Impacts of long-term greywater disposal on soil properties and reuse in urban agriculture in an informal settlement : a case study of Waruku, Nairobi

With increasing urban population exerting pressure on the fast dwindling fresh waterresources, wastewater reuse is gaining popularity in many parts of the world. Greywater,which is household wastewater without input from toilet waste, has attracted specialattention for possibilities of reuse, particularly in urban agriculture, because it has lowpathogen content and can therefore be treated easily for reuse.The trend towards reuse of treated greywater in urban agriculture is driven by the needto maximise limited water resources and to benefit from the nutrients such asphosphorus available in the greywater. However, the sustainable reuse of greywater inurban agriculture is likely to be hampered by the high dissolved solids in greywater, thatinclude soluble sodium that impinges on soil properties, which cannot be removed bythe conventional biological wastewater treatment systems. Only, energy-intensive sizeexclusion systems such as reverse osmosis and membrane technologies can removegreywater dissolved solids but these systems are beyond the reach of developingcountries. Thus, there is a need to study the impacts of greywater on soil properties inorder to gain insights into its long-term potential reuse in urban agriculture.Soil sampling was carried out in a site that had been drained with greywater for over 20years in Waruku urban informal settlements, Nairobi. The greywater from the studyarea had sodium adsorption ratio (SAR) of 8.0 and electrical conductivity (EC) of 1.1dS/m. The results of the study indicated that soils (sampled to 150 mm depth) from thegreywater disposal site were statistically different in terms of soil exchangeable sodium,with p < 0.001 for significance compared with non-greywater irrigated site locatedwithin the study area (taken as the control). The greywater disposal site exhibited 2.1units of higher pH and 326% higher soil sodium adsorption ratio (SAR). It had high soilsalt accumulation (soil ECe), increasing to 4.1 dS/m, which was 569% higher than thecontrol site, indicating potential yield reduction of salt sensitive crops such as cabbagedue to unplanned greywater disposal.Regular monitoring of site-specific greywater and soil, and appropriate managementpractices such as periodic leaching, are needed to mitigate the negative impacts ofgreywater reuse in urban agriculture due to high soil sodium accumulation and salinitybuild up. Despite these challenges, the study also revealed opportunities exist ingreywater reuse in urban agriculture. It is a powerful means for water conservation andnutrients recycling thereby reducing demands for freshwater and mitigating pollution ofaquatic systems