1.0
Free chlorine Ct and contact time 0 9.3 min.mg/L (2 minutes) 22 min.mg/L (5 minutes) 42 min.mg/L (10 minutes) 116 min.mg/L (30 minutes) 217 min.mg/L (60 minutes) 744 min.mg/L (240 minutes)
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decay changed. Results obtained in this study suggest that the occurrence of microbiological monochloramine decay was related to the presence of chloramines in backwash water. However, other factors such as backwash frequency, WTP inlet water quality (including DOC concentration and character) and bacteriological composition may also be important. Further research is required to determine the importance of each factor.
Ct value (mg/L.minute)
4.0
Figure 10. Impact of chlorine addition prior to ammonia on monochloramine decay in Tailem Bend WTP product water: a) normalised monochloramine decay; and b) monochloramine decay rate.
Results confirm that microbial organisms or degradation products arising from microbial organisms in the filter are responsible for the accelerated monochloramine decay observed and that their presence is possibly related to backwashing filters with chloraminated water. It is uncertain if the microbiological monochloramine decay behaviour observed was present prior to the flood events, as monitoring of WTP product water monochloramine decay commenced only when nitrification became an issue in the distribution systems. The combined effects of higher WTP product water DOC concentration and microbiological impacted monochloramine decay was enough to result in nitrification in the outer parts of distribution systems such as the example shown in Figure 3. It is important to mitigate microbiological monochloramine decay to improve monochloramine stability and reduce the risk of nitrification. Based on results obtained, a possible solution would be modification of the filter backwash water strategy where filters would be backwashed with
non-disinfected water, such as at Morgan WTP, or with chlorine as is the case at Murray Bridge WTP. At the WTPs studied this option would require re-design of the filter backwash system and new infrastructure, so another option was investigated. Chlorination prior to ammonia addition was identified as a potential method to overcome the rapid chloramine decay, as chlorine would have the potential to inactivate problematic or causative bacteria and oxidise any SMPs. Laboratory tests were conducted, simulating chlorine contact between two and 240 minutes before ammonia. The effectiveness of free chlorine contact time in improving monochloramine decay rates was proportional to the free chlorine contact time and Ct. Numerical Ct values were calculated from the free chlorine decay profile (concentration verses time) prior to ammonia addition (data not shown). Free chlorine Ct values of 217 to 743 minute.mg/L were found to result in the most stable monochloramine formation (Figure 10) for Tailem Bend WTP filtered water. Similar tests on Summit WTP filtered water determined that a Ct value of 42 minute.mg/L (data not shown) was required. The need for a higher Ct for Tailem Bend WTP product water could be related to a greater concentration of specific bacteria such as AOA and AOB and SMPs that were not quantified. Monochloramine decay tests completed in Tailem Bend WTP product water showed the microbiological monochloramine decay component was present in May 2012 but was absent in June 2013 (Figure 11). This result means that those factors impacting on microbiological monochloramine
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The presence of free ammonia encourages the growth of nitrifying bacteria and, perhaps, other bacteria that result in microbiological monochloramine decay. Pilot-scale trials are currently underway to validate this hypothesis. The presence of AOB and AOA was confirmed in the filter media from Morgan, Summit and Tailem Bend WTPs. Quantification of AOA and AOB would be required to determine their significance, which was not possible at the time the samples were taken. Filter media from Swan Reach WTP was not examined.
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Tailem Bend WTP Product water May 2012 May 2012 (0.2 m membrane filtered) June 2013 June 2013 (0.2 m membrane filtered) Morgan product August 2013 August 2013 (0.2 m membrane filtered)
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Figure 11. Comparison of monochloramine decay in Morgan and Tailem Bend WTP product waters as received and after 0.2 Âľm membrane filtration.
CONCLUSION This investigation found that monochloramine decay behaviour in the network was not only due to water quality change (increase in DOC concentration), but also due to microbiological processes occurring in the media filters at three WTPs. Differentiation of the cause of rapid monochloramine decay can be readily determined by comparing monochloramine decay in 0.2 Âľm membrane filtered and unfiltered samples. Rapid monochloramine decay was determined to be microbiological in nature and possibly related to backwashing of filters with chloraminated water. Further research is required to identify the factors that result in accelerated decay such as the use of chloraminated backwash water, filter backwash frequency and bacterial concentration and/or type. Mitigation of rapid monochloramine decay in WTP product water is important to minimise the risk of nitrification in the distribution system with chlorination prior to ammonia identified as a successful treatment strategy.
APRIL 2014 WATER
WATER QUALITY & MONITORING
115
Technical Papers