The draining search for wet weather solutions
Combined sewer overflows (CSOs) are among the most difficult water pollution challenges confronting older municipalities. During storms, combined sewers often reach capacity and overflow, discharging untreated human, animal and industrial wastes into waterways.
The pollution compromises water quality, which in turn jeopardizes human and environmental health. Quality of life may be affected by odors, unsightly contamination, illness, lost recreational space, fish kills and shellfish contamination. In some cases, CSOs will necessitate costly changes in drinking water treatment.
Recognizing the variety of CSO problems around the country, EPA issued a National CSO Policy in April 1994. At the same time, the agency estimated that annual CSO discharges had reached 1,200 billion gallons and were affecting 43 million people.
EPA’s policy gives more than 1,000 U.S. communities the flexibility to consider the site-specific nature of their CSOs and identify cost-effective means to control them. It encourages municipal and regional authorities to assess the impact of CSOs and solutions on the local environment; the community’s financial capability to implement the solutions; and citizen concerns about water quality. Also, it asks local officials to consider the possibility that their communities should revisit their water quality standards.
Throughout the United States, communities are working within the framework of the National CSO Policy. In-line storage, retention basins, screening and disinfection, vortex separators and sewer separation are among the tools they are using to reduce combined sewer outfall and to diminish its effects.
Getting control options ‘in line’ Storing wet weather flows before they discharge into receiving waters is perhaps the most common means of controlling CSOs. Storage can be accomplished with existing pipes and conduits, holding tunnels, or retention basins and tanks. The storage facilities are used to full capacity, and automatic gating devices prevent the water from flowing to the combined sewer relief outlets.
Cleveland is among the cities using in-line storage with success, says Frank Greenland, planning manager for the Northeast Ohio Regional Sewer District. “During the 1970s and 1980s, we did a lot of CSO control via in-line storage, using our largest combined sewers,” he explains. “Automated regulators that work by inflatable dam or hydraulically operated gates hold the flows back until the wet weather flow recedes.”
Long-term CSO control planning studies are under way across much of the district’s CSO area. Additionally, the district is constructing a new CSO storage and conveyance tunnel to augment its existing in-line storage system, and it is improving the capabilities of its wastewater treatment plants for more efficient treatment of wet weather flows.
Prior to undertaking new construction, communities often look for cost-efficient ways to maximize their existing systems. Such was the case in King County, Wash., where the county’s Department of Natural Resources modified existing infrastructure to maximize in-line storage, saving area residents $50 million over the cost of new construction.
“We took an incremental approach [to improving CSO controls],” says Laura Wharton, senior planner for the department’s wastewater treatment division. “First we perfected our monitoring and control system modeling to determine where we already had in-line storage. We then made improvements to the control system and infrastructure.” The changes included activating out-of-service tunnels, elevating weir heights, and modifying computerized controls on gates and weirs to capture wet weather flows.
King County’s updates are part of a 35-year, $600 million CSO control plan that also includes the addition of storage and treatment facilities. In 2000, construction begins on the largest of the new facilities, which is designed to handle up to 50 CSO events annually. The project will include a 6,200-foot-long, 14-foot-diameter storage and treatment tunnel, costing $156 million and providing up to 7.2 million gallons of storage.
Looking at the long term One of the largest of all current CSO control projects is the ongoing restoration of the Rouge River in southeast Michigan (see the story on page 28). The river’s watershed overlays three counties and 48 cities and towns, including Detroit.
The restoration project involves 59,000 acres with 157 CSO outfalls. Ultimately, it will include 10 retention treatment basins and separation of eight combined sewer lines. Another sizeable program is under way in Syracuse, N.Y., where 6,800 acres contain 66 points that overflow into Onondaga Lake’s tributaries. One creek, traversing the city center, has 45 CSO outfalls.
Responding to a federally ordered consent judgment requiring CSO controls, the Onondaga County Department of Drainage and Sanitation has proposed a combination of control strategies that relies heavily on improvements to CSO capture and treatment. For example, the 15-year, $380 million program will include: * the construction of four regional facilities. The facilities will employ swirl concentrators with CSO treatment capabilities ranging from 10 cubic feet per second to 667 cubic feet per second; * installation of net bags and booms for floatables removal; * more than 15 sewer separation projects; and * storage capacity improvements via in-line retention and reactivation of unused infrastructure.
In addition to making those improvements, the county will strengthen its CSO controls by upgrading pump stations and siphons, and by improving treatment efficiencies at the Metropolitan Sewage Treatment Plant.
Implementing treatment technology In concert with, or as an alternative to in-line and basin storage, local governments are employing a variety of treatment technologies for CSO control. The equipment ranges from the very simple (e.g., a screen) to the complex (e.g., the continuous deflective separator and the vortex separator).
