WC128 JanFeb2023 - Magazine - Page 34
such as bioavailable phosphorus and organic carbon to the water
column, which can contribute to algae proliferation and NOM
transformation in untreated water storage reservoirs and increased
coagulant demand in treatment plants. While some watershed regions are not fire-prone, treatment processes resilient to wildfire-associated organic carbon threats are also likely resilient to disturbances that result in similar erosion-associated water quality changes.
To investigate whether biofiltration could offer treatment
resilience in wildfire-prone systems, duplicate bench-scale biofilters treating agriculturally and municipally impacted source
water (preceded by roughing filtration for solids removal) were
intermittently fed wildfire ash-amended water. Their performance
was compared to that of control biofilters that continued to treat
source water without ash. Interestingly, the wildfire ash did not
seem to affect the removal of NOM by the biofilters. NOM is
complex, but its fractions added after wildfire ash was mixed with
water tended to be smaller and thus more biodegradable, although
some smaller amounts of larger, more aromatic substances were
also added. The biodegradable fractions were removed by the
biofilters, whereas the larger aromatic substances were less well-removed. Since more biodegradable fractions of organic carbon were
added by the wildfire ash, this suggests that biofiltration could be a
promising drinking water treatment solution for wildfire-impacted
source waters. However, the bench-scale results may not be representative of what could be expected at an actual treatment plant,
and more work is needed to investigate how bench-scale biofilters
scale up to full-scale biofiltration.
Promising, but not perfect
Evidence that biofiltration is resilient to changes in NOM
following severe watershed disturbances like wildfires is encourag-
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WATER C AN ADA • JANUARY/ FEBRUARY 2023
ing, especially since they require less operational demand and
resources compared to conventional technologies—which can
be beneficial particularly for small systems. However, benchscale biofilter experiments with wildfire ash showed that NOM
character is an important consideration in assessing whether it
can be removed effectively by biofiltration. With the wildfire
ash and source water used in the experiments, a large fraction
of small and biodegradable NOM was added with ash addition,
which likely contributed to the effectiveness of biofiltration
in removing NOM. However, more aromatic carbon can also
be elevated in source waters after wildfire disturbances. Other
watershed disturbances like floods can also result in elevated aromatic NOM in source waters. These larger and more aromatic
fractions are less biodegradable and thus less effectively removed
by biofiltration. Therefore, biofiltration may not always be resilient to changes in NOM after severe watershed disturbances
because not all organic carbon is easily biodegradable.
Fortunately, biofiltration technologies come in many different forms, and various pre-treatments can either remove NOM
that is more recalcitrant to biodegradation (e.g. with coagulation of aromatic NOM) or increase the biodegradability of
NOM prior to its removal by biofiltration (e.g. with ozonation
to break down recalcitrant NOM into more biodegradable
fractions). These pre-treatment options merit further investigation, though biofiltration remains a promising techno-ecological NBS to address NOM-related treatment concerns due to
climate change-exacerbated threats to water quality, especially
for systems that can benefit from its low operational demand
and energy requirements.
For further reading visit watercanda.net/resources/jf22
WAT E R C A N A D A . N E T
A. J. Blackburn
Preparation of wildfire ash-impacted waters for treatment by bench-scale biofilters.
