By Robert Y.G. Andoh
Up and down Maine’s rocky coast, urban communities are grappling with a common problem — what to do about combined sewer overflows (CSOs). CSOs are common in Northeastern states, including Maine, where infrastructure engineers built sewer systems to collect stormwater in the same pipes through which wastewater flows. During intense rains, these “combined sewers” become overloaded with excessive runoff causing an overflow event in which an untreated mixture of domestic sewage and stormwater is discharged directly into a receiving water body. Pollution from CSO outfalls into local rivers have angered residents and prompted state environmental agencies to set timetables to deal with the problem.
The issue facing Maine communities is how to fix the problem without breaking their budgets. Separating the sewers is one solution, but creating a whole new set of stormwater sewers is expensive and disruptive. Expanding the capacity of the wastewater treatment plant so peak wet weather flows will pass through without offloading at CSO outfalls is also costly because it can mean doubling or even tripling the size of the existing treatment plant.
There are alternative CSO solutions, however, and two of these solutions have been deployed in recent years by Saco and Bucksport, a pair of communities about 130 miles apart on the central Maine coast.
Saco opted for a hybrid solution to its CSO problem by pursuing a plan that eliminated seven of the eight CSO outfall sites by separating the stormwater and wastewater sewers. However, at the eighth CSO site, which used to offload effluent into the Saco River in the downtown area, rather than dig up downtown streets, city officials decided to treat the effluent before it reached the river.
The challenge was twofold: First, how to treat excess stormwater, and second, how to control the flow of water to ensure proper treatment and avoid discharges of polluted runoff. To gauge the storm system needs, the city analyzed precipitation in the Saco River from the 1980s and ’90s, as well as a recent four-year period. Based on the findings, city officials felt confident they had a good gauge on estimating future storm events.
The city implemented a new conveyance line at the treatment plant. To control the flow, the city implemented two vortex flow control valves from Hydro International (Portland, ME). To avoid the cost and performance challenges of conventional tankage systems, the city chose to deploy an alternative system based on Hydro’s advanced vortex separators to treat the excess flow to the same standard.
“Treating stormwater isn’t a finite thing,” said Christopher J. Osterrieder, senior engineer with Deluca-Hoffman Associates Inc. of South Portland, ME, the engineering firm that worked on the Saco sewer project. “We had to be careful to only send enough water so that we could perform within the vortex separator’s rated range.”
The first valve restricts the flow that goes down to the treatment plant and the second valve further splits the flow: the maximum volume goes through the regular secondary wastewater treatment process and the rest goes to the city’s new CSO treatment system based on Hydro International’s Storm King Advanced Hydrodynamic Vortex Separator (HDVS). This results in the treatment of excess wet-weather flow by the vortex separation system prior to discharge.
In the new CSO treatment system, sedimentation, screening and disinfection are all accomplished in the HDVS vessel. The disinfectant sodium hypochlorite (NaClO) is injected into the flow of the HDVS. The disinfectant and combined sewer then mix in the Storm King vessel, where the sewer solids and disinfection flocs are removed by gravitational and rotational forces. These forces increase the amount of time it takes for water to flow through the tank, thereby increasing its contact time with sodium hypochlorite. This increased contact time provides a more efficient “kill rate” for undesirable materials in the water. Because of the improved efficiency of the system, the city was able to use a smaller tank to achieve the required level of treatment, vastly reducing the up-front capital investment and ongoing maintenance costs.
As settleable solids collect in the base of the vessel, the clarified flows are directed upward through the central area of the vortex chamber and then down through the Swirl-Cleanse screen, which captures all floatables and neutrally buoyant material greater than 4 mm in diameter. The screened effluent is discharged from the system through an air-regulated siphon, which also provides an effective self-activating backwash mechanism to prevent the screen from blinding. After screening, the treated sewer flows are discharged into the Saco River. The collected screenings and grit are then pumped a short distance back to the wastewater treatment plant for processing.
The Saco system went online in November 2006.
Another example of an alternative CSO solution is the multi-year Bucksport project, which includes a sewer separation, a moderate expansion of the wastewater treatment plant, and a “satellite treatment” plant to avoid the expense of digging up downtown streets.
The satellite treatment system, which went online in the spring of 2008, is a Hydro International Storm King advanced vortex separator unit. The unit is deployed in a satellite location upstream from Bucksport’s wastewater treatment plant and is the first CSO treatment unit using advanced vortex technology to be installed remotely, away from a central wastewater treatment plant, in the state of Maine.
The treatment unit removes pollutants from the flows to meet the Maine Department of Environmental Protection (DEP) water quality standards and saved the town the expense of having to pump the flows a long distance and upgrade sewer lines along the way.
“We needed a solution that improved the treatment of CSO discharges to Maine DEP specifications and, at the same time, came at a reasonable cost to taxpayers,” Bucksport Mayor Jeff Robinson said. “We looked at a number of solutions, and [chose] the best solution to meet both of those challenges.” uwm
About the Author:
Robert Y.G. Andoh is the director of innovation for Hydro International. He has authored and presented a significant volume of technical papers relating to urban flooding, collection systems and wastewater treatment processes, contributing extensively, on both a national and international basis, at communication events in the field of urban water management. He has served as a visiting professor of Liverpool John Moores University and a member of the Technical Advisory Board for the University of New Hampshire Stormwater Center.
