B & V Water Treatment can supply, install and maintain reverse osmosis plant and equipment. We also have a full range of reverse osmosis antiscalants and membrane cleaning chemicals available.
Removal of dissolved solids from water by reverse osmosis can produce water of similar purity to that produced by ion exchange technology, but with the advantage of not requiring the hazardous chemicals that are required to regenerate an ion exchange deioniser.
Some typical applications that benefit from this process are high pressure steam boiler feeds, high pressure hot water boilers, rinse water for printed circuit boards, rinse water for electronics manufacture, laboratory water, haemodialysis, anodising, rinse water for electroplating, rinsing of metals prior to painting, thinning of water based lubricants, paint manufacture, pharmaceuticals, battery manufacture, process water for chemicals and glass window manufacture.
In order to select the correct model for a particular application, several related factors need to be taken into consideration. The most important of these are as follows:
Reverse Osmosis Services B & V Water Treatment Provide
- Daily usage of treated water
- Quality of water required
- Quantity of water at peak flow rates
- Whether a continuous supply of water is required
- Chemical composition of the raw water to be treated
- Whether the raw water requires pre-treatment by softening and carbon filtration
- Whether a raw water tank is required
- Whether a treated water tank is necessary
- Method of distribution of the treated water to the point of use
Effective reverse osmosis (RO) cleaning usually requires some knowledge of the type of foulant and the cleaning options available. It is essential to clean membranes at an early stage of fouling. It is often difficult to clean excessively fouled membranes and irreversible damage may occur during the cleaning process. Cleaning is recommended when your RO shows evidence of fouling, just prior to a long-term shut down or as a matter of scheduled routine maintenance. As a rough guide a generally acceptable cleaning frequency is once every 3 – 12 months. If you have to clean more than once a month you should be able to justify further capital expenditure for improved RO pre-treatment, or a redesign of the RO operation. If cleaning frequency is every 1 – 3 months you may want to focus on improving the operation of your existing equipment.
The B & V Water Treatment RO product range is designed to enhance cleaning efficiency. RO cleaning frequency, due to fouling and type of fouling, will vary by site. Unfortunately what often complicates matters is that it is common for more than one foulant to be present on membranes which need cleaning. Technical aspects of the system design and operation, along with feedwater and membrane foulant analysis should be considered before designing a membrane cleaning programme. Step-by-step procedures to include pH, temperature and cleaning duration, along with product selection, sequence and concentration can be provided by your B & V representative. This document reviews basic considerations and guidelines for improving onsite membrane cleaning.
The B & V Water Treatment RO Product Range
The B & V Water Treatment RO product range consists of acid- and alkali-based cleaning chemicals, designed to remove most foulants and restore performance in all types of membrane systems. Extensive experience with combinations of these formulations has enabled B & V Water Treatment’s technical specialists to identify a range of cleaning schedules for common types of fouling. These include recommendations for cleaning RO, NF, UF and MF membrane materials in spiral wound, hollow fibre, flat sheet or tubular configurations.
It is not unusual to have to use a number of different cleaning chemicals in a specific sequence to achieve the optimum cleaning. There are times that a low pH cleaning is used, first to remove foulants like mineral scale, followed by a high pH cleaning to remove organic material. However, there are times that a high pH cleaning is used first to remove foulants like oil followed by a low pH cleaning. Some cleaning solutions have detergents added to aid in the removal of biological and organic debris, while others have a chelating agent like EDTA added to aid in the removal of colloidal material, organic and biological material and sulphate scale. An important thing to remember is that the improper selection of a cleaning chemical or the sequence of chemical introduction can make the foulant worse.
B & V Water Treatment RO Product Range Selection Guide
The products listed in Table 1 are suitable for cleaning fouled polyamide and polysulphone membranes. Membrane manufacturers’ recommendations should always be followed with respect to pH, pressure and flow rate.
