Abstract:
Urbanisation has directly impacted the hydrosphere and water resources, causing degradation of water quality and water flow into streams. Pollutants such as heavy metals, sediments and polyaromatic hydrocarbons (PAHs) are key contaminants of urban stormwater, affecting water quality and the aquatic ecosystem. Traditional technologies focussing on contaminant removals, such as ion exchange and reverse osmosis, are either too complicated or costly. More recent approaches involving low-impact designs (LIDs), such as green roofs and conventional raingardens, have shown promise; however, high installation costs have prevented their more comprehensive implementation.
This study proposes suspended raingardens as a plausible, viable option for sustainable stormwater management in urban settings. Suspended raingardens are an engineered system, which includes natural processes that can assist in the removal of stormwater pollutants without using urban space. These systems can be installed as lightweight filtration units that can be suspended under bridges and on the top of existing structures. This study tested three different novel, lightweight media mixes composed of different proportions of compost, coconut coir (CC), activated carbon (AC), vermiculite, perlite and zeolite against compost (only) control.
The current study focused on identifying microbial communities in the above-mentioned three lightweight media mixes intended for suspended raingardens, with a specific focus on managing PAHs, nutrients, solids and metals in stormwater. Naphthalene, phenanthrene and pyrene were introduced to the media mixes, which were regularly dosed with synthetic stormwater (containing nutrients – phosphate and nitrate) for 9 weeks. Media and effluent samples were analysed for PAHs, nutrients and solids. Polycyclic aromatic hydrocarbon (PAH)-degrading bacteria and fungi were assessed by culture and 16 S rRNA gene and ITS2 region sequencing. The degradation of phenanthrene in the presence of a combination of heavy metals (copper [Cu], zinc [Zn] and lead [Pb]) dosed through synthetic stormwater was also monitored to determine the effect of heavy metals on phenanthrene metabolism.
Results suggest that all three media mixes performed better than compost in removing stormwater pollutants. The maximum reduction of total dissolved solids and nitrates was 68% and 65 %, respectively, compared to 36% solids removal and nitrate leaching achieved with the compost control. The mixes' non-soil components showed low water-holding capacity, with
better microbial activity, whereas the media dominated by compost yielded higher water-holding capacity. PAHs introduced into the media mixes were significantly degraded within 68 days. The percentage of phenanthrene degradation ranged from 91% – 98% and was >95% for pyrene. Degradation of naphthalene for media 2 (compost, CC, AC, zeolite) and 3 (compost, CC, AC, zeolite, perlite, vermiculite) ranged from 89% – 95%. Media 1 (compost, CC, perlite, vermiculite) showed complete naphthalene degradation within 30 days and naphthalene formation as a by-product by 68 days. Plate counts showed high but variable microbial activity between different media. Sequencing demonstrated that media mixes support the growth of potential hydrocarbon (HC)-degrading bacterial and fungal communities, which showed negative correlations to HC levels in the media. These results support our hypothesis that microbial activity has a role in PAH degradation in suspended raingarden media. Furthermore, phenanthrene degradation in media 3 in the presence of Cu, Zn and Pb introduced through stormwater suggest that Cu alone positively affects PAH degradation. At the same time, Pb and its combinations (CuPb, ZnPb) inhibit phenanthrene degradation. Zn alone or in combination (CuZnPb, CuZn) had little to no effect on phenanthrene degradation.
This study also analysed the importance of AC, a media mix component, as a site for potential microbial degradation of phenanthrene and regeneration of further sites for binding. We analysed AC test media mixes with a sterile AC control. The results showed complete degradation of phenanthrene in the tests compared to only 40% removal in the sterile AC media. The AC test media also performed better than media 3. Scanning electron microscopy, adsorption kinetics and adsorption isotherms further supported our hypothesis that AC can be a potential microbial degradation site for phenanthrene, and microbial population on the surface of AC may help regenerate adsorption sites. In summary, this study offered an insight into PAHs’ biodegradation potentials of lightweight media mixes suitable for use in suspended raingardens.