Columbia Heights Filtration Plant, Minneapolis, MN, USA

Scheduled to become operational in 2005, the 265,000m³/d ultra-filtration (UF) facility currently being constructed at the Columbia Heights plant will be the largest such installation for potable water in North America and one of the largest in the world. The project requires UF technology to be integrated into the ageing 90-year old plant on the Mississippi River, along with lime softening and coagulant / floc sedimentation, to replace the existing granular media.

ULTRA-FILTRATION

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, included rebuilding the existing filters and the use of ultraviolet or ozone disinfection.

Even before completion, the plant design has already helped Black & Veatch, as main contractors, win a Project Merit award for innovative implementation of membrane systems.

The ultra-filtration technology is being supplied by Norit, with Ionics responsible for the complete membrane system. The overall cost of the project is $60 million, with the membrane system representing $16 million of this.

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 color 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.

When the new filtration plant is completed, these approaches will have been replaced by a low-pressure, cartridge system of polyethersulphone UF membranes capable of purifying 265,000m³ daily, arranged horizontally within the new treatment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 currently softens it to an average hardness of 75mg/l.

This is achieved by the addition of lime, with further dosing of alum to remove color 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 odor. Finally, carbon dioxide is introduced into the water to lower the pH and stabilize the remaining hardness before the water is pumped to one of the two filtration plants - Columbia Heights or Fridley.

FUTURE DEVELOPMENTS

Plans have been made to upgrade the Fridley filtration plant by retro-fitting it with a new membrane ultra-filtration system and constructing a wastewater treatment lagoon. Construction of the new feed water pumping station, strainers and the membrane facility is scheduled to begin in 2005, as the Columbia Heights plant becomes operational.

In order to maintain membrane performance, the deposits which accumulate over time must be 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 must be cleaned using dilute acid, sodium hypochlorite and sodium bisulphite. The wastewater arising is diluted and neutralized by the further addition of sodium bisulphite 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 will be a continuous discharge from the lagoon to the river which will vary up to a maximum of 6,500m³/d, dependent upon the frequency of chemical washing necessary to optimize operation of the plant.

KEY PLAYERS

The plant is owned by the City of Minneapolis and operated by Minneapolis Water Treatment & Distribution Services. Black & Veatch are the main project contractor.

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 are 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.