The impact of irrigation with treated wastewaters on soil and kikuyu grass nutrient compositions

Abstract

A lysimeter study was conducted for 1 year to examine how the source of wastewater for irrigation impacted soil physicochemical properties and kikuyu grass (Pennisetum clandestinum) nutrient composition. The wastewater used included treated wastewater produced by a membrane bioreactor (MBR) and intermittently decanted aerated lagoon (IDAL) treatment systems. No significant differences were observed between the treatments regarding total nitrogen and total phosphorus across the depths of the columns. However, highly significant differences were observed for Na content of the soils at various depths. Remarkable differences were recorded for soil exchangeable K and Na at different depths. In contrast, soil exchangeable Ca and Mg experienced no significant differences concerning the depth of the columns. For kikuyu grass, sodium contents of the grasses irrigated with MBR and IDAL treated wastewaters increased more than 200% and 100%, respectively, when compared with the grass irrigated with tap water. Over the period of monitoring considered in this study, there was no sign of excessive soil salinity/sodicity issues. The MBR treated wastewater has the potential to supply the grass with a constant dosage of valuable nutrients such as N and P without the requirement of using chemical fertilizers. This reduces the risk of contamination of receiving waters and groundwater and enhances the recycling of the nutrients in the wastewater to achieve a circular economy of nutrients. PRACTITIONER POINTS: Application of treated wastewaters revealed no harmful effects on soil and plant nutritional properties over the study period. The membrane bioreactor (MBR) treated wastewater potentially supplies the grass with constant dosage of valuable nutrients in the absence of chemical fertilisers. Sodium contents of the grasses irrigated with MBR and IDAL treated wastewaters increased more than 200% and 100%, respectively. Soil soluble and exchangeable cations showed very similar trends of changes versus the depth of the soil over the study period.

Keywords: Soil exchangeable cations; intermittently decanted aerated lagoon; membrane bioreactor; nitrogen; phosphorus.