America’s Authority in Membrane Treatment Membrane Post Treatment Post treatment for low pressure used for desalting drinking water determined by regulatory requirements, membranes (MF and UF) is typically supplies. the design of the system, finished water minimal consisting of disinfection (as a quality criteria and water chemistry. The MEMBRANE DESALINATION secondary barrier) and sometimes pH need for post-treatment generally de- PROCESSES adjustment and corrosion control, pends on a number of factors, which can Many municipal plants have multiple depending on raw water chemistry. be grouped into several categories, all of process trains installed in parallel, Since these low pressure membranes do which are related to water quality: allowing flexibility in permeate (product not remove dissolved substances and water) production and ease of • Chemical stability water chemistry remains unchanged, the expansion. In some instances it is post treatment approach is similar to • Microbiological Stability possible to bypass a portion of the raw conventional sand filtration and • Palatability and Customer or pretreated water around the therefore not discussed in this fact sheet. Acceptability membrane system and blend that flow Instead, this fact sheet focuses on with the permeate stream to reduce the • Secondary Impacts on membrane desalination. size of the membrane system, improve Wastewater Influent Quality Desalination is intended for the finished water stability, and minimize A recent overview of the current state of removal of total dissolved salts (TDS) capital and operating costs. The 62 full-scale RO/NF plants, 9 greater that generally cannot be removed by maximum allowable blend ratio is than one-million gallons per day of conventional treatment processes alone. determined from an analysis of bypass capacity, used for either seawater Reverse Osmosis (RO), Nanofiltration and permeate water qualities. desalination, brackish water (NF) and Electro-Dialysis Reversal desalination (including ground water, (EDR) synthetic membrane processes surface water and agricultural runoff), produce treated water that requires post or wastewater reclamation provides an treatment before delivery to the insight into post-treatment practices. All distribution system as finished water. of the surveyed facilities reported using These membrane processes produce per- at least one post-treatment meate water depleted in minerals which method for permeate conditioning and often is found to be aggressive towards corrosion control. These included such distribution system components. methods as caustic addition (31%), Different RO and NF membranes have blending with raw, semi-treated or different mass transfer characteristics; Post-treatment processes typically finished water (29%), degasification/ using a membrane with a lower include stabilization, disinfection and decarbonation (25%), and addition of molecular weight cutoff will decrease corrosion control, and can include corrosion inhibitor (14%). Most of the the permeate concentration. EDR degasification and/or air stripping brackish water RO plants responding to processes are impacted by the amount of processes if carbon dioxide and the survey reported using electrical current and electrical hydrogen sulfide gases are present in the degasification/decarbonation and requirements and less effected by the permeate water. Post- treatment is need- caustic addition, with the majority type of membrane. Regardless of ed for municipal water treatment before blending permeate with groundwater. specific membrane formulations, the the membrane-treated water is delivered Permeate disinfection was reported to be water produced by RO, NF and EDR to the distribution system as finished used by 85% of the surveyed membrane processes is incompatible water. facilities that responded, most of which with many components and used chlorine. Other reported appurtenances that comprise water POST-TREATMENT PROCESS disinfection methods included the use of distribution system infrastructure. This OVERVIEW chloramine (24%) and ultraviolet irradi- fact sheet discusses the post-treatment The choice and sequence of ation (4%). of RO and NF membrane processes post- treatment operations are typically Table 1 presents the typical problems when distributed. Many the pH which will be dependent upon categorization of permeate facilities pump desalinated water the buffering capacity and bicarbonate post-treatment depending on source wa- directly into the distribution system alkalinity, calcium, sulfate and chloride, ter type. There are four primary without being mixed or blended with dissolved oxygen, boron, total dissolved issues concerning the post-treatment other finished water supplies that cause solids concentration and corrosion water. These relate to blending, concern with regards to distribution sys- indices. These parameters are interrelat- remineralization, disinfection