Substrate and filter materials to enhance phosphorus removal in constructed wetlands treating diffuse farm runoff: a review

Constructed and restored wetlands have significant potential to reduce nutrient losses in drainage waters from New Zealand farms. While both types of wetland show reasonably good nitrogen (N) removal efficiencies, they are not always so effective at phosphorus (P) removal and their flooded topsoils can be net sources of P. Wetland P-removal efficiency could be enhanced, either by adding a P-retentive amendment to the soil in the bottom of the wetland or installing a porous filter with a high P adsorbency and retention capacity at the end of the wetland. This review was carried out to evaluate a range of materials reported in the scientific literature as having the ability to remove P from water. Materials reviewed include: naturally occurring materials, such as soils, sands, clays and aggregates; processed and modified materials; and waste materials. The reported performance of the materials reviewed varied widely. A simple scoring system based on P-removal characteristics, availability, likely cost and potential reuse or disposal on saturation was used to identify the materials with most promise as soil amendments or filters for constructed wetlands. Allophane, Papakai tephra, limestone and alum were judged as materials with the most potential as soil amendments, while limestone, slag, seashells, shell-sand and tree bark had most potential as filter materials. Another possible approach is to use subsoil or a mix of subsoil and topsoil as the growth media in the base of the wetland to avoid P release on flooding of P-rich agricultural topsoils.