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LONG-TERM IMPACTS OF ORTHOPHOSPHATE TREATMENT ON COPPER
Citation:
SCHOCK, M. R. AND A. M. SANDVIG. LONG-TERM IMPACTS OF ORTHOPHOSPHATE TREATMENT ON COPPER . In Proceedings, AWWA Annual Conference and Exhibition, San Antonio, TX, June 11 - 15, 2006. American Water Works Association, Denver, CO, -, (2006).
Impact/Purpose:
to present information
Description:
Laboratory, pilot, and field data collected support the theoretical "cupric hydroxide" copper solubility model. For the short time frames inherent in laboratory and pilot studies of copper solubility and in initial field monitoring for the LCR from Tier 1 soldered copper sites, cupric hydroxide or a very microcrystalline tenorite should be the dominant solid phase, whereas over time, the pipe will "age" and tenorite or malachite will form and predominate as the surface phase. Orthophosphate treatment will initially lower copper levels, when applied in the proper pH range. Unlike systems optimizing pH and DIC adjustment, the orthophosphate-treated systems tend to see stable copper levels, rather than the "aging" phenomenon. Given enough time (years to decades), in similar systems without orthophosphate, the copper levels will probably eventually drop to below what would be achieved with orthophosphate. Systems employing orthophosphate see rapid reductions initially, but the stabilization long-term could be difficult to overcome if there are later issues with more stringent limits on phosphate or copper levels in wastewater. Compliance monitoring required for the LCR is biased towards sites that may exhibit elevated lead levels rather than elevated copper levels. Current targeting will basically just show that Cu levels will continue to decline, so the discrepancy between the copper levels covered under LCR monitoring and that in the locations with potential for highest exposure to copper will increase over time. The Lead and Copper Rule, promulgated in 1991, required utilities to monitor for copper levels in their system in addition to lead. Samples are to be collected at the tap after a minimum 6 hour standing type from sites that either contain copper pipes with lead solder installed after 1982 or from sites that are served by a lead service line. The USEPA in Cincinnati developed a model to describe copper solubility behavior in domestic plumbing. This model, ‘the cupric hydroxide model’ predicts that the metastable cupric hydroxide (Cu(OH)2(s) with a higher solubility than malachite {Cu2(OH)2CO3(s)} is the temporary controlling solid in oxic drinking waters. There is some evidence to suggest that this mineral is slow to form (20 – 30 years). For systems with high alkalinities or those needing orthophosphate for plumbosolvency control, any of several orthophosphate solids could form that will also reduce cuprosolvency. However the minimum copper levels that can be achieved with orthophosphate addition may be substantially higher than what would have been achieved through natural aging of the copper material, because in certain dissolved inorganic carbonate (DIC) and pH ranges, the orthophosphate scales have a higher copper solubility than the aged copper scales, and can interfere with the normal copper “aging” process.
