Exploring greywater hydroponic vegetable growing and phytoremediation capacity of coleus plants for urban farmers in Mbarara city Uganda

Sustaining the escalating urban population in food and nutrition security remains a global concern. Urban farming, which maximizes production using limited resources, is the right option for urban populations in developing countries. Between July and September 2023, an eight-week experiment to establish the capacity of greywater as a hydroponic medium to grow vegetables for human consumption and test the greywater cleansing capacity of Coleus ornamental plants was conducted. The motivation was to pilot an intervention to the challenges of land, water, space, malnutrition and wastewater disposal in Mbarara. Broccoli, Spring onions and Lettuce were used and greywater was collected from restaurant sculleries in Mbarara. Four treatments: Greywater, Greywater + NPK, Tap water, and Tap water + NPK were prepared and vegetable seedlings assigned in triplicates to each. The pH, EC, TDS, Temperature, Turbidity, Nitrates and Phosphates of the growing media; shoot length and number of leaves for the test vegetables and fecal coliforms and heavy metal in the media and plant tissues were tested following standard procedures. All vegetables survived but the levels of production differed significantly between treatments. Lettuce had the highest production in three treatments except GW+NPK (ANOVA shoot length (F (3, 56) = 2.970, p = 0.039). Spring onions grew best in greywater + NPK significantly differing from other treatments, ANOVA (F (3, 56) =3.328, p=0.026). Extreme EC and TDS values (1001.38 µS/cm and 502ppm; and 982.38 µS/cm 490.75ppm) were recorded in greywater treatments and significantly differed from other treatments (p=0.000). No fecal coliforms were detected in the vegetable tissues. Coleus progressively reduced the TDS and EC and exhibited capacity to extract heavy metals from greywater. Greywater is a viable hydroponic medium to grow vegetables. It is recommended that greywater nutrient enhancement to improve production for specific vegetables and use of Coleus plants for hydroponic greywater remediation.