Biological ion exchange shows promise to deliver safe water for island community

By William S. Chen, Jaycee Wright and Madjid Mohseni

Gillies Bay is one of two communities located on Texada Island, a 300-square kilometre island found off the Sunshine Coast along the southern shores of British Columbia.

While the several hundred visitors that come each year might know the island for its beautiful sunsets along rocky beaches, the 450 residents that call Gillies Bay home have not enjoyed stable access to clean, safe water for years. Several times each year, without fail, the community finds itself under a boil water advisory, requiring that residents boil their water before use due to the risk of harmful pathogens.

However, results from a 16-month pilot study conducted in collaboration between Gillies Bay Improvement District (GBID), which manages the water treatment and distribution infrastructure in Gillies Bay, RESEAU Centre for Mobilizing Innovation (RESEAU CMI), and researchers at the University of British Columbia, may finally point the path forward for the community to have clean and safe water.

Nature Of The Challenge

Gillies Bay faces challenges shared by many small and hard-to-access communities, including a lack of funding. The limited revenue collected from its small user population is not sufficient to finance conventional treatment options like coagulation, sedimentation and filtration, which require large and expensive equipment.

Moreover, the town is separated from Vancouver, the nearest major city, by three ferry crossings, which delays servic- ing and the delivery of parts and chemicals. A suitable water treatment solution therefore needs to be robust, reliable and cost-effective.

The primary treatment challenge for GBID is the removal of natural organic matter (NOM). Water is sourced from Cranby Lake, which sees an annual average dissolved organic carbon (DOC) concentration of 9 mg/l that reaches upwards of 18 mg/l during the summer. NOM in water, which forms from natural processes like plant decay, is not inherently harmful to human health. However, it interferes with disinfection processes and gives water an unpleasant taste and colour.

The current water treatment system at GBID is comprised of only a coarse screen at the inlet and disinfection using sodium hypochlorite (bleach) injection. Disinfectant contact time is provided by the transit time of the water through the distribution network.

When high water demand meets high DOC, as often happens during the summer months, the combined effects of decreased disinfection efficiency and decreased disinfectant contact time result in bacterial detection and the subsequent issuance of boil water advisories.

One of the key stages missing in the treatment train is thus the removal of NOM. However, with conventional meth- ods being prohibitively expensive for the small community, a solution for removing NOM has remained elusive.

Prospects Of Biological Ion Exchange

Ion exchange is a known method for NOM removal. It takes advantage of the negative charge of NOM at the near-neutral pH of raw surface water. Polymer beads (resins) embedded with positively charged active groups are used to bind to negatively charged NOM, thereby removing them from the water.

In the process, anions previously attached to these resins (typically chloride) are released. Once the resins are full of NOM, removal capacity can be restored by rinsing with a salt solution. This forces NOM to be released and chloride to be taken up once more. However, the waste brine produced by the regeneration process can be difficult to treat and dispose of, especially for remote communities.

Biological ion exchange (BIEX) is a new spin on ion exchange, coined through a collaborative effort by researchers at the University of British Columbia and Polytechnique Montréal. By simply reducing the frequency of regenerations and allowing native bacteria to populate the ion exchange resins, BIEX was found to sustain NOM removal for prolonged periods of time.

This method has been successfully piloted in Des Praires, Quebec, and implemented full-scale at the Dzit’lain’li community in Middle River, B.C. (Tz’latzen First Nation) to end a long-term boil water advisory.

Following the success of these past experiences, RESEAU CMI and GBID agreed to conduct a pilot study to assess the feasibility of BIEX filtration for the town of Gillies Bay.

Pilot Study At Gillies Bay

A pilot treatment system fabricated by BI Pure Water Inc. for RESEAU CMI was mobilized to Gillies Bay in May 2020. The system consisted of two parallel trains for biological ion exchange filtration, and granular filtration using activated carbon (coconut shell based), to compare the performance of these treatment methods.

Raw lake water was piped to the pilot system from the existing network, where large particulates greater than 5-μm were removed through a series of bag and cartridge filters before being distributed to the 100-litre BIEX and activated carbon filtration vessels.

Filter effluent was discharged to the environment. Treatment capacity was

3 litres/min (26 min empty bed contact time [EBCT]) for activated carbon filtration and varied from 3.5 to 7 litres/min for BIEX filtration (10 to 20 min EBCT).

The pilot system was operated for 16 months from May 2020 to October 2021.

continued overleaf…

Superior Doc Removal Versus Activated Carbon

Results of the pilot study confirmed the viability of BIEX for NOM treatment. Greater than 50% NOM removal was achieved using BIEX for 70 days of operation, whereas activated carbon only sustained this removal rate for less than one day.

A corresponding improvement in ultra-

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violet (UV) transmittance, an important parameter for UV disinfection systems, was also observed. UV transmittance increased to greater than 80% using BIEX for 70 days, whereas activated carbon filtration again only achieved this performance for less than a day.

Transition of activated carbon to a primarily biological mechanism of DOC removal (biological activated carbon [BAC]) occurred in less than two weeks and resulted in DOC removal averaging 10%. This remained unchanged until the end of the study.

As for BIEX, upon complete resin exhaustion, removal rates also decreased to 10% as biological removal became dominant. Again, this reduction in BIEX performance occurred more than 10 weeks after start-up, compared to less than two weeks for activated carbon/BAC.

Furthermore, two BIEX regenerations were performed over the course of the 16-month pilot study, each time resulting in successful restoration of NOM removal performance. These results confirm the long-term reusability of biological ion exchange media, which has important cost-saving implications for the community.

As part of ongoing efforts to further improve BIEX operation, the pilot study also involved experimenting with different protocols for backwashing. This included the flow of fluid in reverse through the filter vessel to flush out debris and restore flow. Working together with the community’s water operator, UBC researchers discovered that injecting air (air scour) prior to a water flush was significantly more effective at removing suspended solids from the BIEX media. It improved NOM removal from 13%, up to 50%, averaged over four backwashing events.

These results are being further investigated using benchtop BIEX systems in the UBC laboratory, as researchers continue to optimize BIEX operation to make it viable for more communities.

What The Future Holds

Following the pilot study, GBID is pursuing the design and construction of a full-scale water treatment facility that will utilize BIEX. The utility operator has been long expectant of a new treatment system, but needed to ensure that whatever technology they invested in would be effective long term.

The promising results of the pilot study suggest that biological ion exchange may finally offer the solution needed to provide clean and safe water to the residents of Gillies Bay.

William S. Chen, Jaycee Wright and Madjid Mohseni are with the University of British Columbia Chemical and Biological Engineering. For more information, email: wschen@mail. ubc.ca, jaycee.wright@outlook.com, madjid.mohseni@ubc.ca

GBID pilot treatment system comparing DOC removal with BIEX and activated carbon/BAC filtration. Fluctuating DOC removal of BIEX (300-400 days of operation) reflect the effect of air scour backwashing that temporarily increased DOC removal rates.

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