A Bug s Life: Simulating, Planning and Implementing a Temporary Conversion from Chloramines to Free Chlorine for Biofilm Control

Transcription

1 A Bug s Life: Simulating, Planning and Implementing a Temporary Conversion from Chloramines to Free Chlorine for Biofilm Control Simon Horsley, Stantec Consulting, Cochrane Drive West Tower, Markham, ON Canada L3R 0B8, Simon.Horsley@stantec.com Laura Meteer, The Regional Municipality of York, Yonge Street, Newmarket, ON, Canada L3Y 6Z1, laura.meteer@york.ca 1.0 Study Overview The Town of Newmarket (Town) is a large chloraminated water system located in Ontario, Canada. From , the system experienced significant and escalating chloramine decay across multiple pressure zones. After extensive analysis, the cause of the accelerated decay was identified as proliferation of non-nitrifier biofilm. In response, the Town elected to trial a phased free chlorine burn (temporarily system-wide conversion to free chlorine) between October 2012 and January 2013 to control the biofilm. Although free chlorine is typically used for nitrification control, results of a 7- month bench scale trial, in which biofilm was first grown in a simulated water distribution system (annular reactor) fed by Town water quality and then challenged under various disinfection and hydraulic regimes, had indicated that free chlorine burn (followed by unidirectional flushing) was highly effective as a general biofilm control approach. Before initiating the free chlorine burn, the Town developed a simulation model (based on Infowater hydraulic platform) that visually shows conversion of the distribution system, pipe by pipe, from chloramines to free chlorine over time, based on system-specific hydraulics and water quality. The model calculated the breakpoint conversion of each model node over time, providing a dynamic stop animation approach that showed where and when breakpoint reactions would occur. The model was then used to optimize the conversion to generally limit the occurrence of taste-and odour producing breakpoint species. As free chlorine burn is a novel approach for general biofilm control, the Town field-tested the approach within a discrete pilot zone (approximately 10% of the Town system), which was subsequently expanded to two full pressure districts based on the highly positive pilot results. Biological monitoring data (ATP testing) indicated that the free burn resulted in greater than 90 percent reduction in biological activity over 12 week period. This extended abstract provides: a summary of the biofilm annular reactor bench-scale testing; an overview of the free chlorine burn computer model development and output; a review of operational approaches to optimize a free chlorine burn; a review of impact of the biofilm burn on biological activity throughout the system; how the system responded when converted back to chloramines.

2 2.0 What is a Free Chlorine Burn A free chlorine burn is the term for a planned, temporary conversion of a drinking water system from chloramines to free chlorine for a period of several weeks in order to provide a more aggressive biocidal environment for biofilm control. During implementation of a free chlorine burn the water system is converted from chloramines to free chlorine in one of two ways: Displacement of chloraminated water by free chlorinated water; and Breakpoint chlorination. Breakpoint chlorination may be defined as the removal of ammonia from water through the addition of free chlorine. The occurrence of breakpoint chlorination reactions in the distribution is problematic because: The intermediate species that occur during transition from chloramines to free chlorine (dichloramines and trichloramines) are not aesthetically acceptable, being associated with a strong chlorinous taste and odour; and At the moment of breakpoint, disinfectant residual is reduced to almost zero The above conditions are a minor and transient concern when they occur in watermains, but can be of significant concern when they occur in storage facilities immediately prior to entering drain mode. 3.0 Biofilm Annular Reactor Study A seven month bench-scale simulated distribution system study was conducted to examine bacterial growth on small diameter watermain internal pipe wall surfaces under various alternative residual disinfection regimes. The study involved the use of annular reactor (AR) units to cultivate biofilm on PVC coupons under low hydraulic shear conditions. The units were continuously supplied from two alternative treated groundwater source. The impact of various different applied chloramine concentrations on the growth and control of attached (biofilm) and suspended (bulk water) bacteria within the ARs were examined for the initial six month period. The applied disinfectant was then changed from chloramine to free chlorine for a period of two weeks, to simulate a free chlorine burn. Finally, the simulated velocity was increased to simulate the impact of aggressive flushing on biofilm dislodgement, to assess the efficacy of this practice for biofilm removal following a free chlorine burn. Results from the study indicated the following:

3 Biofilm control was not achieved using chloramines alone within the Ontario regulatory limit of 3.0 mg/l total chlorine under low flow (v = 0.1 m/s) conditions. A simulated free chlorine burn at an applied free chlorine dose of 2.2 mg/l was shown to effectively destabilize biofilm, resulting in short term dislodgement of PVC coupon surface biomass to the bulk water lasting approximately one week, followed by a reduction in suspended bacteria to low levels. Simulated pipe flushing following a two week free chlorine burn at an applied velocity of 1.0 m/s was found to be highly effective for the physical removal of biofilm from PVC surfaces, resulting in visible dislodgement of surface biomass and precipitates (iron and silicate) into the bulk water from PVC coupon surfaces. The study results indicated that free chlorine burn followed by unidirectional flushing was more effective than chloramines to address biofilm growth in the Town of Newmarket. 4.0 Computer Modeling the Free Chlorine Burn To minimize the potential for breakpoint reactions and optimize the conversion process, a water quality model was developed to model the conversion process over time. The model used the Town of Newmarket s pre-existing full pipe hydraulic model (developed in Infowater) as a platform. Spreadsheet calculations for breakpoint stoichiometry were developed for the Town s water quality, and used to develop colour coding for different chlorine species that occur during breakpoint (monochloramine, di/trichloramines, free chlorine). Colour coding was then applied to the Town s pre-existing full-pipe extended period simulation hydraulic model, creating an extended period simulation model that would predict the point of all model nodes on the breakpoint curve over time during a free chlorine burn conversion. An hourly screenshot from one phase of the free chlorine burn conversion animation sequence is shown in Figure 1 as an example of the model output, and shows the progress of the free chlorine burn (moving from west to east) 18 hours after initiation, with node colouration based on the following key: Dark green: complete conversion (full free chlorine residual) Light green: partial conversion (partial free chlorine residual) Red: unwanted breakpoint species (di/trichloramines, breakpoint) Grey: unconverted (chloramines)

