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July 3, 2013
Post-treatment Stabilization of Drinking Water

Defining the Public Health and Financial Necessity behind Post-Treatment, and Developing a Roadmap to Stable Water

 Operators should not underestimate the economic benefits associated with post-treatment stabilization, as corrosion can increase distribution systems operating expenses.[2]  According to a recent AWWA report, the distribution pipes that have been installed over the past 120 years will need to be replaced over the next few decades.[3]  What causes these pipes to be replaced?  Replacement is required mainly because of age and corrosion.[4]  It has been estimated that at least 60% of all replacement costs are for water transmission and distribution pipes.[5]  Therefore proper post-treatment methods can extend the life of the distribution system and lower distribution operation cost by reducing corrosion and its damaging effects. Beyond damage to the distribution system, water that is non-compliant with government standards for lead and copper can cost the municipality thousands of dollars and generate unwanted publicity.Post-treatment stabilization of drinking water holds dramatic importance to both public health and cost-effective facility operations.  Effective disinfection treatment lowers the risk of bacterial or viral infection to the public. Properly treating aggressive water lowers the leaching of lead and copper into the water system from old pipes ensuring compliance with local and federal regulations. Post-treatment can also determine the amount sodium or calcium in the finished water. Studies have shown that consumption of calcium is associated with many positive health benefits. Distribution system impacts related to improper or inadequate post-treatment include corrosion events, lead and copper rule impacts, DBP’s, taste and odor complications, pH instability, disinfection residual variability, white water events, red water events, color increase, and biological regrowth.[1]

Stabilization is required to improve the aesthetics of the water. Negative odors, tastes, and color can be mitigated with properly treated water. However, improper stabilization can cause finished water to smell musty, taste bitter, and stain bath tubs, laundry, and even discolor blonde hair.[6]  Balanced and naturally stable water eliminates many of the common causes of customer complaints. The Roadmap to Stable Water Water stabilization is arguably one of the most important steps in water treatment and can be one of the most challenging. Every source of water is different and every treatment process is unique. The post-treatment technology, chemicals, and dosages must be tailored to meet the exact corrosion control needs of each plant. Three parameters that operators can control to generate more stable water are pH, alkalinity, and hardness. Each possesses unique properties that can enhance the quality and stability of the water as it When adjusting pH and alkalinity in post-treatment applications, three primary concerns must be addressed:travels from the treatment plant through the distribution system.

  1. Insuring the additives are high quality, do not introduce contaminants, and are consistent from load to load.
  2. Chemical dosages are accurately controlled.
  3. Unwanted chemical reactions are avoided (mixing concentrated acids and bases at the same feed point without proper mixing can cause precipitation).[7]

Water plants can safeguard the quality of the additives by making sure chemical specifications are strict, checking the delivered product against the specifications, and rejecting suppliers who do not conform. Municipalities also should include references in the chemical specifications and desire to work with companies who use designated tankers and have quality manufacturing standards in place (ISO 9000). Most post-treatment chemicals will dose accurately with standard tanks and metering pumps. However, there are a few chemicals that need special attention. Adding lime or calcium hydroxide in post-treatment applications can be difficult. Operators should pilot calcium hydroxide feed technology to make sure the dose is accurate and the equipment does not require large amounts of maintenance. Make sure chemicals like hydrofluoric acid and calcium hydroxide have separate dosing points with adequate mixing at each delivery point. Separating acids and bases will lower the likelihood of a chemical reaction causing an unwanted precipitation. One of the most common measurements in water treatment is pH. However, more understanding on how the equilibrium of pH affects other chemical equilibriums in the water can shed light on the role of pH in post-treatment. For example, as the pH of water rises above 8.4, the bicarbonate-carbonate equilibrium begins shifting from the soluble bicarbonate to the insoluble carbonate. The shift in solubility is the reason higher pH values achieve a higher LSI or CCPP values. The higher the pH rises above 8.4 the more carbonates precipitate out of the water which affects the metal ion dissolution potential. Raising the pH has been found to lower the dissolution potential of metal ions.[8]  Lowering the dissolution potential of metal ions reduces the corrosion of iron pipes in the distribution system. Understanding the finished water chemistry is important because the pH value must be one that limits ferrous ions from dissolving, but does not cause excessive carbonate scale.[9]  Although, in most cases higher pH lowers metal ion dissolution in low alkalinity environments, it can actually increase the dissolution rate. Higher pH values have shown to increase copper pitting in waters with low alkalinity.[10]  Therefore while higher pH values can help lower corrosion, the alkalinity and other factors must also be taken into consideration. Another equilibrium that is affected by pH is one that exists between corrosion inhibitors. The finished water pH can impact whether a polyphosphate or orthophosphate corrosion inhibitor are successful in reducing corrosion. An AWWA survey in the early 1990’s showed that polyphosphate corrosion inhibitors could actually increase lead release at certain pH values. Overall the study demonstrated the value of corrosion inhibitors increased with low pH in low alkalinity waters.[11]   If municipalities have low alkalinity and low pH, corrosion inhibitors offer municipalities the greatest benefit. However, as pH and alkalinity increase, the effectiveness of corrosion inhibitors can diminish. Finally, pH can indirectly affect the chlorine residual levels and their effectiveness in the distribution system. Studies have shown that raising the pH of water over 7.8-8.0 greatly diminishes the effectiveness of chlorine as a disinfectant.[12]  However, ammonia-oxidizing bacteria’s optimum pH is below 8.0.[13]  Studies have also demonstrated that supplying a distribution system with water pH values higher than an 8.5 will inhibit nitrifying bacteria. For example, a Nova Scotia water plant demonstrated that the addition of calcium hydroxide to post-filtration proved to be more effective than adding 3-4 mg/L of chorine residual due to the rise in pH in controlling certain bacteria in the distribution system.[14]Understanding the role of pH in disinfection could impact the decision on whether a municipality needs to periodically shift from a chloramine to a free chlorine treatment process to purge the distribution system of bacteria that cause nitrification. If the water plant produces water with an average pH of 9.0 or higher, the likelihood of a biofilm growth is less and periodically treating with free chlorine may not be required. However, if the pH is consistently below 8.5, then the chloramines could provide the needed ammonia for nitrification and a seasonal purge using free chlorine may be a wise option.

