Biochar is a way to improve the performance of the composting process and the quality of compost. This study was aimed to investigate the optimum ratio of khat straw (Catha edulis) biochar and organic municipal solid waste mixtures to improve the quality of the resulting co-composts. Khat-derived biochar during pyrolysis at 350 °C was added to organic municipal solid waste mix and four co-composting treatments were prepared with the compositions (% w/w): control compost (no biochar) and 5%, 15%, and 25% co-composted biochar in three replicates. The total organic carbon, organic matter, total nitrogen, available phosphorus, and potassium values ranged as 16.76–21.45%, 30.77–40.26%, 0.97–1.68%, 0.58–0.76%, and 12.72–15.29%, respectively. The results confirmed that 5% and 15% co-composted khat biochars had significantly reduced (p < 0.05) organic matter loss and increased the contents of cation exchange capacity, pH, phosphorous, potassium, calcium, magnesium, and zinc compared to the control compost, while some heavy metals (Fe, Cu, and Mn) and EC values in co-composted biochars are lower than the control compost. Khat-derived biochar could be added to municipal organic waste mix at 5–15% (w/w) in order to get better quality of compost, which can be used as biofertilizer.

The effects of organic fertilizer to improve soil fertility and crop yield depend on the quality of organic fertilizer. The aim of this study was to test the short-term effects of the applications of municipal solid wastes (MSW) compost, co-composted biochar and biochar on soil quality and faba bean (Vicia faba L.) grain yield and protein content compared with mineral fertilizer. The study was conducted in a field experiment prepared in randomized complete block design with three replicates of each treatment from February 2019 to June 2019. The eight treatments were; control compost, 5%, 15% and 25% w/w co-composted biochars, recommended rate of mineral N & P fertilizer (NPF), biochar, compost + 50% NPF, and control soil. Results showed that the soil pH, organic carbon, total nitrogen, available phosphorus, exchangeable K+, Ca + 2 and Mg + 2 CEC and moisture in the residual soil were significantly increased in the municipal solid waste compost, co-composted biochar and khat-derived biochar amendments compared to the mineral NPF and control soil treatments. The faba bean grain yield was increased significantly (p < 0.05) by 34.2%, 33.7% and 30.% compared to the control soil and by 11.3%, 10.9% and 7.8% compared to the mineral NPF for the 5% co-composted biochar, compost + 50% NPF and control compost respectively. The organic fertilizer amendments with or without NPF applications were better than NPF alone with promoting nutrient concentrations, faba bean grain yield and protein contents. Thus, MSW compost and co-composted biochars are recommended because of their easy availability, sustainability and sufficient for proper growth of faba bean.

Health risk assessment of heavy metals is crucial before large scale applications of municipal solid waste (MSW) as organic fertilizer. This study aimed to estimate Pb, Cd and Cr accumulation and health risk in faba bean (Vicia faba L.) grow on soils amended with MSW compost, biochar and co-composted biochars (5%, 15% and 25% w/w). The heavy metal concentrations were measured by flame atomic absorption spectroscopy. Bioaccumulation Factor (BAF), Estimated Daily Intake (EDI), Hazard Quotient (HQ) and Hazard Index (HI) were calculated. Results showed that Pb, Cd and Cr concentrations in the amended soils and faba bean grain were lower than WHO permissible levels. The EDIs of Pb, Cd and Cr in grains for all amendments were much lower than the reference doses established by USEPA and FAO/WHO. HQs and HIs of heavy metals were < 1 for all amendments indicating that consumption of faba bean grow on MSW compost, khat biochar and co-composted biochar collected from Jimma city was safe related to human health risks. Low accumulation of heavy metals in faba bean indicating that the low fraction of e-waste in the MSW. Since faba bean grain is not accumulator, it’s potential of heavy metals excluder needs further explorations.