The 5 mgd water treatment plant in Council Bluffs, Iowa, uses low-pressure ultrafiltration (UF) membranes ahead of high-pressure reverse-osmosis (RO) membranes to treat groundwater that has high levels of iron and manganese. HDR, Inc.
A new facility on a growing side of Council Bluffs, Iowa, treats water laced with iron and magnesium using multiple membrane processes.
July 9, 2013—The recent start of operations at a 5 mgd water treatment plant in Council Bluffs, Iowa, marks a milestone in the growing city’s efforts to ensure adequate water supplies for the future. Completed at a cost of $35 million, the new Council Point Water Treatment Plant features the use of low-pressure ultrafiltration (UF) membranes ahead of high-pressure reverse-osmosis (RO) membranes to treat groundwater having high levels of iron and manganese.
Since 1952 Council Bluffs has received its drinking water from a 20 mgd conventional lime softening facility that treats surface water from the Missouri River. During the past decade the city has experienced significant growth through residential, commercial, and industrial development, says Douglas Drummey, the chief executive officer and general manager of the Council Bluffs Water Works (CBWW). Recognizing the need to expand its treatment capacity to keep pace with demand, the CBWW evaluated whether to expand its plant or build a new one. Again growth influenced the decision. With most of the new development occurring on the south side of the city, the CBWW opted to construct a new plant there rather than expand its existing facility, which is located on the north side of town. In this way Council Bluffs obviated the need to construct costly new transmission mains through its city center, Drummey says.
But the decision conferred other benefits as well. For example, constructing the plant on the south side of Council Bluffs increased pressure in the distribution system and enabled the CBWW to take advantage of a nearby aquifer. Adding groundwater to the utility’s mix of drinking water sources meant improved system redundancy, which could prove useful given the rapidly changing water quality characteristics of the Missouri River. Because it is more stable with respect to quality, groundwater better lends itself to automated treatment, another goal of the CBWW in developing its new facility. “It became apparent to us that building a new plant and developing a new source of water supply on the south side would have the greatest benefits overall for our water utility,” Drummey says.
Click this image to see the complete integrated membrane
system used in the new water treatment plant. HDR, Inc.
However, treating the groundwater would not come without challenges. Located between 1,000 and 2,000 ft from the new treatment plant, the five wells built to tap the aquifer provide water containing high levels of iron and manganese. Other characteristics requiring careful attention were total organic carbon, naturally occurring ammonia, and hardness. “It is not the easiest groundwater to treat,” says Teresa Konda, P.E., a process engineer for HDR Engineering, Inc., which has its headquarters in Omaha, Nebraska. Along with assisting the CBWW in siting the well field and the treatment facility, HDR designed the wells and well field facilities, including well pumps and motors, pipelines, valve vaults, access roads, and electrical systems. The firm also oversaw well field construction, designed the treatment plant, and handled construction administration services for the treatment plant.
Before selecting a treatment method for the new facility, the CBWW and HDR compared conventional lime softening with a variety of configurations involving different pretreatment and membrane processes. The evaluation concluded that membrane treatment would offer a lower 20-year present-worth value than would conventional lime softening. Although membrane treatment would mean higher costs for equipment and energy usage, lime softening would entail high costs as well because of the need for more land and larger structures, greater use of chemicals, and the disposal of residuals. After conducting a pilot test, the CBWW opted for the use of submerged low-pressure UF membranes to remove iron and manganese from the water before the water passes through high-pressure RO membranes, even though such an approach is “relatively new” for drinking water plants in the Midwest, Konda says. This combination of membranes, provided by GE Water and Process Technologies, of Oakville, Ontario, was then the subject of a 90-day pilot test mandated by the Iowa Department of Natural Resources.
The treatment train begins with two automatic strainers and is followed by two induced-draft aerators that strip carbon dioxide and oxidize the soluble iron present in the groundwater, a process that is completed in two pretreatment basins located ahead of the UF membranes. After entering the pretreatment basins, the water receives sodium permanganate to facilitate the oxidation of soluble manganese. Once in the basins, the oxidized iron and some of the soluble manganese settle to the bottom. The water then flows to the UF membranes for further treatment. At its current 5 mgd capacity, the Council Point plant includes three UF units housed in concrete tanks, one of which remains on standby. Two additional UF tanks are available to house future UF membranes if the plant is expanded.
Two induced-draft aerators strip carbon dioxide and oxidize the
soluble iron present in the groundwater before it moves through
the UF membranes. HDR, Inc.
Filtrate from the UF units then enters a break tank before proceeding to the RO system, which comprises three units, each with a treatment capacity of 1.67 mgd. However, it is not necessary for all of the UF filtrate to undergo RO treatment for the finished water to meet the plant’s water hardness targets. Therefore, approximately 60 to 80 percent of the UF filtrate passes through a cartridge filter and is then pumped through the RO system, whereas the remaining flow bypasses the RO system. The two streams are subsequently blended and disinfected by means of sodium hypochlorite before entering an on-site reservoir. Before entering the distribution system the treated water receives additional chemicals for further disinfection and to ensure that the water closely matches the finished water from the city’s other water plant.
Waste streams from the UF units are sent to a plate settler, and water is then decanted from the settler to the UF treatment system. Solids from the plate settler are discharged to the city’s sewer system, along with waste streams from the automatic strainers, the pretreatment basins, and the plant’s floor drains. Solutions that are used to clean the membrane processes are neutralized and discharged to the sewer system as well. Residuals from the RO treatment units, however, are discharged directly to the Missouri River.
Membranes offer a level of automation that is often not available with lime softening, Konda says. In fact, the new facility can be monitored and operated by staff at the CBWW’s existing plant. Designed with future expansion in mind, the Council Point plant was built with sufficient room to accommodate equipment that will enable the facility to treat up to 10 mgd. Indeed, the site is large enough for the plant to be expanded to a capacity of 20 mgd.
The Foley Company, of Kansas City, Missouri, began constructing the Council Point facility in March 2010. Construction was delayed in the summer of 2011, when flooding from the nearby Missouri River affected the site. Although the facility has been operational since mid-May of this year, fully automated operation did not begin until late June. So far the CBWW has been “very pleased” with the quality of the finished water from its new drinking water facility, Drummey says.