Managing Reservoir Water Quality, Protecting A Company’s
Good Name
As in any industry, a municipal water company’s reputation is a precious
commodity. Built carefully over time through hard work, the mastery of appropriate
technologies, and a commitment to service, a company’s good name can,
nevertheless, be tarnished overnight due to an equipment malfunction, human
error, or happenstance.
Drinking water that doesn’t look, taste, or smell just right has left many water companies scrambling to solve the problem and preserve its standing in the community. Water company executives who have encountered such water quality issues agree that the damage to their companies’ reputations was worse than the fines levied by local government authorities.
Now, Severn Trent Services has developed a solution to one of the primary
causes of poor or uneven water quality – and tests of the new system
in the United States and the United Kingdom show impressive improvements
in water quality and consistency while providing long-term cost savings.
The Problem: Maintaining Uniform Chlorine Levels
The culprit is chlorine or, more precisely, maintaining uniform levels of
chlorine throughout reservoirs and ground storage tanks. Many water system
operators struggle to maintain appropriate, consistent levels of chlorine,
encountering common problems such as thermal stratification, stagnation,
insufficient blending of different water qualities, and ineffective analysis
techniques.
The most common methods of maintaining uniform levels of a disinfectant
residual such as chlorine or chloramines has been to either reintroduce treatment
chemical into the water as it enters the reservoir or as it exits the reservoir
on its route to the water distribution system. Both methods have drawbacks.
Water systems that reintroduce chlorine or chloramines into the inflow of
water entering the reservoir must often contend with water coming from several
sources and which, consequently, have different levels of disinfection residuals.
Simultaneously and continuously monitoring, analyzing, and treating water
from various sources to reach and maintain the required disinfection residual
level is complex at best. Even the most technologically advanced automatic
systems are stretched to the limits of their capabilities under such circumstances.
And if human intervention is part of the maintenance scheme, it also becomes
a part of the potential problem through human error.
Systems that reintroduce chlorine or chloramines into the water as it leaves
the reservoir in order to meet the required disinfection residual level
for the distribution system must be exceedingly accurate in order to monitor,
analyze, and treat the wide range of flows. Few “post-treatment”
systems can consistently meet that standard.
Other means are sometimes used to solve the problem of maintaining residual
levels, including filling reservoirs to less than half capacity to increase
cycle frequency and prevent thermal stratification. This method is accompanied
by continuous monitoring of chlorine residuals. Such a strategy, of course,
does not take full advantage of the reservoir’s full capacity, resulting
in a less efficient operation and greater potential expense through the
need to use additional reservoirs.
An Innovative Solution
The ClorTec™
Reservoir Management System (RMS) is an economical solution to manage,
maintain and control residual levels across varying municipal, commercial,
and industrial applications. The system uses real-time water quality samples
from inside the reservoir, automatically detecting and feeding the level
of chlorine or chloramines required to maintain a consistent predetermined
residual. The system’s integrated analyzer provides continuous, accurate
measurement of free chlorine and chloramines. Consisting of circulators,
analyzers, and PLC controls, the RMS is compact and operates independent
of reservoir level, input flows, or output flows. A portable RMS Rover is
also available and can easily be dropped into position within a storage
tank or reservoir location to replace the circulator, providing the same
benefits and features of the traditional RMS system.
Unlike conventional mixing systems, Severn Trent’s technology offers
cost-efficiencies through decreased power consumption and faster installation
and service time. Installation of an RMS does not require plumbing changes
and can be completed while the reservoir system is still in operation.
For reservoir systems requiring chloramines as a disinfectant, an economically
priced ammonia storage and addition system to refrigerate and cool the ammonia/water
mixture below boiling temperature is available. A vacuum ammonia system
can be provided, in addition to a controller, to automatically control the
5:1 chlorine to ammonia ratio required for chloramination. If needed, Severn
Trent Services offers chlorine feed technology including ClorTec
on-site hypochlorite generation that generates 0.8 percent NaOCl or
a Capital Controls™ gas feed system to dispense 100 percent CI2 gas under
vacuum, to accompany the RMS unit.
Once installed in the reservoir, the submersible pump and eductor moves
water through a mixer, which, in turn, moves the body water within the reservoir
upward. The circulatory motion of the water thoroughly mixes the contents
of the tank, breaking the temperature stratification. A small stream emanates
from the pump exit and is sent to the chlorine residual analyzer. When the
analyzer senses a drop in chlorine residual, it injects the disinfection
chemical until proper residual levels are reestablished. The circulation
pump runs continuously, providing a fresh sample to the analyzer and continually
blending the various flows in the reservoir. The system operates independently
of the reservoir level, input flows or output flows and only responds to
a predetermined residual set point. For some large reservoirs or multiple
reservoir systems, an optional PLC-based control system is provided where
multiple circulators run continuously with each tank’s sample stream
and injection lines are sequenced at timed intervals.
Maintaining Water Quality From Different Sources in England
According to Mike Ashley, director of United Kingdom/European operations
for Severn Trent Services, an RMS quickly solved the problems of water quality
fluctuation at a facility in the east midlands of England. “At Severn
Trent Water’s Diamond Avenue facility in Mansfield, inflow from different
water sources required significant differences in rechlorination,”
Ashley said. “But after the installation of an RMS, the plant operators
achieved a very consistent level of water quality within three days –
much faster than we’d anticipated,” Ashley said.
“The Diamond Avenue operator previously had no automatic system of
chlorine dosing,” he continued. “Chemical was added manually,
and while Diamond Avenue was well-operated and in full compliance with government
regulations, the manual dosing is less precise and prone to occasional errors.
After testing RMS at one of its two reservoirs, the company now is installing
an additional system at the second site.”
Maximizing Water System Resources in California
The Moulton Niguel Water District (MNWD) in south Orange County, Ca. conducted
a six-month pilot program using the ClorTec RMS in 2001 at its four million-gallon
Nellie Gail Reservoir. According to Robin Hamilton, water district supervisor,
the system has ensured better water quality and has enabled MNWD to maximize
the use of its reservoirs.
“It has been far easier to maintain the quality of the water since
we installed the ClorTec RMS,” Hamilton said. “We’ve been
able to maintain the chlorine residual at a consistent rate of 1.5 to 2.0
ppm, compared to under 1.0 ppm prior to installation.
“In the past, we maintained lower reservoir levels in order to combat
thermal stratification. Now, we’re consistently maintaining higher
reservoir levels, which enables us to utilize our system to greater capacity.
This is particularly important for MNWD because of the potential for earthquakes
in Southern California. We receive our water from a treatment plant in Yorba
Linda, and the ability to use our reservoirs to the greatest capacity possible
is especially important in case of any kind of disruption to our water supply.”
As a result of the successful pilot program, MNWD now is using the ClorTec
Reservoir Management System throughout its 21-reservoir, 28-tank system.
For more information on Severn Trent Services' innovative Reservoir Management System, email agiti@severntrentservices.com.