In addition to testing retention basins in the Rouge watershed, Detroit plans to screen and disinfect overflows along the Detroit River. Its facilities will incorporate rapid mixing, with 10 minutes of detention time for chlorination, and 4-millimeter mesh mechanical screens.
“Detroit’s CSO program includes a series of demonstration projects with a variety of control technologies,” says Gary Fujita, assistant director of wastewater operations for DWSD. “In each of the demonstration projects, we are trying to answer the question, ‘Is a smaller facility as effective as a larger presumptive size?’ We are using this approach to manage the total cost of CSO control and monitoring.”
Although the screening process is effective for many systems, it is not the answer for all CSO problems. That is why the Louisville and Jefferson County (Ky.) Metropolitan Sewer District is testing a continuous deflective separator (CDS), previously used only in Australia.
The CDS works on a principle of deflection and screening, whereby the flow entering the unit slows and eddies into a whirlpool inside a circular, screened chamber. The CSO circulates slowly inside the chamber, where solids and floatables are deposited. The clarified flow exits through the surrounding screen.
Units range from 6 feet to 41 feet in diameter, and they can manage from 1.2 mgd to 123 mgd. Richard Galardi, CSO program manager for Louisville, says a small footprint makes the CDS attractive to his city.
Others are optimistic about the new technology but are awaiting more performance data. “These CDS units are a new technology a lot of people are waiting to see,” says Charles Noss, research director for the Water Environment Research Foundation in Alexandria, Va. “The counter-current flow technology gets away from clogging, and you don’t have to clean the screen.” Noss notes that “a number of small CDS units are being tried in different cities,” but the foundation has not yet compiled data from those projects.
Like the CDS unit, a vortex separator basin is a promising treatment option. Incorporating “whirlpool” action, it captures and slows CSOs, and it settles the solids in a conical basin. Communities are drawn to it primarily because of its efficient treatment rate (up to six times that of conventional separators) and its few moving parts.
In Columbus, Ga., the vortex separation basin has proven effective in a demonstration project, says Clifford Arnett, senior vice president for Columbus Water Works. “We did side-by-side comparisons with baffled settling basins and determined the vortex to be much superior for solids removal from a loading perspective,” he says.
As part of the Chattahoochee River watershed, Columbus needs to protect the river from high fecal coliform counts and solids carried by CSOs. According to Arnett, the vortex separator basin removes about 55 percent of the CSO’s total suspended solids and provides disinfection for 90 percent of annual rainfall.
While he notes that the vortex technology has saved Columbus $45 million over alternative treatment methods, Arnett adds that the city is testing additional treatment techniques. The tests include demonstrations of: * dissolved air to make floatables more buoyant and to promote solids removal; * high-rate filtration with synthetic “fuzzy” filters capable of higher particle removal than sand filters (with treatment rates up to 20 gallons per square foot); * ultraviolet and chlorine disinfection combinations (chlorinating the first flushes, followed by UV disinfection of the remaining flows, appears most effective); * chlorine dioxide disinfection; * dechlorination with sodium bisulfite; and * paracedic acid testing. (The disinfectant is well-known in the food industry and for medical sterilization, but it is generally untested in wastewater and drinking water treatment.)
Examining disinfection techniques As some communities experiment with new treatment technology or equipment, others are testing disinfection methods. Prompted by EPA’s 1998 passage of its Enhanced Surface Water Treatment Rules and stage 1 of its Disinfectants/Disinfection By-products Rule, communities are taking a new look at the chemical and biological processes used to disinfect CSOs.
In New York, the city’s Department of Environmental Protection has tested several high-rate disinfection methods at its Spring Creek CSO storage and treatment facility. There, ultraviolet disinfection and ozonation were tested, as was the use of chlorine dioxide. (The latter process proved unfeasible because New York prohibits transportation of chlorine gas, which is needed to produce chlorine dioxide.)
Ultimately, chlorination/dechlorination with sodium hypochlorite and sodium bisulfite emerged as the most efficient and cost-effective processes for the 13-mgd facility. Although operations currently rely on high-efficiency chlorination techniques – e.g., flash mixing with high-energy diffusers – the facility has the capability to upgrade to dechlorination in anticipation of possible future regulations restricting chlorine residuals.
Dividing and conquering Storage and treatment are the most common methods of attacking CSO problems because the final choice – separating stormwater and sanitary sewer conduits – often carries a prohibitive price. “We’ve decided to do it only in smaller sewershed areas (neighborhoods of 200 units or less) where combined sewer exists,” Louisville’s Galardi says. “I don’t know if it’s worth it for larger CSO drainage areas when compared to other alternatives. We are also looking at in-line storage as well as storage basins in our long-term control plans.”
