Efficacy of heated tourmaline in reducing biomass clogging within woodchip bioreactors

Woodchip bioreactors can effectively remove waterborne nitrates from subsurface agricultural drainage and prevent the eutrophication of receiving water, but rapid biofilm growth can severely reduce water flux and denitrification efficiency of this practice within a few years. Tourmaline minerals with thermal excitation could generate reactive oxygen species which would inhibit bacterial growth. In this study, laboratory scale woodchip bioreactors were set up to test the anti-clogging and denitrification efficiency of heated woodchips with tourmaline, heated woodchips without tourmaline, and unheated woodchips. The results showed that the heated tourmaline treatment could reduce the clogging and optimize the nitrate removal rate (47.6 g N/m³/day) under all three hydrologic retention times tested (1, 4, and 8 h). Dissolved oxygen and pH values fluctuated with the removal rate and temperature change, while temperature was identified as the key factor impacting the tourmaline treatment. The heated tourmaline treatment had the lowest biofilm growth (lowest DNA concentration), while the 16S rRNA and a higher abundance of nirS-, nirK-, and nosZ-encoding denitrifying bacteria (based on qPCR) confirmed the higher denitrification efficiency of the heated tourmaline treatment.