The Role of Alkalinity and Hardness

Alkalinity is a measure of the general buffering capacity of water and one of critical importance related to water stabilization.[15]  Therefore increasing alkalinity increases the buffer capacity of the water.[16]  Also, increasing alkalinity in most cases mitigate large pH swings in the distribution system.[17]  Alkalinity (as CaCO3) over 80 mg/L has shown a strong correlation to reducing the release of color in the distribution system in unlined and galvanized-iron pipes.[18]  When municipalities in the study maintained higher values of alkalinity they saw decrease in customer complaints of red water.[19]  Iron corrosion rates at a fixed pH have been shown to decrease as total alkalinity increases.[20]  In one study, when alkalinity levels were decreased from 30-35 mg/L to 10-15 mg/L at a constant pH, the results demonstrated an immediate increase of 50-250% in iron release.[21]

Plant Pilot Study Stabilizing Membrane Permeate

Conversely, low alkalinity can be a problem even in high pH waters. Studies have shown that one of the most aggressive types of water related to pitting of copper pipes is low alkalinity, high chlorine residual, high pH water.[22]  Higher alkalinity allows municipalities to take advantage of conditions where some of the calcium bicarbonate is converted to calcium carbonate coatings in the distribution pipes. Some of the earliest studies on corrosion control showed how an increase in pH formed a thin protective layer of carbonate around the metallic surface of the pipe.[23]  Forming a thin protective coating that lines the pipe is one additional safeguard to reduce corrosion in the distribution system. Consequently, municipalities should closely watch alkalinity levels when changing source water or treatment processes. If alkalinity levels drop due to raw water or treatment process changes, negative impacts in the distribution system may occur years later with no warning.[24] End user changes in the distribution system can also affect the stability of the water. For example, in areas where solar panels are used for heating water for homes, the circulating water can be exposed to an increased flow rates, higher temperatures, and diverse piping materials which can dramatically affect the corrosiveness of the water or change the effectiveness of phosphates, the level of chlorine residual, bacteria growth and other factors that could cause a higher rate of corrosion. Alkalinity is a tremendous weapon in a municipality’s arsenal to combat changes outside the water plants control. Without sufficient alkalinity changes in the distribution system could dramatically affect corrosion rates only surfacing as customer complaints or compliance violations.

Case Study – South Blount Tennessee

South Blount Municipality was constructed in 2004. The municipality was the first membrane plant in the state of Tennessee. The plant produces an average of 5.5 million gallons a day. The plant used liquid caustic for final pH adjustment and added 3 mg/L of orthophosphate for corrosion control. No initial problems were detected in the distribution system for several years after the plant was commissioned. All the samples of lead and copper came back within compliance. However in the summer of 2006, the municipality noticed a rise in lead and copper levels.  Then in 2007, the plant began failing lead and copper samples. They were cited as being out of compliance on both lead and copper. The utility superintendent was very surprised because the plant and distribution system was so new and had not been abused or neglected. The municipality decided post-treatment stabilization was needed. Several options were proposed. In 2007, five different corrosion inhibitors were piloted, but did not produce desired corrosion control.   The plant’s next option included pairing post-treatment chemicals to achieve and increase in both pH and alkalinity. The reviewed chemical pairs included liquid caustic/sodium bicarbonate, CAL~FLO slurry/CO2, or liquid caustic/CO2as potential options.

