Precipitation of solids in plumbing systems (i.e., scaling) is a major problem in both traditional tank (electric and gas) and tank-less building hot water systems. Scaling can cause energy inefficiencies, flow reduction, pressure loss, and erosion corrosion damage. Consumers are also concerned with unsightly soap scum, cloudy water, discolored glassware, and failing infrastructure including appliances and fixtures. There are many treatments available that claim to ameliorate scaling problems, and several efforts have been made to develop standardized test protocols to verify and quantify their performance.
This work critically evaluated previous testing efforts and revealed limitations in terms of reproducibility and a need to measure all key aspects of scale deposition including quantity, location, aesthetic and other issues. A Standardized Scaling Test Protocol (SSTP) was conceived and vetted to address these deficiencies and measure key parameters of calcium carbonate scaling throughout a model premise plumbing system, while using a synthesized test water that could provide reproducible results in any laboratory. This synthetic water and methodology was able to produce significant scaling in a model hot water system within the targeted 5-day experimental time frame. The average amount of scale recovered for the triplicate control tests (with no scale reduction device) was 25.1 grams of calcium carbonate with a 95% confidence interval of 20.3-29.8 grams of calcium carbonate. The approach also worked in recreating scaling in natural waters and was used to verify the performance of a wide array of scale reduction technologies including cation exchange softeners, electrochemical deionization, physical magnets or electric field generators, media induced precipitation, sacrificial media (phosphate), and sacrificial media (citric acid).
While calcium carbonate precipitation within a water distribution system is generally undesirable; it was recently discovered that calcium carbonate particles are sometimes naturally clogging leaks in pipes and extending the lifetime of aging infrastructure. Corrosion inhibitors, mainly phosphates, have been increasingly dosed (up to 3.0 mg/L as PO4) into water to inhibit the corrosion of lead and copper pipelines in potable water systems since the advent of the Lead and Copper Rule (LCR) in 1991 by US Environmental Protection Agency (EPA). Phosphate corrosion inhibitors are now used at over 50% of water utilities in the United States and they can affect calcium carbonate scaling kinetics. In bench-scale experiments, the critical concentrations of phosphates that could inhibit leak repair over the short-term in one water tested were: tripolyphosphate (0.05 mg/L as P) < hexametaphosphate (0.1 mg/L) < orthophosphate (0.3 mg/L). The results prove that dosing of phosphates for corrosion control will also affect the kinetics and likelihood of calcium carbonate precipitation, with both beneficial and adverse consequences for pipes and consumers. Specifically, increased use of inhibitors for corrosion control is expected to reduce the likelihood of all calcium carbonate scaling problems while reducing the likelihood of autogenous pipe leak repair.
In Providence, RI the dosing of orthophosphate at relatively high pH to control a lead corrosion problem, caused formation of a white precipitate, consumer reports of white water, clogging of aerators and loss of the added soluble phosphate corrosion control inhibitor due to precipitation. The precipitate was identified as a calcium phosphate solid. Field and lab scale tests suggest that at doses below 2 mg/L as PO4, precipitation did not occur in water at pH 10.4 even when the water was heated to 48°C. However, if the water was dosed above 2 mg/L as PO4 precipitation occurred within 5 minutes, and once pre-existing particles were formed precipitation tended to continue even at much lower phosphate doses. Virtually all of the phosphate precipitated within 4 hours at the upper range of 60°C that is commonly found in water heaters. Thus, dosing of phosphate can actually increase scaling problems in some circumstances.
Prior work has highlighted a need for a simplified bench-scale test that can be used to rapidly screen for qualitative trends in scaling. The SSTP and practical experience showed that the vast majority of scaling occurred in the water heater. Therefore, a simplified bench-scale test consisting of a heating element in a small volume of water could be used to focus on the most sensitive aspect of scaling. A 3-hour bench-scale test was developed to quickly examine scaling with orders of magnitude less volume, time, labor, cost, and space requirements. This approach was used to evaluate aspects of scaling in water heaters for the following illustrative examples: (1) scale impacts of combined phosphate corrosion inhibitor addition and partial water softening at centralized treatment plants, (2) role of silica concentration in scaling propensity and deposit durability, (3) effects of phosphate addition on scaling in a water known to cause erosion corrosion pipe damage.
This dissertation reveals the complexity of scaling for consumers and water utilities and provides tools to systematically study and resolve these practical problems. Dosing of phosphate corrosion control inhibitors can increase scaling from calcium phosphate, decrease scaling of calcium carbonate, and in other cases will have little or no effect on scaling. Both calcium carbonate and calcium phosphate can contribute to scaling as controlled by pH, temperature, hardness, phosphate dose, and other circumstances. The standardized bench and pilot scale approaches developed herein, can serve as a basis for building knowledge reproducibly in any modern laboratory. These methods can also be used to verify performance claims for a wide range of scale reduction technologies, test treatments that could be applied at centralized treatment plants, and optimize water heater design dependent on water chemistry. / Doctor of Philosophy / Precipitation of solids in plumbing systems (i.e., scaling) is a major problem in both traditional tank (electric and gas) and tank-less hot water systems. In addition to scale build up within the hot water system, consumers are also concerned with unsightly soap scum, cloudy water, discolored glassware, and failing infrastructure including appliances and fixtures. There are many treatments available that claim to mitigate scaling problems, and several efforts have been made to develop standardized test protocols to verify and quantify their performance.
This work evaluated previous testing efforts to determine limitations in their methodology. A Standardized Scaling Test Protocol (SSTP) was developed to address these deficiencies and measure key parameters of calcium carbonate scaling throughout a model home plumbing system, while using a test water that could provide reproducible results in any laboratory. The test water was able to produce significant scaling within a 5-day test period with reproducible results.
While calcium carbonate precipitation within a water distribution system is generally undesirable; it was recently discovered that calcium carbonate particles are sometimes naturally repairing leaks in pipes and extending the lifetime of aging plumbing systems. An increasing number of water treatment plants are adding corrosion inhibitors to water to prevent the corrosion of lead and copper pipelines. Small scale lab experiments were run to determine how effective this natural leak repair was when there were corrosion inhibitors in the water. The results showed that most corrosion inhibitors also prevented or delayed calcium carbonate precipitation which reduced the likelihood of pipe repair through clogging leaks.
In Providence, RI the addition of a corrosion inhibitor caused a white precipitate to form in the water which led to consumer complaints of white water and clogging of aerators. This was due to the uniquely high pH of the water. The precipitate was identified as a calcium phosphate solid. Field and lab scale tests suggest that there is a critical inhibitor dose, below which no precipitation occurred in the high pH water. However, if the water was dosed above this critical limit, precipitation occurred immediately and continued as time went on.
Prior work has highlighted a need for a simplified bench-scale test that can be used to rapidly screen for qualitative trends in scaling. A 3-hour bench-scale test was developed to quickly examine key aspects of scaling with orders of magnitude less volume, time, labor, cost, and space requirements.
This dissertation reveals the complexity of scaling for consumers and water utilities and provides tools to systematically study and resolve these practical problems.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/112095 |
| Date | 14 April 2021 |
| Creators | Devine, Christina Laura |
| Contributors | Civil and Environmental Engineering, Edwards, Marc A., Parks, Jeffrey L., Dove, Patricia M., Knocke, William R. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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