Combined sewer overflows (CSOs) are gaining the attention of environmentalists and regulators. Communities are taking steps to decrease the number of overflow events and to reduce the amount of pollutants discharged in these events. Like most communities in the Northwest, Mount Vernon, Washington, has a mandate to reduce CSOs to less than one event per year. Engineers, regulators, and City staff members investigated cost-effective means to implement
CSO reduction programs. This paper summarizes the cost-effective CSO reduction program implemented by the City of Mount Vernon. The program involves storage, treatment, and blending as methods to achieve the desired level of less than one CSO event on average per year.
Mount Vernon, Washington, like many Northwest communities, is faced with a population explosion. The population increase is due to an influx of people desiring affordable housing, cleaner air, less traffic, and a slower-paced life than is offered by the large West Coast cities. Preserving the pristine waters that have attracted these environmentally friendly people presents challenges for the communities that are experiencing this growth. The challenge shared by all communities has been to reduce the number of combined sewer overflows (CSOs) that discharge into streams, rivers, and Puget Sound.
Mount Vernon’s original sewer conveyance system was developed as a combined system that discharged into the Skagit River. Mount Vernon is unique in that the older portion of the city is below the flood level of the river. As a result, all flows are pumped either to the wastewater treatment plant or are diverted to stormwater pumping stations that ultimately discharge to the Skagit River.
The Washington State Department of Ecology’s (Ecology’s) guidelines for CSO facilities are to reduce the number of untreated events to one or less per year. Along with the CSO reduction requirements, many communities are also required to reduce the amount of ammonia discharged. Since Mount Vernon discharges to a pristine salmon stream, it has elected to reduce the ammonia in its discharge in addition to reducing its CSO events. An aging facility originally designed for carbonaceous biochemical oxygen demand (BOD) reduction is being taxed to treat the increase in flows, let alone to provide nitrification.
Flows to Mount Vernon’s Wastewater Treatment Plant (WWTP) do not follow the typical flow pattern observed at other treatment facilities in the Puget Sound area. During rain events, typical flows in the Puget Sound Basin are characterized by a gradual increase, then maintenance of steady state flows for several hours after the rain event passes, and a gradual decrease in system flows. In some King County drainage basins to the south of Mount Vernon, flows may continue for a day or more after the rain event passes. Mount Vernon’s system flows are atypical; the flows spike soon after a rain event, and then subside very soon after the event passes, as illustrated in Figure 1. The flows in the Mount Vernon system are rain-influenced infiltration.
As illustrated in Figure 2, average flows in the system are less than 5 million gallons per day (MGD) with occasional spikes to 10 MGD. The reason the spikes are not higher than 10 MGD is that the plant did process flows over this amount prior to 1993. When flows approached the 10-MGD mark, the influent gate to the plant was slowly closed to limit flows. The remainder of the system flows was bypassed to the Skagit River through three CSO overflow points. Storm events have a significant effect on the system flows. Flows can increase from an average of 5 MGD to over 35 MGD in just a few hours, as illustrated by the typical system flow graph in Figure 1.