Evaluation of High pH Infiltration Water and Recycled Bedding Material

ABSTRACT for presentation to the Water Environment Federation: Collection Systems Conference 2017

This evaluation was meant to determine the cause and source of high pH (alkaline) wastewater in the CBI Southlands sanitary sewer in Council Bluffs, Iowa. This newly installed sanitary filled with a high pH wastewater and its first discharge to the city wastewater treatment plant resulted in a rise in the pH of the total wastewater stream.

This sewer consists of 48-inch diameter HOBAS pipe and 21-inch diameter PVC pipe. Recycled crushed concrete was used for bedding support during the pipe installation.

To determine and evaluate the source of the high-pH wastewater, we visited the area and held discussions with site personnel. We then reviewed the sewer site and potential water and contamination sources, requested water samples be tested, and researched papers and reports on related alkaline sources.

We explored the potential that something in the pipe bedding could cause such an increase in pH and calcium either by leaking joints or infiltration of the pipes. To confirm or eliminate this theory, a sample of the recycled concrete pipe bedding was collected; distilled water was poured into the sample and allowed to rest for 24 hours after which there was a white precipitate in the sample. The liquid was poured off and sent to a laboratory for testing. Test results showed calcium levels were over 2,000 mg/l as CaCO3 in the distilled water. This indicates that groundwater encountering the recycled pipe bedding has a strong potential for becoming saturated with calcium as it flows through the bedding. There is also potential to raise the pH and alkalinity. This is similar to the lime-softening process used in a water treatment plant.

Test results also showed that the recycled concrete added significant amounts of potassium and sodium to the water which was also observed to be high in the sewer samples. This is another indicator of the potential for groundwater as the source of high pH water infiltrating the pipeline.

The figure below contains results from a sample test of the bedding material and distilled water to determine pH and alkalinity levels.

Even though the recycled concrete had the same structural strength and pipe lateral support as natural rock, it did not prove to provide the same long lasting support. Based on testing and confirmation of high alkalinity wastewater the sewer line was completely removed and replaced. To avoid future problems, installation was completed with new bedding material of natural rock.

FOX Engineering is an environmental engineering firm based in Ames, Iowa. We specialize in water and wastewater solutions for our diverse municipal and industrial clients. Our work varies in size and scope and can be found throughout the Midwest and beyond.