and the tem water quality. Consequently, evalu- ed in the final treatment process selected materials used for storage and transport ation of water quality parameters for use for post- treatment, depending on of the water to the tap. in determining appropriate application and source water (i.e. ocean post-treatment actions is required for surface versus brackish ground water Desalinated water is often blended with desalination membrane treatment supplies). other sources that contribute minerals to applications. Factors that should be the final blended water. Seawater as a Alkalinity, Scale and Red Water included when referring to the quality of source for blending is limited due to Alkalinity in water is a measure of the desalinated waters include the issues related to corrosivity and taste if general buffering capacity or stability of chemical and biological stability of the blending levels exceed about 1%. the water. Increasing the alkalinity gen- water and its interaction with the Blending of permeate water with erally leads to lower and corrosion rate distribution system. Permeate streams seawater results in the addition of and results in less changes in the pH of from sea- water and brackish water sodium, potassium, calcium, and distributed water; however, excess desalting processes are primarily a magnesium to the drinking-water but alkalinity can cause excessive scale dilute solution of sodium chloride. also will contribute bromide and iodide deposition where calcium may be Untreated permeate from sea or which are DBP precursors, and is present. Alkalinity is thus directly brackish water reverse osmosis plants limited in quantity due to the related to the buffering capacity of does not conform to the drinking water significant concentrations of these water and is considered an important standards of such organizations as WHO constituents. Consideration should be parameter affecting the pH. Alkalinity or the EPA. Due to the low TDS values given to the natural minerals present and depends on the concentration of RO permeate water can be unpalatable, whether these will result in the bicarbonate, carbonate, and hydroxide corrosive, and suspected as unhealthy. finished water having unacceptable ions in water. For a given pH value, the water qualities in addition to To stabilize the water, and to prevent higher the alkalinity value, the higher unacceptable taste and odor. corrosion (metal release) of piping the ability of the water to withstand a systems and domestic plumbing, change in pH due to release of H+ and PERMEATE WATER QUALITY post-treatment is necessary to return OH- ions to the water. A higher CONSIDERATIONS some calcium hardness and bicarbonate alkalinity at a given pH translates into a The chemical composition of permeate alkalinity to the water. In many higher dissolved inorganic carbon (DIC) water produced by RO or NF when situations, post-treatment also includes concentration of the carbonate species blended with other source water can the removal of carbon dioxide to raise (CO2- ). However, too high of an alka- cause water quality and infrastructure the pH, hydrogen sulfide removal when linity at higher pH levels may accelerate required, and the addition lead and copper metal release. It is also of fluoride which is known that red water prevention can be removed during the accomplished by maintaining the alka- desalting process. linity in the system when considering a Corrosion control is a subsequent pH shift if treatment was to priority when either be employed. A non-stabilized finished directly pumping desalted water can experience fluctuations in pH finished waters into the in the distribution system as scale is distribution system or deposited (scale) or dissolved when blending different (corrosion). It is desirable to maintain water sources from the alkalinity concentration in distributed membrane process. The water above one mill-equivalent of constituents of concern alkalinity, or 60 mg/L (as calcium when establishing a post carbonate). treatment process include Dissolved Oxygen WHO has recently published guidelines + - H S (gas) = H + HS (aq) pK = 7 (4) The oxygen concentration can have for boron value of 2.4 mg/L for human 2 1 HS1- = H+ + S2- (aq) pK = 14 (5) varying effects on iron corrosion. The health perspective. 