Inorganic precipitation during autotrophic denitrification under various operating conditions

The effects of hydraulic retention time (HRT, 7 h, 13.5 h and 18 h) and pH (7.5 and 8.0) on the denitrification performance and the extent of inorganic precipitation, in a H2-driven non-porous membrane biofilm reactor for autotrophic denitrification were investigated. The membrane was cleaned periodically to evaluate the solids contents generated in the system and identify the precipitants. The reactor that was operated under the longer HRT contained more inert solids than when operated under shorter HRT, and showed lower specific nitrate removal rates, even though the generation of volatile solids was almost identical in all tested cases. A lower pH level at shorter HRT, due to higher influent dilution rates, resulted in improved denitrification rates. However, aggressive pH control, by adding additional phosphate buffer, was not beneficial because of extensive precipitation of up to 76% of total solids. Inorganic precipitation was strongly and linearly related to the denitrification performance, indicating the inhibitory effect of inorganic precipitants on the transfer of gases or substrates in a membrane biofilm reactor. The sloughed-off solids from the reactors were characterized by ICP analysis and revealed an abundance of various calcium-related precipitants, which were found to be responsible for the increase in the inert solids content. Calcium phosphate precipitation was identified as the major reason for the inhibition of system performance in this study. Operation under long HRT increases the potential to generate substantial amounts of diverse, inhibitory inorganic precipitants.