FIELD OF THE INVENTION

[001] The present invention relates to a method for treating membranes used in water treatment. The membranes may be, for example, reverse osmosis membranes, nanofiltration membranes, ultrafiltration membranes and microfiltration membranes.

BACKGROUND TO THE INVENTION

[002] Securing urban water supply will be one of the major challenges in Australia and many parts of the world in the decades to come. The current, low cost method of harvesting water through rainwater catchments is reaching its limits as a result of increasing population growth, unreliable rainfall patterns, environmental concerns and political sentiment against construction of new dams. Recycled water and desalinated water are universally acknowledged as alternative climate-independent sources of water that need to be further exploited. In South East

Queensland, three water recycling plants and one desalination plant have been constructed and become operational in the last two years, with a combined capacity to provide >30% of the total water demand in this region. Across Australia, six desalination plants will be operational within the next two years. This trend is global.

[003] Membrane filtration processes are at the heart of most water recycling and desalination schemes producing high quality water, since they have an exceptionally low area footprint and high throughput, are modular by nature, provide excellent physical barriers against water born pathogens, and are relatively cost-efficient. These features apply to low pressure (microfiltration and ultrafiltration) and high pressure (nanofiltration, reverse osmosis) membrane filtration.

Reverse osmosis in particular provides excellent rejection of salt (approx. 99%) and organic pollutants (>90% for most micropollutants), and is therefore indispensible for the production of potable water.

[004] Membrane fouling is a major operational problem for membrane filtration systems, which substantially increases the costs of membrane filtration through increased energy consumption, the requirement of regular chemical cleaning of membranes and membrane replacement. According to a large water reuse application case study, chemical consumption and membrane replacement (both related directly and indirectly to membrane fouling and its control) account for approximately a quarter of the total treatment cost associated with reverse osmosis, including capital expenditure. This is even higher than the cost of energy (20-25% of total cost of produced water). In smaller schemes, the relative indirect costs associated with membrane fouling via chemical consumption and membrane replacement are even higher.

[005] There are three types of fouling that occurs on water treatment membranes, namely, inorganic fouling, organic fouling and biofouling. Biofouling is caused by the development of biofilms on the surfaces of membranes. Biofilms form on membrane surfaces initially through colonisation of microorganisms present in the feed water and the subsequent growth and formation of an extensive matrix of extracellular polymeric substances (EPS). The EPS matrix along with other organic and inorganic materials reduces the membrane permeability and increases energy consumption. Chemical cleaning is regularly required to restore the membrane fluxes. The commonly used cleaning agents are alkalis, acids, metal chelating agents, surfactants, oxidising agents and enzymes. All these chemicals are non-renewable products and their production, transport and disposal incur significant costs, both economically and

environmentally. A further problem is that some of the most biocidal oxidising agents such as chlorine and chlorine dioxide are incompatible with polyamide membranes. Even the commonly used, but weaker disinfectant chloramine has to be kept to low concentrations (l-5mg/L) to avoid membrane damage. This in turn limits its biocidal activity particularly once biofilm formation has been initiated.

[006] Water utilities that operate desalination and water recycling facilities that form an adjunct to water supply from dams or natural watercourses also operate their desalination and water recycling facilities in a stand-by mode when the dams or other natural watercourses are at high capacity due to good rainfall. At such times, it is economically sensible for the water utilities to minimise the production of water from their desalination or water recycling facilities. However, in stand-by mode, it is also desirable to keep moist or wet the membranes used in the

desalination or water recycling facilities in order to prevent degradation or cracking of the membranes, which can occur if they dry out. However, simply keeping the membranes in contact with water can lead to biofilm growth.

BRIEF DESCRIPTION OF THE INVENTION

[007] In a first aspect, the present invention provides a method for treatment of a membrane used in water treatment, comprising the steps of contacting the membrane with a solution containing free nitrous acid.

