The Future of Clean Drinking Water: A New Treatment Method

While the debate regarding whether clean drinking water is a basic human right or a commodity continues around the world, few can argue the increasing need for ways to improve existing water treatment methods.

It is with this in mind that WSP initiated exploratory work on the use of ion exchange resins for primary treatment of organic carbon, and in doing so, helped develop an ‘outside-the-box’ method, which could change current treatment for organics in drinking water.

Organics are compounds that exist in all natural water and are fundamental to the viability of a natural ecosystem. Unfortunately, they typically cause brown discolouring of drinking water, react with chlorine disinfection to produce toxic disinfection by-products, interfere with the disinfection of water using UV light, and support biological regrowth in the water distribution system. As such, they must be removed during the treatment of drinking water.

For many communities, there is a growing desire to minimize chemicals used during the treatment of drinking water and the overall system complexity. However, this is often difficult to achieve when removing organic carbon from water, as the standard approach is to apply a metal-based coagulant and filter the resulting agglomerated particles from the water. The next most common organic removal approach is oxidation using powerful oxidants, such as ozone, followed by biologically activated carbon (BAC) filters and, lastly, granular activated carbon (GAC) and nanofiltration using membranes.

The performance of chemical treatment requires balancing the overall plant performance for backwashing with turbidity and organic removal - in many cases, reducing the overall effectiveness of the system. Plants that can achieve >50% removal of organic carbon through this method are considered effectively operated. Oxidation and biological carbon filtration have seen little wide-scale adoption within the industry, likely due to the added complexity of using ozone. Additionally, GAC filters and nanofiltration membranes can become a costly long-term strategy for many communities.

These challenges motivated WSP to start looking at water treatment differently, with a focus on using ion exchange resins for organics removal rather than the typical cation treatment. In order to evaluate the operational performance of the ion exchange media using natural waters, WSP approached the University of British Columbia (UBC) to evaluate tests, which could be completed in the field to estimate full-scale performance. It was proposed that a comparison between several available benchscale test procedures should be reviewed. Surprisingly, the results of the available test procedures did not follow the anticipated behaviour for cation ion exchange breakthrough patterns.

Discovery and Development

The test columns were operated with a continuous supply of typical surface water and a relatively long retention time. Organics concentration measured at the outlet, unexpectedly, took over 50 days to break through the column. In fact, where columns typically break through in a matter of weeks, one of the test waters maintained over 60% removal after 100 days of continuous operation.

In order to determine why the columns operated for such a long period without requiring any regeneration, a biocide was added to some of the test columns. Organic breakthrough of the column was achieved in days (around 25) of operation instead of months. The results suggested that a biological environment was forming within the column and providing beneficial organic removal, significantly extending the amount of time required between filter regeneration. It was hypothesized that by running the ion exchange media without regeneration, biological communities were allowed to grow on the media, altering the organic removal mechanism from an ion exchange to a supplemental biological process – thereby coining the term biological ion exchange (BIEX).

To better understand the performance of BIEX, UBC undertook an additional study in collaboration with the RES’EAU-WaterNET. A comparison study was done where BIEX, ion exchange (IX) and BAC columns were operated in parallel using the same source water and loading rates. Over an 11-month period of operation, where BIEX was not regenerated and IX was regenerated weekly, the BIEX and IX consistently removed between 40% to 60% of the influent organics. The BAC only removed approximately 20% of the organic carbon..

The RES’EAU-WaterNET recognized the potential benefits of this treatment process for many of the smaller, remote communities they are engaged with, to improve access to high quality drinking water. In 2017, through a coordinated project with École Polytechnique, a BIEX filter system was installed at the Pont-Viau water treatment plant in Laval, Quebec. Similar operating conditions were applied for BIEX, IX and BAC as at the UBC laboratories, with the exception that the BIEX was backwashed with air and water on a weekly basis to remove particle build up and prevent the bed from becoming compact, but not regenerated with salt brine to maintain the biological activity. This additional step was undertaken to mitigate potential fouling of the resin and loss of hydraulic efficiency.

Results from this work confirmed that BIEX and IX systems remove organics efficiently. During the first 56 days of operation, the influent organic concentration was consistently reduced by over 70% -- neither BAC nor GAC consistently reduced the carbon levels by even 20%. Moreover, BIEX and IX both worked equally well at reducing organic carbon, reducing the formation of disinfection by-products and increasing the clarity of the water for UV disinfection. When a detailed comparison of the organic removal was completed on the two systems, several differences were apparent.

