Columbia Heights Filtration Plant, United States of America
The Columbia Heights filtration plant began operations in 2005 with a capacity to produce 265,000m² per day of potable water through ultra-filtration (UF) technology. It was constructed at the Columbia Heights plant and is the largest such installation for potable water in North America and one of the largest in the world.
The project required UF technology integrated into the ageing old plant built during 1913-18 on the Mississippi River, along with lime softening and coagulant / floc sedimentation to replace the existing granular media. The plant can now produce about 78 million gallons of potable water per day, sufficient for half a million residents.
Ultra-filtration water plant
A state-of-the-art facility, the ultra-filtration water plant in Columbia Heights initially processed up to 70 million gallons of Mississippi River water per day. It removes particles that are undetectable by the microscope.
The plant conforms to all the federal drinking water standards and removes impurities.
Ultra-filtration plant features
The ultra-filtration plant uses hollow fibres to take particulate matter out of the water. The fibre walls are porous, letting water through and keeping particles behind. A cross section reveals thousands of hollow fibres packed inside. Even the tiny holes in the walls of a fibre can be viewed by a powerful microscope. They act as a sieve, whereby larger material is left and cleansed.
There are about 9,600 fibres in one vessel and four vessels in one long module. There are 40 vessel units in the new plant and an estimated 43,008,000 fibres cleaning the city’s water.
The fibres create a surface area of 1,669,000ft². If they were laid out end to end the fibres would stretch 40,000 miles, or about 1.6 times the circumference of the world.
The ultra-filtration plant can produce up to 70 million gallons of clean, drinkable water in a day.
Impurities of 0.03 micrometres can be removed, and a fraction that cannot be completely screened are mitigated by chlorine treatment. Dissolved salts and minerals are small in ratio and so remain in the water after ultra-filtration and contribute to the taste of the water. Mineral content is also added to the water to prevent pipe corrosion.
After a careful review process involving both a technical panel and a citizens' advisory committee, the City of Minneapolis chose UF membranes to improve treatment processes, in anticipation of increasingly stringent regulations. Driven largely by the need for higher levels of microbial removal, other options were considered, but ultimately rejected after an extensive UF pilot trial, including rebuilding the existing filters and the use of ultraviolet or ozone disinfection.
Even before its completion, the plant design helped Black & Veatch, which is one of the main contractors, win a Project Merit award for innovative implementation of membrane systems.
The ultra-filtration technology was supplied by Norit, with Ionics (acquired by GE Infrastructure Water & Process Technologies) responsible for the complete membrane system. The overall cost of the project was $60m, with the membrane system representing $16m of the total.
Chemical dosing and filtration system design
The existing Columbia Heights plant uses traditional chemical dosing and filtration methods to purify the raw water drawn from the upper Mississippi. Chlorine and ammonia are added for initial disinfection, ferric chloride is added as a coagulant to remove remaining colour and turbidity and the water is then fluoridated. It then enters a series of coagulation / sedimentation basins, followed by filtration through single, dual or mixed media filters.
Blended poly / ortho phosphate is subsequently added to inhibit corrosion and post chloramination used to adjust disinfectant residuals before the water is delivered for use.
With the new filtration plant, these approaches were replaced by a low-pressure, cartridge system of polyethersulphone UF membranes capable of purifying 265,000m³ daily, arranged horizontally within the new treatment building.
Although the main driver in the plant upgrade was the desire for enhanced exclusion of microbes, the city authorities' requirement to retain soft water was another criterion in the overall technology selection process. The average hardness of Mississippi River water is 170mg/l; the Minneapolis softening plant softens it to an average hardness of 75mg/l.
This was achieved by the addition of lime, with a further dose of alum to remove colour and turbidity. As a result, a dilute lime and alum slurry settles out, which is removed and ultimately applied to farmland in Minnesota and Wisconsin. Powdered activated carbon and the occasional addition of potassium permanganate are used to control taste and odour. Finally, carbon dioxide is introduced into the water to lower the pH and stabilise the remaining hardness before the water is pumped to one of the two filtration plants - Columbia Heights or Fridley.
Plans were made to upgrade the Fridley filtration plant by retro-fitting it with a new membrane ultra-filtration system and constructing a wastewater treatment lagoon, but these were cancelled in 2009. Construction of the new feed water pumping station, strainers and the membrane facility was scheduled to begin in 2005, when the Columbia Heights plant became operational.
In order to maintain membrane performance, the deposits that accumulate over time are removed periodically. The feed strainers are backwashed using strained water, which is then recovered through the treatment process itself, thus avoiding discharge to the environment. The membrane units themselves also undergo periodic backwashing with membrane filtered water, which is also reclaimed in the same way.
In addition to backwashing, the membrane units are cleaned using dilute acid, sodium hypochlorite and sodium bisulphate. The wastewater arising is diluted and neutralised by the further addition of sodium bisulphate or sodium hypochlorite as necessary.
The planned 30,000m³ lagoon, which is to be built in two separate cells adjacent to the existing seven lagoons on the Fridley site, would accept the wastewaters produced by cleaning both of the new membrane ultra-filtration systems, and settling and storing residual solids.
There would have been a continuous discharge from the lagoon to the river, which will vary up to a maximum of 6,500m³ per day, dependent on the frequency of chemical washing necessary to optimise operation of the plant.
The plant is owned by the City of Minneapolis and operated by Minneapolis Water Treatment & Distribution Services. Black & Veatch provided the study, design, construction and start-up services for the plant. The main contractor of the project was Adolfson & Peterson Construction.
Norit is supplying the ultra-filtration technology and the X-Flow membranes to Ionics, who in turn is responsible for supplying the complete membrane system. HDR Engineering is acting as evaluation contractor and providing technical services. Other contractors include SPI, contributing technical expertise and test protocols, and Progressive Consulting Engineers who performed tracer studies and hydraulic analysis.