Nitrogen and phosphorus dynamics in plants and soil fertigated with decentralised wastewater treatment effluent

Municipalities in South Africa face problems in providing sanitation to unserved informal settlements in peri-urban areas and rural nodes. The Decentralised Wastewater Treatment System (DEWATS) connected to community ablution blocks can be an option, with the treated effluent then applied to agricultural land. However, the management of treated wastewater through irrigation of crops must be environmentally sustainable. This study therefore investigated nitrogen (N) and phosphorus (P) dynamics in soil irrigated with DEWATS effluent. A field study with banana and taro in a randomised complete block design with three blocks and two irrigation treatments (DEWATS effluent without fertiliser vs tap water + fertiliser) was carried out over a period of 992 days at the Newlands-Mashu Research Site, Durban, South Africa. Data were collected on crop N and P uptake, soil chemical properties, and nutrient leaching together with groundwater monitoring. Nitrogen and P uptake was not significantly different (p > 0.05) between the two irrigation treatments. Irrigation with DEWATS effluent increased soil N and P concentrations within the upper 0.3 m implying its importance as a fertiliser source. Leaching of N and P from DEWATS effluent treated plots was comparable to that from the tap water + fertiliser treatments. However, to manage excess water in the soil, practices such as irrigation to meet crop water requirements with room for rainfall and installation of subsurface drainage when possible can be employed.