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THE IMPACT OF ORTHOPHOSPHATE ON COPPER CORROSION AND CHLORINE DEMAND
Citation:
LYTLE, D. A., J. Liggett, AND J. Conover. THE IMPACT OF ORTHOPHOSPHATE ON COPPER CORROSION AND CHLORINE DEMAND. Presented at AWWA WQTC, Nov 2011, Phoenix, AZ, November 13 - 17, 2011.
Impact/Purpose:
To inform the public.
Description:
In 1991, EPA promulgated the Lead and Copper Rule, which established a copper action level of 1.3 mg/L in a 1-liter, first-draw sample collected from the consumer’s tap. Excessive corrosion of copper can lead to elevated copper levels at the consumer's tap, and in some cases, can lead to pinhole leaks and pipe failure. Water chemistry has a large impact on the type of corrosion that takes place on copper solubility, and on oxidant demand. Orthophosphate is used to control lead solubility in many drinking water systems. The usefulness of orthophosphate to reduce copper release has also been recognized. The relationships between orthophosphate, dosage, water quality, and copper solubility, however, are not well defined, nor are the mechanism(s) by which orthophosphate works. The objective of this work was to investigate the impact of orthophosphate and pH on copper corrosion and release, oxidant (chlorine) demand due to corroding copper surfaces, and the properties of corroded copper pipe surfaces. A recirculating copper pipe loop system was used to meet the study objectives. The pilot-scale treatment system consisted of plastic reservoirs, pumps, copper pipes, tubing, and a chiller (Figure 1). Experiments were initiated by adding 10L of deionized water to each of the plastic reservoirs. An appropriate amount of sodium bicarbonate, sodium sulfate, sodium chloride, free chlorine, hydrochloric acid, and sodium phosphate (Na3PO4) were added to the designated reservoir to meet desired experimental conditions. Pumps brought the water from the reservoirs into the copper loops through the influent plumbing. Once the water went through the loops, it exited through the effluent tubing and into a chiller to keep the water at the desired temperature. Water was recirculated through the pipes for 48 hours before being replaced with new water, during which time a computer automated dual titration system was used to maintain the target pH within 0.1 units. Copper and other water chemistry variables were monitored during each 48-hour period. In addition, one pipe section in each loop was inserted into a clear Tygon® tube sleeve for in-situ observation. At the completion of each test run, the pipes were analyzed using a variety of solids analysis approaches, which included XRD and SEM. Tests for chlorine and orthophosphate were measured using the Hach test kits every day for effluent water and every other day for influent water. A sample for total metals analysis was taken every day for effluent water and every other day for influent water. Temperature, pH, DO, free chlorine, and orthophosphate were processed immediately following collection. All samples were taken from the reservoirs through either the spigot or through pipettes. Some important results include: At pH 9.2, the presence of 3 mg PO4/L reduced total chlorine consumption by approximately 33% over a 6-month period of time. Interestingly, over the same period of time, the total copper released into the water nearly doubled in the presence of orthophosphate. At pH 7.2, the presence of 3 mg PO4/L reduced total chlorine consumption by nearly 50% and decreased total copper release by nearly 70%. Copper levels remained very steady over the entire test runs starting from day 0 at all pH values. In the absence of orthophosphate, copper levels started high then decreased with time to eventually level off. The shift in copper levels corresponded to a visible change in the appearance of the pipe wall. Copper surfaces were dramatically mineralogically impacted by the presence of orthophosphate. In the absence of orthophosphate, malachite and tenorite were present at pH 7.2 and 9.2, respectively. In the presence of orthophosphate, no Cu(II) mineral phases were identified by X-ray diffraction nor was there any obvious visual appearance of any Cu(II) passivating solids. Data collected at other pH values as well as the practical implications of the findings will also be discussed.