Vol 3 No 3
Summer 2005

Focus On
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Reclaimed Water Corrosion Issues
By Jose L. Villalobos, P.E.

V&A has conducted many investigations involving the use of reclaimed wastewater. Over the years several issues have surfaced on these projects regarding the performance of the materials of construction. These issues fall into the three main groups:

  1. Water Quality
  2. Operational Parameters
  3. Material Selection

Water Quality

To evaluate a reclaimed water system, the quality of the original source water needs to be determined in order to better understand how the reclaimed water will impact the various materials of construction.

One method of evaluating the corrosion potential of water is to calculate the Langelier Saturation Index (LSI). The LSI is an evaluation of the calcium carbonate (CaCO3) saturation capabilities present in the water. There are five water quality factors that are used to calculate the LSI: pH, Total Dissolved Solids (TDS), Water Temperature, Calcium Hardness, and Total Alkalinity. The following equation calculates the LSI.

LSI = pH - pHs

where pH = measure pH in water sample
pHs = pH at which water is saturated with CaCO3
LSI > 0 water is saturated with CaCO3 and will continue to form scale
LSI = 0 water neither forms CaCO3 scale nor removes CaCO3 scale
LSI < water is undersaturated with CaCO3 and will remove existing scale
 
The formation of the CaCO3 scale on water exposed surfaces is both good and bad. LSI values more negative than -1 tend to expose metallic surfaces to corrosion or will leach out the minerals from concrete. LSI values greater than +1 indicate conditions where excessive formation of CaCO3 scale will affect the performance of water treatment equipment, heat exchanging equipment, and other water-exposed surfaces. Generally the optimal range for LSI values is between +0.1 and +0.5.

Photos 1 and 2 show examples of how CaCO3 scale formation relates to the LSI.

Photo 1. Interior surface corrosion on a steel pipe
installed in a negative LSI water environment		 Photo 2. Light scale on the interior surfaces of a
steel pipe installed in a positive LSI water
environment.
Photo 3. Major leaching of minerals from the
cement mortar lining in a negative LSI wastewater
environment.		 Photo 4. Minor leaching of minerals from the cement mortar lining in a slightly negative LSI wastewater environment.

Operational Parameters

The California Code of Regulations, Title 22 Division 4, Chapter 3 Water Recycling Criteria, Section 60301, states limits on total coliform, bacteria, viruses, turbidity, and retention times in a chlorine contact basin. However it does not give a specific number for the chlorine residual in the recycled water as it leaves the treatment plant and enters the distribution system. Typical chlorine residuals might range from 1 mg/L to 5 mg/L and anything higher may damage vegetation if it is sprayed on directly. In addition, concentrations greater than 5 mg/L may have a corrosive effect on the interior surfaces of bare steel pipes or cement mortar-lined pipes.

In terms of the chlorine residual, there has to be a balance between preventing the growth of bacteria and preventing the corrosion of the distribution system. Typically, reclaimed water that has undergone secondary treatment with biological nutrient removal has Total Dissolved Solids ranging between 500 and 700 mg/L, and sulfates and chlorides ranging between 100 mg/L and 200 mg/L. At these concentrations, it would be prudent to conduct a corrosion impact study on the distribution piping system because the water may cause internal corrosion.

Material Selection

Reclaimed water applications are growing beyond the typical wetlands restoration, or golf course, agricultural, and highway landscape irrigation. Power plants and oil refineries are using reclaimed water for cooling towers. Facilities such as animal shelters may use it as a water source for cleaning kennels, and office buildings use reclaimed water for restrooms.

Engineers and architects should consider the following factors when specifying pipe materials to be used in a reclaimed water environment:

  1. Verify the concentrations of Total Dissolved Solids, Calcium Hardness, Total Alkalinity, and water temperature to calculate the Langelier Index.
  2. Review the concentrations of chlorides and sulfates together with the pH of the water.
  3. If the water is considered to be corrosive, specify thicker-walled pipes to allow for corrosion or dose the water source with a corrosion inhibitor.
  4. Recommend the use of stainless steel materials wherever applicable.
  5. Recommend the use of protective coatings for concrete or steel in chlorine contact basins or other areas where high concentrations of chlorine will be present.

V&A’s Experience with Reclaimed Water Corrosion

V&A has conducted many internal corrosion investigations related to potable water, reclaimed water, and wastewater. V&A provides material selection and corrosion inhibitor recommendations for potable water, reclaimed water, and wastewater. Contact V&A at the number below if any further information or guidance is needed with respect to these services.

V&A Consulting Engineers
1999 Harrison Street, Suite 975, Oakland, CA 94612
Phone 510-903-6600     Fax 510-903-6601

One Riverway, Suite 1700, Houston, TX 94612
Phone 713-840-6490

www.vaengr.com
Copyright 2005