Seasonal operation of dual-mode biofilters: The influence of plant species on stormwater and greywater treatment
Graphical abstract
Introduction
With effective design and careful construction, stormwater biofilters (also known as bioretention systems or raingardens) are a desirable infrastructure for urban areas. While their primary purpose is to treat stormwater, they can also attenuate floods (Batalini de Macedo et al., 2019; Mei et al., 2018), are scalable, passive, low maintenance, provide greening of urban areas, increase biodiversity (Kazemi et al., 2009) and decrease the urban heat island effect (Coutts et al., 2013). However, extended dry periods caused by the irregular characteristic of stormwater diminish the treatment reliability of biofilters, with vegetation and microbe die-off (McDowell et al., 2008) and contaminants released from the systems when stormwater inflows resume (Baldwin and Mitchell, 2000; Barron et al., 2019; Hatt et al., 2007). Biofilters can also be perceived as ugly, due in part to a lack of diversity in vegetation, which can reduce community acceptance of them in the urban landscape (Dobbie, 2016; Funai and Kupec, 2017; Suppakittpaisarn et al., 2019). Unfortunately, these negatives seem to be hampering the adoption of biofilters as legitimate urban water infrastructure.
To maintain system health and treatment performance, alternative water sources to stormwater are required during dry conditions. This is particularly important in tropical and semi-arid regions that have seasonal rainfall patterns, such as the Mediterranean coast (e.g. Spain, Israel), parts of California, Western Australia and the Arabian Peninsula. The successful use of groundwater for this purpose has previously been reported in Israel (Aloni and Brenner, 2017). However, a stormwater-groundwater biofilter is not possible in all situations (e.g. deep aquifers, steep topography). In this regard, light greywater (greywater), which includes wastewater from showers, baths and bathroom sinks (Friedler and Hadari, 2006), could be exploited as it is relatively clean compared to other wastewaters (Eriksson et al., 2002) and is an underutilised and abundant water source in urban areas (Grant et al., 2012). Recent work has reported the successful treatment of greywater using biofiltration technology (Chowdhury, 2015; Fowdar et al., 2017) in a single-mode operation (i.e. using only greywater inflows), highlighting the potential to direct greywater to biofiltration systems when it is not raining. Not only would this sustain biofilter health, it would also maximise the treatment capacity of the system, with greater volumes of water being treated. In turn this would reduce the volume of polluted water requiring conveyance to water treatment plants and provide a fit-for-purpose water source.
While dual-mode systems for combining rainfall/stormwater and greywater are available, they have predominantly used different technology to biofiltration (Leong et al., 2018; Rozos et al., 2013; Sapkota et al., 2015; Stratigea and Makropoulos, 2015). Stormwater and greywater differ significantly in terms of concentrations, volume and frequency of generation (Duncan, 1999; Eriksson et al., 2002) and it is has been hypothesised that dual-mode biofilters may not be able to provide effective treatment due to these stark differences (Barron et al., 2019). In their initial work on dual-mode biofilters, Barron et al. (2019) utilised a mixed-mode operation, where stormwater and greywater inflows were alternated almost daily, and noted significant fluctuations in outflow concentrations across sampling events. The regular switching between water sources seemed to impact the system's ability to provide treatment to a consistent level. Treatment performance could potentially be improved utilising a different operational mode. Perhaps water sources could be swapped less regularly, following seasonal rainfall patterns that are very common (and predictable) in semi-arid regions (e.g. we can expect no rainfall for months in Israel, Southern Spain, and Western Australia). Hence, we propose a seasonal operation of dual-mode biofilters, where stormwater would be treated in wet periods, and greywater in dry periods.
