Charlotte D. Smith, Charlotte Smith & Associates, Inc.
Background: Nitrification in Alameda Reservoir
The Alameda County Water District (ACWD) operates and maintains the Alameda Reservoir,
a 15 MG 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. Year 2002 and 2003 data
illustrate 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.
Figure 1: Total Chlorine Residual & Nitrite Values in 2002 & 2003
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 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 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 degrees Celcius
- 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 MG would require multiple RMS units. Prior to the study at ACWD, the largest
application of an RMS was a 10 MG tank operated by the Olivenhain Water District
located in northern San Diego County, California.
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 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 sampling ports in differing locations
in the reservoir. The RMS unit was installed on September 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 until the end of the study
on December 21 st.
Temperature Assessment
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 August 1 through December
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 restarted.
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.
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 October 16th to November 11th. 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 11th). Additionally,
measurements of total chlorine, free ammonia-N and nitrite-N levels throughout the
reservoir showed no significant variation in the levels from location to location
leading to the conclusion that the disinfectant was evenly distributed (Figure 3).
Figure 2: Alameda Reservoir Breakpoint and Booster Chloramination Results for Cl2
and NO2
Figure 3: Total Chlorine Residual by Port and Depth
Conclusion
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.
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.
For more information, email
info@severntrentservices.com