In Seattle, sewer separation during the 1960s and 1970s actually added to King County’s CSO problems. As the city grew, so did its impervious surfaces; and that, along with newly constructed separated sewers, generated increasing amounts of stormwater runoff flowing directly into metropolitan area waters. The increased runoff, as well as the prohibitive cost of large-scale sewer separation, led to declining use of separation as an alternative in King County.
On the other hand, Boston has found that separating sewers can be beneficial in lowering contamination caused by stormwater runoff. In a section of the city served by the Stony Brook Sewer System, Boston’s Water and Sewer Commission (WSC) found sewer separation preferable to construction of a treatment and disinfection facility. In 1997, the WSC had a plan for a disinfection facility that would treat wet weather flows consisting mostly of stormwater. The facility would have reduced bacteria and solids but not other pollutants found in stormwater.
As an alternative to the facility, the commission decided to separate the stormwater conveyances upstream. The move would not eliminate all wet weather pollution, but it would reduce non-sewage flows enough to decrease the instance of CSOs farther downstream.
According to Paul Keohan, project manager for the commission, the separated system has proven to be the better alternative in the Stony Brook project. “Separation removes pollutants, such as total suspended solids and biochemical oxygen demand, that would not be removed by a screening and disinfection facility,” Keohan told attendees at the 1998 conference of the Water Environment Federation, Alexandria, Va. Additionally, the commission is relying on public education, source controls, the removal of illegal connections, catch basin cleaning and other best management practices to improve the quality of stormwater.
The National CSO Policy of 1994 has fostered a wide variety of CSO solutions. Using site-specific technology and disinfection processes, communities are producing valuable technical information for future CSO control. As projects are completed and data is compiled and shared, local governments are in the strongest position ever to reduce stormwater contamination and improve the quality of their water resources.
Carl Johnson is vice president (in the Detroit office), and Andrew Brengle is senior communications specialist, for Cambridge, Mass.-based Camp Dresser & McKee.
In 1997, Pearland, Texas, implemented a program encouraging developers to purchase acre-feet in city-owned retention basins as compensation for increased water runoff caused by new construction. Twenty months later, nine developers have made such purchases, and the resulting revenue (nearly $30,000) has been used to offset costs of retention-related capital projects.
Pearland, located south of Houston, may seem an unlikely place for flooding. But, since the early part of this decade, seasonal rains have regularly forced area creeks over their banks and into streets, homes and businesses. In October 1994, when record rainfall flooded more than 100 homes, Pearland’s city council implemented a master drainage plan and coordinated it with a similar county plan.
City Manager Paul Grohman spearheaded the city’s flood control efforts, which included a $2 million investment in landacquisitions, adding 613 acres for stormwater retention. (Six regional basins were planned; one is open, and three are being excavated this year.) At the same time, Senior Engineer John Hargrove drafted the retention purchasing program for developers.
As part of the program, developers are asked to mitigate flood hazards created by “construction on land that creates new surface disturbances that result from filling, excavating or surfacing activities on the land.” To do so, developers purchase 0.2-0.6 acre-feet of retention space for every 1 acre of development. The actual amount of space purchased depends upon the shape of the development; the amount of grading, filling and paving required; and the hydrologic condition of the site before development.
Prior to implementing the program, the city directed builders working in special flood hazard areas to provide stormwater retention – usually in the form of on-site holding ponds. However, with the implementation of its new plan, the city extended the retention requirements to all builders, anywhere within the city limits. Since then, developers have begun purchasing space in city retention facilities because doing so is less costly than building retention ponds.
Pearland is excavating the detention basins, which range in size from 30 acre-feet to 100 acre-feet, from the acreage it purchased as part of its master drainage plan. The excavating contractor is selling the dirt to local builders, meaning that the jobs pay for themselves and cost the taxpayers nothing.
Land acquisition and basin engineering costs have been paid with certificates of obligation and annual operating funds. The retention fees and revenue generated by land use (e.g., one site will accommodate multi-story office buildings; others will accommodate parks) will help the city recover some of those costs.
Last fall, when Washington County, Ore., planned a road-widening project for urban Hillsboro, grading and paving were the least of its worries. In addition to expanding a portion of East Main Street from two lanes to three, the county planned to install sidewalks and bicycle lanes, making the area multi-modal and pedestrian friendly. The new construction meant that officials had to consider stormwater management; and, with tight space, they had to consider a variety of options.
As the county examined potential stormwater solutions, it was guided by four factors: land area, land cost, construction costs and maintenance. With the assistance of W&H Pacific, a Portland, Ore.-based engineering firm, officials determined that the project’s space restrictions eliminated the possibility of retention ponds and swales. Rather than install the storage facilities, they opted for in-line filtration as a means of managing the stormwater’s effects.
