Water treatment plant features microfiltration, solar power and gas chlorination
Early this year, the city of San Benito, Texas, completed construction of a new 6-million-gal-per-day (mgd) microfiltration plant to replace a plant that had been in operation since 1928. 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–about 20 miles from Brownsville and 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 both the old and new San Benito water treatment plants.
The resaca is fed by the Rio Grande, the fourth longest river system in the United States. The river serves as a natural boundary along the border between Texas and four Mexican states. Among the treatment challenges for San Benito is the fact that many maquiladoras–Mexican factories along the U.S. border that import parts for assembly and manufacturing and re-export the assembled product–operate along the river. The water effluent standards to which these factories must adhere are not as strict as U.S. Environmental Protection Agency (EPA) standards.
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 needed 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 that would be ready for operation by early 2009. In the long term, the project would help San Benito ensure that there would be sufficient water treatment and storage capacity to meet the demands of and comply with Texas state regulations through 2025.
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, so the operation of the facility requires little ongoing, hands-on operation. The disinfection system features the Rio Grande Valley’s first microfiltration disinfection system. The old San Benito plant and most other water treatment facilities in the region use conventional sand filters. The microfiltration system, which is fully automated and adjusts to the flow entering the treatment plant, 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, is performed on each filter.
A unique feature of the plant is a supplemental solar power system. Made possible by an EPA 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 clean, dependable electricity while providing 10% to 20% 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-gal 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 time so the solids will settle more thoroughly, improving turbidity levels. After treatment, the water is stored in a new one million-gal 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 (mg/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.
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 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 three months of operation, the new San Benito Water Treatment Plant is operating very well. 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.”