Dual-mode stormwater-greywater biofilters: The impact of alternating water sources on treatment performance

The intermittent nature of stormwater runoff impacts the treatment performance of biofilters, also known as stormwater biofiltration or bioretention systems and raingardens. During extended dry periods, which are common even in temperate climates, plants can perish, creating unattractive and non-functional systems that might leach pollutants during the next rainfall event. The current solution is to irrigate during long dry spells, which is costly and unsustainable as biofilters become more widespread. This paper presents the development of dual-mode biofilters, where stormwater and greywater are treated within the same system. Fifty columns, utilising eight plant species, including understory and climbing ornamentals, and designs with and without a carbon source in the submerged zone, were subjected to alternating greywater and stormwater inflows over five months. Six sampling events investigated treatment performance across these switching inflows and an extended dry period (atypical event). Good removal of total suspended solids (>83%), biochemical oxygen demand (>86%) and some heavy metals (e.g. lead >96%) were reported irrespective of design. Plant species selection was critical for the removal of nitrogen (2 to 79%) and phosphorus (12 to 75%) under dual-mode operation. However, following the extended dry period, plants with the lowest nutrient outflow concentrations also experienced some of the highest sediment and carbon concentrations, suggesting that a mixture of plant species may be beneficial for withstanding abnormal conditions. Differences between the treatment performance of designs with and without a carbon source were negligible, with potential benefits possibly negated due to the increased root mass that comes with age (systems were approximately two years old) and the release of carbon from root exudates. The results demonstrate the potential for dual-mode stormwater-greywater biofilters as an alternative to single-mode systems as they can provide effective treatment, along with greater volumes of treated water, while maintaining system performance throughout the year.