Significance of intermittent wetting and drying periods and the organic material in the filter layer on nitrate removal in bioretention basins

Nitrogen is an important nutrient that can impact the quality of aquatic environments when present in higher concentration. Primary mechanisms that remove nitrogen in treatment systems require specific environmental conditions such as aerobic (nitrification) and anoxic (denitrification) conditions. In addition, organic matter is often added to the filter sand to provide electron donors to assist the process of denitrification. Even though lower concentration levels of ammonium nitrogen have been observed in laboratory studies in bioretention basins, poor removal or even production of nitrates within the filter is often recorded in such studies.

Ten Perspex bioretention filter columns of 94mm (internal diameter) were packed with a filter layer (PS-Particle size < 1mm) – height: 800mm), transition layer (1mm < PS < 2mm – 20mm) and gravel layer (2mm < PS < 5mm – 200mm) material and operated with synthetic stormwater in the laboratory. The filter layer contained 3% organic material by weight. A free board of 350mm provided detention storage and head to facilitate infiltration. Synthetic stormwater was prepared in the laboratory by adding NH₄NO₃ (Ammonium nitrate) and C₂H₅NO₂ (Glycine) and a mixture of kaolinite and montmorillonite clay, to tap water. The columns were fed with synthetic stormwater with different antecedent dry days (ADD) (0-56days) and constant inflow concentration (2ppm: nitrate-nitrogen, 1.3ppm: ammonium-nitrogen, 2.5ppm: organic-nitrogen and 100ppm: total solids) at a feed rate of 100mL.min (85.7cm/h). Samples were collected from the outflow at different time intervals between 2.5 – 150mins from the start of outflow and were tested for total organic carbon (TOC) and nitrate-nitrogen.

Concentration of TOC was very high at the beginning of outflow (higher than inflow), and then steadily decreased to settle at about 10ppm after 30-60mins of outflow during each event. This indicates wash-off of organic carbon from the filter itself. Moreover, the concentration of organic carbon was observed to vary depending on ADD and age of filter. At the same time, very low concentration of nitrate-nitrogen was recorded at the beginning of outflow indicating effective removal of nitrate-nitrogen. The removal percentage of nitrate in the columns were observed to vary between 60-90% at the beginning of the outflow depending on ADD and then decreased steadily to settle at 0-20% removal after 30mins of outflow.

Here we conclude that removal of nitrate-nitrogen is insignificant during the wetting phase of an event and the process of denitrification is more pronounced during the drying phase of a rainfall event, accounting for removal of nitrate-nitrogen from stormwater that was retained in the filter layer following an event. The phenomena known as Birch Effect, results in breaking the complex-structured organic material into labile organic carbon that can be readily consumed by microorganisms and depletion of dissolved oxygen by aerobic microorganisms leading to development of a micro-anoxic environment in the filter layer, facilitate the process of denitrification. Therefore, intermittent wetting and drying is crucial for removal of nitrate-nitrogen in bioretention basins. In addition, a significant number of rainfall events are short and hence produce limited runoff. Hence, retention of runoff in bioretention filter layer accounts for a highly significant fraction of runoff from such events. Therefore, performance of bioretention basins for removing nitrate-nitrogen during the drying phase is instrumental where ADD and organic content of the filter layer play a crucial role.