The effect of vegetation in stormwater biofiltration systems: influences on infiltration rate and nutrient removal

Population growth and human activity have caused changes to the environment, such as climate change, deforestation and degradation of natural landscapes. The consequence is that it is difficult for the earth to support other life forms. As the primary cause of environmental destruction, it is our duty to address and mitigate environmental problems. WSUD aims to alleviate some of the environmental concerns, and in particular, reduce waterway degradation. This investigation sought to improve our understanding of WSUD technology, focusing on vegetative effects on biofiltration performance. Biofiltration systems (also known as biofilters, bioretention systems and rain gardens) are a vegetated basin or trench containing a sand-based filter media. Past research has demonstrated that biofilters are able to attenuate flows as well as reduce concentrations of various pollutants, including sediment, nitrogen, phosphorus and heavy metals. However, one aspect of biofilter design which has not been thoroughly investigated is vegetation selection and its influence on infiltration rates and nutrient removal. This research investigated the effect of plant selection on infiltration capacity and nutrient removal performance of biofiltration systems. A large laboratory column study that included 22 plant species that represented a wide range of plant types (lawn grasses, grasses, sedges, rushes and shrubs). The results emphasised the influence of seasonal effects on infiltration and pollutant removal capacities. The study also found that design elements can influence biofiltration performance. The inclusion or exclusion of submerged zones, presence or absence of vegetation and vegetation selection influenced results observed. Plant traits were more influential to infiltration rate and nitrogen removal than phosphorus removal. Plants with large root systems in the study tended to impede the flow of water, hence lowering the infiltration rate and lengthening the detention time in the columns. However, plants with larger and faster growing root systems were able to removal nitrogen more efficient than smaller and slower growing ones. While the infiltration rates and nutrient removal results appear to conflict, further investigation showed that other factors act to ensure that the nutrient outflow concentration is minimal during winter and spring seasons. In practice, the findings indicate that: 1. A submerged zone should always incorporated into biofiltration system 2. Plant type was generally not an indication of plant species performance (except for Lawn Grass). Poor-performing or best-performing plant species were often within the same plant type. 3. Plants that have large root systems are preferable to plants with smaller root systems of the same plant type. Plants with larger root systems are likely to provide better treatment as they are able to reach more areas in the soil matrix to directly uptake nutrients.