Sustainable treatment technologies using mixed waste media to mitigate agricultural contaminants in land drainage

Abstract

Intensification of agriculture in the European Union has resulted in nutrient losses from farms, which have contributed to a deterioration in water quality. As soil in drainage water ditch networks has limited capacity to attenuate nutrients leaving farms, efforts to reduce nutrient loads have been unsuccessful. Therefore, innovative solutions and experimental approaches are needed to intercept nutrients in ditches before final discharge to receiving waters. The existing ditch networks on farms may offer an opportunity for implementation of nutrient attenuation measures, by combining the natural attenuation capacity of the ditch with in-ditch engineered structures containing media capable of adsorbing nutrients. Although these structures have gained in popularity as a mitigation option, their configuration or optimal placement in the landscape has not yet been fully considered. The selection of appropriate media depends on the type of nutrient losses, the nutrient loads, media adsorption capacity and lifetime. In addition, the identification of an optimal location for the placement of in-ditch engineered structures is crucial for successful implementation as such structures are capable of only removing a proportion of nutrient loads exiting the farm, so the natural attenuation capacity of the ditch is important to further reduce the load.

This thesis proposes two innovative mitigation techniques to remove both nitrogen (N) and phosphorus (P) in an agricultural drainage system: an in-ditch engineered system filled with reactive media and a natural solution which utilises soil chemistry of the ditch network for nutrient removal.

In order to develop the first technique, a novel, internationally applicable decision support tool (DST) was developed to select locally sourced media for single or dual mitigation of N and P. The developed DST was validated in several case studies and it was then used to select an optimal combination of media for the removal of ammonium (NH4+) and P in water draining from an intensive dairy farm in south-east Ireland. Normally, large-scale column tests need to be conducted to develop design criteria for engineered structures, but as these are time consuming and expensive, rapid small-scale column tests (RSSCTs) were used, for the first time, to assess the media performance and longevity in simultaneous N and P removal in comparison with large-scale columns. The adsorption capacity and lifetime of the selected media in large- and small-scale column studies were consistent and the generated data using RSSCTs were successfully used to model P and N removals in the large-scale filters. This indicated that RSSCTs may be used to accurately and quickly develop design criteria for in-ditch engineered structures. In the second technique, the natural P remediation capacity of the ditch network of the study site was investigated with a view to identifying the optimum location for the placement of an engineered structure and to examine the capacity of a ditch in retaining or mobilising P. Experimental analyses indicated that the ideal location for installation of an in-ditch structure was at the point where a sharp increase in nutrient concentration was observed, which was due to discharges from the farm yard. The results also showed that P inputs into the drainage network accumulated in the sediments and bankside over time. This not only contributed to degradation of water quality leaving this farm, but the stored nutrients in the ditch network, as a result of decades of application, had also changed the chemistry of sediments to act as a secondary source of P, adding to the already polluted water.

Arising from the findings of this thesis, in order to limit nutrient losses from intensive farms into drainage waters, implementation of an enhanced remediation technique is essential where natural attenuation is insufficient to eliminate pollution. An efficient mitigation measure starts with characterisation of the type of nutrient losses and then the development of appropriate in-ditch engineered structures filled with media to remove the identified nutrients. However, cognisance must also be taken of potential pollution swapping as a result of using the media, appropriate structure dimension and optimal location, and the nutrient remediation or immobilisation capacity of the ditches. This thesis provides a design framework that will contribute to sustainable, environmentally friendly farm management.