As with the UBC findings, no salt brine was generated when using the BIEX, as the system did not break through over the test duration, whereas the IX was regenerated with a brine solution on a weekly basis as per traditional IX operating conditions. For smaller water systems, which have no sewer connection or opportunity to dispose of brine waste, this is quite significant. Subsequent tests suggest that BIEX will require brine regeneration every 3-4 months, which is well suited for off-site truck disposal options, rather than fixed infrastructure. This would also allow for third party maintenance contracting.

A second variation between the BIEX and IX was observed when the organic components in the water were characterized. The source water organics were characterized into groups ranging from long to short molecular compounds-- the latter being a major contributor to reduced efficiency of UV disinfection in drinking water and reactivity with chlorine to form disinfection by-products. Neither the BIEX nor IX significantly removed the longer molecules. For the shorter molecules, which were the largest component of organics in the source water, the BIEX consistently removed the vast majority of these components (>90%), whereas the IX was limited to removing approximately 50%. 

img-BIEX System

Full-scale BIEX system installed in Tl’azt’en Nation with a ‘valve nest’ for maximum operational flexibility and further process development and optimization.

Implementation

Indigenous Services Canada (ISC), one of the original partners in the RES’EAU-WaterNET network, became more engaged in the BIEX research, based on the promising results from the pilot studies. ISC was actively monitoring the work being completed by the team and agreed to provide support to install another full-scale pilot system at a First Nation water treatment plant within British Columbia. This pilot plant was operated between July 2017 and April 2018.

Throughout the test, the organic carbon levels were reduced by around 70%. Removal of organics fell to approximately 50% in January and February, at which time the water temperatures were around 5°C. Reduced biological activity is expected during this time, which suggests the organics are in fact being removed biologically. Backwashing of the filter columns was completed using treated water only, and no brine regeneration was undertaken during the entire 270-day pilot test.

Following decommissioning of the pilot in 2018, ISC and RES’EAU-WaterNET funded the construction of a full-scale facility for this First Nation. In 2017, WSP was engaged to complete the process design and provide technical and engineering input for the construction of a full-scale pilot facility, which was constructed by BI Pure Water in the spring of 2018. The commissioning was completed in August-September 2018 and is currently supplying treated water to the local residents. RES’EAU-WaterNET and the design engineers continue to provide monitoring and operational support. Consequently, a long-term boil water advisory has been lifted for this community.

img-Backwash

Pilot filter columns on site in Tl’azt’en Nation’s existing water storage building.

Future Opportunities

As operation of the first full-scale system is monitored over 2019, the potential benefits of the BIEX system could be more widely realized. These include avoiding chemical treatment for the removal of organics in drinking water and the subsequent increase in the performance of UV disinfection. The ability to bypass chemical treatment, other than chlorination, for the treatment of drinking water will also provide direct operational benefits for communities, including operator safety, reduced chemical deliveries to remote sites, increased public perception and process stability.

Additional investigative work is needed to evaluate the impact of filter design parameters, including loading rates, bed depth, empty bed contact time and filtration velocity; these become more significant as the treatment facilities increase in size. Unpublished testing completed on the UBC laboratory columns indicated that a short-term doubling of the flow rate through the column did not negatively affect the performance, although further work is necessary to better evaluate this.

The speed and success with which this project has been achieved involved identifying, researching, testing, designing and implementing a new process operation. This could only have happened through the alignment of the project team -- from community members, research institutions, engineering consultants and contractors. This includes the university researchers at UBC and École Polytechnique, the Tl’azt’en Nation, Alegro Consulting, funding partners including RES’EAU-WaterNET and the Department of Indigenous Services Canada, and the engineering and construction teams from WSP Canada and BI Pure Water.


Authors


img-Claire Bayless

Claire Bayless, P.Eng.
Project Manager, Infrastructure

img-Walt Bayless

Walt Bayless, M.A.Sc., P.Eng.
BIEX Development Manager, Infrastructure

img-Megan Wood

Megan Wood, M.A.Sc., P.Eng.
Engineer – Water, Infrastructure 

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