Outbreaks of waterborne diseases, more stringent disinfection byproduct requirements, increasing economic viability of membrane-based treatment technologies, and aging infrastructure have generated ever-increasing interest in membrane technologies. As communities across the U.S. take a greater interest in ultrafiltration, microfiltration and reverse osmosis—increasing the number of membrane treatment facilities and projects—the size and complexity of new and planned facilities are also on the rise.
UF meets the need
With a current capacity of 70 mgd and ultimate capacity of 78 mgd, the Columbia Heights membrane filtration plant near Minneapolis is the world’s second largest potable UF plant. This facility is second in size only to an award-winning 72-mgd membrane facility in Singapore. Both of these facilities were designed by Black & Veatch.
The Columbia Heights facility exemplifies the Minneapolis Water Works’ (MWW) commitment to providing high-quality drinking water as well as the growing importance of membranes. The decision to replace existing granular media filters with UF to further treat lime-softened, clarified water from the Mississippi River was prompted by multiple goals and considerations. The MWW needed to address future contamination threats, continue to comply with regulatory requirements and replace nearly century-old sand filters. The new membrane filtration plant is producing low-turbidity water that is better than regulatory standards require, and it was designed to protect MWW customers from waterborne disease by providing high-log removal of microbial pathogens such as Cryptosporidium.
“Ultrafiltration was a logical choice for physical pathogen removal and multiple-barrier protection at a reasonable cost,” said MWW Interim Director Shahin Rezania, Ph.D. “Successfully applying a relatively new technology at such a large scale to existing operations presented many challenges, but the collective managerial skills and technical knowledge of everyone involved in this project enabled us to rise to those challenges and ensure the people of Minneapolis and nearby suburbs a continued supply of good-quality drinking water well into the future.”
Rising to the challenges
The MWW selected Black & Veatch to evaluate pretreatment alternatives and provide retrofit design, construction, training and startup services. The project, launched in 2002, entailed detailed evaluation of a full range of water treatment technologies through a progressive review process that included a peer review panel of nationally recognized experts.
Cost-effectively and successfully integrating a large membrane filtration system into an existing operation necessitated special consideration on many fronts. It was challenging to apply a still-emerging technology at such a large scale to effectively treat a variable water source, and process control and waste disposal required special attention. It was essential to optimize capital and operations and maintenance costs without overly complicating the system.
After identifying the best approach through pilot testing that included performance life-cycle costing, the team applied the pilot results to full-scale performance.
Following a performance evaluation bid to select the most appropriate technology, the MWW decided to continue pilot testing. Assisted by the design engineer and membrane equipment supplier, the MWW conducted extended pilot testing to optimize the design and confirm operational settings for a successful plant startup. To help achieve accurate scale up, the pilot plant experiments mimicked full-scale operation.
Snow melt in the spring of 2002 changed the water quality entering the plant and caused a corresponding change in membrane performance that required adjustment to chemical cleaning procedures. With membrane filtration, filtered water turbidity remains low during these events.
However, increases in head loss across the membrane surface lead to more frequent backwashing and cleaning of the membranes, which decreases effective filtration time and output of treated water. The project team developed a strategy to address this issue and modified the design accordingly. Additional chemical feed pumps were incorporated to supply alternative chemicals when fouling occurs.
Use of 3-D and architectural rendering software helped the project team develop equipment, structures and piping layout. The software was used to develop a model of the plant that enabled the MWW to clearly visualize how the plant would look. Various layout alternatives were assessed using the 3-D design software to optimize both equipment orientation and available space.
The new treatment facility is housed in a single structure consisting of administration and process areas. The administration area includes demonstration, conference and control rooms; offices; a laboratory; and a lobby furnished with educational displays for the visiting public. The membrane gallery features 40 UF units supplied by Ionics, Inc. (now part of GE Infrastructure, Water & Process Technologies), and associated storage and feed equipment for membrane cleaning. Each membrane unit comprises 28 fiberglass pressure vessels approximately 28 ft long.
Aesthetically, the new plant features brick facades, archways and tile roofs to complement the existing, turn-of-the-20th-century architecture. Extensive landscaping provides screening and security.
“Working with a client of this caliber on a project of this significance was very rewarding,” said B&V Water President and CEO Dan McCarthy. “Working with the MWW to effectively utilize and advance membrane treatment not only benefits area residents but also the water industry as a whole.”
The newly operational MWW plant further establishes membrane filtration as an economically viable technology.
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