THREE-STEP PROCESS TO SUCCESS IN CONTROLLING WASTEWATER COLLECTION SYSTEM ODORS IN GASTONIA, NORTH CAROLINA

The City of Gastonia's municipal wastewater system serves a population exceeding 80,000, with a large industrial base that consumes over 40% of its total system capacity. The service area now covers more than 50 square miles, including most of Gaston County and extending beyond county borders into several surrounding counties as well as South Carolina. The City's wastewater collection system covers parts of three major drainage basins, which flow away from the city's center. The system consists of over 620 miles of collection lines and 12 pump stations. Recently one of the City's three wastewater treatment plants (WWTPs) was converted to a pump station. The wastewater from this pump station is conveyed via force main about two miles to the wet well of another major pump station. From the second pump station, wastewater is conveyed via force main about three miles to a manhole near the county hospital and a busy street. From that manhole, wastewater flows by gravity through a 48-inch interceptor to one of the City's two remaining WWTPs.Odor complaints related to the City's wastewater system had been infrequent until the pump station change, conveying wastewater from one basin in the city's system over 10 miles through two major interceptors and two force mains. Odor complaints increased dramatically from two neighborhoods close to the second pump station (now the City's largest). The addition of wastewater from the decommissioned WWTP and its longer residence time in the collection system were the likely causes. Odor complaints also increased from a commercial area near the force main discharge point from this pump station, and from a residential neighborhood surrounding the 48-inch interceptor that conveyed this wastewater to the WWTP.Three levels of odor control were implemented in response to the odor complaints:Liquid-phase treatment through the addition of chemicals and enzymes to the wastewater stream.Mechanical improvements to the major wastewater pump station.Vapor-phase treatment at strategic points in the system, via both chemical scrubbing and biofiltration.Several types of liquid-phase treatment were employed, including iron salts, a nitrate inhibitor, and a biological enzyme. The chemical that worked best was a combination of the three. For reasons of cost and effectiveness, however, liquid-phase treatment was not sufficient to reduce the odor complaints to an acceptable level.Several mechanical improvements to the City's largest wastewater pump station were made, including submergence of the force main discharge point, and installation of a submerged mechanical mixer to break up the wet-well scum layer. A third recommendation (variablespeed pumping) was not implemented due to cost. Submergence of the force-main discharge point and installation of the mixer resulted in reduction of odor complaints from the neighborhoods surrounding the pump station. The mixer installation paid for itself in less than two years by eliminating the City's need to remove the wet-well scum layer by vacuumtruck every two to three months. Odor complaints increased in the downstream neighborhoods, however, likely because of the increased organic load of the wastewater conveyed from the pump station as a result of the mixer installation.Even with the partial success of liquid-phase treatment and mechanical improvements, odor complaints continued to a point that the City decided to invest in the third level of vaporphase treatment. Three areas were selected for odor control, summarized below:The City's largest wastewater pump station (discussed above): Odor complaints had diminished but were still not down to an acceptable level, and the cost of liquid-phase treatment warranted installation of a vapor-phase system. Biofiltration was selected.The force-main discharge point from the above-mentioned pump station: Large amounts of hydrogen sulfide were released at this location due to exposure of the anaerobic wastewater to air at the force main discharge point, so a dry scrubber was recommended with its media targeted for hydrogen sulfide removal.A neighborhood surrounding the 48-inch interceptor on its way to the WWTP: Mercaptans were the prevalent odorant in this area, believed to be due to biological decay of the wastewater prompted by long residence time in the system. A dry scrubber was recommended, with its media targeted for mercaptans and total reduced sulfides.The vapor-phase odor control devices described above have been effective and resulted in a significant decrease in odor complaints.