A Comparison of Greywater Irrigation and Hydraulic Loading Rate on Growth and Treatment Performance With Various Ornamental Plants

Qatar, due to its arid climate, high water demand, and limited freshwater resources, faces significant challenges with water scarcity. Almost all household water is provided by desalination, which has high economic and environmental costs. To combat this issue, recycling wastewater through separation and treatment of the greywater portion for irrigation can provide a solution to conserve precious freshwater resources. Recent interest has developed in the use of green walls as a nature-based treatment system that fits within the urban environment. Green walls are a sustainable building technology, that simultaneously offer a natural and appealing method to enhance air quality, mitigate ambient temperature, promote biodiversity, and improve human well-being. Additionally, they contribute to reducing carbon emissions and enhancing the aesthetic and livability of buildings and dense regions.

The study aimed to compare the plant growth and greywater treatment performance for five ornamental plant species grown in a substrate consisting of recycled spent coffee grounds and date seeds that could be used in a typical green wall treatment system. The use of waste materials as growing media promotes waste reduction through circular economy. Specifically, the study investigates the differences in plant growth and development between greywater irrigation and freshwater irrigation (both at 200 mL/d), as well as the effects of greywater hydraulic loading (at 200 and 400mL/d).

A synthetic greywater made from household personal care and cleaning products was applied in the pot test. When averaged across all plant types under the 200 mL/d greywater irrigation regime, its applicated resulted in increases of 27.3±11.7 cm in plant height, 6.3±2.1 in number of pinnula, 55.3±6.7 in number of leaves (excluding Portulaca grandifolia), 13.06±8.83 g shoot dry matter weight, 115.1±10.9 cm root length, and 70±4% and 74±3% in moisture content of the root and shoot system as compared to the same hydraulic loading with freshwater irrigation. The same greywater condition removed 46% organic carbon, 87% TN, 98% NH4+-N, 35% PO₄²--P, and 88% isobutyric acids averaged across the tested plants. Cyperius alternifolius showed maximum pollutant removal performance achieving 70% TOC, 98% NH₄⁺-N, 44% PO₄^2--P, and 84% isobutyric acid removal, while Portulaca grandifolia had the best TN removal performance (93%). The higher greywater irrigation volume led to a reduced TOC and PO₄^3- removal but showed no substantial difference in the removal of TN, NH₄⁺-N, and isobutyric acid. The study shows that Alternanthera amoena have the greatest height, and foliage values, while Cyperus alternifolius had maximum biomass yield in root length, root and shoot dry matter weight, making them promising plant selections to flourish in the system. The system suffered a decrease in chlorophyl content across all irrigation conditions except for Pennisetum rubrum which had improved foliage chlorophyll content (3.13±1.49 spad units) at 200 mL/d greywater irrigation. Over the final three weeks of the study, odor assessments were conducted, focusing on odor-emitting compounds such as ammonia and isobutyric acid. The effluent's ammonia concentration was notably low, as evidenced by the absence of its characteristic pungent odor across all irrigation and hydraulic loading conditions. However, a significant leaching of isobutyric acid (exceeding 70 mg/L) under the freshwater loading rate led to an unpleasant odor being detected. Different plants had different leaching values of iso-butyric with Pennisetum rubrum contributing mostly to the leaching concentration. In contrast to this, greywater applications at 200 mL/d and 400 mL/d rates did not result in any noticeable odor emissions from the collected effluents throughout the study.

These findings suggest that greywater irrigation is suitable for green walls, promoting an integrated climate and water solution. Appropriate hydraulic loading is necessary for achieving targeted effluent concentrations, and plant selection is an important design choice for optimal performance, with Althernanthera amoena, Cyperus alternifolius, and Portulaca grandifolia showing great promise.