New Missouri Membrane System Helps Water District Meet Goal

Public Water Supply District #7 of Cass County is the first facility in the nation to combine the Kruger Actiflo clarification system and Koch Membrane Systems? membrane ultrafiltration technology at one facility.

The clarification system provides an extremely low turbidity water to the membranes, which improves run times and should extend the life-span of the membranes, according to Leonard Whiting, superintendent of the district.

?Membranes have only been used in water for the last decade, and Actiflo is a recent addition to U.S. technology in the same timeframe,? said Gary Lee, president of Archer Engineering, the firm which helped design the system. ?The Cass #7 facility provided the right environment to develop the marriage of these technologies.?

The district began construction of its new one mgd facility in mid-September 1998. The project took 10 months. The facility went online in mid-July 1999.

A large portion of the $2.129 million project was completed by members of the district?s staff. The project included purchase and installation of the packaged Actiflo and membrane systems, modification of the existing presedimentation and clarifier basins, plus purchase and installation of a 500 kw generator and an adjacent 300,000 gallon in-ground storage tank.

As required by the Missouri Department of Natural Resources (DNR), the presedimentation and clarifier basins were modified to serve as chlorine contact basins. Water flows from the Actiflow clarifier into the basins and then to the membrane filtration unit.

?Missouri DNR requires that membranes withstand up to 200 ppm chlorine,? Whiting said. ?At the time we bid this project, Koch was the only unit on the market that could withstand this criteria.?

The DNR also required that all equipment be maintained in the basins so they can return to their original function, if needed. The existing sand filters also were preserved to serve as a backup filtration system.

Raw water for the district is drawn from the South Grand River and pumped to a 23 acre storage reservoir. Water drawn from the reservoir is high quality and has an average turbidity of 6-7 NTU, Whiting said.

The first phase of treatment is the Actiflo unit. A high-rate clarifier, Actiflo uses a recycled micro-sand to develop a weighted ?floc.? After clarification, the water typically has a turbidity value of about 0.6 NTU, Whiting said.

?The flocculation technology, which causes particles in the water to clump together and settle, results in better algae control and operates much better in cold climates,? said Lee. ?In addition, the rapid clarification it provides allows for reduced work on the part of the membrane unit, extending its life.?

Phase two, and the heart of the process, is the membrane filtration unit, which consists of 32 membranes eight inches in diameter and six feet long. Each membrane cartridge contains approximately 10 miles of fiber and is able to process 27 gpm of finished water. The ultrafiltration membranes serve as a positive barrier to large-diameter protozoa, bacteria like E. coli and some viruses.

With low turbidity water flowing into the membranes, the cycle times between chemical cleaning can reach 60 days, with transmembrane pressures of less than 10 psi, Whiting said.

The filtered water flows into the clearwell where it receives a final dose of chlorine before being pumped to consumers.

?We are consistently going to distribution at 0.040 NTU, which is nine times better than what will be required in 2003,? Whiting said. ?We?re very proud of that.

?The water quality provided by this system will prepare Cass #7 to surpass new regulations and demand well into the future as both units are estimated to provide 25 years of service,? he said. ?Membranes do have an estimated life of about 5 years, but the replacement of a membrane is much more cost-effective than replacing an entire system.?