In the 1960’s the first reverse osmosis (RO) membrane was developed for the removal of dissolved salts from water. It was made from cellulose acetate (CA). Later, in the 1970’s, thin film composite (TFC) membranes were introduced. These were made from polyamide (PA) and offered higher water permeability (lower operating pressure) and lower salt permeability (higher salt rejection) as compared to CA membranes. Over time, TFC polyamide membranes have become the predominate RO module although many CA membranes are still in service.
Whether CA or TFC type, eventually membrane performance declines due to fouling of the flow channels and membrane surfaces with organics, colloids, suspended solids, and biofilms. This results in an increase in pressure differential, decrease in normalized permeate flow, and an increase in percent salt passage. At this point, it becomes necessary to clean the RO membranes to restore operating performance and extend their useful life.
RO DESIGN AND PRETREATMENT CONSIDERATIONS
Since RO membranes account for approximately two-thirds of the cost of the reverse osmosis system, taking steps to protect the modules from physical and chemical degradation is an important consideration early on in the design phase. This begins with proper design of the RO array and pretreatment system.
The rate of membrane fouling is a function of permeate flux (gallons of permeate produced per square foot of membrane surface area per day, GFD). Studies have shown a direct relationship between fouling rate and flux. In general, an increase in fouling rate with higher flux is a result of higher concentration of organics at the membrane surface and a higher drag force across the membrane. For these reasons, it is best to limit the flux to less than 15 GFD. Lower flux requires more membrane surface area at a given permeate flow rate. Although more membrane surface area equals higher capital cost, this is a key economic design consideration when one factors in the cost of replacing a set of membranes due to premature failure caused by irreversible fouling.
Another important design consideration is the best method for efficient removal of foulants from the RO feedwater. Designers use a standard water analysis, Langelier Saturation Index (LSI), and Silt Density Index (SDI) to determine the pretreatment requirements for a particular RO system. Filtration of colloidal and suspended solids is indicated if the turbidity exceeds 1.0 NTU and/or the SDI is greater than 4. The limitations on organic content are less clearly defined. A practical guideline is a Total Organic Carbon (TOC) level greater than 3 ppm. The Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) should not exceed 6 ppm.
Since water supplies vary considerably from one location to another, each pretreatment scheme is different. However, several pretreatment methods are commonly employed for this purpose.
Multimedia filtration is used to remove suspended solids and some colloids down to approximately 10 microns. These filters contain at least four types of media. In one design, the light coarse material is on top of the filter bed followed by progressively less coarse material in the lower layers. This design permits the unit to function as a depth filter by removing the filterable solids throughout the depth of the bed rather than just at the surface. A properly designed multimedia filter will have a flow rate of less than 7 gpm per square foot of surface area and a particle size cutoff of 10 microns.
At times it is necessary to inject a polymer coagulant upstream of the multimedia filter to enhance the removal of the colloidal material. These polymers are high molecular weight materials, normally cationic (positively charged), which coagulate the fine colloids and suspended solids into larger particles that are more-efficiently removed in the filter bed.
Carbon filtration is used to remove chlorine and organic molecules (TOC) from the RO feedwater.
Arrays that are designed with cellulose acetate (CA) membranes are susceptible to attack by bacteria that feed on the membrane material. To minimize this problem, CA membranes require 0.3 to 1.0 ppm chlorine residual in the feedwater for bacteria control. CA membranes have a chlorine tolerance of 26,280 ppm-hr.
By comparison, systems that use thin film composite (TFC) polyamide membranes are intolerant of chlorine. The chlorine tolerance is approximately 1,000 ppm-hr. As a result, the RO feedwater should be dechlorinated to less than 0.1 ppm. That said, some operators report that for ROs suffering from biofouling issues, a pulse of 0.25 ppm chlorine for four hours per day resulted in a reduction in chemical cleanings by a factor of 10 with no measurable loss in salt rejection. Some passage of chlorine through the membrane occurs at up to 50% of the feedwater dosage.
In any event, if dechlorination is desired, carbon filters accomplish this task easily. Carbon filters have an infinite capacity for chlorine and produce a final effluent of less than 5 ppb total chlorine. Activated carbon is also effective in removing total organic carbon (TOC) and silt, which often cause fouling problems on the membrane surface.
On the downside, carbon filters are known to be a viable habitat bacteria. Some designers, therefore, chose to avoid the use of carbon for dechlorination; choosing instead to inject a chemical agent into the feedwater such as sodium bisulfite or sodium metabisulfite. These reducing agents readily react with chlorine and other oxidizing agents like peroxide and permanganate.
Softening the RO feedwater by ion exchange is a popular method for reducing mineral scale formation on the membrane surface and in the water flow channels. Sodium softening exchanges sodium for scale-forming ions such as calcium, magnesium, barium, strontium, iron and aluminum. Sodium forms very soluble salts that are less likely to produce mineral scales within the membranes modules.
As an alternative to sodium softening, acid injection is frequently used to reduce alkalinity and control pH, which minimizes the scale-forming tendency of the feedwater. Acid injection is indicated if the Langelier Saturation Index (LSI) of the brine (aka concentrate or reject) is above +3.0. Either sulfuric or hydrochloric acid is used for this purpose.
