Background
The Alameda County Water District (ACWD) operates and maintains the Alameda Reservoir, a 15-million gal finished water storage facility located in Fremont, California. The reservoir experienced loss of disinfectant residual as a result of nitrification even during the winter as shown in Figure 1. Nitrification is a biological process in which ammonia-oxidizing bacteria convert free ammonia to nitrite. Data from 2002 and 2003 illustrates the historic difficulty of maintaining a residual in this reservoir. The graph shows the total chlorine residual declining as the nitrite concentration increases. Fluctuations in the nitrite levels resulted from actions taken by the district to control nitrification, specifically, breakpoint chlorination.
Nitrification commonly occurs in storage facilities like Alameda Reservoir that have low turnover rates. In order to maintain adequate storage for pressure and fire flow, the district is often limited in its ability to draw down the reservoir and exchange enough water to control nitrification. Historically, ACWD has responded to the loss of disinfectant residual by controlling nitrification with breakpoint chlorination. Both calcium hypochlorite and sodium hypochlorite have been used to achieve a free chlorine residual, thereby destroying ammonia-oxidizing bacteria in the reservoir.
The objective of the ACWD – Reservoir Management System Evaluation Project was to evaluate the ability of Severn Trent Services’ RMS™ Water Storage Management System to mix and uniformly distribute sodium hypochlorite and aqueous ammonia throughout the Alameda Reservoir. The district’s ultimate goal was to eliminate nitrification and maintain a 2.0 mg/L combined chlorine residual within the entire reservoir. The following criteria were used to judge whether the objectives were met:
- Temperature—plus or minus 0.2?C; and
- Total Chlorine Residual—plus or minus 0.2 mg/L
Data from five horizontal locations and two vertical locations (depths) were analyzed and subjected to the evaluation criteria. One of the objectives of the ACWD study was to determine whether facilities above 10 million gal would require multiple RMS units. Prior to the study at ACWD, the largest application of an RMS was a 10-million gal tank operated by the Olivenhain Water District located in northern San Diego County, Calif.
The patented RMS (US patent 6,811,710) is designed to add and mix chlorine and ammonia in drinking water distribution system storage facilities. On-line monitoring of the resulting total chlorine (chloramine) residual is an integral component of the system. The RMS includes a submersible pump (with a chemical feed eductor), PLC controller system with operator (SCADA) interface, recorder, and associated control valve. Chlorine can be applied with the on-site generation units, gas chlorine or sodium hypochlorite. Ammonia can be applied as gas or aqueous ammonia. The ACWD evaluation utilized on-site generation of 0.8% sodium hypochlorite and bulk storage of 19% aqueous ammonia. The ammonia tank included refrigeration.
Schedule
Beginning June 2, 2004 (prior to RMS installation), samples were collected and analyzed from water drawn from two depths at Ports 2, 4 and 5 to characterize water quality within the reservoir. Ports 1 to 4 were placed in the four corners of the reservoir, and Port 5 was placed in front of the inlet/outlet pipe. The RMS unit was installed on Sept. 27, 2004. One significant convenience of the unit is that dewatering of the storage facility is not required for installation. Samples were collected a minimum of once per week from all ports beginning shortly after the unit’s installation and continuing until the end of the study on Dec. 21.
Two of the leading obstacles that water system operators encounter in their effort to maintain appropriate, consistent levels of chlorine are thermal stratification, which can lead to stagnation of upper layers of the water in the storage facility, and insufficient mixing of fresh water into the tank or reservoir. To analyze the effectiveness of the RMS in overcoming these challenges, temperature probes were placed at the bottom, middle and top of the water level, and on a float (to ensure that water just below the surface was always measured). At the end of the study, the temperature probe data was downloaded for the period Aug. 1 through Dec. 14, 2004. This period encompasses a time before the RMS was installed, after it was installed, when it was temporarily shut down and after it was re-started.
The temperature data showed that the Alameda Reservoir was not subject to thermal stratification. This result is related to the fact that inlet momentum is adequate, and it is an underground storage facility (which helps maintain the temperature). The lack of evidence for “insufficient mixing” as the cause of nitrification supported the hypothesis that the loss of residual in this reservoir needed to be solved with adjustments to the chemistry not improvements in mixing.
Conclusion
The ACWD – Reservoir Management System Evaluation Project was undertaken to evaluate the effectiveness of booster chloramination as an alternative to breakpoint chlorination to control nitrification in a large reservoir. Samples were drawn on a weekly basis from five locations distributed throughout the Alameda Reservoir. Severn Trent Services’ RMS system provided sustainable total chlorine residual in the reservoir, distributing disinfectant evenly throughout the reservoir. The target level of 2.0 mg/L was effective in suppressing ammonia-oxidizing bacteria and preventing disinfectant loss from nitrification.
Acknowledgment
This work was supported by Severn Trent Services and Alameda County Water District. The efforts of Cris Pena, project manager, and ACWD engineers, operators and laboratory staff are gratefully acknowledged.
