La Vergne, Tenn.,
is one of the fastest growing communities in the country. Located about 20
miles southeast of Nashville and the intersection of three interstate highways,
the city has attracted several international retail chains in recent years
and a number of major industrial companies. Since 2000, the population has
increased by more than 40 percent, standing now at about 27,000 residents.
La Vergne’s drinking water is provided by the La Vergne Water Plant,
a 9.8-mgd surface water treatment plant that utilizes multimedia filtration, on-site
sodium hypochlorite generation for disinfection, sodium permanganate
addition for manganese removal and polyphosphate injection for corrosion
control. Anticipating the city’s growth shortly after the 2000 census,
the La Vergne Water Department doubled the treatment plant’s capacity
in 2004 and 2005.
While the availability of drinking water was not a problem after the plant’s
expansion, the Water Department was fielding an average of 20 complaints
a month from customers regarding their water’s taste and odor, drawing
citations from the Tennessee Department of Environment and Conservation.
In 2007, the city sought to improve the operation of the treatment plant
to ensure high quality water for residents and businesses by entering into
a public-private
partnership with Severn Trent Services for operation of the plant.
Significant maintenance needs were identified at the start of the relationship,
and Severn Trent and the city worked diligently to return the equipment to
optimum condition. A filter profile was performed that showed the filters
were frequently running low on the granular activated carbon media. In some
cases, as much as two feet of media was missing. Excessive backwashing of
the media was identified as the primary cause of media loss, so Severn Trent
upgraded the filters by adjusting their valve timing and restored the proper
media level, which reduced the amount of backwashing and associated water
usage.
Severn Trent operations and maintenance staff also discovered that higher-than-expected
levels of organic material were remaining in the water as it emerged from
the plant’s clarifiers. So, the staff drained, cleaned and inspected
two of the plant’s four clarifiers, which significantly increased the
efficiency of the initial particle removal process. In addition, the improved
effectiveness of the clarifiers enabled the use of less chemical disinfectant.
The plant had been using aluminum sulfate to supplement the work of the clarifiers,
but switched to less costly poly-aluminum chloride shortly after the two
clarifiers were cleaned. Work continues on the remaining two clarifiers.
Process modifications also were implemented to improve water quality and
restore compliance. Plant personnel determined that increasing chlorination
midway through the treatment process would effectively address taste and
odor problems and decrease the need for activated carbon. Then, in early
2009, the staff substituted sodium permanganate for the chlorine, which enabled
the plant to comply with disinfection byproduct regulations. The effectiveness
of this approach has virtually eliminated taste and odor complaints.
The plant staff determined that other chemicals had been overused. In one
instance, Severn Trent was able to cut the alum dosage in half without negatively
impacting water quality. The city benefited by keeping its chemical costs
stable, even during a year in which costs increased. Since it pays directly
for all chemicals, the city essentially avoided the cost of the price increases.
In addition, turbidity has been reduced by 50 percent because of reduced
chemical usage and the filter upgrades.
Overall the professionalism of the plant staff and the quality of the plant’s
process and safety procedures have improved since the beginning of the public-private
partnership. Two of the plant’s operators have increased their certification
to Grade 2 and Grade 4. And the addition of needed health and safety equipment
and supplies and the implementation of a new safety training program has
brought the facility up to OSHA standards.
Current maintenance projects include the rehabilitation of a reverse osmosis
system used to recover backwash water and a coupon study to confirm the effectiveness
of the plant’s corrosion control program.
While maintenance expenditures during the first two years of the plant’s
operations have been significant, the city fully expects future maintenance
requirements to normalize through implementation of Severn Trent Services’ predictive
and preventive maintenance programs. The city has rewarded Severn Trent’s
performance by renewing its contract eight months in advance of the initial
contract’s expiration and has expanded the scope of work to include
cross connection inspections.
For more information, e-mail info@severntrentservices.com.