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MINNEAPOLIS WATER WORKS' Ultrafiltration Plant Gets Off to a Big Start

Posted on: Thursday, 5 January 2006, 06:00 CST

By Pressdee, Jonathan; Rezania, Shahin; Hill, Chad; Hensel, Sarah

CAREFUL PLANNING, EVALUATION, AND CONTINUOUS TESTING MAKE FOR SUCCESSFUL IMPLEMENTATION OF A NEW MEMBRANE SYSTEM.

After 15 months of design and two years of construction, the largest ultrafiltration (UF) plant in North America is producing high-quality drinking water for the residents of Minneapolis, Minn., and nearby suburbs. The plant was constructed to replace existing sand filters installed in the early twentieth century and to provide additional protection against pathogens such as Cryptosporidium and other chlorine-tolerant organisms. The plant utilizes many state-of- the-art technologies, the heart of which is a UF membrane system with a 70-mgd (265-ML/d) capacity. The design team used the latest three-dimensional (3-D) design tools to create a facility that enables ease of access for operations and maintenance (O&M) staff. Fully automated and complete with sophisticated technology, the plant provides a safe and secure environment for this most essential resource-water.

MEMBRANE FILTRATION WAS DETERMINED NECESSARY TO PROTECT AGAINST CONTAMINATION THREATS

Minneapolis Water Works (MWW) supplies water to businesses and residents of the city of Minneapolis and surrounding suburbs. This utility operates two facilities-a 95-mgd (360-ML/d) plant located in Fridley, Minn., and a 70-mgd (265-ML/d) plant in Columbia Heights, a suburb of Minneapolis. Drawing water directly from the Mississippi River has posed many challenges over the years because of rapid changes in water quality, particularly during the spring when snow melts in the watershed. In the mid-1990s the MWW embarked on an ambitious program to improve treatment to meet increasingly stringent regulations and also to replace obsolete infrastructure that was becoming increasingly expensive to maintain. The Milwaukee Cryptosporidium outbreak in 1995 was a major influence in the decision-making process. Because the Misissippi is a direct source of supply, MWW is vulnerable to Cryptosporidium because of surface runoff in the watershed and wastewater treatment plant discharges from other utilities farther upstream.

The design engineer for the Columbia Heights ultrafiltration project used three-dimensional modeling for the plant layout. This rendering shows the arrangement of the basement piping, which connects to the membrane units on the operating floor above.

The Mississippi River receives numerous discharges in the watershed, including agricultural runoff, power plant cooling water, and effluent from sewage treatment plants. These threats have driven improvements to the treatment plant over the years. Notable treatment challenges include variable organic concentration, turbidity, and temperature. The annual snowmelt period in the spring also creates treatment challenges for the pretreatment process because of rapid changes in raw water alkalinity. Typical water quality values are shown in Table 1.

TABLE 1 Raw water quality

MWW has planned extensively for asset renewal and improved treatment at both of its water treatment plants. An exhaustive study and pilot project identified suitable treatment technologies that met MWW's goals and ensured that anticipated changes to water regulations could be met. This report was completed in June 1997. Study recommendations were reviewed in depth as part of a value engineering study, published in 1999. This gave MWW the confidence to secure funding for the desired developments at its Columbia Heights and Fridley facilities.

The study concluded that membrane UF was the most appropriate technology for MWW based on the following factors:

* compliance with current and anticipated future regulations,

* reliability to produce high-quality water,

* compatibility with future regulatory changes and technological developments,

* increased utility effectiveness because of reduced staffing levels, and

* costs associated with levels of reliability.

Various technologies were reviewed as part of the initial treatment alternatives screening process. Evaluation encompassed

* ozone and granular activated carbon (GAC) adsorption,

* Ultraviolet irradiation, and

* micro- and ultrafiltration.

Following pilot-testing, extensive bench-scale tests, and a value engineering study, UF was selected as the most appropriate technology because it met MWW's criteria at an acceptable cost.

FIGURE 1 Minneapolis membrane plant process flow

TABLE 2 Settled water quality

The study team narrowed treatment technologies to two options: (1) ozone in conjunction with GAC, or (2) membrane filtration. The team concluded that membrane filtration afforded better protection against contamination threats in the watershed at an acceptable cost. Team members also recognized that optimization of pretreatment would be necessary to successfully integrate membrane filtration. Such improvements included optimization of powdered activated carbon (PAC) dosing and use of potassium permanganate to control taste and odor.