While seasonal operation of dual-mode biofilters may provide more consistent outflow concentrations, the physical design of the system may impact its treatment performance. The effectiveness of biofilters centres on the interactions between surface area, filter media, vegetation and micro-organisms (Payne et al., 2015), and have been optimised for single water source inflows. Successful design of these elements allows biofilters to meet their treatment objectives, however, these elements also present a number of challenges. For instance, while the use of low nutrient media, usually with high percentages of sands, aids both hydraulic and treatment performance, it also limits the diversity of vegetation that can survive in raingardens (Funai and Kupec, 2017). This combined with the intermittent nature of stormwater inflows, sometimes with extended dry periods, has seen the widespread use of a limited number of effective plant species. While the inclusion of a saturated zone (SZ) in more recent designs has minimised the impacts of extended dry periods on plant survival and treatment performance, the dearth in variety of plant species has remained. Therefore, as part of this study we investigated the ability of a range of ornamental plants and vines to treat stormwater and greywater. The optimal characteristics for raingarden vegetation are well understood; high growth rates and dense root systems (Payne et al., 2018; Read et al., 2010). However, the ability of plants to thrive in a dual-mode stormwater-greywater biofilter under seasonal operation and provide effective treatment of both water sources is unknown. We hypothesise that plant species selection may become even more vital under seasonal operation, with some plants effective at treating both water sources, some preferencing a particular water source and others providing no advantage over barren filter media.
The importance of including a carbon source in the SZ of dual-mode stormwater-greywater biofilters also requires investigation. Kim et al. (2003) suggested that a carbon source promotes biological denitrification of stormwater while Fowdar et al. (2015) hypothesised that greywater has sufficient biodegradable organics that a carbon source may not be required for greywater biofilters. Barron et al. (2019) reported minimal practical differences in treatment performance between biofilters with and without a carbon source when employing a mixed operational mode. In contrast, a carbon source may be necessary for dual-mode biofilters under seasonal operation to ensure adequate treatment of stormwater inflows by promoting denitrification.
To optimise biofiltration systems for regions that have distinct wet and dry periods, we investigated how plant species selection influenced treatment efficiency under seasonal operation, mimicking a real-life situation where stormwater would be treated in wet months and greywater in dry months. Research questions for this project were:
- (1)
What plant species provide effective removal of key contaminants for biofilters under seasonal operation?
- (2)
How does switching between stormwater and greywater impact treatment performance?
- (3)
How does the inclusion of a carbon source in the SZ influence treatment performance?
This experiment forms part of a larger study on dual-mode stormwater-greywater biofilters, which included testing of mixed-mode operation (see Barron et al., 2019). To the best of the authors' knowledge, this is the first biofiltration study to test the same plant species under separate stormwater and greywater inflows.
Section snippets
Experimental design
The experiment was conducted in an open-air greenhouse consisting of a transparent roof and mesh walls. This allowed water inflows to be controlled, but not other climatic aspects. Forty-five 240 mm diameter PVC columns with consistent media type and depth were used (see Fig. 1). All the columns contained a 440 mm SZ. Thirty-five of the columns were planted with a single plant, utilising seven plant species (see Table 1), including a mixture of understory and climbing ornamentals, along with
Infiltration rates across the experiment
Most designs experienced significant (FMT, p < 0.05) changes in infiltration rate across the experiment. As shown in Fig. 3, there was a decrease in median infiltration rate through the middle of the experiment, across all planted designs, with the slowest rates recorded in winter and early spring. At the beginning of the experiment, infiltration rates ranged from 76 ± 93 mm/h (non-vegetated) to 392 ± 28 mm/h (P. tenax). By the sixth measurement (13 Sep), values had dropped to between
Conclusion
This paper focused on the treatment performance of seven plant species, for their ability to remove key contaminants from stormwater and greywater within a dual-mode biofilter. The results suggest that sediment, phosphorus, nitrogen, carbon, heavy metals and BOD5, can be successfully removed from both water sources utilising a seasonal operation for biofilters. However, under greywater inflows, outflow phosphorus concentrations did increase, potentially an artefact of phosphorus saturation of
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was undertaken with an Australian Government Research Training Program (RTP) Scholarship. We appreciate the financial support of the Cooperative Research Centre for Water Sensitive Cities, Australia (C4.1) to undertake this work as part of the Graduate Research Interdisciplinary Program for Water and Sustainability in Asia at Monash University. A special thank you to Christelle Schang, Harsha Fowdar, Emily Payne, Perran Cook, Amirhossein Mohammadizadeh, Richard Williamson, Matthew
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