Community of Windigo Island uses hollow fibre nanofiltration technology

By R.G.Guenther

The Windigo Island Community is a part of the Animakee Wa Zhing 37 First Nation and is located within the area known as Lake of the Woods. There are thousands of inhabited and uninhabited islands in this water body, and water quality is fairly consistent throughout the region.

From a drinking perspective, the main difficulties in using this lake as a drinking water source are elevated levels of turbidity (<3 by average but >50 NTU during upset events) and dissolved organic carbon (DOC) and colour, which can cause trihalomethane (THM) concerns when chlorinated

The 71 residents of Windigo Island live on a small, remote island at the southwest corner of Lake of the Woods. The closest mainland point to Windigo Island is actually in Minnesota.

During the summer, most land access to and from Windigo Island is through the cartographic anomaly that is the Northwest Angle or just “The Angle” to locals. In the winter, there are established ice roads through Lake of the Woods, which makes Canadian access to the island more feasible. The island is far enough offshore to make conventional electrical/communication access impractical.

There have long been issues with drinking water quality on Windigo Island. At the beginning of the project, the drinking water quality advisory in place was one of the longest in Canadian history. By virtue of its remote location, it was not practical, or reliable, to supply water treatment service and support from the mainland.

Conventional treatment solutions, such a simple sand media filtration and/ or traditional small-scale reverse osmosis/nanofiltration treatment skids had proven unreliable and fallen into disuse and abandonment. The residents have historically relied on filtration with a sand filter and chlorination to make the lake water potable.

The three aspects of the water treatment challenge that most needed to be addressed, primarily to satisfy potability requirements were: • Certified log removal credits for micro-organisms and disinfection credit. • A treatment solution that could minimize the presence of dissolved organic carbon. • Minimal treated water flow rate.

Initially, a conventional ultrafiltration (UF) membrane solution was considered. Due to the technology’s references, modular design, and capability of satisfying the requirement for log removal credits, ultrafiltration seemed to be a viable solution. However, after further deliberation, it was unlikely to reduce the elevated levels of DOC/colour in the raw water to a level that would have made the treated water compliant with THM regulations and sodium hypochlorite dosing.

Instead, a dual system technology (e.g., ultrafiltration followed by nanofiltration) would have had to be considered.

Along with considering different treatment solutions, the disinfection process was also a point of discussion. Due to the small system and the remote nature of Windigo Island, it made more complicated disinfection solutions, such as chloramination, or carbon dioxide, impractical. Therefore, disinfection by sodium hypochlorite, was the only practical solution. UV disinfection could be added to further extend disinfection log credits if required.

What was ultimately proposed was an innovative hollow fibre nanofiltration element. Specifically, one with long, holcontinued overleaf…

Drinking water treatment challenges included elevated levels of turbidity, dissolved organic carbon and colour, which can cause THM concerns when chlorinated.

low membrane fibres, which give a large filtration surface area and a membrane separation layer which, with testing, can provide documented micro-organism log-removal credit.

This combined the functionality of an ultrafiltration membrane, with the dissolved constituent removal of a loose nanofiltration membrane. In essence, this was two products in one – ultrafiltration and nanofiltration.

The module is identical to other ultrafiltration modules in the same form factor. The technology uses an inside-out flow path. Water enters the membranes on the inside of the hollow fibre and flows out of the individual tubes into the barrel of the module. Product water in the barrel of the module exits the top of the module through a specially manufactured end cap.

The individual hollow fibres inside each module receive special processing that places a thin charged membrane separation layer on the inside of the tube. This charged surface repels dissolved organic molecules, which have a similar charge, and the membrane achieves DOC rejection by this charge repulsion phenomenon.

The efficacy of this method has proven to be high. In results from pilot testing on Windigo Island with hollow fibre nanofiltration treatment it is achieving 83% rejection of DOC. In practi-

(Left) The existing backwash system. (Right) Comparison of permeate water to raw water.

cal terms, this means that a raw water concentration of 12 mg/L DOC can be reduced to 2 mg/L, which generally would cause no THM compliance issues.

As an interim solution, at the immediate crisis of getting Windigo Island off a drinking water quality advisory, it was proposed that a test-scale system be manufactured and placed in the existing treatment building, as a trial of the hollow fibre nanofiltration technology. If the system worked as intended, a treatment skid with hollow fibre nanofiltration membranes would acquire the necessary log removal credits required for making potable water from a surface source.

The hollow fibre nanofiltration membrane had performed challenge testing to establish its log removal capability. It reduced DOC from the elevated levels in the lake source water, to a level that is consistent with no compliance issues for THMs.

An added benefit was that a pilotscale project could be implemented for minimal cost, as the proposed delivery would include no civil works and could be mechanically and electrically configured to work in the existing building. This effort proceeded in 2018 and was brought online in early 2019.

The immediate results were transformative. Raw water turbidity and DOC were reduced to levels compliant with regulations. The membrane module had challenge test results that made it compliant with log removal credit rules. A commercial grade UV system was

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Municipal Representatives:

included in the pilot equipment and that satisfied disinfection requirements.

During commissioning, there were issues with getting the treated water storage tank clean enough to use as a backwash source for the membranes. After years of operation with just the sand filter, sediments were found in the tank. Once commissioned and operating began, there were obstacles to getting the plant operation certified. But those challenges were overcome, and Windigo Island came off the drinking water quality advisory list.

The pilot-scale membrane plant was operated continuously beyond the commissioning stage. The long-term question of how a membrane solution would fare on this surface water source was answered with continuing months of operation with no degradation.

The pilot system operated successfully through variations in water quality due to the changing of the seasons, and water temperature that ranged from 1°C to 19°C. The automatic cleaning system built into the pilot skid was able to maintain membrane performance without human intervention for cleaning.

Over the two years of operation of the pilot skid, it only required membrane service once. This occurred when a built-in alarm sensed integrity loss, due to a pressure decay test. The required repair was minor and was completed with minimal effort. Once back online, it continued to operate successfully with no incidents until it was decommissioned in November 2021.

Success of the pilot skid on Windigo Island prompted the First Nation to proceed with a design of a full-scale hollow fibre nanofiltration system, in concert with the construction of a new water treatment plant and reservoir on the island.

The full-scale system was designed with significantly more treatment capacity than the pilot-scale system, which had one train with three modules in it. The full-scale system has two trains (one fully redundant) with fourteen modules per train. It was commissioned in 2021 and is operating successfully, replicating pilot-scale results.

R.G.Guenther is with Delco Water, a Division of Delco Automation Inc. Email: gguenther@delco-water.com. Delco Water is represented in Ontario by ACG-Envirocan. www.acg-envirocan.ca

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