[008] In a second aspect, the present invention provides a method for treatment of a membrane used in water treatment, said membrane having a biofilm thereon, the method comprising the steps of contacting the membrane with a solution containing free nitrous acid to thereby disrupt the biofilm.

[009] In a third aspect, the present invention provides a method for maintaining a membrane using water treatment in a stand-by mode, the method comprising the steps of contacting the membrane with a solution containing free nitrous acid. This method is a subset of the method of the first aspect of the present invention. In this aspect, the membrane may be stored in the solution containing free nitrous acid. The membrane may be stored in a solution containing free nitrous acid for an extended period of time.

[0010] In some embodiments, the solution containing free nitrous acid may contain free nitrous acid at a level of at least 0.1 ppm, more preferably from 0.1 ppm to 10000 ppm, more preferably from 0.3 ppm to 5000 ppm, even more preferably between 1 and 1000 ppm, or even between 10 and 100 ppm.

[0011] In some embodiments, the solution containing free nitrous acid may comprise a solution containing nitrite and having a pH of less than 7. Preferably, the solution containing nitrite has a pH of less than 6.5. In some embodiments, the solution containing nitrite may have a pH of between 1 and 6.5, for example, between 2 and 6, or even between 2 and 4. Accordingly, in another aspect, the present invention may be considered to involve contacting the membrane with a solution containing nitrite and having an acidic pH.

[0012] The method of the present invention may involve contacting the membrane with the solution containing free nitrous acid for a period that is sufficiently long to disrupt a biofilm that has formed on the membrane. The contact time may vary, with times of between 1 minute and 1 day being useful. However, the present invention encompasses all possible contact times.

[0013] The method may involve the steps of taking the membrane off-line in a water treatment plant and subjecting the membrane to contact with the solution containing free nitrous acid.

[0014] The method of the present invention may be used as replacement for present cleaning methods or may be used in conjunction with present cleaning methods for cleaning water treatment membranes. For example, cleaning with other acids (such as hydrochloric or phosphoric acid) could be complemented by the method of the present invention. Alternatively, treatment of the membrane with the solution containing free nitrous acid may be able to replace cleaning with hydrochloric or phosphoric acids. [0015] In some embodiments of the present invention, the method comprises the steps of contacting the membrane with a solution containing free nitrous acid and one or more other chemicals. The one or more other chemicals may comprise, for example, hydrogen peroxide, other sanitising agents, other cleaning agents, or other acids. The other chemicals are desirably compatible with the membrane being contacted by the solution. The other chemicals are desirably compatible with maintaining a solution of free nitrous acid.

[0016] In some embodiments, the solution containing free nitrous acid may come into contact with the membrane under conditions of high shear. For example, the solution may be provided at high velocity into a housing containing the membrane, or an agitator may be used to agitate or stir the solution that is in contact with the membrane. The high shear conditions are likely to enhance biofilm disintegration. If spiral membranes are being used, high velocity may be used to generate high shear conditions.

[0017] If the biofilm forms particularly problematic extracellular polymeric substances (EPS), it may be necessary to also treat the membrane with other treatment methods that facilitate EPS removal. It is expected that this would still result in more cost-effective cleaning methods.

[0018] The solution containing free nitrous acid may be formed from a nitrite containing solution generated in a water treatment processing plant. In this manner, formation of the solution containing free nitrous acid may occur at relatively low cost. Furthermore, in this embodiment, large quantities of solution containing free nitrous acid can be formed. Formation of the free nitrous acid stream can be achieved using biological processes. The nitrite containing solution may be generated in accordance with the process as described in our international patent application number PCT/AU2011/000482, the entire contents of which are incorporated herein by cross reference.

[0019] The present invention is based upon a discovery by the present inventors that free nitrous acid can deactivate microorganisms and cause substantial cell death and biofilm detachment. The present inventors have found that solutions containing free nitrous acid are biocidal at free nitrous acid contents in the parts per million range and thus can kill the microorganisms responsible for biofilm formation on membranes. Further, solutions containing free nitrous acid are also acidic and thus can also dissolve some inorganic precipitates in or on the membrane.