2 corrosion rate increases with increasing dissolved oxygen. Dissolved oxygen is BRACKISH GROUND WATER As shown in equation (4), since at pH of also responsible for the ability of POST-TREATMENT 7 only 50 percent of hydrogen sulfide buffering ions, including phosphates, to CONSIDERATIONS exists in the gas form and is available inhibit corrosion. The primary desalination water plant for stripping pH adjustment is typically post-treatment unit operations for used to improve removal efficiency. pH potable water supplies reliant upon Since the pK for hydrogen sulfide is 7, Various studies have been done to brackish ground waters are the half of the sulfide speciation is present correlate the effect of pH on corrosion following: as a gas and strippable. Hence, in pipes. The pH in a system is directly 1. Carbon dioxide removal hydrogen sulfide gas can be effectively related to the alkalinity, Ca2+ and (degasification or decarbonation) removed at pH levels of 6.0 or less CCPP in the system. The pH 2. Hydrogen sulfide removal without the formation of turbidity determines the buffer capacity of the (stripping) and odor control (elemental sulfur). However, all of the water sources. When different water treatment (scrubbing) carbon dioxide in the permeate water sources are blended the chemical will also be removed. If stripping of 3. Alkalinity recovery, pH adjustment, stability of the blend is significantly de- sulfide occurs at pH 6.3 (bicarbonate stabilization and corrosion control termined by the buffering capacity of pKa1) some buffering capacity will 4. Disinfection the original waters. A higher pH will remain. Unless carbonate is added or a usually result in a lower buffer Carbon dioxide is easily removed from significant amount of alkalinity passes capacity, which also can be noted to be brackish permeate water with the use of the membrane, there will be no associated with low corrosion rates and aeration (degasification or carbonate (alkalinity) buffering in the prevention of red water episodes, decarbonation). Carbon dioxide exists in permeate, a possible problem with however most studies have shown pH to equilibrium with other carbonate species respect to stabilization and corrosion be a isolated single parameter. as defined by equations (1), (2) and (3). control even if pH adjustment with Boron The pH of the permeate water will sodium hydroxide is practiced. Better Boron removal is more costly and determine the amount of carbon dioxide methods are required to resolve this difficult to process than other ions, available to be removed from the water. common post-treatment issue; an especially when dealing with seawater. increase in the pH entering the tower CO2 + H2O = H2CO3 pK = 2.8 (1) + prior to air stripping to recover 1 to 2 This is due to the fact that seawater is H2CO3 (gas) = H + HCO3 (aq) pK1 = 6.3 (2) not drinkable or useable for irrigation. HCO3 (aq) = H + CO3 ( aq) pK2 = 10.3 (3) meq/L of alkalinity would be beneficial. Treatment must be taken into The use of carbonic acid pH adjustment consideration in order to decrease Many of the brackish ground waters prior to air stripping of hydrogen sulfide boron, and correct any other corrosive used as feed streams to RO or NF plants has proven to be beneficial with regards minerals that may be in the water. contain hydrogen sulfide. Conventional to buffering loss of finished water. The Boron rejections depend on pretreatment (acid addition, scale alkalinity of water is a measure of its temperature, pH and salt rejections. inhibitors, cartridge filtration) will not capacity to neutralize acids. Due to low boron levels required for remove hydrogen sulfide nor will the Bicarbonates represent the major form irrigation, several stages of reverse membrane process, as hydrogen sulfide of alkalinity in water, since they are osmosis treatment are used. The first will permeate the membrane as a gas. formed in considerable amounts from the stage will use antiscalants on both pass- Aeration and oxidation are the two action of carbon dioxide upon basic es to avoid risk of salt precipitation. The primary means for removing hydrogen materials in the soil. Temperature, pH and second stage will use caustic soda to sulfide. Incomplete chemical reactions the concentration of bicarbonate are transform boric acid to borate, which is in the process are often responsible for important in the formation of CaCO3 feed much easier to remove. Residual boron formation of polysulfide complexes and water (equation 6). is related to the pH levels in this stage. elemental sulfur, which manifest Ca2+ (dissolved) + 2(HCO) (dissolved) 3/4 The degree of boron removal depends themselves as turbidity in the finished CaCO ( solid) + H O + CO ( gas) (6) on the finished water quality goals, water. Hydrogen sulfide dissociates in 3 2 2 some countries and municipalities water according to equations (4) and (5). having more strict standards than others. TREATMENT METHODS FOR adjustment alters the concentration of calcium carbonate and iron stability. For CORROSION CONTROL dissolved inorganic carbonate (DIC) in lower alkalinity waters, sulfate can also The primary options for stabilization and the source water. Alkalinity adjustment precipitate calcium and cause scale. post-treatment of membrane permeate and can be accomplished with lime, soda Odor Control EDR product water include: ash, sodium bicarbonate, sodium Some ground waters may have high hydroxide, potassium hydroxide and • pH adjustment concentration of H2S, which being a carbon