Greywater irrigation and soil quality: An assessment framework adjustment and application

The on-site use of greywater is increasingly popular for alleviating water stress in various parts of the world, particularly as a water source for irrigation. However, greywater can contain a range of pathogenic bacteria that may compromise public health as well as substances with the potential to induce environmental consequences, such as soil hydrophobicity, accumulation of salts, and damage to plants. While the health issues are being addressed by greywater legislation, its environmental risks are largely ignored. Therefore, the main objective of the current study was to quantify the impacts of greywater irrigation on soils by developing a soil quality index (SQI) using a 14-month planter experiment. The sum of the absolute value of all indicator scores represents the final score of the integrated SQI, which ranges from 0 to 100. Three threshold values were used: <30 represents deteriorated soil quality, 50–70 indicates intermediate quality, and >70 represents optimal quality. The results based on the planter experiment revealed that, after 14 months, the SQI of all raw greywater-irrigated soils was lower than 70, indicating soil functions and plant health might be compromised. The use of scoring functions was a useful tool for quantifying and comparing the effects of greywater irrigation on different soil quality indicators. Integration of all indicator scores into a single SQI quantifies and summarizes the overall beneficial and detrimental effects of greywater irrigation. However, for better understanding and management decisions, SQI scores should be used and interpreted in conjunction with the scores of the single indicators constituting the index. In our experiment, treated ​greywater ​did ​not compromise ​soil quality even after 14 months of irrigation. As such, based on the fact that irrigation with raw greywater might compromise soil quality, treatinggreywater prior to its use is recommended.