THE QUEST FOR ZERO ODOR COMPLAINTS

A Systemwide Odor Evaluation was conducted for wastewater flows that are conveyed to the Noman M. Cole, Jr. Pollution Control Plant (NMCPCP) in Fairfax County, Virginia, in order to quantify the sources of odors at several key locations, identify appropriate control measures, and reduce odor complaints. The collection system tributary to the NMCPCP consists of over 60 pump stations and force mains, one grinder pump station, and a major portion of the County's 3,100 miles of gravity sewers. The plant has a capacity of 54 mgd and an average daily flow of 45 mgd. The investigation of the odor problems included:Review of existing data such as complaint records and previous gas monitoring results;A sampling, testing, and monitoring program designed to quantify sources of odor at representative locations in the collection system during a period with conditions most likely to produce severe odor problems;Development of sulfide generation and corrosion rate estimates for the collection system;Evaluation of alternative sulfide control technologies;Development of recommendations for both the plant and the collection system.Five pump station/force main systems, one gravity interceptor sewer, and the influent trunk sewers to the NMCPCP were selected for monitoring. From the monitoring results, a specific sulfide flux coefficient was developed, and the Pomeroy-Parkhurst equation was used to estimate sulfide generation for force mains and gravity sewers not monitored. The estimates were used to rank force main and gravity sewer systems by sulfide concentration and corrosion rate so control measures could be prioritized. Control measures examined for the collection system included air injection, oxygen injection, nitrate addition, pigging, periodic sodium hydroxide slug addition, chlorination, hydrogen peroxide addition, potassium permanganate addition, iron salt addition, turbulence reduction, and continuous caustic addition. Foul air treatment alternatives considered as odor control measures for the plant influent included wet chemical scrubbing, carbon adsorption, biofiltration, and scrubbing in the aeration basins. Advantages and disadvantages of each alternative were considered. Recommended control measures included nitrate or iron salt addition in the pump station/force main systems with the highest sulfide generation potential, and two- and three- stage chemical scrubbing at several locations within the plant.