Batch Test Evaluation of Four Organic Substrates Suitable for Biological Groundwater Denitrification

Abstract

Nitrates pollution represents nowadays a serious issue related to the quality of groundwater; continuous growth of industrial-scale agricultures lead to an increase of nitrates content in groundwater in the last years. Several technologies have been validated as capable to promote in situ biological nitrates remediation, such as permeable reactive barriers (PRB), biotrench, biobarriers etc. These technologies are all characterised by the use of organic substrate that act as a slow release carbon source. In free dissolved oxygen absence, such organic carbon is further oxidised, by heterotrophic bacteria naturally present in soil, in compliance to anoxic metabolism by using nitrates bound oxygen. Such dissimilatory reaction converts nitrates in elemental nitrogen. Organic substrates capable to sustain this reaction are various and easily recoverable (e.g. sawdust, cotton, woodchips etc.); thus, several carbon source has been already tested. The present paper reports the results of batch test carried out on four organic substrates used to promote biological denitrification; in details the organic matters tested were: sawdust, pine bark, cork and olive pomace. The first step of experimental study was focused to evaluate the organic carbon release capability of each substrate; particularly, organic matter has been keep in contact with tap water for almost 10 days; thus, samples of water has been periodically collected to measure Total Organic Carbon (TOC). Further, microcosm batch test has been carried out to reproduce in situ biological groundwater denitrification. In details, each batch reactor was prepared with a mixture of organic matter and agricultural soil, used to provide heterotrophic bacteria capable to promote biological denitrification, kept in contact with tap water artificially spiked at 60 mg NO3-N L-1. Batch test were realized in slow agitation condition, by using a vibratory plate, and were carried out for 12 days. All organic matters tested provided good results, in terms of removal efficiency; further, specific denitrification rate has been computed and ranged from 0.06 (olive pomace) to 0.51 (sawdust) mg NO3-N L-1 d-1g-1sub. Column test are actually in progress to evaluate the biological denitrification in continuous.