Background: Nitrification in Alameda Reservoir
The Alameda County Water District (ACWD) operates and maintains the Alameda Reservoir, a 15-million gallon finished water storage facility located in Fremont, Calif. The reservoir experienced loss of disinfectant residual as a result of nitrification even during the cold temperature season (Figure 1). Nitrification is a biological process whereby ammonia-oxidizing bacteria convert free ammonia to nitrite. Year 2002 and 2003 data illustrate the historic difficulty of maintaining a residual in this reservoir. Figure 1 shows the total chlorine residual declining as the nitrite concentration increases. Fluctuations in the nitrite levels primarily result from actions taken by ACWD to control nitrification.
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 the levels of ammonia-oxidizing bacteria in the reservoir.
The district expressed interest in exploring in-situ booster chloramination as an alternative to breakpoint chlorination for nitrification control and maintenance of the disinfectant residual in the Alameda 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 maintain a 2.0 mg/L combined chlorine residual within the reservoir to suppress ammonia-oxidizing bacteria. The following criteria were used to judge whether the objectives were met:
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 distribution tanks and monitor the resulting combined chlorine (chloramine) residual. 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 RMS unit was installed on Sept. 27, 2004, and one significant convenience of the unit is that dewatering of the storage facility is not required for installation. Samples were collected at least once per week from the all ports, beginning shortly after the unit’s installation until the end of the study on Dec. 21 (N=17). An additional 17 weeks of samples were analyzed from water drawn from ports 4 and 5 and an additional 12 weeks of samples from Port 2 before the RMS unit was installed to characterize water quality within the reservoir before the unit was installed.
Maintenance of Total Chlorine Residual
Figure 2 illustrates the success of the booster chloramination project. After installation of the booster chloramination system, total chlorine stayed within the target range (2.0 +/- 0.2 mg/L) and nitrite was below the method detection limit. The vertical line on the chart enables a side-by-side comparison of the total chlorine and nitrite levels before and after installation of the booster chloramination system. Of special note on this graph is the degradation of water quality when the booster chloramination facility was off from Oct. 16 through Nov. 11. During this period the total chlorine decreased and the nitrite level increased until the booster system was re-started and water quality improved (after November 11).
Many water system operators struggle to maintain appropriate, consistent levels of chlorine, encountering common problems such as thermal stratification, stagnation and insufficient blending of water containing different quality characteristics. 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 of Aug. 1 through Dec. 14, 2004. This period encompasses the time before the submersible mixer was installed, after it was installed, when it was temporarily shut down and after it was restarted.
The temperature data showed that the Alameda Reservoir was not subject to thermal stratification. This result may be partially related to the fact that it is an underground storage facility. Additionally, measurements of total chlorine, free ammonia-N and nitrite-N levels in various sections of the reservoir showed that disinfectant was evenly distributed throughout the reservoir, with no significant variation in the levels from location to location. The lack of evidence for “insufficient mixing” as the cause of nitrification supported the hypothesis that the loss of residual in this reservoir could be solved with adjustments to the chemistry.
The 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.
Click here to view an animated version of the RMS process