method remains the most common and cheapest option – the main driver for pretreatment technology is reduction in operational costs. This is especially important in countries where water charges must be kept low.
There is a potential market demand for ‘new and better' conventional pretreatment solutions. The effectiveness of measures will not correlate with the extent of disinfection in pretreatment, as biofouling occurs as long as conditions are favourable for even a very few surviving bacteria. According to Robert Huehmer, desalination technology leader at CH2M Hill, “Even the best pretreatment in the world will not solve biofouling problems if the SWRO environment is already suited for bacteria to cause biofouling.”
Biocides are agents used to inactivate microorganisms and inhibit biofouling. Most biocides used in SWRO systems are chlorine or chlorine-based chemicals, and they are often generated onsite using seawater due to its economic efficiency. Chlorine is the cheapest, most common and the most effective form of disinfection.
Typical doses in desalination plants range between 0.5 and 2 mg/l. Due to its strong oxidation potential and the possible degradation of polyamide RO membranes, free chlorine remaining in feedwater must be removed, or ‘dechlorinated', using sodium metabisulfite (SMB). However, chlorination followed by de-chlorination may cause bacterial after-growth; of major concern is the biological response to chlorination/dechlorination processes, which create nutrients for surviving bacteria to thrive. Application of biocides ‘stresses' bacteria and causes them to produce EPS which is the principal cause of biofilm formation. Changes in pH can also cause bacteria to produce more EPS.
Another problem associated with chlorination is the formation of carcinogenic by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs). In theory, trichloramine should not pass through RO membranes, but in practice the level of contamination is uncertain and levels of such by-products must be carefully monitored. Two other types of biocides, chloramine (NH2Cl) and chlorine dioxide (ClO2), are gaining recent attention as ways to alleviate the problems of chlorine dosing. Both compounds can be produced on-site using chlorine, and tend to reduce the negative impacts on RO membranes and by-product formation. However, they are less effective as disinfectants than chlorine.
Coagulants are widely applied in SWRO pretreatment. Most existing plants apply either ferric chloride (FeCl3) or ferric sulfate (FeSO4) as a primary coagulant/ flocculent in pretreatment systems. When these are added to water, a hydrolysis reaction precipitates out insoluble ferric hydroxide, which binds non-reactive molecules and colloidal solids to form larger aggregates or ‘floc' for filtration. Dissolved organic substances are associated with the floc particles formed. This effectively reduces the chance of adsorptive fouling of the membranes. The resulting ferric hydroxide floc is backwashed periodically.
Organic polymer can be applied, in addition, as coagulant aid.
Generally, membrane pretreatment is less reliant on coagulants and polymers in comparison with conventional pretreatment. This is due to the fine pore size rating of a pretreatment membrane, which makes it less dependent than a media filter on the formation of large robust particles for good removal efficiencies, allowing the application of low coagulant dose concentrations with shorter contact time.
Over-dosage of coagulants and/or insufficient mixing can also cause coagulant fouling. Rather than using the water quality test, some plants apply dynamic coagulant control, by which the dosage of coagulant is determined by changes in RO permeability. This can minimize the risk of coagulant fouling.
Prospects for ultrafiltration
With poor quality and/or highly variable feed water, UF pretreatment is becoming the technology of choice. Until 2004, the installed UF pretreatment capacity was less than 200,000 m³/d. However, it began to increase rapidly from 2005 and exceeded 1,000,000 m³/d by 2008. Norit estimates that the UF pretreatment market share today has reached 10-20% of the total pretreatment market, in terms of the total SWRO production capacity. Aside from Norit, the market for UF (and MF) pretreatment is dominated by Hyflux, Siemens and GE (see pie chart above). The main advantages of UF over conventional pretreatment are that
1) it produces higher quality feed water,
2) the quality of UF permeate is stable, and
3) it is simpler to operate.
In general, is often an appropriate choice in large-scale plants dependant on open-intake systems, particularly in regions with poor local qualities. Despite a growing number of installations and advantages, however, UF alone does not provide a complete solution against biofouling. In theory, UF is capable of removing large viruses, all bacteria and eukaryotic microorganisms in water. However, UF does not provide protection against many dissolved organic compounds, including some sugars. Typical removal of Transparent Exopolymer Particles (TEP) by UF is 30-60%, depending on the seawater composition and membrane type. Insufficient removal of nutrient sources leaves the feed water prone to bacterial growth if the UF treatment fails to achieve complete disinfection (which is often the case in reality). Extensive research on TEP removal in UF pretreatment systems has been carried out in the Netherlands by UNESCO-IHE and Delft University of Technology, with major sponsorship from Norit. Their major research focus is on ‘nutrient management' and a better understanding of microbial activities in biofilm formation, rather than simple ‘sterilization' of SWRO systems.
