Abstract:
Anaerobic digestion is a critical technology for converting organic waste to renewable biogas
energy. But high nitrogen substrates like poultry manure (C:N = 8.5:1), fish waste (C:N =
6.85:1), and beef waste (C:N = 7.3:1) have high methanogenic potential (0.35-0.45 m3 CH4/kg
VS) yet ammonia inhibition (>3000 mg NH3-N/L) and overproduction of volatile fatty acids
(>5000 mg/L). To this end, thermodynamically-optimised co-substrate formulations striving to
achieve C:N ratios of 20:1-25:1 were developed, ASPEN Plus models to predict the behavior of
gaseous phases based on Peng Robinson equations were established, and the synergistic interactions
that enhanced the activity of the methanogens were quantified. Mesophilic conditions
(35±0.5◦C, pH 7.0±0.2) were used with five co-substrate combinations in controlled batch
anaerobic digesters (1.2 L): PM+FW+CW (50:20:30%), PM+FW (50:50%), CW+FW+PM+BW
(25:25:25:25%), BW+CW (60:40%), and PM+BW (50:50%). The characterisation of substrates
was done in terms of total solids (52.3-67.8 g/L), volatile solids (44.6-62.1 g/L), chemical
oxygen demand (58,400-72,600 mg/L), and total Kjeldahl nitrogen (2,840-4,270 mg/L). ASPEN
Plus V12.1 simulations used Peng-Robinson thermodynamic models, which used Monte
Carlo sensitivity analysis (10,000 iterations, n=3 replicates per treatment). The best co-substrate
formulation (PM+FW+CW: 50:20:30%) produced significantly more biogas (0.847 m3/kg VS,
p <0.05) with a methane concentration of 58.3±2.1% (20.98 MJ/m3), CO2 concentration of
38.7±1.8% and H2S concentration of 1,200±150 ppm and NH3 concentration of 680 ppm. At
308.15 K and Gibbs free energy of −394.2 kJ/mol, thermodynamic modelling revealed that
methanogenic reactions were optimally represented. Substrate interactions yielded synergistic
interactions leading to 52.7% higher yields of methane than in mono-digestion (p<0.01). Commercial
viability was shown by economic analysis with a net present value of $2.34M to $1.8M,
an internal rate of return of 24.6 to 1.9 and a levelised cost of energy of $0.063-0.065/kWh
over 20 years of operation. A combination of thermodynamic modelling and overall technoeconomic
analysis of high-level nitrogen substrates is a new way of streamlining anaerobic
co-digestion systems. Findings show that strategic co-substrate formulation has the capability
to frustrate conventional nitrogen inhibition constraints without being economically competitive
in renewable energy markets.
