Nitrifying moving bed biofilm reactor (MBBR) biofilm and biomass response to long term exposure to 1 °C

•MBBR nitrifying biofilm, biomass viability and community are investigated at 1 °C.•Ammonia removal rate after long duration at 1 °C was 18 ± 5.1% of removal at 20 °C.•Thickness of biofilm attached to MBBR carriers increased with exposure time to 1 °C.•Viable counts of cells embedded in biofilm increased with exposure time to 1 °C.•No population shift in AOB or NOB observed with long exposure to 1 °C.

This study aims to investigate moving bed biofilm reactor (MBBR) nitrification rates, nitrifying biofilm morphology, biomass viability as well as bacterial community shifts during long-term exposure to 1 °C. Long-term exposure to 1 °C is the key operational condition for potential ammonia removal upgrade units to numerous northern region treatment systems. The average laboratory MBBR ammonia removal rate after long-term exposure to 1 °C was measured to be 18 ± 5.1% as compared to the average removal rate at 20 °C. Biofilm morphology and specifically the thickness along with biomass viability at various depths in the biofilm were investigated using variable pressure electron scanning microscope (VPSEM) imaging and confocal laser scanning microscope (CLSM) imaging in combination with viability live/dead staining. The biofilm thickness along with the number of viable cells showed significant increases after long-term exposure to 1 °C. Hence, this study observed nitrifying bacteria with higher activities at warm temperatures and a slightly greater quantity of nitrifying bacteria with lower activities at cold temperatures in nitrifying MBBR biofilms. Using DNA sequencing analysis, Nitrosomonas and Nitrosospira (ammonia oxidizers) as well as Nitrospira (nitrite oxidizer) were identified and no population shift was observed between 20 °C and after long-term exposure to 1 °C.

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