Helping Mother Nature
The cascading creeks flowing from the peaks of the Rocky Mountains provide a good source of drinking water for the residents of East Shoshone County, Idaho. For decades, drinking water for approximately 1,400 households came from that surface water, which was treated only by chlorination.
Unfortunately, spring runoff each year caused turbidity to increase, and residents experienced noticeably dirty water flowing from their faucets.
By the early 1990s, the county’s old, privately owned water treatment system was falling into disrepair. Additionally, the unfiltered water did not meet the health and safety standards of the Surface Water Treatment Rule as required by the Safe Drinking Water Act. To address these problems, the East Shoshone County Water District was incorporated in 1993. The newly formed district decided they also needed a system that would meet the proposed limits for Cryptosporidium.
A solution was developed with the help of the U.S. Department of Agriculture – Rural Development (USDA – RD) agency, which provided funding for the purchase of the infrastructure, the replacement of the raw water intake and leaking mains, and the filtration of water to meet the new standards. The plan called for the installation of two filtration systems, a 2 million gal per day (mgd) system to filter the creek water from the mountain located above the town of Wallace, and a 0.6-mgd system seven miles away at the mountain creek above the town of Mullan. The district needed a filtration method that could operate reliably without 24/7 onsite staffing at both locations.
Membrane filtration a viable solution
The district’s engineering consultant designed a rigorous two-and-one-half-month pilot test of the ROMICON PMPW ultrafiltration (UF) membrane system from Koch Membrane Systems, Inc. (KMS). The pilot test showed that the UF membrane system consistently removed Giardia- and Cryptosporidium-size particles with 3.3- and 3.5-log efficiency, respectively, and consistently produced water with turbidity between 0.026 and 0.028 NTU from raw water with turbidity ranging from 0.35 to 5 NTU.
“We chose KMS membrane technology because it delivers excellent turbidity removal in a compact system and because it requires minimal operator monitoring and maintenance,” said Carl Scheel, district manager of the East Shoshone County Water District.
The hollow fiber geometry of the UF membranes provides a large surface area contained in compact cartridges that produce high flow rates while requiring minimal space. The self-supporting membrane can be backflushed to remove accumulated debris and maximize the efficiency of the crossflow filtration process. This operating feature, combined with chemical stability, ensures reliability and long membrane life.
When the treatment plants were commissioned in 1998, they were the first membrane filtration systems financed by USDA - RD in Idaho. The KMS membranes have consistently and reliably produced high-quality potable water that is low in turbidity, and have provided a mechanical barrier against Cryptosporidium oocysts, Giardia, bacteria, viruses and larger organic molecules.
Low maintenance and long life
“The KMS membranes require almost no maintenance,” Scheel said. “We perform a chemical cleaning once a month, but we could probably go for a year without observing any significant pressure change.”
Each system also automatically performs a periodic chlorinated backwash to prevent raw water algae from decreasing filter flow.
“We have very little interaction with the system, mainly just to collect data once a day,” Scheel said. “The systems run themselves, which is important because we have a total staff of five employees, including myself, at our two facilities.”
The membranes have proven remarkably durable and long-lasting. “Originally, KMS told us that we will get 10 years of life from each cartridge, but we now expect that the cartridges will last much longer than that,” Scheel said.
Gravity does the work
The district hired KMS last year for a “tune up,” and the KMS engineers reconfigured the stages to operate in a single-pass mode. Previously, the Wallace and Mullan plants used a recirculation mode of operation. This required an additional pump running on each stage.
Now, reengineered in the single-pass mode, these pumps are off during water production. The pumps have been programmed to be available for use during high turbidity spikes in the spring and whenever the customer takes the stage off-line for a cleaning.
Another interesting aspect is that the mountains provide not only the raw water for the systems, but also the energy to operate them. Thanks to the elevation difference between the infiltration galleries at the creeks and the intake at the water treatment plants, gravity provides the necessary feed pressure. Because UF is a low-pressure process, pressure-reducing valves restrict the plant intake pressure to 56 psi at both locations.
“The KMS membranes are so efficient that we can happily let Mother Nature do all the work,” Scheed said.