RE~MIN PROCESS® Technology (CAL~FLO System® and CO2 System)

Initially, the municipality chose to run a trial of sodium bicarbonate and liquid caustic. However, due to dust and safety concerns, operators chose not to manufacture sodium bicarbonate slurry from dry bags inside the water plant.  The trial was never performed because the feed equipment was difficult to operate.  Then in December of 2009 the municipality chose to run the CAL~FLO slurry and CO2 trial. CAL~FLO slurry is a high grade calcium hydroxide slurry utilized in water and waste water treatment for pH and alkalinity addition. Burnett Inc. supplied a pilot unit for both the slurry and CO2 delivery. The results of the trial were positive. Not only did the pilot unit easily dial in the precise pH, but it effectively raised and accurately controlled the alkalinity. The pilot successfully raised the finished alkalinity from 0-3 mg/L to 51 mg/L and raised the pH from 7.3 to 8.4. The results allowed the municipality to achieve acceptable values in their corrosivity index. After reviewing the data on all three available options, the municipality chose to install a CAL~FLO® System and CO2 System. The municipality chose CAL~FLO slurry over caustic because it was non-hazardous, did not gel in the feed lines, and added needed calcium to the water while adjusting the pH. The municipality chose CO2 over sodium bicarbonate because the cost of CO2 was substantially less and CO2 is easier to store and feed. The municipality installed both the CAL~FLO® System and a CO2 system in 2011. After installation, the municipality began moving back into compliance.  Positive reduction in lead and copper were noticed after the first sample was collected.  After collecting the second lead and copper sample, South Blount moved back into compliance. The municipality has not experienced any more corrosion related issue since the installation of the CAL~FLO® System and CO2system. Raising the pH and adding needed alkalinity to South Blount’s water dramatically decreased the corrosion, increased the life of the distribution system, and moved the municipality out of non-compliance. Whether a plant is brand new, has changed raw water sources or treatment processes, or just struggles with issues like lead, copper, red water, bacteria, chlorine residuals, or nitrification, make sure to evaluate the post-treatment strategies. Even small changes to the finished water can make all the difference.

 


[1] Duranceau, S., “Maximize Post-treatment to Stabilize Desalinated Water,” AWWA Opflow, (2012), 38:2
[2] Ripp, K.M., “Causes and Cures of Distribution System Corrosion,”  AWWA Opflow, (2000), 17:5
[3] Baird, G.M., “The Silver Bullet for Aging Water Distribution Systems,” Journal AWWA, (2011), 103:6
[4] Baird, G.M., “The Silver Bullet for Aging Water Distribution Systems,” Journal AWWA, (2011), 103:6
[5] Baird, G.M., “The Silver Bullet for Aging Water Distribution Systems,” Journal AWWA, (2011), 103:6
[6] Ripp, K.M., “Causes and Cures of Distribution System Corrosion,”  AWWA Opflow, (2000), 17:5
[7] Duranceau, S., “Maximize Post-Treatment to Stabilize Desalinated Water,” AWWA Opflow, (2012), 38:2
[8] Sarin et al, “Iron Release from Corroded, Unlined Cast-Iron Pipe,” Journal AWWA, (2003), 95:11
[9] Cantor, A. F. et al, “Effect of Chlorine on Corrosion in Drinking Water Systems,” Journal AWWA, (2003), 95:5
[10] Sarver, E. et al, “Copper Pitting in Chlorinated, High-pH Potable Water,” Journal AWWA, (2011), 103:3
[11] Dodrill, D. M. et al, “Corrosion Control on the Basis of Utility Experience,” Journal AWWA, (1995), 87:7
[12] Cantor, A. F. et al, “Effect of Chlorine on Corrosion in Drinking Water Systems,” Journal AWWA, (2003), 95:5
[13] Martin, R. S. et al, “Factors Affecting Coliform Bacteria Growth in Distribution Systems,” Journal AWWA, (1982) 74:1
[14] Martin, R. S. et al, “Factors Affecting Coliform Bacteria Growth in Distribution Systems,” Journal AWWA, (1982) 74:1
[15] Duranceau, et al, “Guidance and Recommendations for Posttreatment of Desalinated Water,” Journal AWWA, (2012), 104:9
[16] Duranceau, et al, “Guidance and Recommendations for Posttreatment of Desalinated Water,” Journal AWWA, (2012), 104:9
[17] Duranceau, et al, “Guidance and Recommendations for Posttreatment of Desalinated Water,” Journal AWWA, (2012), 104:9
[18] Imran, et al, “Red Water Release in Drinking Water Distribution Systems,” Journal AWWA, (2005), 97:9
[19] McNeill, L. S. et all, “Iron Pipe Corrosion in Distribution Systems,” Journal AWWA, (2001), 93:7
[20] Sarin et al, “Iron Release from Corroded, Unlined Cast-Iron Pipe,” Journal AWWA, (2003), 95:11
[21] Sarin et al, “Iron Release from Corroded, Unlined Cast-Iron Pipe,” Journal AWWA, (2003), 95:11
[22] Sarver, E. et al, “Copper Pitting in Chlorinated, High-pH Potable Water,” Journal AWWA, (2011), 103:3
[23] Duranceau, et al, “Guidance and Recommendations for Posttreatment of Desalinated Water,” Journal AWWA, (2012), 104:9
[24] Rissel, J., “Corrosion Controlled with Carbon Dioxide Addition,” AWWA Opflow, (1999), 25:1

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Author: Dallas Burnett

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