Nitrogen removal in stormwater bioretention facilities: effect of drying, temperature and submerged zone

Removal of ammonium‑nitrogen (NH 4- N), nitrite/nitrate‑nitrogen (summarized as NO x —N) and total nitrogen (TN) was examined in pilot-scale bioretention columns with and without a submerged zone under varied temperature and length of antecedent dry periods. The experiment was divided into wet and dry periods and samples were collected to represent continuously wet conditions and conditions with antecedent dry periods. Removal percentages and main effects of the examined factors as well as the effect of their two-way interactions were evaluated. Generally, low temperature favored NO x- N and thus TN removal independent of bioretention column design. Interestingly, bioretention columns with a submerged zone achieved the highest nitrogen removal at low temperature (1.5 °C). For standard bioretention columns, outflow concentrations of NO x- N and TN increased with increasing length of antecedent dry period, whereas a submerged zone mitigated this effect.