Sorptive Media Biofiltration for Inorganic Nitrogen Removal from Storm Water

Passive biological filtration for nitrate removal from storm-water drainage is challenged by highly transient mass loadings, the need to adequately supply an electron donor, and potential inhibition by dissolved oxygen (DO). An approach to optimizing nitrate removal is to employ a filter medium containing a mixture of ion exchange and electron donor particles, where the former serve to retain nitrate at high loadings and enable biological denitrification to be more effective. Bench scale filtration experiments were conducted using a 50:50 volume mixture of expanded clay particles (Filtralite P) and elemental sulfur pastille. Nitrate reduction was 98% under steady flowrate operation at 30?min residence time and 2.1?mg/L influent NO3–N. Step increases in flowrate by factors of 5.2, 11, and 25 resulted in maximum effluent NO3–N of 0.93, 1.54, and 1.87?mg/L, respectively. Substantial nitrate breakthrough occurred even when effluent DO remained close to zero. The results suggest methods by which mixed media denitrification filters can be more effectively designed and operated.