B & V Water Treatment RO Product Range Summary
It is important for the operator to refer to the membrane manufacturer’s instructions before starting any cleaning procedure. All B & V Water Treatment RO range products are liquid formulations. They are produced mainly in acidic or alkaline form and are often classified as ‘hazardous’. For this reason, as with other industrial chemicals, the operator must be aware of all the safety procedures before using any of the B & V Water Treatment RO range of products. Safety data sheets are available for every product.
If you system has been fouled biologically you may want to consider the extra step of introducing a sanitising biocide chemical after a successful cleaning. Biocides can be introduced immediately after cleaning, periodically (e.g. once a week) or continuously during service. You must be sure however that the biocide is compatible with the membrane, does not create any health risks, is effective in controlling biological activity and is not cost prohibitive.
The nature and rapidity of fouling depends on a number of factors, including:
- Quality of the feedwater
- System recovery rate
- Element flux
Typically fouling is progressive and, if not controlled early, will impair the RO membrane element performance in a relatively short time.
General Precautions in Cleaning and Chemical Selection and Usage
Use the least harsh cleaning regime to get the job done. This includes the cleaning parameters of pH, surfactants such as RO281 and RO280 for organic debris removal. RO315 is also used for online shock dosing to control microbial growth in non-potable membrane applications. RO315 may be used as a long-term preservative for membranes under storage for 24 hours up to 6 months. RO315 is compatible with all membrane types.
RO325 is a non-oxidising micro biocide based on DBNPA which has a broad spectrum of activity. RO325 will destroy both planktonic- and biofilm-creating micro-organisms in membrane systems, pipework and pre-treatment plants. RO325 is fast acting and highly effective. It is ideal as an online treatment for non-potable water applications. RO325 is compatible with all membrane types.
Foulants on the membrane surface can cause an increase in differential pressure (IP), an increase in product water conductivity, a flux loss requiring an increased feed pressure to maintain output or a combination of these effects.
The surface of the RO membrane is subject to fouling by foreign materials which may be present in the feed water. Examples are:
- Calcium Carbonate scale
- Sulphate scale of Calcium, Barium or Strontium
- Hydrates of metal oxides (iron, manganese, copper, nickel, aluminium etc.)
- Polymerized silica scale
- Inorganic colloidal deposits
- Mixed inorganic/organic colloidal deposits
- Natural Organic Matter (NOM) organic material
- Man-made organic compounds (e.g. antiscalant/dispersants, cationic polyelectrolytes)
- Biological (bacterial bio slime, algae, mould or fungi)
The term fouling used here includes the build-up / deposition of all kinds of layers on the surface of the membrane, including scale formation.
Biofouling due to microbiological deposits is the most common cause of poor membrane performance. These organic based deposits can also be difficult to remove, particularly if the feed path is plugged. Biofouling is primarily due to the accumulation of extra cellular polysaccharide substances (EPS) secreted by microorganisms (bacteria, fungi and yeasts) that enter the membranes in the feed water, or growth within the system.
Plugging of the feed path makes it difficult to introduce and distribute the cleaning solutions. This phenomenon may even occur with chlorinated feed waters; the use of chlorination is no guarantee in the prevention of membrane biofouling. Excess sodium-bisulphate (which is used for the neutralisation of free chlorine) may accelerate biofouling. To inhibit additional growth it is important to clean and sanitise, not only the RO system, but also the pre-treatment, piping and deadlegs etc. Recommended cleaning programmes need a combined sanitisation and cleaning procedure that is usually carried out in stages. High pH cleaners in association with biocide treatments are most effective against this type of problem.
Due to the increasing salt concentration of the feed/brine as it passes over the membrane surface, scale formation is most likely to occur at the concentrate end of the plant. The use of an effective antiscalant will prevent this from occurring. Scales found in RO membranes include calcium carbonate, calcium and barium sulphates and calcium and magnesium silicate.
