THE EFFECT OF VARIOUS INTERMITTENT DRY-WET CYCLES ON NITROGEN REMOVAL CAPACITY IN BIOFILTERS SYSTEMS

Elevated levels of nutrients in urban runoff are detrimental to urban waterways and bays. Total Nitrogen (TN) in stormwater may contain up to 91% dissolved nitrogen, often dominated by nitrate (up to 47%) which is highly harmful to aquatic ecosystems [Taylor, et al., 2005]. Biofilters, also known as bioretention systems, are promising technology that may be able to remove nutrients from stormwater [Lloyd, et al., 2001; Wong, 2006]. Some studies suggested that the role of plants in nitrogen removal is crucial. For example, Henderson et al’s [2005] study of vegetated biofiltration mesocosms demonstrated 67 and 52% higher TN removal by vegetated mesocosms in sand and loam (respectively) than in unvegetated biofilters. Some researchers suggested that there is a need to enhance anaerobic processes (i.e. denitrification) in biofilters in order to boost TN removal [Davis, et al., 2001; Gerardi, 2003; Hunho, et al., 2003]. This was confirmed by Hunho et al (2003) and recently by Zinger et al (2007), who found that creation of a submerged anoxic zone (SAZ) (that contains a carbon source) at the bottom of biofilters could substantially improve NOx removal. Biofilters, like all other stormwater treatment facilities, are subjected to a high level of variability in the inundation frequency and the length of intervening dry periods. However, evaluation of their performance to date, has focused on treatment of actual storm events with little consideration given to the effects of cyclic rewetting/drying periods on their behaviour. 

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