Biological filtration is resilient to wildfire ash-associated organic carbon threats to drinking water treatment

Elevated/altered levels of dissolved organic matter (DOM) in water can be challenging to treat after wildfire. Biologically-mediated treatment removes some DOM; its ability to remove elevated/altered post-fire dissolved organic carbon (DOC) resulting from wildfire ash was therefore investigated. The treatment of low, medium, and high wildfire ash-amended source waters by bench-scale biofilters was evaluated in duplicate. Turbidity and DOC were typically well-removed during periods of stable operation (effluent turbidity ≤ 0.3 NTU in 93% of samples, average DOC removal ~20% in all biofilters during periods of non- impaired DOC removal). Daily DOC removal across all biofilters was generally consistent, suggesting that the wildfire ash and associated water extractable organic matter did not reduce the DOC biodegradation capacity of the biofilters. DOM fractionation indicated that this was because the low molecular weight neutral (which are known to be readily biodegradable) and biopolymer fractions of DOM were reduced; however, humics were largely recalcitrant. Thus, biological filtration may be resilient to wildfire ash-associated DOM threats to drinking water treatment. However, operational resilience may be compromised if the balance between readily removed and recalcitrant fractions of DOM change, as was observed when baseline source water quality fluctuated for brief periods during the investigation.