Assessing the Amending and Fertilizing Properties of Digestates From Anaerobic Digestion of Faecal Sludge in Burkina Faso, West Africa

The aim of this work is to contribute to the agronomic utilization of digestate from the anaerobic treatment of faecal sludge at the Office National de l’Eau et de l’Assainissement (ONEA) Kossodo plant in Burkina Faso. Digestates were sampled and dried in the laboratory (25±2 °C). Physico-chemical and microbiological parameters were determined using standard methods. The degree of maturity of the digestate was determined using respirometry and humification tests. The effects of digestate on plants and on environment were studied using phytotoxicity, emergence, growth tests and microbiological analysis. The results revealed a pH of 8.54, electrical conductivity of 7300 µS/cm, 25.42% dry matter, 83.31% organic matter, and 489.8 g/kg total organic carbon. The C/N ratio was 9.7 and the NO3/NH4 ratio was 23.34, indicating mature compost despite other indicators suggesting that the digestate was only partially stabilized. Nutrient contents were 50.48 g/kg total nitrogen, 0.02 g/kg total phosphorus, and 4.72 g/kg total potassium. Water-soluble minerals included 1.97 g/kg ammonium, 45.99 g/kg nitrate, 0.02 g/kg nitrite, 0.35 g/kg phosphate, and 5.90 g/kg sulfate. Phytotoxicity tests showed that cabbage seeds had a germination rate of 89% with digestate filtrate compared to 100% in the control, while tomato seeds had a 100% germination rate with digestate filtrate. The germination indices were 137.58 for tomato and 82.10 for cabbage. Tests on tomato plant growth showed significant effects on biomass, height, leaf number, and branch number, with the 50% digestate proportion demonstrating the highest values. Microbiological analysis detected total aerobic flora at 4.75E+07 CFU/g, yeasts and molds at 7.77E+04 CFU/g, and coliforms at various levels, with potential pathogens such as S. aureus and Salmonella sp. reaching 6.14E+04 and 2.65E+06 CFU/g respectively. The presence of these pathogens suggests a need for further treatment of digestate to ensure safety before field application.