Evaluations point to UF as the optimum technology. Once MWW decided to add membrane filtration, the utility prepared procurement documentation for the testing and purchase of filtration equipment. Bidders were required to submit costs for membrane filtration equipment in a sealed envelope in conjunction with their technical submittal. Technical evaluations were kept separate from financial evaluations to ensure the best-performing membrane system was not unduly influenced by financial considerations. The financial information was to be opened only after a membrane filtration system provider had

* complied with the technical requirements,

* successfully completed the pilotphase testing protocol, and

* demonstrated the requisite 4-log virus-rejection criterion.

Two membrane systems prequalified for the testing phase of the project, but only one system1 using a specific membrane2 completed the pilot-test phase satisfactorily, so the financial information was opened only for this system. After successful negotiation, the company1 was awarded a contract to supply membrane filtration equipment to Minneapolis.

A panel of membrane filtration technology industry experts was formed to provide input for the procurement documents and also to review the pilot-testing that accompanied the bidding process.

Additionally, a representative from the largest wholesale purchaser of MWW's treated water attended key meetings, including selection committee meetings for the membrane filtration equipment supplier and the design engineer.

Design of the Columbia Heights UF facility commenced in January 2001 following selection of the design engineer2 and the membrane equipment supplier.1

Community involved throughout planning and design. MWW worked closely with local citizens' groups to prepare the community for these necessary improvements. A Citizens Advisory Committee (CAC) was formed with community representatives so that the treatment upgrades could be effectively communicated and discussed with the citizens. The CAC was a well-informed group of individuals who understood the importance of good-quality drinking water. One of the committee members had closely followed the Milwaukee incident that led to an unprecedented outbreak of cryptosporidiosis in the mid- 1990s. The CAC was involved throughout the planning and design stages of the project.

The use of three-dimensional design software enabled integration of the basement piping design with structural support columns in the new Columbia Heights ultrafiltration plant building, the exterior of which was designed to complement existing treatment facility buildings.

TREATMENT COMPRISES PRETREATMENT, BACKWASH, AND POSTTREATMENT

Pretreatment. Water is extracted by means of a raw water lift station located near the banks of the Mississippi River. Water is fed to lime precipitators into which PAC and aluminum sulfate are dosed. Potassium permanganate is used intermittently for additional taste and odor control, depending on water quality. Additional PAC is added immediately after lime softening. Softened water is recarbonated in two chambers using carbon dioxide fed from evaporators. One chamber feeds the Fridley Filtration Plant; the other chamber supplies a pumping station for lifting softened water to the Columbia Heights Membrane Filtration Plant.

Recarbonated water is then subjected to further coagulation and clarification through the addition of ferric chloride at the Columbia Heights. Six horizontal-flow sedimentation basins are provided in three hydraulically connected pairs. Chlorine and ammonia are added with the coagulant as part of the disinfection process. The process flow schematic is shown in Figure 1, and settled water quality supplying the membrane filtration process is shown in Table 2.

Sludge from the horizontal-flow sedimentation basins is manually removed once per year during annual maintenance. Sludge is transported to a dewatering facility comprising thickeners and centrifuges.

The membrane filtration process is fed by five variable frequency- driven pumps (four duty and one common standby). Water is d\istributed to four rows of membrane units. Every row comprises nine UF membrane units, each with a gross capacity of 2.2 mgd (8.3 ML/d) operating at a flux of 57 gfd (97 L/m^sup 2^/h). Feed water to each membrane unit is pre-filtered by automatic backwash strainers (four duty, one common standby) with 150-pm mesh. The strainers are a candle filter design and are washed periodically by a rotating wash arm. Strainers remain in service during backwashing. Each wash takes approximately two minutes.

The membrane units1 use UF membrane cartridges4 (Table 3). Each membrane unit comprises 28 fiberglass pressure vessels, similar to those used in reverse osmosis systems, and is 28 ft (8.5 m) in length. Each pressure vessel contains four membrane cartridges connected in series. Bypass channels within each cartridge allow for even flow distribution throughout the pressure vessel.

Filtered water is used for backwashing the membrane units ahead of fluoride and polyphosphate dosing.

Backwashing and CEB. Backwashing is completed every 25 minutes. Sequencing is arranged so that a complete row of membrane units can be washed in cascade fashion at maximum plant flow. This means that all nine units are washed in sequence without restarting the backwash pump between washes. This approach minimizes pump start- and-stop sequences and enables coordination of the backwashing process, which is an important consideration with as many as 36 individual units in operation at any one time.

