Determination of the optimal substrate ratio during co-digestion of poultry manure with fruit and vegetable waste for biomethane production: bloemfontein case study

Abstract

The anaerobic digestion (AD) of organic waste for biogas production is an important waste-to-energy route; however, it is often constrained by poor yields during mono-digestion. While co-digestion offers a solution, it requires the optimization of substrate ratios for each specific combination of co-substrates identified. This study determined the optimal substrate blends applicable to codigestion of poultry manure (PM) and fruit and vegetable wastes (FVW) by evaluating the biomethane potential (BMP) of these cosubstrates’ blends. For this study, substrates were sourced from Bloemfontein, a small town in South Africa, although results obtained can be extrapolated to other locations or regions. The BMPs derived from models that are based on the substrate’s organic elemental composition showed that FVW had higher BMP values, ranging from 294 to 334 mLCH 4 /gVS(VS–Volatile Solids), than those of PM which, ranged from 130 to 139 mLCH 4 /gVS, although the FVW values seemed overestimated due to the high lignin proportion in the VS of FVW when compared to that in PM. FVW also reported lower protein and higher lipid fraction values compared to PM. BMPs based on the latter methods ranged from 190 to 406 mLCH 4 /gVS for FVW and 266–369 mLCH 4 /gVS for PM. The experimental BMP assays revealed a lower BMP value of 133 mLCH 4 /gVS for FVW monodigestion compared to 289 mLCH 4 /gVS for PM monodigestion, confirming the negative impacts of lignin in AD and how this high lignin effect renders the BMP prediction models unreliable. The metagenomic profile studies in each substrate blend’s digesters revealed differences in microbial abundances of Actinomycetota , Bacillota, Pseudomonadota , and Cyanobacteriota phyla across substrates. The study results allowed us to better understand the challenges and opportunities presented by the specific substrates studied and helped us to identify the blend containing 20% wet weight PM and 80% wet weight FVW, codenamed PM20FVW80, as the best blend for codigestion. This determination procedure can be applied across other combinations of substrates that were not investigated in the current study to broaden our scope of choices in running digesters from an informed point of view.