Calcium carbonate scale: calcium carbonate is a mineral scale that may be deposited from almost any feed water is there is a failure in the antiscalant / dispersant addition system or in the acid injection pH control system. An early detection of calcium carbonate scaling is essential to prevent damage caused by the crystals on the active membrane layers. Calcium carbonate scale detected early can be removed by lowering the feed water pH to between 3 and 5 for one or two hours. Longer resident accumulations of calcium carbonate scale can be removed by a low pH cleaning with a citric acid solution.
Calcium, barium and strontium sulphate scale: sulphate scale is a much “harder” mineral scale than calcium carbonate and is therefore more difficult to remove. Sulphate scale may be deposited if there is a failure in the antiscalant/dispersant feed system or if there is an over feed of sulphuric acid in pH adjustment. Early detection of the resulting sulphate scaling is essential to prevent damage caused by the crystals on the active membrane layers. Barium and strontium sulphate scales are particularly difficult to remove as they are insoluble in almost all cleaning solutions.
Calcium phosphate scale: This sale is particularly common in municipal wastewaters and water supplies which may contain high levels of phosphate. This scale can generally be removed with acidic pH cleaners.
Metal oxide / hydroxide foulants: Typical metal oxide and metal hydroxide foulants are iron, zinc, manganese copper, aluminium etc. They can be the result of corrosion products from unlined pipes and tanks; from oxidation of the soluble metal ion with air, chlorine, ozone, potassium permanganate; or from a pre-treatment filter system upset that utilises iron or aluminium based coagulant aids. These can generally be removed with low pH cleaners.
Polymerised silica coating: Colloids are inorganic or mixed inorganic / organic based particles that are suspended in water and will not settle out due to gravity. Colloidal matter typically contains one or more of the following major components: iron, aluminium, silica, sulphur or organic matter. High pH cleaners are generally more effective against this type of foulant.
Dissolved NOM / organic foulants: These are generally derived from the decomposition of vegetative material into surface waters or shallow wells. The chemistry of organic foulants is very complex, with the major organic components being either humic acid or fulvic acid. Dissolved NOMs can quickly foul RO membranes by being absorbed onto the membrane surface. Once absorption has occurred then a slower fouling process of gel or cake formation begins. It should be noted that the mechanism of fouling with dissolved NOM should not be confused with the mechanism of fouling created by NOM organic material that is bound with colloidal particles. High pH cleaners are generally more effective against this type of foulant.
Membrane Cleaning Stages
The selection of the most suitable cleaning products depends on the foulants present. It is often advisable to use a combination of B & V Water Treatment RO range products in one or more cleaning stage. In some situations the sequence of applying B & V Water Treatment RO range products is important. The table below provides a quick reference chart for removing specific foulants from membrane systems. Laboratory research has shown that if the membrane is contaminated with certain organic foulants such as humic acid, the use of an acid before an alkaline detergent may result in irreversible flux decline. For this reason an alkaline surfactant should be used as the first part of the cleaning cycle if the fouling type is unknown or is likely to contain organics.
When to Clean the Membrane
Monitoring overall plant performance on a regular basis is an essential step in recognising when membrane elements are becoming fouled. Performance is affected progressively and in varying degrees, depending on the nature of the foulants. It is essential to clean membranes at an early stage of fouling, since it may be difficult to clean excessively fouled membranes. If fouling is allowed to accumulate irreversible damage may occur during the cleaning process. If normalised membrane performance drops to 30-50%. It may be impossible to restore the performance back to baseline conditions. Cleaning is recommended when one or more of the following parameters change by 10-15%:
- An increase in IP across the plant
- An increase in feed pressure
- A decrease in normalised permeate flow
- An increase in salt passage
Regular maintenance cleaning is recommended to keep the membranes in good condition. The frequency of cleaning can vary from monthly to annually dependent on the process involved and the degree of fouling. It is important to clean the membranes at an early stage of fouling. If performance deteriorates by more than 30% it may be impossible to recover plant performance by routine cleaning practises since flow restrictions may cause channelling inside the membrane element. Provided cleaning is carried out before the fouling becomes a severe problem, the membranes can usually be recovered and no irreversible damage will be done.
