Greywater treatment by ornamental plants and media for an integrated green wall system

Highlights

• Six media and six ornamental plants tested for green wall-greywater treatment.

• Removal of organics, solids and nutrients improved with time exceeding ninety percent.

• Microbial biodegradation was a key process and hydrophobic organics were removed.

• Media type was much more influential than plant type on pollutant removal.

• Nitrification is dependent on the presence of plant roots.

Abstract

Greywater recycling through natural vegetated systems provides a low cost and sustainable approach to manage urban water scarcity but is space intensive. This study aims to treat greywater naturally using ornamental plant species that can be integrated vertically into green building structures and elucidate the main removal mechanisms occurring therein. To this end, various regional ornamental plants and growing media were tested in column experiments, with a bi-daily application-rest cycle of 500 mL. d⁻¹ per plant. Removal percentages greater than 90% were achieved for all contaminants monitored (organics, solids, nitrogen and phosphorus) when using high surface area, small-diameter media such as coco coir, spent coffee grounds and sand. Media selection was found to be a much more dominant factor than plant selection for treatment performance. Using media replacement and plant removal studies, it was found that nitrification was linked with the presence of plant roots in the media. The removal rates of hydrophilic and hydrophobic organics were also analyzed and effluent specific UV absorbance measurements demonstrated hydrophobic compounds, which are typically more toxic in nature, were successfully removed from the system. This study demonstrates the complex interactions between the media, microbes, plant roots and greywater in natural treatment systems and their potential as an innovative approach for greywater treatment in the urban built environment.

Introduction

Rapid urbanization has occurred over the past century, with more than half the global population living in urban areas today (Desa, 2014). Urbanization has significant impacts on the environment and in particular on the hydrologic cycle, characterized by the reduction in vegetation coverage, increase in local atmospheric temperature via heat island effect, reduction in groundwater recharge through the formation of increased impervious surfaces, increase in water pollution via runoff, and increased pressure on local water resources to meet increasing population demand (Gong et al., 2012). The use of high-quality potable water in urban areas for various non-drinking purposes, including irrigation, toilet flushing, vehicle washing and fire-fighting puts additional and unnecessary demand on local water resources. For such low-quality water uses, treated greywater is a promising and sustainable water supply source, which typically accounts for approximately two thirds of household wastewater flow (Masi et al., 2016). Treatment systems for greywater vary widely, ranging from nature-based systems, such as constructed wetlands, to advanced package-plant systems such as membrane bioreactors (Ghunmi et al., 2011). For low-quality uses that are more generally acceptable to the public, low-energy nature-based technologies are favorable owing to their low cost, low technical requirement and low energy usage. Such systems provide additional hydrological benefits such as increased urban greening, mitigation of heat island effect, and stormwater detention and pollution control (Nolde, 2000). However, most nature-based treatment systems are land-intensive. A recent area of development is in the integration of greywater treatment with vegetated building walls (green walls, living walls and green facades) that can be applied in high-density urban areas (Fowdar et al., 2017; Masi et al., 2016; Musy et al., 2017; Prodanovic et al., 2019).

A few recent studies have demonstrated reasonable treatment with these types of systems at pilot scale using green facades (Fowdar et al., 2017), horizontal-flow green wall systems (Gattringer et al., 2016) and vertical-flow green walls (Masi et al., 2016; Prodanovic et al., 2019), though the primary focus of the studies was on removal performance rather than removal mechanisms. Prodanovic et al. (2019) assessed the removal performance of media and showed that fine media provided improved removal performance; however, because of clogging, a blend of coir and perlite was recommended. In their most recent study, they evaluated the performance of various ornamental plants for pollutant removal and concluded that nitrate uptake and phosphorus uptake by plants was important, with the total nitrogen (TN) removal being 7–10% higher in planted systems than in media-only systems (Prodanovic et al., 2019). Different plants showed different total phosphorus (TP) removal performance, and while TP removal improved with time, it was relatively low for all plants tested (no more than 53%). However, it is necessary to study the mechanisms involved in the removal process to determine the roles of media, microbe biodegradation and plant uptake. This is particularly true for the removal of organics, as this contaminant class contains a number of xenobiotic priority substances associated with personal care products and other categories. Gattringer et al. (2016) reported excellent removal of a number of these specific compounds; however, the mechanisms involved in the removal process were not explained, and the removal performance reported included the contribution of an aerated pretreatment system.

The objective of this study was to evaluate the removal of both nutrients and organic compounds in a planted green wall system and identify the main removal processes. Moreover, the study was aimed at evaluating a number of alternative ornamental species and media from those studied by Prodanovic et al. (2019) and understand the changes in treatment performance with time through focused laboratory-scale planted-pot studies.