Practical Evaluation of Corrosion Before Water Quality Changes
- Becki Rosenfeldt, Robert Sharp, Gary Iversen, Mark Bishop - Hazen and Sawyer
- Drew, Glaser
In an effort to comply with the USEPA Stage 1 and Stage 2 D/DBP rules, an increasing number of drinking water systems are altering their treatment processes, resulting in changes to finished water quality. These finished water changes are often times subtle, making it difficult to predict their effects on water quality within the distribution system.
Concurrent with changes in finished water quality has been an increase in lead corrosion in some drinking water systems. Recent studies investigating lead corrosion problems experienced by water utilities after modifying their treatment process are still somewhat speculative despite hindsight into the possible causes. Part of the problem is that wholesale changes to a variety of water quality parameters take place once a utility is in the “headlines” for a LCR violation.
Recent lead corrosion problems in several utilities, for example, have resulted in a variety of suspect or causative factors including change in coagulant type, change to chloramine as a residual disinfectant, slight change upward in finished water pH, and removal of more organic matter in order to reduce disinfection byproduct levels. Opinions as to the exact causative factor are many in several cases.
Given recent revelations regarding the potential impacts of treatment process changes on lead leaching, a thorough investigation into any deleterious impacts that water quality modifications may have in a given water system is warranted prior to full scale implementation. By examining the possible impacts before employing these changes, water utilities may be able to protect public health while avoiding violation of water quality regulations and associated adverse publicity.
Traditionally, the evaluation of corrosion has been done with the calculation of industry accepted indices, performing pilot loop or coupon studies, or the evaluation of full scale system samples. The use of full scale evaluations can be problematic in that the resulting data may be required to be used in compliance analysis. The use of either pilot or loop studies are less problematic from a regulatory compliance standpoint and allow for a variety of water quality conditions to be evaluated. However these types of studies can be expensive and need to be tailored to ask specific fundamental and applied questions regarding corrosion. The challenge is to provide a test that is representative of the expected conditions in the distribution system and still obtain meaningful results that address the specific problem.
This paper provides a practical approach to evaluating the corrosive effects of finished water quality changes with the use of corrosion coupons, and copper, lead, and lead soldered copper pipes. Corrosion pilot tests for both small and large utilities will be discussed. The advantages and disadvantages of each approach will be presented.
The construction of the pipe loop and coupon racks will be discussed as well as the testing procedures used to assess comparative corrosion rates. The test apparatus is designed to allow for both general corrosion rates and well as comparative first draw stagnation samples to be evaluated side by side. The advantage is the ability to conduct time based studies of subtle water quality changes at a remote location prior to the implementation of full scale changes. The benefit is to allow for a much more educated decision with regard to lead and copper corrosion and inhibitor or water quality changes that may be needed. Both testing procedures also allow utilities to study the effects of water quality changes on the corrosion of household plumbing (lead soldered copper pipe), as well as a full range of distribution system materials such as cast iron, steel, brass, and bronze.
Example results will be presented for three utilities that are now evaluating finished water changes to disinfection strategies (chlorine verses chloramines), inhibitor selection and dose, and final pH. The studies are conducted in order to better understand the effects of change in finished water quality in order to comply with new disinfection byproduct levels anticipated as part of Stage 2 DBP levels of 80/60 for TTHM and HAA. These studies present an approach to testing the effects of switching to chloramines that will allow a municipality to make technically based decisions regarding specific changes in treatment and water quality, and how they will impact lead and copper leaching.
For a copy of the full paper, please contact the author at email@example.com
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