It is not unusual for B & V Water Treatment to recommend the use of a number of different cleaning chemicals in a specific sequence to achieve the optimum cleaning. Typically a low pH cleaning is used, first to remove foulants like mineral scale, followed by a high pH cleaning to remove organic material. There are times that a high pH cleaning is used first to remove foulants like oil or biological matter, followed by a low pH cleaning. Some cleaning solutions have detergents added to aid in the removal of heavy biological and organic debris, while others have a chelating agent like EDTA added to aid in the removal of colloidal material, organic and biological material and sulphate scale.
An important thing to remember is that the improper selection of a cleaning chemical, or the sequence of chemical introduction, can make the foulant worse.
In-Situ Membrane Cleaning
Membranes may be cleaned in-situ by taking the plant ‘offline’ and circulating the cleaning solution through the pressure vessels in parallel. With multi-staged plants each stage should be cleaned in isolation with the other stages disconnected, so the cleaning solution from the stage being cleaned returns directly to the cleaning tank. Even under low pressure a small amount of product water will be produced, which should be returned to the CIP tank. It is generally advisable to clean with the product water valve open and the plant making water. Care should be taken when the recycle pump is turned off to avoid creating an osmotic pressure that can cause ‘suckback’. Reversing flow across the membrane may permanently damage the membrane. Advice on this should be obtained from the system’s manufacturer.
General Guidance Notes for Cleaning
- A centrifuge pump of corrosion resistant material designed to deliver the cleaning solution to all the pressure vessels in the first stage at a pressure drop 4.1 bar (60 psi) per pressure vessel (multi 8” membranes)
- One 5-micron cartridge filter to prevent suspended solids re-entering the membrane system on recirculation
- Pressure control valves, flow meters, sample and drainage points
- Always refer to the membrane manufacturer’s specification concerning flow rates, pressure, pH and temperature.
- Ensure all hoses, connections and fittings can withstand the required temperature, pressure and pH
- All cleaning solutions should be prepared with chlorine-free product water
Always add cleaning chemicals slowly to water not water to chemicals, and make any necessary changes to pH and temperature gradually
- Use good quality sodium hydroxide or hydrochloric acid for pH adjustment
- Beware: small volumes of acids and alkalis can rapidly change the overall pH of the cleaning solution
- Do not use sulphuric acid for cleaning or pH adjustment
- Some chemical reactions are exothermic and generate heat. Monitor pH and temperature throughout the cleaning programme
- Do not use seawater for making up alkaline cleaning solutions
One RO design feature that is commonly overlooked in reducing RO cleaning frequency is the use of RO permeate water for flushing foulants from the system. Soaking the RO elements during standby with permeate can help dissolve scale and loosen precipitates, reducing the frequency of chemical cleaning.
Cleaning System Components
B & V Water Treatment Cleaning Tests
- A cleaning tank, usually plastic, sized to give 5 minutes retention time (or 55 litres [14.5 gallons] cleaning solution per 8” element), complete with a heating/cooling coil and stirrer
- Temperature and pH indicators are essential
- An inlet and outlet manifold system. Product and reject flows should be returned to the tank to minimise changes in pH and cleaning solution strength
- Reject return line should be submerged below cleaning solution level to reduce foaming
- Rinse membranes thoroughly between cleaning steps with chlorine-free water
- Before returning the system to service ensure that all residual cleaning products are removed from the membrane surfaces by thoroughly flushing and discharging all permeate during this period
- Be aware that cleaning programmes, such as those using high pH, may cause short term changes to product water conductivity
Your B & V Water Treatment representative, in conjunction with the B & V technical support team, is available to carry out site reviews, cleaning supervision (if requested) or arrange for membrane autopsy and chemical analysis to assess plant fouling problems and recommend solutions, in addition to standard water analysis and membrane autopsy.
