Conference Agenda

Erratic climate patterns coupled with aging sewer infrastructure has caused many utilities to worry about their wet weather management strategies. One of the major contributors to wet weather flows is Infiltration and Inflow (I/I) which is a function of pipe material, pipe age, groundwater level and precipitation. In some cases, when groundwater level is above the invert elevation of the sewer collection pipes, I/I can also result in increased dry weather flows. With many utilities across the country planning major infrastructure sewer network upgrades due to ageing infrastructure, areas with highest I/I are usually a low hanging fruit to minimize wet weather flows. However, quantifying the benefits of such sewer upgrades is often difficult due to limited flow measurements typically available across sewersheds where upgrades are performed. The lack of flow measurements coupled with climate variability year over year render the use of simple ‘before’ and ‘after’ comparisons ineffectual. To avoid the bias and uncertainty due to climate variability, a different methodology is required to compare data and quantify impacts. This study investigates an alternate methodology of using pump station energy consumption data in lieu of flow measurements for quantifying the benefits for approximately $20 million in investments made by a utility¹ for rehabilitation of 210,000 linear feet of sewer pipe across six (6) sewersheds. This paper will review the approach of using a “control sewershed” i.e. a sewershed not having undergone any repairs, with similar characteristics as that of the sewershed undergoing rehab, to eliminate bias and using this alternate approach to quantify reduction in flows as a result of the I/I repairs. This paper will also review the approach for quantifying the return on investment for the rehabilitation as well as cost savings as a result of deferred treatment capacity expansions for the receiving wastewater treatment facility.

Note: 1. Approval to disclose the utility name is anticipated to be received ahead of the conference presentation.

This presentation will dive into a case study from Gresham, OR where deep infiltration was used to reduce the strain on an overwhelmed MS4 system. This site had shallow perched groundwater but beneath the silt layers were permeable sands and gravels that were perfect for infiltration. By drilling deeper, the stormwater can infiltrate and reduce the burden on the existing MS4 system.

This deep infiltration system was also installed in an existing residential street, which had tight existing utilities. By minimizing the footprint of construction and surgically targeting the location of the drywell, the construction timeline is shortened, and the risk of damaging existing infrastructure is reduced.

These deep infiltration systems are designed to protect groundwater quality by having a minimum of five feet of vertical separation between the bottom of the drywell and the high seasonal groundwater level. Results from monitoring this deep infiltration system will also be shared.