Quantification of nitrification and denitrification rates in algae and duckweed based wastewater treatment systems

Nitrification and denitrification rates at three different depths (0.1, 0.45 and 0.9m from the water surface) in two series of four algae and duckweed based waste stabilisation ponds (ABPs and DBPs) were measured using nitrate reduction techniques in laboratory batch incubations. The effects of temperature and BOD5 loading were investigated. In situ measurements over the ponds' depths were also done for confirmation of laboratory results. Higher dissolved oxygen (DO) concentrations in ABPs, especially during the warm season, favoured higher nitrification in ABPs over DBPs. Organic surface loading also affected the rate of nitrification in the ponds. Nitrification rates did not increase along the treatment line despite the decrease in organic matter content. Adsorption of nitrifiers to available suspended particles and subsequent sedimentation was assumed to be the main reason for the similar nitrification rates in most ponds. In both systems, the presence of DO in the water column resulted in very low denitrification rates (5-45 mg-N m(-2)d(-1)). Higher denitrification rates (160-560 mg-N m(-2)d(-1)) were measured in the sediments when anoxic conditions prevailed in the overlaying water. The absence of nitrite or nitrate accumulation suggested sufficient nitrite and nitrate diffusion within the water column to allow full denitrification. The nitrification and denitrification rates in both systems were higher at high temperature. The range of nitrogen loss via denitrification in ABPs and DBPs corresponded to 15-25% of total influent nitrogen.