Public Water Supply District #7 in rural Cass County, Mo., has been reliably and cost-effectively producing quality water since it was commissioned on July 29, 1999.
“At 11:56 in the morning, to be precise,” said Leonard Whiting, the superintendent of the plant, who has recounted the plant’s successes during more than a hundred tours that he has hosted for water treatment professionals from all over the country.
Visitors come to learn how effective pretreatment optimizes the efficiency and longevity of membranes filtering surface water.
“Some people doubt whether membrane systems can stand up to challenges of treating highly turbid surface water, such as the water we pump from the South Grand River,” Whiting said. “But we have proven that with proper design and maintenance, membrane technology performs extremely well, at a reasonable cost.”
High quality water
At the core of the 1 mgd plant is an ultrafiltration (UF) membrane system developed by Koch Membrane Systems, Inc. (KMS).
“We chose this membrane technology to achieve our goal of producing consistently high quality potable water that meets current regulatory requirements and anticipates future regulations,” said Whiting.
The Romicon 8 x 72-in. PMPW hollow fiber UF cartridges from KMS ensure the plant exceeds the Safe Drinking Water Act guidelines for turbidity.
Whiting explained, “Current regulations require us to meet the 0.3 NTU threshold for turbidity 95% of the time, but we think we owe it to our customers to exceed that standard more than 99% of the time. We continually meet that objective due to the physical barrier of the membranes.”
With a nominal molecular weight cut-off of 100,000 Daltons, the PMPW membranes also remove 99.99% of Giardia, Cryptosporidium, bacteria and viruses.
In addition, Cass County #7 is meeting regulations pertaining to the formation of harmful trihalomethanes and haloacetic acids, which occur when small, dissolved, naturally occurring organic molecules react with chlorine.
Membranes’ efficiency
There is little question concerning the capability of membrane technology to produce high quality water. The challenge is from the high turbidity of surface water, which consists of algae and other organics that can foul membranes and reduce their efficiency.
Cass County #7 has met that challenge with pretreatment in two stages. River water is initially pumped into a pre-sedimentation basin large enough for 16 months of storage. The basin is aggressively treated for algae—a critical step in pretreatment, according to Whiting.
The second stage of pretreatment is an Actiflo conventional-type water clarification system, supplied by Kruger, Inc.
Actiflo uses ballasted sand in a flocculation and coagulation process. This process significantly lowers the turbidity of the feed water entering the membrane system.
“For the past two-and-a-half years, we have used an enhanced coagulation process, which has been a real plus in lowering TOC,” said Whiting. “This process involves changing water parameters by lowering pH and injecting coagulant. We carry the 6.3 pH water straight through the membrane. Just prior to distribution, we stabilize it with sodium hydroxide to raise the pH, lower alkalinity and precipitate out some of the hardness.”
Cost-effective
“With our pretreatment process, the membranes operate efficiently with minimal fouling,” said Whiting. “And the hollow fiber membranes last longer.”
Membrane replacement costs are a significant contributor to the lifecycle costs of a water treatment system. Whiting has calculated the cost of the membranes during their first six years of operation and reported, “We were able to provide a superlative product to our customers, for a reasonable price.”
Whiting is also proud of the low maintenance requirements. Except for refilling chemical containers, Whiting claims he could leave the system unattended for months at a time.
“The system virtually runs itself, and that is a big testament to the confidence we have in the technology,” he said.
As the first plant in the U.S. to combine the Actiflo pretreatment with the KMS ultrafiltration membranes, Cass County’s Public Water Supply District #7 serves as a model for municipal treatment of surface water.
RSS: WWD Articles
