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This study, initiated by the Fairfax County, Virginia Department of Public Works and Environmental Services, is a Systemwide Odor Evaluation Study for wastewater flows that are conveyed to the County's Noman M. Cole, Jr. Pollution Control Plant (NMCPCP). 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 3,000 miles of gravity sewers. The average daily flow conveyed to the plant is 45 MGD.

Despite numerous odor control measures that have been implemented at the NMCPCP and at several pump stations in recent years, odor complaints have continued. Thus, a full-scale evaluation of the entire collection system was necessary to quantify the sources and extent of odors.

The investigation of the odor problem is being conducted in several steps, as follows:

Review of existing data related to odor problems, such as odor complaints in the system and hydrogen sulfide gas monitoring results at the plant;

A sampling, testing, and monitoring program designed to quantify sources of odor at representative locations in the collection system;

Development of sulfide generation and corrosion rate estimates in the system tributary to the NMCPCP;

Evaluation of alternative sulfide control technologies;

Development of recommendations for a plan of action to address odor problems in the system tributary to the NMCPCP.

The monitoring program was designed to assess and quantify the contribution of sulfide odors at representative facilities in the collection system. Seven facilities, specifically five pump station/force main systems, one gravity interceptor sewer, and the influent trunk sewers to the NMCPCP, were selected for monitoring. Each monitored facility (except at the plant influent) included three sampling points to ascertain upstream and downstream conditions and to allow computation of net sulfide generation. Each facility was monitored for 80 hours in September and early October 1999. A rotating schedule was implemented over the monitoring days to characterize the diurnal fluctuations of sulfide at each facility. Both wastewater and sewer gas were monitored during this period.

This extensive data obtained during the Monitoring Program will be used in subsequent project tasks in evaluating the odor problem systemwide and in decision-making for odor control measures. Empirical values used in estimating sulfide generation in the County collection system will be developed. The data will be used to develop the specific sulfide flux coefficient for the entire collection system, so that sulfide generation can be computed for force mains and gravity sewers not monitored. The Pomeroy-Parkhurst equation is the basis for the methodology and the development of sulfide generation estimates.

Next, sulfide generation estimates will be developed for all pump station/force main systems that are tributary to the NMCPCP. In addition, sulfide loss through volatilization in the gravity sewers downstream of the force main outfalls will also be estimated. Using these estimates, pipeline corrosion rates will be estimated for each gravity sewer segment downstream of the force mains tributary to the NMCPCP. Sulfide and corrosion estimates will be developed for existing average and future average flow conditions. The results of the estimates will be used to assess the relative effects of potential sulfide production and corrosion in the system tributary to the NMCPCP. Rankings by relative severity of sulfide concentration and corrosion rate estimates will be developed, so the County can prioritize control measures.

On the basis of these results, all technically viable alternative technologies for sulfide control will be evaluated. Criteria for evaluation will include capital and operating costs, safety, ease of operation and maintenance, effectiveness in actual use, ease of implementation, and effects on the treatment plant.

Recommendations will be developed and included in a plan of action to address odor problems in the system tributary to the NMCPCP. This will include recommendations for sulfide control technologies for further consideration by the County and for control locations.

Document Type: Research Article


Publication date: January 1, 2000

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