TABLE 3 Characteristics of the X-Flow membranes installed at the Minneapolis, Minn., plant

Routine chemical cleans are performed by modifying the backwash sequence and adding chemicals. This is termed a chemically enhanced backwash (CEB). A number of alternate CEBs can be performed depending on influent water quality and its effect on membrane performance; the aim is to maintain a high permeability and low transmembrane pressure (TMP). The following chemical cleaning regimes are used:

* sodium hypochlorite,

* sodium bisulfite at low pH, and

* hydrochloric acid.

The CEB sequence comprises a wash in, a soaking period of 20 minutes, and then a wash out. CEB solutions are fed via separate, fully segregated piping systems to prevent cross-connections with permeate or filtered water. CEBs were developed during extensive pilot-plant trials that continued during the design phase of the project. The design of the chemical storage and dosing systems was kept flexible to provide a variety of cleaning regimes in the event of water quality change.

A chemical clean-in-place (CIP) system is also provided as an emergency backup. This facility comprises a 6,000 gal (22,710 L) tank fitted with a heater and fed with a drychemical eductor or liquid-chemical drum pump. This enables cleaning solutions to be made with either powder- or liquid-form chemicals. A CIP supply pump recirculates liquid in the tank to perform mixing in addition to feeding the tank contents to the membrane unit. This process is performed manually only if CEBs do not restore permeability to acceptable values. This system is not expected to be used on a regular basis, but it is available as an alternative means of cleaning the membranes should an event occur that causes major fouling of the membrane cartridges.

Posttreatment. Filtrate is dosed with fluoride and polyphosphate. In addition, ammonia and chlorine may be added to maintain disinfection residuals. Treated water flows via an effluent wet well that provides back pressure on the membrane process to offer surge protection. From the effluent wet well, treated water enters a system of reservoirs that provide the required contact time and dosage (i.e., C T) for disinfection and treated water storage for the distribution system.

CLOSEWORKING RELATIONSHIPS HELPED OVERCOME CHALLENGES

Integration of a new, complex treatment process into existing treatment facilities always presents challenges, and the Columbia Heights UF plant was no exception. Challenges that necessitated extra attention from the project team included

* flow control and hydraulic balancing through the existing pretreatment process to meet the demands of the UF system;

* treatment, recovery, and discharge of waste streams such as chemical cleaning waste, backwash water from the membrane units and feed strainers, and integrity test draindown water;

* construction of new chemical feed processes to replace existing facilities while maintaining plant operations; and

* ease of access for anticipated future maintenance activities such as replacement or repair of valves and membrane fibers.

Flow balancing is achieved by improved process monitoring and instrumentation that provides much finer control than was available with the previous process. Close monitoring of inlet reservoir level, influent gate position, and sedimentation basin level ensures sufficient flow is available for the membrane feed pumps without creating an overflow condition. A triple-validated pressure set point for the membrane feed pump control is maintained for smooth feed flow operation, adjusted by monitoring TMP across each membrane unit to minimize pump operating head and therefore optimize energy consumption.

After carefully evaluating life-cycle costs and process considerations, the project team decided to recycle backwash water and integrity drain-down water back to the 75-mil gal (285-ML) inlet reservoir that feeds the existing pretreatment process. This solution was developed in close cooperation and with the approval of the state regulatory agency, the Minnesota Department of Health (MDH). Chemical cleaning solutions are neutralized at the plant and then discharged to a lagoon located alongside the Mississippi River. After storage, the neutralized water is decanted into the river under a National Pollutant Discharge Elimination System permit, as developed with the Minnesota Pollution Control Agency.

The administration building lobby provides visitors with a good view of the ultrafittration membrane system and demonstration area without the need to enter operational areas of the facility.

Changeover to the new chemical feed processes (chlorine, ammonia, fluoride, and polyphosphate) was achieved during a planned maintenance shutdown. The changeover proceeded smoothly as a result of close working relationships developed among the MWW O&M staff, the contractor, and the design engineer.

Building on the design engineer's previous experience with membrane system design, the decision was made to lay out the facility using integrated 3-D design software. This approach is particularly beneficial for membrane filtration plant design because of its modular construction and complex piping arrangements required for not only conventional backwashing but also chemical cleaning procedures.

A 3-D rendering (page 57) shows how piping to each membrane unit was arranged. The structural design of the building was integrated with the plant layout in mind; the columns provide the dual function of supporting the floor and providing anchorage for the pipe hangers. To facilitate access by plant operations personnel, this part of the building is clear-spanned so that no columns hinder access to the membrane pressure vessels, essential for easy removal of the membrane cartridges.

