Feasibility of two low-cost organic substrates for unducing denitrification in artificial recharge ponds:batch and flow-through experiments.

Abstract

Anaerobic batch and flow-through experiments were performed to assess the capacity of two organic substrates to promote denitrification of nitrate-contaminated groundwater within managed artificial recharge systems (MAR) in arid or semi-arid regions. Denitrification in MAR systems can be achieved through artificial recharge ponds coupled with a permeable reactive barrier in the form of a reactive organic layer. In arid or semi-arid re- gions, short-term efficient organic substrates are required due to the short recharge periods. We examined the effectiveness of two low-cost, easily available and easily handled organic substrates, commercial plant-based compost and crushed palm tree leaves, to determine the feasibility of using them in these systems. Chemical and multi-isotopic monitoring (δ15NNO3, δ18ONO3, δ34SSO4, δ18OSO4) of the laboratory experiments confirmed that both organic substrates induced denitrification. Complete nitrate removal was achieved in all the experi- ments with a slight transient nitrite accumulation. In the flow-through experiments, ammonium release was ob- served at the beginning of both experiments and lasted longer for the experiment with palm tree leaves. Isotopic characterisation of the released ammonium suggested ammonium leaching from both organic substrates at the beginning of the experiments and pointed to ammonium production by DNRA for the palm tree leaves experi- ment, which would only account for a maximum of 15% of the nitrate attenuation. Sulphate reduction was achieved in both column experiments. The amount of organic carbon consumed during denitrification and sul- phate reduction was 0.8¿ of the total organic carbon present in commercial compost and 4.4% for the palm tree leaves. The N and O isotopic fractionation values obtained (εN and εO) were −10.4¿ and −9.0¿ for the commercial compost (combining data from both batch and column experiments), and −9.9¿ and −8.6¿ for the palm tree column, respectively. Both materials showed a satisfactory capacity for denitrification, but the palm tree leaves gave a higher denitrification rate and yield (amount of nitrate consumed per amount of available C) than commercial compost.