Simultaneous removals of nitrate and sulfate and the adverse effects of gravel-based biofilters with flower straws added as exogenous carbon source

The feasibility of adding straws of ornamental flowers as carbon source into gravel-based biofilters to enhance removals of nitrate and sulfate simultaneously, and the concomitant adverse effects were evaluated in this study. The straws were added into a perforated PVC pipe vertically inserted into each biofilter. Results showed that compared to the control biofilter with almost no removal capacity, the experimental systems effectively co-eliminated nitrate and sulfate from the wastewater, with a mean removal rate of 1.29–2.11 g NO3−-N m−3 d−1 and 1.03–2.16 g SO42− m−3 d−1, and higher efficiencies achieved for carnation and lily straws than rose and violet. A significant positive correlation was detected between nitrate and sulfate removal efficiency, with the nitrate removal notably more efficient. Nitrate and sulfate removal rates both decreased gradually with the operation time due to the declining carbon supply, and could be augmented again by fresh straw addition even in winter season. The phosphorous removal capacity of the biofilter was low and just influenced limitedly by the straw addition. High contents of organic carbon and colored matter were determined in the effluent after the straw addition but sharp drops to low levels were followed, resulting in insufficient carbon source for the removals of nitrate and sulfate. Mild accumulations of nitrite and ammonium occurred, averaging 0.39–1.30 mg L−1 and 0.46–0.93 mg L−1 in the effluent, while the content of dissolved sulfide was low. The biofilter added with rose straw showed the lowest relative adverse effects despite of its low removal capacities, and good performance with moderate adverse effects were obtained when adding with the carnation straw. The biofilter can be potentially applied to co-alleviate nitrate and sulfate pollutions although further optimization of straw addition is still required.