Biofiltration for drinking water treatment has several water quality benefits but questions remain about biofilter start-up in temperate climates where temperature and water quality changes can impact performance. A year-long fullscale demonstration was performed with biofilters operated in parallel to chlorinated filters to (O1) monitor the development of the biofilter microbiota across the acclimation period with respect to removal of Mn and total organic carbon, and (O2) determine the impact of depth and backwash events on the microbial community composition and function. Biofilter media biomass increased for the first 6 months of operation and after about 9 months the biofilters achieved comparable Mn removal to the chlorinated filters. Prokaryotic diversity decreased when the water temperature dropped. Backwash resulted in an increase in microbial diversity deeper in the biofilter. By the next sampling (30 h post backwash), 16S rRNA gene copies were again more abundant in the upper layers. Metagenomic sequencing confirmed the presence of moxA and mofA, Mn oxidizing genes. The results presented can help inform expected performance of full-scale biofilters in temperate climates.

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Key Takeaways Cyanotoxin occurrence is difficult to predict and detect, yet cyanotoxins at high concentrations are acute health concerns, raising the stakes for utility response. There's no one‐size‐fits‐all method to respond to harmful algal blooms. Utilities should customize their response plans to their specific circumstances and characteristics. Two water utilities in the same US region developed different yet equally comprehensive, reliable plans.