“The county didn’t have to buy additional property to build water quality facilities,” says Bill Ihly, project manager for Washington County. “We were able to take care of all the water quality requirements for 1.8 miles of road widening using stormwater filters.” Provided by Portland-based Stormwater Management, the filtration system consists of an underground concrete vault housing rechargeable cartridges filled with a variety of filter media. For the East Main Street project, the county installed two vaults, aligning them with existing storm drains. As stormwater flows through the filters, it is discharged into collection pipes.
Having successfully installed in-line filtration at the East Main site, Ihly says he is considering it for use in other projects. For example, the county is planning an extension of Brookwood Avenue, which would run through an environmentally sensitive area containing woods and a creek. “The county wants to be sensitive to the environment, so going out and creating ponds and chopping trees to make swales is not going to be a good thing,” Ihly says. “I can use the stormwater filters to the public’s advantage.”
When Woody Creek, Colo., recently planned the construction of a fire station and post office complex, officials faced stringent design demands. After all, the community is part of Aspen, which depends on its natural beauty as an economic catalyst.
“In Pitkin County, we pride ourselves on protecting the environment and making sure that any development, even our own, fits within the natural surroundings,” says County Engineer Bud Eylar. “We got involved in the Woody Creek project because of comments we received from people in the neighborhood and the Woody Creek Caucus (the neighborhood’s governing body).”
Pitkin County has a comprehensive project review process based on Colorado State Senate Bill 1041, which gives counties the right to review projects of state and local interest. Additionally, Woody Creek has a master plan that governs all development. During the design and approval processes, local concerns were twofold. First, the Woody Creek Caucus wanted a buffer to shield the project from the road and the view of passersby. Caucus members also felt that a retention pond would be unsightly and unsafe, so the group sought an alternative method of stormwater control.
Schmueser Gordon Meyer, Glenwood Springs, Colo., installed a landscaped berm as a project buffer and enlisted the assistance of Aspen Earth Moving for the stormwater system design. Because Pitkin County regulations require that any stormwater system be designed for a 100-year storm (equating to 2,800 cubic feet of storage space at the project site), the companies chose to use a subsurface system.
The stormwater management system consists of 175 polyolefin storage chambers from Infiltrator Systems, Old Saybrook, Conn. The chambers have open bottoms and louvered sidewalls that allow infiltration into surrounding stone and soil. Three inspection ports extend through the 3-foot berm, which surrounds the site and partially covers the system. (Because of traffic in the area, designers were careful to create the berm with plants that would not entice wildlife – especially deer.)
The stormwater system, which took five days to install, was completed in March. Construction of the fire station and post office is under way, and Woody Creek plans to open the facility this summer.
Last year, Detroit officials unveiled an 11-year plan to increase the city’s sewage transportation and treatment capabilities. The project will include the implementation of several measures to reduce the impact of combined sewer overflows (CSOs) on public health and natural resources.
Detroit has more than 70 storm-related CSOs that discharge 20 billion gallons of untreated sewage into the Detroit and Rouge rivers annually. The contamination threatens water quality by introducing bacteria, floatables and solids, and by depleting dissolved oxygen. With the assistance of the Michigan Department of Environmental Quality and local engineering firm Wade-Trim, the Detroit Water and Sewerage Department (DWSD) evaluated the city’s CSO problem. Using a capacity stress test and modeling, DWSD examined unit processes and support facilities such as transport piping and pumping, staffing, laboratories, maintenance, operations and plant management.
The model, based on EPA’s Stormwater Management Model, allowed DWSD to simulate how the Greater Detroit Regional Sewer System would respond to specific rainfall events. It incorporated hydraulic risk assessment to produce risk contours and rate the collection system’s transport capacity.
Once the treatment plant’s performance and maximum capacity were established, DWSD identified the changes necessary to handle excess flows. They are: * Rainwater control. Actions will include disconnecting downspouts; using catch basin covers with fewer holes; storing rainwater in sub-surface pipes; planting trees and grass to absorb rainwater; and demolishing abandoned buildings and planting grass on the lots. * In-system storage. Actions will include installing gates and inflatable dams in sewer pipes to regulate wastewater flow during and after storms. * Treatment plant improvements. Actions will include expanding the existing plant by adding two primary clarifiers and one pump to increase capacity. * End-of-pipe treatment. Actions will include building facilities – including basins, tunnels, and screening/disinfection treatment sites – to store and treat combined sewage.
Detroit spent more than $21 million building the project’s first demonstration facility and developing the city’s overall CSO control plan. It paid for the project with local share bonds and federal grants. With additional federal funds, the city plans to build a total of 10 retention basins, each uniquely designed to demonstrate the effectiveness of the plan’s elements.
Pilot programs for detention basins, screening/disinfection facilities, double-leaf storage gates, inflatable dams and rainwater control are under way. They will be monitored and evaluated, and the results will form the basis for control plan amendments, which are scheduled for 2002.