[0020] In some embodiments of the present invention, the solution containing free nitrous acid is used as a non-oxidising biocidal cleaning agent, compatible with reverse osmosis membranes, nanofiltration membranes and other membranes used in water treatment processes, for biofouling control on these membranes. In some embodiments of the present invention, the membranes are treated with the free nitrous acid at predetermined periods of time. Treatment of the membranes with free nitrous acid may take place before formation of any biofilm on the membrane has progressed to a stage where performance of the membrane is compromised. Those skilled in the art will understand that the period between treatments may vary from plant to plant. Similarly, the time at which it is necessary to contact the membrane with the free nitrous acid may vary. A person skilled in the art will be readily able to determine the necessary contact times and periods between treatments that are required to maintain satisfactory membrane performance in any particular plant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 shows a graph of bacteria % (% live and % dead) following treatment of a membrane that had been exposed to a reverse osmosis influent.

EXAMPLES

[0022] In order to develop biofouling on reverse osmosis membranes, a series of experimental runs were conducted on a bench-scale cross flow filtration unit in which a secondary effluent pre-treated with coagulation and microfiltration and spiked with 100 μgC/L of sodium acetate was recirculated through the membrane. The following conditions given in Table 1 were used:

Table 1:

Parameter Runs 1 2 Runs 3 4

Feed flow rate 35-45L/h 45-50L/h

Feed Pressure 6-l lbar 5-7bar

Flux 26-36L/(m ² .h) 17-30L/(m ² .h)

Temperature (°C) 22-27 22-27

Operational time About 1 week about 2 weeks

Final permeability 50 - 60% 85-90% [0023] In Table 1 , the final permeability is given as a percentage of the original permeability. The thin film composite reverse osmosis membrane used in these experiments had a size of 14.6cm x 9.5cm and an effective membrane area of 0.014 m ² .

[0024] In order to clean the membranes resulting from the Runs 1, 2 ,3 and 4, the following cleaning procedure was followed:

[0025] A stock solution of lOOOmgN/L NaN0 ₂ was prepared. This NaN0 ₂ solution was then diluted to the required concentration. The pH was adjusted to the required value by adding 1M HC1.

[0026] The cleaning of the reverse osmosis membranes obtained from the fouling trials was performed by first adding 25 mL of the cleaning solution to a beaker. Coupons of the reverse osmosis membrane were then cut to size (3cm x 3cm), submerged in the cleaning solution containing free nitrous acid and shaken for 24 hours.

[0027] For the analyses the reverse osmosis couponswere removed from the beaker after cleaning. The cleaning solution was retained for later analysis. The biofilm was then detached by brushing the surface of the membrane coupons. The detached biofilm was dissolved into l OmL of milliQ water. Half of the biofilm sample was then treated for live/dead staining.

[0028] Figure 1 shows the live/dead percentage of bacteria plotted against the cleaning conditions used. In figure 1 :

[0029] UPW denotes ultra pure water was used for cleaning at the specified pH;

[0030] TN10 specifies that a nitrite solution containing lOmgN/L was used for cleaning at the specified pH;

[0031] TN50 specifies that a nitrite solution containing 50mgN L was used for cleaning at the specified pH; and

[0032] TNI 00 specifies that a nitrite solution containing lOOmgN/L was used for cleaning at the specified pH.

[0033] The results shown in Figure 1 demonstrate that a solution containing free nitrous acid can be used to control membrane fouling in reverse osmosis systems. In particular, the solution containing free nitrous acid can kill the bacteria growing on the membrane and significantly contribute to the removal of biofouling from the membrane. [0034] Those skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. It will be understood that the present invention encompasses all such variations and modifications to fall within its spirit and scope.

[0035] Throughout the specification, the term "comprising" and its grammatical equivalents shall be taken to have an inclusive meaning unless the context of use indicate otherwise.