Use of Nitrifier Growth Rate Testing as an Operations and Capacity Planning Tool

The City of Gastonia owns and operates two wastewater treatment plants (WWTPs), the 16 mgd Long Creek WWTP and the 6 mgd Crowders Creek WWTP. Approximately 35% of the influent wastewater to both WWTPs consists of industrial discharges, and nitrification upsets occur occasionally. As part of a master planning project, process evaluations were conducted for both plants to examine their existing treatment performance and to identify capacity needs. Based on the historical operating data, it was concluded that a 15 day aerated solids retention time (SRT) was needed to ensure that nitrification was maintained during the winter months. This is longer than the 8 to 9 days SRT normally needed for reliable winter nitrification of domestic wastewater in this geographical area. Both plants have adequate aeration basin capacity to provide a 15 day SRT at the flows anticipated through 2010, but not for the 2020 projected flows. However, the proportion of domestic and industrial wastewater in Gastonia is gradually changing, which could favorably affect nitrification performance and enable future operation at a lower SRT. To assist with planning for future capacity needs, nitrifier growth rate testing was conducted to determine the required aerated SRT and monitor whether it changes over time.Following the master planning project, treatment performance at both WWTPs was excellent for several years. During the winter of 2007 the Crowders Creek WWTP experienced process upsets possibly as a result of an industrial discharge. High concentrations of chlorine were measured in an industrial discharge that represents approximately 20% of the total plant influent flow. To further examine the issue with the apparent SRT requirement as well as potential nitrification inhibition from the industrial discharger to the Crowders Creek plant, the City conducted two sets of laboratory tests in June 2007. This paper will present the detailed results of this testing and examine the impacts on both design and operational requirements.