Optimizing Phytoremediation to Enhance Treatment Efficiency of Suspended Raingardens

Abstract

The current research focused on the performance of lightweight soilless media, suitable for use in suspended raingardens, that support satisfactory growth of the plant’s root and shoot systems while also reducing the concentration of typical stormwater pollutants such as suspended solids, nutrients, heavy metals, and emerging contaminants. To achieve the objective, the present study was conducted in two stages. The first stage involved a laboratory-scale unplanted column study to investigate nutrient leaching occurrence in several soilless lightweight media mixes. The second stage pilot-scale phytoremediation study in a suitable lightweight media mix was carried out to investigate the effectiveness of treating semi-natural stormwater using biocolumns. The 20-week greenhouse experiment incorporated twenty-four columns; half of which were unplanted controls, and half planted with the compact, fine-leaved Australian grass, Lomandra ‘Evergreen Baby’™ (dwarf lomandra). The pollutant-removal efficiency of columns with a saturated zone (SZ) or non-saturated zone (NSZ) was tested for five different lightweight media bases: perlite, vermiculite, activated carbon, zeolite, and coconut coir. All columns were added with compost as a source of organic matter. Every column had a media depth of 300 mm, while SZ columns had a bottom depth of 150 mm as saturated. Nutrient leaching test on individual media revealed compost is the main source of leaching for NO₃-, PO₄³⁻, and heavy metals (Zn, Cu, and Pb). Other than compost, activated carbon and coco coir also leached PO₄³⁻. In the unplanted column study, layered media was statistically significant better (p < 0.05) than mixed media configuration in reducing nutrient and heavy metal leaching. Though layered media performed better than mixed media, due to limitations to maintaining such media in real-world conditions, two best-selected media mixes (MM1 and MM2) from the mixed media configuration were chosen for the stormwater dosing study. The composition of MM1 was 50% (v/v) coco coir, 10% compost, 20% zeolite and 20% activated carbon. MM2 also has the same percentage of coco coir and compost as MM1 but 10% each for zeolite, activated carbon, perlite, and vermiculite. Both media mixes, MM1 and MM2, were equally efficient and achieved satisfactory removal in total suspended solids (TSS) (75 – 85%), excellent removal in nutrients (> 96%) and Pb (97%) but average removal in Zn (40 - 50%) and low removal in Cu (< 20%) at the end of the experiments. However, a high degree of PO₄³⁻ leaching was observed in MM1 and MM2 media mixes due to the residual amounts present in some of the soilless media components as mentioned above. Phosphate leaching decreased over time. In the phytoremediation study, effluent performance analysis revealed that saturated zone (SZ) columns performed better in the removal of TSS, NH3-N, NO2-N, NO₃-, N, TN, Zn, Cu, and Pb than non-saturated zone (NSZ) columns. Both NSZ and SZ columns experienced leaching of total nitrogen (TN), total phosphorus (TP), and PO4-P, with the leaching of PO4-P and TP being prevalent in SZ columns than NSZ columns. The presence of plants did not significantly improve the removal efficiency of TSS, NH3-N, TN, Zn, Cu and Pb but improved removal of NO₃-N was observed. Loss of N via effluent from unplanted columns was higher than planted columns. Mass balance analysis showed that increment of N mass distribution in the plant was mainly due to root growth. Total N mass in the media for SZ columns was lower than NSZ columns due to denitrification in the saturated zone. The distribution of P mass was found to be higher in the media than in plant and effluent. Plants reduced the P leaching into effluent by uptaking P for root development than actively transferring it to the shoot. The presence of a saturated zone, while is beneficial for N removal, caused higher P loss from the media into the effluent. Retention of Zn, Cu and especially Pb for all columns mostly occurred in the media. Plant assists in Zn, Cu, and Pb uptake via root with planted SZ columns performed better than NSZ columns. Hence, Zn and Cu loss in the effluent from SZ columns were reduced. Preference in heavy metal uptake via root by dwarf lomandra follows the order: Zn > Cu > Pb. Accumulation of Zn, Cu and Pb in plants’ shoot did not differ with study duration. Retention efficiency for heavy metals in the media improved with the saturated zone. Removal of ibuprofen and bisphenol A (BPA) in all columns were excellent with most effluent samples showing ibuprofen below detection limit and bisphenol A below 0.4 ppm. Media was speculated to be the primary retention. Further removal of ibuprofen could occur in the saturated zone which provides sufficient hydraulic retention time for its biodegradation. Overall, the inclusion of planted biocolumns, modified with saturated zone and optimised lightweight media mix has the potential to significantly increase pollutant removal in terms of TSS, nitrogenous compounds, heavy metals, and emerging contaminants for effective stormwater runoff treatment.