Assessing the fertiliser value of co-composted biochar compost made from black soldier fly larvae faecal residue.
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The Black Soldier Fly Larvae (BSFL) technology can treat faecal sludge emptied from full Urine Diverting Dry Toilets (UDDT’s). A residue containing residual mineral elements such as nitrogen (N), phosphorus (P), potassium (K), and high organic matter, pathogens, and potentially heavy metals is left behind. Improper disposal of the residue can pose a challenge and lead to environmental pollution and health threats. However, there is potential for recycling BSFL Residue. The fertiliser value of BSFL Residue as an organic fertiliser has not been studied in South Africa. This study was carried out to evaluate the use of co-composted biochar compost made from BSFL Residue as a plant nutrient source for maize production. The residue was pyrolysed for 120, 90, and 60 minutes at 300 °C, for 60 and 45 minutes at 400 °C, and 60, 45, and 30 minutes at 500 °C. Biochar pyrolysed at 300,400, and 500 °C for 60, 45, and 30 minutes, respectively, was not burned and was analysed for physico-chemical, and biological characteristics. Biochar yield decreased significantly with increasing pyrolysis temperature. Surface area, pH, extractable phosphorus (P), exchangeable bases, trace metals significantly increased with pyrolysis temperature. Pathogens were destroyed with pyrolysis. Biochar pyrolysed at 500 °C for 30 minutes was chosen based on its characteristics as a bulking agent in the co-composting experiment. Co-composting of the residue was carried out, and chemical and physical characteristics of BSFL Residue composts (COMBI (compost with biochar) and (COMP (compost without biochar)) were compared to chicken manure (CM) and BSFL Heated Residue (HR). pH and exchangeable bases in BSFL Residue COMBI were higher than BSFL Residue Compost but less than BSFL HR and CM. Composting with biochar significantly increased trace elements, water holding capacity, total P, and total N in the BSFL Residue COMBI compared to BSFL Residue Compost. The BSFL Residue COMBI, BSFL Residue COMP, BSFL Residue, HR, CM, Chemical Commercial Fertiliser (CCF), and control were incubated in a sandy Cartref soil over a 112-day incubation period to determine phosphorus and nitrogen release patterns. Phosphorus decreased in all treatments during the first 21 days except for the control. Phosphorus release started after day 21, and the pattern for BSFL Residue COMBI and BSFL residue Compost were comparable, indicating the potential of using these amendments for crop production. Chicken manure had the least phosphorus released at the end of the incubation. Ammonium decreased with a concomitant increase in nitrates for all treatments. Nitrate release was lower for BSFL Residue COMBI compared to BSFL Residue Compost. An additional source of N is needed if BSFL Residue COMBI is to be used as a fertiliser based on P. The highest nitrate release was observed in CM. Amendments used in the incubation were used at the recommended and double recommended application rate to grow maize in a greenhouse pot trial. Statistically similar yields were obtained in grain harvested from COMBI, COMP CCF, and CM. More researchshould be carried out on the residual effect of BSFL COMBI on subsequent maize growth to establish possible residual fertility on the second cycle of growth. Keywords: biochar; biochar co-compost; black soldier fly larvae;faecal matter; fertiliser-value; nutrient recycling; phosphorus; pyrolysis; crop growth.