dioxide. Sodium bicarbonate gas, will not be removed by membrane • Alkalinity adjustment addition is preferable for alkalinity processes. Post treatment consisting of a adjustment. Sodium hydroxide • Calcium adjustment single or sometimes two stage odor contributes little alkalinity to the water, control system may be required to • Corrosion inhibitors but can cause dramatic increases in pH. remove H2S and other produced gases The primary disadvantages of alkalinity • Blending such as CO2 (if excessive pretreatment adjustment include capital, operation acids are added). pH Adjustment and maintenance cost and increased Adjustment of pH is used to induce the carbonate scaling on pipe walls. The formation of insoluble compounds on primary benefit of alkalinity adjustment the exposed pipe walls. Passivation is is increasing the buffering capacity for the operating mechanisms for this the source water. This helps to prevent corrosion control strategy. pH wide fluctuations in pH throughout the adjustment is accomplished with the distribution system. addition of chemicals, such as lime, Calcium Adjustment soda ash, sodium hydroxide, potassium The mechanism for this corrosion hydroxide and carbon dioxide. pH Corrosion Inhibitors control strategy is the adjustment of the adjustment is most suitable for source Inhibitors have found wide spread use equilibrium for the calcium carbonate waters with low to moderate hardness as a method of corrosion control. The system for the source water. The and alkalinity levels (between 80 and most prominent forms of inhibitors used objective for this treatment technique is 150 mg/L as CaCO3). Frequently, this are polyphosphates, zinc phosphates, the precipitation of a protective film of treatment technique is used in lieu of and silicates. The inhibitors control calcium carbonate onto the pipe walls. calcium carbonate precipitation. Some corrosion by several mechanisms, Calcium addition or removal is not concerns with pH adjustment include including sequestering of the corrosion necessary for the precipitation of increased formation of disinfection by-products, specifically lead and calcium carbonate and is accomplished by-products at pH levels above 7.8, copper, scale inhibition, development of with pH and alkalinity adjustment of the decreasing chloramines disinfection a coating film on the pipe walls and source water. The key to this treatment efficiency with pH values below 7.8, buffering the water at the desired pH. technique is to provide the conditions and a higher potential for calcium Operating data indicate that the choice necessary for achieving calcium carbonate scaling in the distribution of inhibitor depends upon pH, alkalinity, carbonate saturation. Adjustment of the system pipe at pHs above 7.9. calcium and total hardness, chloride, pH/alkalinity is done to create sulfide, iron concentrations, and Alkalinity Adjustment conditions necessary for the calcium and dissolved oxygen levels of the source Alkalinity adjustment frequently is used carbonate ions to exceed their solubility water. to induce the formation of insoluble limits in water. The concerns with using compounds on the pipe walls of the calcium carbonate adjustment include Blending distribution system. Passivation is the precipitating a uniform protective film Adding or blending pre treated source operating mechanism for this corrosion throughout the distribution system, water into the (permeate) product water control strategy; carbonate passivation reduction in the hydraulic capacity of can help in stabilizing the product water is achieved by incorporation of pipe ma- the water lines, and scaling in thereby reducing the impact of the terials into a metal hydroxide/carbonate mechanical systems (boilers and hot before mentioned issues. However, protective film. This corrosion control water heaters). Scaling is of particular blending introduces the need for strategy is most suitable for source concern for those water systems with disinfection of the pre treated water waters with minimum alkalinity, and is high levels of non-carbonate hardness prior to or after blending. Unfortunately, frequently used in lieu of calcium and sulfate. Adjustment of the pH is blending will not stabilize the product carbonate precipitation. Alkalinity necessary for the precipitation of water completely hence permeate will still need to have some level of calcium system as it reduces the amount of water Alkalinity recovery needs to be and alkalinity (alkalinity being the more that needs to be treated and thereby considered when selecting scaling important parameter) present. This can reduce the operating costs of the system. control options, and is dependent on be accomplished by employing either When integrating into an existing how much carbon dioxide and lime or limestone treatment. If the system, control over corrosion inhibitors bicarbonate is in the raw water. source of the water to