In the process of overcoming these challenges, the public works department became one of the first in the nation to use submerged membranes for filtration.
The $52 million project to renovate the older plant and add a submerged membrane system to the newer plant was the city's most expensive, necessitating a bond issue to be paid off through higher rates. But the investment is paying off in lower manganese, trihalomethanes, and haloacetic acids levels, not to mention regulatory compliance.
Meanwhile, the older plant has been modified to send treatment residuals to the sanitary sewer. As a bonus, the membranes doubled the newer plant's capacity to 50 mgd without having to build into adjacent wetlands the city had bought in anticipation of a physical expansion.
The department looked at three membrane-filtration systems, two small-footprint flocculation/sedimentation systems, ozone, and granular-activated carbon filtration before choosing the ZeeWeed system. Made by GE-Zenon Environmental Inc., the system was ideal for the plant expansion project because it eliminates the need for sedimentation. To reduce the likelihood of finger-pointing on issues related to the membranes' 10-year warranty, the company also supplied all major upstream pumps and mixers as well as the computer control system, which uses GE-Fanuc human-machine interface software.
“Pilot testing is essential because different membrane systems respond to different waters differently,” says Donald Bach, PE, senior civil engineer for the department. “Visit other plants to see how different installed systems operate and how their operators view their systems.”
Today, the wetlands remain untouched, the Pearl River no longer receives untreated residuals, operators spend much less time physically monitoring operations, residents drink better water, and the membrane system is less expensive to operate than the conventional system. That's a win-win situation for everyone.
— Stephanie Johnston