A NEW (NOW PROVEN) APPROACH TO COMBINED SEWERED AREAS: KEEPING BASEMENTS DRY AT HALF THE COST

Wastewater utilities in combined sewered areas face two general challenges: 1) reduction of pollutants entering waterways and 2) reduction of the frequency of backups of combined sewers into residents' basements. Whereas the former challenge is often imposed by regulatory agencies, the latter is imposed by customer wrath after major rainfall events.This paper focuses on the basement backup issue. It presents the results of a large scale project in Evanston, Illinois that has utilized an innovative approach to attempt to reduce the frequency of basement backups down to a target of once in 100 years, AND at roughly 50% of the cost of traditional solutions that would have reduced the same frequency to once in 10 years.Evanston is now in the ninth year of a phased construction program and is more than 60% complete with the total program. Further, some of the drainage basins within the city have been completely finished and several have been in operation for more than eight years. As a result, there is now the opportunity to review the cost and performance of this innovative program, not only from the theoretical perspective, but also from a practical standpoint based on a significant record of actual operation.The innovative approach taken by Evanston includes the concept of inlet control of wet weather flows into the combined sewers. The concept is to limit the entry of wastewater to the sewer system to just that quantity that matches the conveyance capacity of the system. For the last 15 years, the general concept of inlet control has been on the fringes of the set of options that utilities consider to mitigate basement backups. Although the potential benefits of the concept may have been attractive, invariably the inlet control option has been discarded because of the absence of a large scale proven application.The paper describes the inlet control option as it was tailored to the Evanston situation. More importantly, the paper examines the performance of the program against the following evaluation criteria: technical hydraulic performance (basement backup and street ponding performance), citizen perception of performance (based on postcard surveys and complaint calls), maintenance concerns and costs, project capital costs, level of construction disruption, and public acceptance.Each application of the inlet control concept needs to be tailored to the specific unique situation faced by a community. Nonetheless, based on the success in Evanston, it is believed that the concept has great potential to be extended to many cities with a combined sewer system and extensive basement backup problems. Several communities, including the City of Chicago, are currently vigorously pursuing programs that use the inlet control concept.