4 Figure 1 Snapshot from Computer Simulation of Free Chlorine Conversion Animation sequences were developed for each Phase of the free chlorine burn, showing the movement and conversion of chlorine species throughout the system in one hour increments, with the moving front of breakpoint reactions clearly visible, until complete conversion was achieved. 5.0 Operational Approaches to Optimize a Free Chlorine Burn Optimization strategies for the free chlorine burn conversion were evaluated using the model, and ultimately resulted in significant reductions in breakpoint reactions relative to the baseline model run. The most important element of optimizing the conversion was management of water quality in the Town s storage facilities. Options that were reviewed and ultimately employed for management of storage facility water quality included: Managing storage facility volumes to ensure they are at minimum operating level (i.e. about to enter fill mode) when free chlorine residual arrives. This was the most common strategy, and required coordination with operations staff to regulate reservoir levels in the hours prior to the start of the free chlorine burn conversion;

5 Overflowing storage facilities, in order to hydraulically isolate them until the desired free chlorine residual target was achieved. This option required consideration of a safe and regulatory compliant discharge of the overflow volume; Drain down and isolate the storage facility prior to the conversion, and then open up the isolation valve to allow re-fill only once a free chlorine residual is established in the watermains at the base of the facility. This option required review of fire flow storage needs, to confirm that the storage facility volume could be removed from active service; Use booster chlorination at a storage facility to speed up the conversion process. Standalone stoichiometric spreadsheet calculations were developed for individual reservoirs, to calculate chlorine metering pump dosage rates and projected effectiveness. This option required confirmation of metering pump capacities and available volume of sodium hypochlorite solution; Other operational optimization criteria that were reviewed in the model and applied at full scale were: Treatment optimization prior to the free chlorine burn (increase chlorine-to-ammonia ratio, reduce total chlorine concentration); Treatment optimization during the free chlorine burn (increase applied chlorine concentration) Optimization of hydrant flushing locations, rates and duration. 6.0 Effectiveness of the Free Chlorine Burn for Biofilm Control Sampling pre-conversion determined the presence of a mature biofilm containing anaerobic species. In order to monitor the effectiveness of the free chlorine burn it was necessary to design a thorough monitoring plan. Disinfection conversion monitoring, on-going residual monitoring as well as ATP for a measure of biological activity was implemented. Conversion monitoring clearly showed the collapse of the chloramine residual and resulting free chlorine formation. Pre-conversion ATP levels were quite high, but showed a rapid decrease with the application of free chlorine (Graph 1). Individual locations showed positive residual and ATP response to unidirectional flushing. Per Graph 1, it can be observed that with the application of the free chlorine there was a rapid biological response with steady increase in total and free residuals with time. Upon reversion, the residuals did maintain at a slightly lower levels and ATP sampling indicated biological activity was minimal.

6 Chlorine Residual (mg/l) Pilot Area ATP, Residuals catp (pg/ml) /07/ /09/ /11/ /12/ /02/ /04/ /05/2013 Axis Title Total Cl (mg/l) Free Cl (mg/l) catp (pg/ml) Graph 1 - Pilot Area ATP and Residuals with time. Records were kept regarding, taste, odour, and visual quality by staff working in the field. An extensive customer communication program was also followed and complaints mapped. All of the preceding data enabled the use of targeted uni-directional flushing and swabbing to rid areas of concern of biofilm. 7.0 System Response Following Reversion Back to Chloramines. Upon reversion to chloramines, the following actions were taken to improve operational management of the system: a level of service agreement including best management practices, protocols, triggers and responses was developed; continuous monitoring of chlorine residuals and ATP as well as spot monitoring of anaerobes; hydraulic and water quality optimization study. Initial monitoring indicated that the free chlorine burn control strategy had achieved the desired reduction in biofilm and residuals were stable. As summer progressed and temperatures

7 increased, chlorine levels decreased as ATP levels increased in the absence of anaerobic bacteria. In combination, this indicated the formation of immature biofilm in the system. Flushing and valving per the level of service agreement have been pursued to control the water quality issue while the hydraulic and water quality optimization study is completed. Acknowledgements: The authors would like to thank the following individuals for their support and involvement in this initiative: York Region Operations Staff: Jeremy Hunt, Beth Weir, Hub 3 and 4 Operators Newmarket Operations Staff: Bill Wilson, Alison Day and Operators Stantec Consulting Ltd. Staff: Glenn Wallis Susan Andrews, Drinking Water Research Group, University of Toronto Glen Howard, Cromer Industries Garry Palmateer, Garry Palmateer Consulting Inc. Rick Knight, MMM Group