UF pretreatment has become ‘fashionable' in desalination despite being more expensive than conventional methods. UF pretreatment might be a more accepted option in Middle Eastern countries such as Saudi Arabia, where energy considerations are not a major issue. “UF pre-treatment may require slightly higher capital investment than conventional pretreatment depending on circumstances, typically with 5-10% higher capital costs; however, there is the possibility to reduce operational costs, and the price of UF membranes has dropped significantly, by about 20% since 2000” says Dr. Graeme Pearce, President of Membrane Consultancy Associates. “Although the technology is still at an early stage in seawater applications, there is a potential market globally as a better and cheaper alternative to conventional method.
With UF pretreatment, the total cost of pretreatment chemicals can be reduced by 40%. In particular, UF may enable a 50-100% reduction in coagulant dosage, depending on the intake water quality and the temperature.” If this trend continues, an increase in installed UF capacity could put a dent in the pretreatment chemicals market.
Current focus of R&D activity
There has been significant progress in gaining a primary understanding of the mechanisms of biofouling over the past decades, but much more effort is required to understand the operational and environmental factors which trigger biofouling. A major research effort has been carried out on the phenomenon of ‘concentration polarization (CP)' on SWRO membranes. CP is a flux-driven phenomenon in which concentration gradients of salts are formed on the feed and downstream sides of the membrane, due to the difference in permeate rates of feed mixture components. Biofouling can increase CP of downstream elements, as bacterial cells on the membrane delay the back diffusion of salts. The increase in CP leads to an increase in nutrient availability at the membrane, promoting further biofouling. Suggestions for future research often cite the need to shift from the traditional approach of attempting total disinfection. “Killing all the bacteria should not be the main focus, as it is not possible”, argues Robert Huehmer. “Future R&D should be focusing on controlling the physical response of surviving bacteria so the level of EPS is acceptable”.
Fairleigh Dickinson University and Singapore Membrane Technology Centre have identified a chemical compound effective in controlling bacterial activities responsible for the formation of biofilms. The chemical is naturally produced by organisms and cheap to produce, and applicable in SWRO, as well as in other industries. A number of demonstrations and a pilot study (in the Caribbean) showed very positive results, with no major issues. This substance can be patented, and is expected to be commericalised soon.
Market forecast for pretreatment chemicals
Broadly speaking, the market drivers for pretreatment mirror those for desalination – water scarcity and increasing industialisation. The continued increase in size of desalination plants means open intakes which are more vulnerable to biofouling are being used more widely. Cost is an over-riding factor for operators when selecting chemicals. For daily operational purposes, cheaper chemicals such as chlorine are used and are usually supplied by local traders. Operators are willing to pay more for speciality chemicals and those used in non-continuous applications, such as membrane cleaning and backwashing. Some of the major competitive players in the pretreatment chemicals sector include American Chemicals, PVS Technologies, Nalco, Kemira, GE, BKG Water Solutions, ACIDEKA, Solvay Chemicals and Cross Chemical, Avista, Dow and Genesys. An important competitive factor is the level of technical support – plant managers tend to select chemicals based on price and brand recognition, particularly in the absence of information on the molecular structure of the chemicals. The trend is shifting towards using fewer (intermittent) or no pretreatment chemicals in RO pretreatment, which has been shown to be more cost effective. There are some plants which do not apply disinfectants in pretreatment.
As more successful cases get reported, it is possible that the trend will shift towards reduction or omission of disinfection chemical applications. The increase in the global RO production capacity implies a significant growth opportunity for the water pretreatment chemicals market. The expected compound annual growth rate in the volume of new RO capacity contracted each year is expected to be around 15% (compared to 6% for thermal desalination). The graph on page 41 shows the estimated market in 2006-2016 for membrane desalination chemicals for SWRO chemicals by type. It should be noted that the SWRO chemicals market accounted for about 30% of the total RO chemicals market in 2006, but it is predicted to account for about 60% of the total market in 2016. The data might appear to underestimate the market size for biocides – principally sodium hypochlorite (NaOCl) – that is used in desalination. Typically this is produced on-site from seawater brine using electrochlorination.
Scrubbing up well
References and more Information:
1. Tackling the biofouling challenge, Global water Intelligence, Vol 12, Issue 4 (April 2011)
Compiled by Rami E. Kremesti M.Sc.