Membrane Manufacturers Cleaning Instructions
Most membranes respond to an alkaline detergent wash at high pH, followed by an acidic wash at low pH or vice-versa. Increasing or decreasing the cleaning solution pH is often the simplest and the most effective means of cleaning fouled membrane systems.
Increasing the cleaning solution temperature enhances most chemical reactions. All membranes have a maximum temperature limit for a particular pH which should never be exceeded.
Pressure and Flow
Membrane manufacturers give advice on cleaning pressures and flow rates, which should be closely followed. The high-pressure feed pump should never be used to circulate cleaning solutions. The feed to concentrate flow path should always be followed unless otherwise stated.
Most cleaning programmes require a turbulent flow to help remove debris from the membrane surface, which means maintaining a minimum water velocity. Excessive pressures must be avoided to prevent telescoping and particulate matter being forced into the pores in the membrane surface.
Membrane manufacturer’s cleaning instructions must always be followed with respect to pH, temperature, flow rate and differential pressure (IP). Guidelines for these parameters appear in the table below. Membranes should always be flushed out thoroughly with good quality, chlorine-free water between each cleaning stage.
Top Ten Tips for Effective Cleaning of Reverse Osmosis plant
1. Clean membranes on a regular basis, or when differential pressure (IP), normalised permeate flow, salt passage or feed pressure changes by 10-15% from the design limits. Regular and careful membrane cleaning is necessary and should not shorten the membrane life.
2. i) Organic foulants: clean with an alkaline surfactant such as RO281 or RO280 to break down and remove organic matter and biofilms. Acid flushing may follow this programme if necessary.
ii) Scale deposits: calcium carbonate, iron oxide and iron hydroxide clean with a low pH cleaner. Calcium sulphate, strontium sulphate, barium sulphate and calcium fluoride clean with RO260 at alkaline conditions.
*If there is uncertainty if the type of fouling always start with an alkaline cleaning product.
3. Flow rates during cleaning must be sufficient to remove foulants from the membrane element but not exceed manufacturer’s limits. Flow rate should not exceed the feed pressure and pressure drop (IP) limitations determined by the membrane element manufacturer. Typical flow rates for membrane cleaning are shown in the table below.
4. The maximum recommended pressure drop during membrane cleaning of 8” membranes should not exceed 1.4 bar [20 psi] per element or 4.1 bar [60 psi] for a multi-element pressure vessel.
5. A cleaning solution volume of 55 litres [14.5 gallons] is recommended per 8” x 40” membrane element; this excludes pipework volumes. A minimum of 40 litres [10.5 gallons] of cleaning solution is advised for each membrane element.
6. Where practicable, warm the cleaning solution to the highest temperature allowed by the membrane manufacturer. Typical cleaning solution temperatures should be 25 - 35°C [77 – 95°F] although some membrane elements can tolerate even higher temperatures.
7. Soak the membranes in cleaning solution for a minimum of 15 minutes before recirculation. This procedure should be repeated regularly throughout the cleaning.
8. Flush the pipework, membranes and cleaning tank thoroughly with chlorine-free water between each cleaning cycle when returning the plant to normal operation.
9. When cleaning multi-staged plant clean each stage individually.
10. Don’t panic when the plant returns to service and operating conditions are not improved or are even worse than at the start of the cleaning. Many of the cleaners used temporarily affect the membrane or polysulphone support structure and routine operation for 4 – 24 hours may be necessary to stabilise operating conditions.
Membrane manufacturers’ recommendations should always be followed with respect to pH, temperature, pressure and flow rate.
We have the experience to advise our customers on the correct type of Reverse osmosis plant and equipment to serve their specific needs. We can also provide technical back-up and advise when selecting the correct reverse osmosis antiscalant to use and the correct method and cleaning chemicals to use when cleaning reverse osmosis membranes.