be blended with and pH adjustment should be optimized Regardless, permeate water will require the product water from the reverse for maximum efficiency. chemical disinfection. Selection of post- osmosis system is from a ground source treatment processes may not completely Blending Multiple Source Waters from a limestone or chalk geological consider the impacts on the distribution Blended waters from coastal and formation, the amount of lime treatment system, particularly when blending estuarine areas may be more susceptible will be substantially reduced. Blending multiple varying supplies. Although to contamination with petroleum of variable and differing water supplies pilot studies are often conducted for RO hydrocarbons or algal toxins, which where desalted water serves as one of and NF process design considerations could give rise to taste and odor the supplies is becoming more frequent. related to pretreatment, process problems. Some ground waters or Concern has also been expressed about optimization and operation surface waters, after suitable treatment, the impact of extremes of major ion considerations, these pilot studies often may be employed for blending and may composition or ratios for human health. do not include adequate consideration of improve hardness and ion balance. It is There is limited evidence to describe the post treatment processes focused on necessary to model the affects of health risk associated with long-term specific distribution system related different blends to prevent the release of consumption of such water, although issues that are specific to that system. It red water in the distribution system. mineral content may be augmented by has been recommended by researchers Should multiple sources be used, the stabilization processes typically used by that water purveyors mandate studies to utility should consider the need to utilities practicing desalination (WHO, evaluate the secondary impact of develop a unidirectional flushing 2003). Desalinated waters are commonly permeate post treatment (or lack program or distribution system blended with small volumes of more thereof) on water quality and subsequent rehabilitation (including replacement) mineral-rich waters to improve their compliance related topics: disinfection prior to the incorporation of a desalting acceptability and particularly to reduce and residual maintenance, the formation process into existing infrastructure. In their aggressive attack on materials. of disinfection by-products, addition, the water purveyor may also Blending water should be fully potable; maintenance of lead and copper need to increase storage reservoir size to where seawater is used for this purpose, corrosion control, bacteriological be able to control the blending location the major ions added are sodium and re-growth and coliform impacts. of multiple source waters. In most cases, chloride. This does not contribute to the water purveyor (water utility) should improving hardness or ion balance, and This material has been prepared as an expect to see an increase in its only a small amount (typically no more educational tool by the American Membrane operational and maintenance expenses. than one to two percent) can be added Technology Association (AMTA). It is designed without leading to problems of SUMMARY for dissemination to the public to further the acceptability. The need to stabilize water in order to understanding of the contribution that membrane prevent metal corrosion and concrete water treatment technologies can make toward Bypass Blending of Source Water dissociation has been recognized for improving the quality of water supplies in the US Blending can improve the stability of decades. Permeate typically is adjusted and throughout the world. the product water by increasing the chemically in order to prevent corrosion alkalinity and calcium in permeate to of pipes in the distribution network and For more information, please contact: reduce the corrosiveness of the water. control, pH value and carbon dioxide American Membrane Technology The water to be used for blending may content for scaling prevention. A buffer Association (AMTA) be the source water used for the reverse intensity greater than 0.5 milli 2409 SE Dixie Highway osmosis process or from another source, equivalents per pH unit is indicative of a Stuart, Florida 34996 but is limited to brackish waters having balanced, stabilized source water. The moderate to low TDS with no Phone: (772) 463-0820 purveyor should focus on producing significant DBP precursor content. Use Fax: (772) 463-0860 finished water having an adequate of bypass blending or reliance on Email: [email protected] alkalinity and buffer intensity with a multiple source waters for blending will target that falls between one and three or visit our website at: reduce the stress on the membrane www.amtaorg.com meq/L of bicarbonate alkalinity. (FS-16) Oct. 2010