Assessment of the agronomic value of digestate from cashew nut shell and cow dung anaerobic digestion

Abstract This study investigated agronomic characteristics of digestates from cashew nut shell and cow dung anaerobic digestion. General characteristics and agronomic value of digestates were determined using standard methods. Gaseous emissions (biogas, CH4, CO2) were evaluated. Microbiological quality of digestates was evaluated, as well as phytotoxicity on maize, okra, tomato and lettuce seeds. Higher conductivity indicated a greater potential for salinity to affect germination and plant growth. High C/N ratio and degree of humification greater than 0.7 are indicative of immature digestate. Total nitrogen, organic nitrogen and phosphorus contents in g/Kg were 11.26, 0.49 and 5.35 for cashew shell digestate and 18.15, 17.12 and 0.16 for cow dung digestate, respectively. Potassium content was 0.32 and 0.98 g K/Kg in cashew shell and cow dung digestate, respectively. Mineral nitrogen content of cashew nut shell digestate was 0.19 g NH4+/kg, 1.28 g NO2−/kg, and 0.0016 g NO3−/kg. These characteristics showed amending and fertilizing effect of digestates. Physical parameters indicate digestate can improve soil structure. Both digestates are significant source of greenhouse gas. Microbiological analysis revealed spore-forming bacteria and coliforms, with proportions that are acceptable for spreading. Germination test on okra, tomato and lettuce seeds indicated high phytotoxicity. Maize showed significant results for seed germination, root elongation, germination index and germination speed with values of 100%, 100%, 118.63% and 67.21% respectively. The study indicates that digestates present root growth-promoting properties that can be advantageous for plant development. Digestates could be improved by an integrated system in which digestates are composted downstream of anaerobic digestion.