Early this year, the City of San Benito, Texas, completed construction
of a new 6-mgd microfiltration plant. The new facility is considered one
of the most modern and innovative water treatment plants in the region
and has been recognized for its use of microfiltration and solar power.
Its chlorination system, while more traditional than these technologies,
was selected for its cost efficiency, reliability and flexibility.
San Benito, with a population of around 26,000, is located at the southern
tip of the state – only eight miles from the Mexican border. The
City is also known as the Resaca City, due to the resaca, or irrigation
canal, that runs through it. The 400-ft-wide resaca provides irrigation
for local agriculture and is the water source for the new San Benito water
plant.
The resaca is fed by the Rio Grande River, which serves as a natural boundary
along the border between Texas and Mexico. Among the treatment challenges
for San Benito is the fact that many maquiladoras – Mexican factories
along the U.S. border – operate along the river. The water effluent
standards to which these factories must adhere are not as strict as USEPA
standards.
Population growth spurs plant expansion
In 2000, the area’s steadily growing population began to take its
toll on both the City’s water and wastewater treatment plants. The
water treatment plant exceeded its maximum permitted treatment capacity
of 5.97 mgd during the summer. The wastewater treatment plant also was
in need of upgrades.
To address the water treatment plant concerns, the construction of a 10-mgd
plant was planned, which would effectively double the City’s treatment
capacity. The project was to be spread out over 23 years, with a 6-mgd
plant built in the first phase, ready for operation by early 2009. The
long-term construction project would help San Benito ensure that there
would be sufficient water treatment and storage capacity to meet demand
and comply with Texas state regulations through 2025.
Innovative technologies alongside the tried-and-true
Ground was broken for the new $15 million plant in May 2007, and construction
was completed in early 2009, with operation beginning in February. The
new plant is fully automated with a SCADA system, requiring little in
the way of ongoing, hands-on operation. The disinfection system features
the Rio Grande Valley’s first microfiltration disinfection system.
The system is fully automated and adjusts to the flow entering the treatment
plant. It has three filters, each containing seven trains with 152 membrane
modules to each filter. Every 24 hours, a pressure decay test, required
by the state of Texas, is performed on each filter.
A unique feature of the plant is a supplemental solar power system. Made
possible by a USEPA grant, the 45 kw solar array is the largest of its
kind on the Texas-Mexico border. The system utilizes 224 ground-mounted
solar panels that tilt to follow the sun during the course of the day,
producing more energy than fixed-tilt systems. The system will produce
75,000 kwh per year of dependable electricity while providing 10 to 20
percent of the power used to treat water at the plant. The Texas Renewable
Energy Industries Association named the San Benito project as Texas Renewable
Energy Project of the Year for 2008.
Another improvement to the new plant was the construction of a 15-million-gallon
reservoir on site. The old San Benito plant pumped water from the resaca
directly into the treatment plant. With the new plant, water is pumped
from the resaca into the reservoir before it is pumped to the treatment
plant. This increases detention, improving turbidity levels. After treatment,
the water is stored in a new one million-gallon storage tank.
For chlorination, the plant utilizes Capital
Controls Series® WX4100 gas feeders for pre- and post-treatment.
Both feed systems are equipped with microprocessor-based automatic controls
to meet the plant’s needs.
The gas
feed systems are wall cabinet-mounted and vacuum-operated. The Series
WX4100 comes equipped with an automatic valve that opens and closes in
proportion to a signal received from the controller. The controller receives
electrical input signals from a flow meter, causing the controller to
automatically reposition the control valve to maintain the required chlorine
residual – as low as 0.5 and as high as 7.0 ppm (µg/L) total
chlorine residual. By state law, the chlorinated water is combined with
liquid ammonia to prevent reaction with organic matter in the water that
would form trihalomethanes or other disinfection byproducts.
Capital Controls Series NXT3000 vacuum regulators operate at sonic conditions,
eliminating the need for regulating differential pressure across the
rate control valve. The vacuum regulator is mounted on a ton container,
and a liquid trap and heater are provided to prevent liquefied gas from
reaching the regulator.
Confidence in system that will “serve needs for years to
come”
Severn Trent Services, the manufacturer of both the WX4100 and the NXT3000,
has been involved in the gas feed chlorination market for nearly 50 years.
According to Romulo Garza, water treatment plant supervisor for the City
of San Benito Water Department , that experience gave the City confidence
in the gas feed systems. “When we adjust our water flow, we know
these units will respond to whatever amount of chlorine we need.
“During the first six months of operation, the new San Benito Water
Treatment Plant is operating very well,” Garza continued. “The
combination of new technologies – such as the microfiltration system
and the supplemental solar power system – with many traditional,
reliable technologies – such as the gas feed systems – provides
the residents of San Benito with a water treatment system that will serve
their needs for many years to come.”
For more information, e-mail info@severntrentservices.com.