The performance of plant species in removing nutrients from stormwater in biofiltration systems in Cape Town

Abstract

In 2009, the City of Cape Town (CoCT) adopted a stormwater policy which mandates that new and existing developments should reduce the concentration of phosphorus and suspended solids in stormwater runoff by 45% and 80%, respectively, but offered no explicit guidance about how these water quality targets might be achieved. This study aims to contribute to the limited knowledge that exists about the performance of local plant species to treat stormwater. A large nursery-based study was conducted to investigate the performance of 9 locally-occurring plant species to remove orthophosphate ), ammonia (NH 3 ) and nitrate (NO 3 -) found in urban stormwater. Synthetic stormwater was applied to each species as well as a control consisting only of soil (Malmesbury shale). The discharge was collected from a drainage pipe at the base of each of the 150 containers. The results show that all species (excluding Ficinia) reduced the average concentrations of PO 4 -3 by 81% and NH 3 by 90%. By contrast, NO 3 -was reduced by an average of 69% (excluding by Elegia and Phragmites) with 8 of the 9 species removing significantly more than the control. The species that performed well for all three nutrients include Agapanthus and turf grasses, Stenotaphrum and Pennisetum. The results of the study highlight three important factors in the design of biofilters: that a substantial proportion of nutrients can be captured or absorbed by plants; that the soil medium is an important factor in the removal of PO 4 -3 and NH 3 ; and that plant choice is essential in the removal of NO 3 -. Future research should test plant species in both the laboratory and field settings, and should include additional contaminants such as household detergents, heavy metals and bacteria.

The potential use of plant species within a Renosterveld landscape for the phytoremediation of glyphosate and fertiliser

“…The removal efficiency ranged between 65 and 96% (average removal 87%), whilst the soil control removed 59%, on average. In contrast to the findings of previous research (Bratieres et al, 2008;Read et al, 2008;Milandri et al, 2012), this study showed far greater NO 3 -N removal. These contrasting findings may be the result of the specific plant species under study, micro-organisms, the establishment of bacterial assemblages in the rhizosphere, and the drip irrigation applied as opposed to rapid high volume irrigation (Bratieres et al, 2008;Read et al, 2008).…”

“…The IAP species were selected for their current use in constructed wetlands, wastewater treatment facilities, SuDS and biofiltration treatment trains. These species (Table 2) have proven excellent remediators of polluted water, and are commonly used internationally (Schachtschneider et al, 2010;Milandri et al, 2012).…”

Effects of Substrate Material on Plant Growth and Nutrient Loss

“…It was also demonstrated that higher content of water and fertilizer does not benefit plant growth. Furthermore, plant type is an important factor that affects nutrient runoff, absorption of nutrients, permeability and porosity of the substrate material, and the effects of plant roots on the soil microbial community [44]. In the second year, plant biomass of A. fruticosa becomes relatively stable, the plant's demand for nutrients is reduced, and the major elements accumulated in the substrate are gradually released in the runoff [45].…”

Green Infrastructures in Stormwater Control and Treatment Strategies

“…Studies in China [33] showed that bioretention systems led to reductions in water volume and peak flow rates of 59-68% and 72-86%, respectively. Additionally, there is indication [34] that some plant species (mostly grasses) can reduce average concentrations of phosphates by 81%, ammonia by 90% and nitrates by an average of 69%. Most phosphate and ammonia treatment occurred within the soil medium.…”

“…For the case of TP, the removal performance is effective in both laboratory and site studies, with a removal rate of 65 to 97%. For TN, removal performance rates present too wide ranges, from 1% (Hsieh and Davis, 2005) to 99% (Milandri et al 2012). Investigations [36] indicated that high nutrient and metals removal rates can be achieved over a range of hydraulic conductivities using designed mixes of recycled organic and mineral materials.…”