MULTIPHASE PILOT-TESTING WAS KEY TO SELECTING AND MODIFYING THE BEST SYSTEM

Extensive piloting has been conducted throughout the procurement, design, and postconstruction phase of this project. Initial pilot testing was performed as part of the procurement process to identify the most economical membrane system that met strict performance goals. Membrane manufacturers were given an optimization period to establish operating parameters at the start of the initial piloting phase. At the end of the optimization period, the membrane suppliers had to identify their operating parameters and conduct extended piloting without any further adjustments. For the Columbia Heights project, two membrane systems made the short list following the initial advertisement. Ultimately, the Ionics system performed best and was selected for installation at the Columbia Heights facility. Additional' piloting was conducted following selection to further optimize system parameters.

During the extended piloting phase, a major fouling event occurred. It is believed that the fouling coincided with a major change in water quality because of snowmelt in the spring. This has been known to cause a reduction in the alkalinity of the Mississippi River water source, and it has been postulated but not proven that the nature of organic species may have been altered at this time. An alternative chemical cleaning procedure has been developed to restore permeability caused by the fouling event. After successful piloting, adjustments were made to the design of the chemical feed and neutralization systems. These design adjustments would not have been made if piloting had ceased following procurement.

Membrane feed pumps and strainers (above) are easily accessible in the pump room. The new Minneapolis Water Works filtration plant features 40 ultrafiltration membrane units (right), each composed of 28 fiberglass pressure vessels.

The importance of long-term pilot-testing is a lesson to be gleaned from MWW's experience integrating membrane filtration. Changes to the chemical feed system are relatively inexpensive when caught during the design phase of the project, but they can be very expensive to modify once the plant has been constructed. For utilities with rapidly changing surface water quality, long-term pilot trials can be beneficial.

It is MWW's intention to continue pilot testing throughout the lifetime of this plant. A demonstration-scale unit will be installed in the near future to enable utility staff to conduct offline testing. Such testing may encompass alternative chemical cleaning regimes, additional virus testing, and alternative membrane formulations as the\y become available to compare performance with the existing plant.

PARAMETERS ARE MEASURED THROUGH ONLINE INSTRUMENTATION AND CONTROL

Online instrumentation is provided for measuring particle counts, turbidity, temperature, pH, conductivity, and chlorine residual. Temperature values are used to automatically calculate flux set points according to an algorithm refined during pilot-testing. Particle counts are used to complete comparative log rejection values for each membrane unit and total system particle rejection. Conductivity is used as a guide to ensure membrane units are flushed out effectively.

Permeate water is sampled for conductivity, particle counts, pH, turbidity, and chlorine residual. Individual membrane units are provided with turbidity and particle counters to monitor performance at the unit level.

The neutralization process comprises instrumentation for monitoring redox potential, conductivity, and pH. Redox potential is used to ensure that residual oxidant and reducing agents are properly neutralized before discharge to a lagoon for ultimate disposal. Conductivity is used as a rapid measurement for checking neutralization of acidic and alkaline wastes to check the progress of the neutralization process, with pH used as the final measurement before discharge to the lagoon. The lagoon system is located at the Fridley plant, and effluent discharges at a controlled rate to the Mississippi River.

Flow meters are provided for influent water supply to the membrane units, but control is achieved by utilizing pressure in the feedwater manifold. A desired flow set point is entered into the supervisory control and data acquisition (SCADA) system, and the correct number of units is selected automatically to achieve the desired output flow. The number of units will vary for a given flow according to water temperature-more units will be necessary to achieve the same water production at lower water temperatures. Feed pressure set points are calculated based on TMP . measurements from each unit-the highest values are used to ensure that sufficient pressure is maintained in the feed piping to overcome this head loss and to provide sufficient additional pressure to overcome friction losses in piping and hydrostatic head into the effluent wet well that collects filtered A' water. Membrane feed pumps are brought online, and speed is adjusted automatically to meet the desired pressure set point. Pressure readings are a critical measurement and are triple-validated to ensure the correct values are achieved. Triple validation helps avoid instrument errors by cross-checking values. Instrument drift is easier to identify if two instruments provide similar values with the third instrument providing significantly different results.

The SCADA system uses the same software5 used throughout MWW, and programmable logic controllers6 (PLCs) were installed for process control.

AFTER INTEGRITY TESTING, THE PLANT BEGAN PRODUCING WATER

Integrity testing is performed daily using a pressure-hold technique that measures water displacement. A 4-log rejection capability is maintained for the system, with the alarm point set at a conservative 4.5-log rejection. MWW has worked closely with MDH to develop reporting software and monitoring of the membrane plant. Reporting of turbidity, chlorine residuals, and integrity test results forms the basis of the monthly reports, all of which can be generated by the SCADA system.

