Comprehensive Pilot Testing Guides CSO Treatment in New York City
When it rains in New York City, the umbrellas unfurl, the traffic snarls, and the combined sanitary and storm water collection system surges -- sending dilute, untreated wastewater into the city's waterways. To provide storage and treatment of wet weather flows, known as combined sewer overflows (CSOs), New York City's Department of Environmental Protection (DEP) built the Spring Creek auxiliary water pollution control plant in the early 1970s. This prototype facility, located on the north shore of Jamaica Bay, was one of the first of its kind to deal exclusively with the water quality problems resulting from the release of CSOs.
The Spring Creek plant, which serves portions of Brooklyn and Queens, provides 23 million gallons of storage capacity and is designed to disinfect the CSOs with a chlorination process, and return the treated overflows back through the collection system to local wastewater treatment plants once the storm flows have subsided. During times of extremely heavy rainfall events -- approximately 15 times a year -- the storage capacity of the Spring Creek plant is surpassed and disinfected CSOs are released to Jamaica Bay via Spring Creek.
In 1990, the DEP hired the Cambridge, Massachusetts-based consulting firm of Camp Dresser & Mckee Inc. (CDM) to review the design and operation of the 20-year-old facility and to address odor control needs. In 1995, acting on CDM's recommendation for upgrades and modifications to the plant's design and operations, the DEP expanded CDM's role to include pilot testing of CSO disinfection and alternative basin cleaning systems, with particular emphasis placed on potential overflow toxicity.
Disinfection Pilot Testing
The Spring Creek facility currently disinfects with sodium hypochlorite, but DEP officials had been concerned that future limits on effluent chlorine residuals might require either the addition of dechlorination facilities or the adoption of an alternative disinfection technology.
For four months, a CDM team pilot tested four high-rate disinfection technologies -- ultraviolet light (UV), ozone, chlorine dioxide, and chlorination/dechlorination -- to determine their effectiveness in reducing bacteria levels in water representative of Spring Creek's CSO wastewater. In addition, through a grant from the Electric Power Research Institute (EPRI) and the New York Power Authority, CDM also pilot tested the effectiveness of electron beam disinfection. "This is the first time that several of these disinfection technologies have been applied to wastewater with such variable water quality characteristics as CSOs," explained CDM project manager Sorin Goldstein.
During the concurrent side-by-side testing, samples of the influent wastewater and the treated effluent from each pilot run were collected and analyzed for bacteria and conventional wastewater quality parameters. Capital and operating costs of each technology were also analyzed to determine the most cost-effective treatment option.
Chlorination/dechlorination with sodium hypochlorite and sodium bisulfite - an enhancement of the technology now being used -- was found not only to be the most effective treatment, but also the most cost efficient. The chlorine dioxide process also proved to be a very effective approach, but the transportation of chlorine gas, required to generate chlorine dioxide, is not permitted within New York City because of safety concerns. Alternatives for chlorine dioxide generation that are safe and effective, such as the UV/sodium chlorite process, are available but will require further testing. This is planned by the DEP and will partially funded by a grant from the U.S. Environmental Protection Agency.
Basin Cleaning
The Spring Creek facility has six storage basins, each 500 feet long and 50 feet wide, that are cleaned with a high-pressure spray water system from a traveling bridge after each storm event. The cleaning process removes residual solids from the bottom of the tank. These are then then pumped back to the treatment plant. This stage is the source of the facility's odor problems. CDM is currently pilot testing five types of automatic basin cleaning systems to ease the cleaning process and reduce odors -- tipping buckets, flushing gates, high-pressure/low-volume sprayers, low-pressure high-volume sprayers, and submersible mixers that agitate the tank contents and suspend settled solids. According to CDM project engineer Tom Schoettle, "Early results indicate that the tipping bucket and low-pressure spray system provide the most effective cleaning of the basin."
Odor Control Studies
In conjunction with improving the basin cleaning system, CDM has conducted an extensive air sampling and monitoring program. To mitigate odors that may affect a housing complex being planned near the Spring Creek facility and to provide adequate ventilation during basin cleaning, CDM is designing four packed-tower air scrubbers. The CDM and DEP teams also are investigating the possibility of lowering the facility's concrete roof to cover the tanks and reduce the volume of the air being treated.
On the Horizon
In addition to the disinfection pilot testing and the upgrade of the Spring Creek facility, CDM is assisting -- under a separate contract with Hydroqual Inc. -- with the assessment of more than 80 structural and non-structural CSO abatement technologies to control floatable materials and settleable solids discharged into New York City's water bodies. Structural technologies to be evaluated include screens, swirl concentrators, netting, and booms. Non-structural controls such as programs for catch basin cleaning, street sweeping, recycling, and public education will also be reviewed. The resulting plan will guide the DEP's program to eradicate the effects of floatables and CSOs on water quality.
"Much of the knowledge and experience gained at the Spring Creek plant can be applied to other CSO control programs," said Goldstein. "The technologies that are being pilot tested and evaluated today not only will have a profound impact on the city's water quality, but will help advance the design and implementation of practical, cost-effective CSO controls nationwide."
Editor's Note: This is a slightly modified version of an article that appeared in the November, 1997 issue of CDM News, a publication of Camp Dresser & McKee Inc.