Choosing Right Methods for the Chlorine Disinfections to Meet Daily E. coli Limit in a Large Nitrification Plant

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Indiana is one of a few states that has replaced a weekly Fecal limit with a daily E. coli limit as an indicator organism for wastewater disinfection. It has been accepted that E. coli is a better organism indicator. However, there is still not an EPA approved method for enumerating E. coli in wastewater. Indiana has adopted several different methods for use by wastewater treatment plants for effluent monitoring. These different E. coli methods might have an impact on the disinfection process performance and efficiency. In addition, for the nitrification plant there will be a challenge for them to meet the daily limit because of the low effluent ammonia and the possible interference of the organic nitrogen. In this study, a full-scale test was designed and performed at a large nitrification plant to compare the impact of three E. coli methods, i.e., Colilert®, m-TEC and modified m-TEC, on the disinfection process performance and efficiency. In addition, an ORP online analyzer was installed side by side with the total chlorine residual analyzer to investigate the correlations between the process performance and ORP and other chlorine control parameters. The results demonstrate that the Colilert® method yields a higher E. coli density compared to the m-TEC methods. The enumeration difference of these methods was found to be much larger for the effluent E. coli density than for the inflow E. coli density. It was difficult to meet the daily E. coli limits using the Colilert® method. The E. coli results of m-TEC method and the modified m-TEC method are significantly correlated. They can be exchangeable without significant impact on the results. ORP was identified as the most significant factor correlated to both the effluent E. coli and the E. coli removal rate, which indicates the ORP is the promising chlorine control parameter for achieving the daily limit goal. Finally, Fecal coliform counts were compared to the E. coli data determined by the Colilert® method and the m-TEC method. It was found the m-TEC enumeration results have a much stronger relationship with the Fecal coliform counts than the Colilert® results. The ratio of the m-TEC E. coli counts to the Fecal coliform counts is about 0.5.

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