Startup has progressed well as an additional testament to the close partnerships developed during the project. Disposal of membrane preservative and software debugging and optimization have consumed the bulk of the commissioning hours, which is expected with this type of facility. Integrity testing, chemical cleaning, chemical washout, and neutralization are the most complex sequences to coordinate. The plant is now producing water, and at the time this article was written it was part way through a 28-day acceptance test.

COSTS INCLUDED EQUIPMENT, CONSTRUCTION, AND INSTALLATION

The membrane equipment bid cost was $17 million for a 70-mgd (265- ML/d) system in 2001. This scope comprised the following principle components:

* 36 UF membrane units,

* 5 automatic backwash feed strainers,

* 4 backwash pumps with variable-speed drive units,

* all CEB and neutralization chemical feed pumps,

* neutralization recirculation and discharge pumps, and

* master and bank level PLC cabinets and associated programming.

A separate contract was created in 2003 for the membrane facility construction and installation of the membrane filtration equipment at a cost of $34 million. The building comprises an administrative area, small visitor center, training room, new chlorine and posttreatment chemical storage, and a pilot-plant room for future research. The building is constructed with a full basement that houses membrane cleaning chemicals and all interconnecting piping and pumps. Traditional brick and natural stone accents have been used to create a building that blends into its environment. The new building complements other treatment plant buildings that were constructed in the early 20th century.

THOROUGH PLANNING AND COMMUNICATION WERE KEY TO SUCCESS

The utility has found that careful planning has paid dividends and led to a smooth sequence of events without surprises. Clearly identifying the best treatment solution and effectively communicating the need for rate increases to citizens and council members enabled MWW to raise funds and increase rates as necessary to fund improvements. Other recommendations are to:

* continue piloting after bid award, especially if the water source is of a changeable nature,

* allow for flexible chemical cleaning regimes to address unexpected fouling events,

* maintain and enforce rigid protocols for membrane pilot testing to gather high-quality data to accurately determine the best- performing membrane filtration unit,

* use 3-D design because it is an excellent tool for developing a well-designed plant that facilitates O&M work,

* coordinate and cooperate with regulatory agencies to enable smooth passage through the approval process, and

* develop close working relationships among the owner, contractor, and design engineer to ensure a successful design, startup, and handover process.

ACKNOWLEDGMENT

The authors thank Adam Kramer, retired director of Minneapolis Water Works, Minneapolis, Minn., and Dale Folen and Annika Bankston, project managers for Minneapolis Water Works. Information presented is based on a forthcoming Integration of Membrane Filtration Into Water Treatment Systems project sponsored by the AWWA Research Foundation and the Bureau of Reclamation.

If you have a comment about this article, please contact us at journal@awwa.org.

The importance of long-term pilot-testing is a lesson to be gleaned from Minnesota Water Works' experience integrating membrane filtration.

This article discusses the planning, design, and startup of a large-scale membrane plant, integrated with existing infrastructure. The Minneapolis Water Works (MWW) plant produces drinking water for Minneapolis, Minn., residents and draws water directly from the Mississippi River, which has posed challenges in the past because of rapid water quality changes. An exhaustive study and pilot project identified suitable treatment technologies that met MWW's goals and ensured that anticipated changes to water regulations could be met. Ultrafiltration (UF) was selected as the best technology for the plant. Careful planning, good communication, and continuous pilot- testing and assessment of design feasibility were key to the successful integration of membrane technology into the plant-now the largest UF plant in North America.-SH

FOOTNOTES

1 Ionics Inc., Watertown, Mass, (now part of GE Infrastructure Water & Process Tech nologies)

2 Xiga(TM) Norit Membrane Technology, Enschede, The Netherlands

3 BIack & Veatch Corp., Minneapolis, Minn.

4 Norit X-Flow, Enschede, The Netherlands

5 GE Cimplicity, GE Fanuc Automation Inc., Charlottesville, Va.

6 Alien Bradley ControlLogix, Rockwell Automation Inc., Milwaukee, Wis.

ABOUT THE AUTHORS

Jonathan Pressdee (to whom correspondence should be addressed) is a project manager and membrane practice leader with Black & Veatch Corp., Minneapolis, Minn. Pressdee has 15 years experience in the water industry and has a degree in chemical engineering from the University College Swansea, University of Wales, UK. He can be reached at pressdeejr@bv.com. Shahin Rezania is interim director of Minneapolis Water Works, Minneapolis, Minn., and Chad Hill is a project director with Black & Veatch Corp., Minneapolis, Minn.

Copyright American Water Works Association Dec 2005

Source: American Water Works Association. Journal

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