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%.