Effect of greywater treated by horizontal subsurface flow wetlands planted with Chrysopogon zizanioides and Andropogon gayanus on the germination of tomato (Lycopersicon esculentum Mill.) seeds under Sahelian climate

Abstract

Greywater reuse for agricultural irrigation may contribute to reduce pressure on scarce freshwater sources and the use of chemical fertilizers in Sahelian countries. However, treatment methods that reduce the harmful effects of untreated greywater are essential for sustainable agriculture. This study aims to evaluate greywater treatment capacity by Horizontal Subsurface Flow wetlands (HSSF) planted with Chrysopogon zizanioides (HSSF-C) and Andropogon gayanus (HSSF-A), and to investigate the suitability of the effluents on the germination of tomato seeds. Three pilot-scale HSSF (planted with Chrysopogon zizanioides or Andropogon gayanus and an unplanted (control)) were designed and their removal efficiencies of organic matter, nutrients and fecal bacteria were evaluated. Principal component analysis showed planted systems provided higher removal efficiencies than the unplanted system. HSSF-A showed the best performance for removal of organic (COD, BOD5) and mineral (NO3−, NH4+) matter while HSSF-C showed the best performance for removal of phosphorus compounds and fecal indicators (thermotolerant coliforms, E. coli). HSSF-UP showed the lowest performance for removal of all parameters. The reduction of thermotolerant coliforms and E. coli were significantly greater in the two planted systems and exceeded 2 log10 removal. However, no significant difference was observed between the planted and unplanted HSSF for fecal indicators. Furthermore, germination rates obtained with greywater treated by HSSF were higher than 80%, approaching that obtained using distilled water, while the lower germination rates were observed with seeds irrigated with raw greywater (58%). This is a clear indication of the beneficial effect of treated greywater by HSSF wetlands on seeds germination. In addition, residual content of fecal indicators and physicochemical characteristics of the treated greywater are compliant with WHO reuse guidelines for restricted irrigation. The results demonstrate the possibility of using constructed wetlands for greywater treatment and its reuse in gardening in Sahelian countries.