In addition to acid injection for pH control, chemical anti-scalants are sometimes effective in extending the intervals between chemical cleanings. These products are formulated to include inorganic phosphates, organophosphonates and dispersants. These antiscalants contain negatively-charged (anionic) polymers that are compatible with the negative surface charge of the polyamide membrane. However, they are not compatible with positively-charged (cationic) polymers that may be injected upstream prior to the multimedia filters.
Final filtration of the RO feedwater is accomplished by a 5 micron cartridge or bag filter. This removes any last traces of suspended solids or biomass that pass through the multimedia or carbon filters. A 5-micron, nominal-type filter is generally used for this purpose.
MEMBRANE CLEANING METHODS
In the early days of RO operation, little was known about the impurities that cause membrane fouling. Today, many of these impurities have been identified. Autopsies of failed membrane modules have revealed a build up of mineral scales like calcium carbonate, colloidal materials like clays and silica, dead and living microorganisms, carbon particles and chemical attack by oxidizing agents such as chlorine, ozone or permanganate. Likewise, dissolved metals like iron and aluminum, whether naturally occurring or added as a coagulant, will cause premature fouling and failure of the membrane.
Despite all efforts to protect the RO system from fouling and loss of flux, eventually the membranes require chemical cleaning. Continuous monitoring of the RO operating parameters is required to pinpoint when cleaning is required. The following guidelines are useful in determining the best time to clean:
When the pressure differential increases by 15%
When normalized permeate flow decreases by 15%
When the salt rejection rate decreases by 15%
Under ideal conditions, assuming the RO pretreatment system is properly designed and operated, the frequency between membrane cleanings should be 6 months or longer. Cleaning every 1 to 3 months is considered a fair performance and suggests that some improvement in the pretreatment system may be necessary. Cleaning every month or more indicates a change in raw water quality, a problem within the pretreatment system or difficulties with the RO unit.
A well-designed RO system will include provisions for a cleaning skid. The skid should include a chemical tank, solution heater, recirculating pumps, drains, hoses and all other connections and fittings required to accomplish a complete chemical cleaning. Specifications and drawings for a cleaning skid are available from the equipment manufacturer, membrane supplier or your water consultant.
Fouling impurities fall into one of two categories: inorganic deposits and organic foulants. Inorganic deposits include calcium salts, metal oxides, colloids and silica. Organic foulants include biofilms and organic molecules. The chemical nature of the foulant determines the best cleaning chemicals for the job. Acid cleaners are used to remove inorganic deposits and alkaline cleaners are required for organic deposits.
The cleaning method is best determined by an assessment of the chemical nature of the deposits. An autopsy of a sacrificial membrane module is often required the first time around to verify the quality and quantity of the fouling material. The most effective cleaning solution for that specific foulant can then be applied.
Each cleaning procedure is unique depending on the nature of the foulant and the system operation. In general, it is best to remove organic foulants first with an alkaline (high pH) cleaning solution followed by an acid (low pH) cleaning for inorganic scale and metals removal. For multi-stage arrays, it is best to clean one stage at a time to avoid “pushing” foulants from one stage into another. However, successful cleanings have also been achieved by cleaning all stages at once. Experience and common sense is the best teacher in this regard.
Begin the high pH cleaning by filling the CIP (clean in place) tank with permeate. Circulate the permeate through the RO reject line and back to the tank at low pressure (40 to 60 psig). Heat the water to approximately 80 to 90 F and slowly mix the cleaner to the desired strength. Alternate “circulating” and “soak” cycles at 5 to 30 minute intervals for up to 4 hours, if necessary. Repeat with a fresh solution of cleaner if the membranes are severely fouled.
After the alkaline cleaning is done, flush the system with permeate and neutralize any residual alkaline cleaner with mild acid, if necessary. The pH should be neutral (pH 6 to 8). Refill the CIP tank with fresh permeate and heat to temperature. Mix the acid cleaner to the required concentration and alternate “circulating” and “soak” cycles at 5 to 30 minute intervals as described for the alkaline cleaner. Monitor the cleaner pH and add more acid to “sweeten” the solution if the pH drops by 0.5 pH unit. Once the acid cleaning step is complete, flush the system with fresh water and neutralize to pH 6 to 8.
Start the RO and operate normally with the permeate discharged to drain until the conductivity of the permeate returns to normal operating level. Place the system back into service after the specified water quality is achieved.
SUMMARY AND CONCLUSIONS
Reverse osmosis is a useful technology for producing high purity water for many industrial, commercial and institutional applications. Proper design, operation and maintenance of the RO system is required to insure minimal problems with membrane fouling, shortened membrane life and increased operating and maintenance costs. Proper pretreatment is required to prolong the useful life of the membranes. Eventually, despite all best efforts, cleaning is necessary to restore the design flow rates and pressures. If this is done properly, a reverse osmosis system will provide a continuous source of high purity water for 7 years or more before membrane replacement is required.