Abstract
SummaryAnaerobic digesters produce biogas, a mixture of predominantly CH4 and CO2, which is typically incinerated to recover electrical and/or thermal energy. In a context of circular economy, the CH4 and CO2 could be used as chemical feedstock in combination with ammonium from the digestate. Their combination into protein‐rich bacterial, used as animal feed additive, could contribute to the ever growing global demand for nutritive protein sources and improve the overall nitrogen efficiency of the current agro‐ feed/food chain. In this concept, renewable CH4 and H2 can serve as carbon‐neutral energy sources for the production of protein‐rich cellular biomass, while assimilating and upgrading recovered ammonia from the digestate. This study evaluated the potential of producing sustainable high‐quality protein additives in a decentralized way through coupling anaerobic digestion and microbial protein production using methanotrophic and hydrogenotrophic bacteria in an on‐farm bioreactor. We show that a practical case digester handling liquid piggery manure, of which the energy content is supplemented for 30% with co‐substrates, provides sufficient biogas to allow the subsequent microbial protein as feed production for about 37% of the number of pigs from which the manure was derived. Overall, producing microbial protein on the farm from available methane and ammonia liberated by anaerobic digesters treating manure appears economically and technically feasible within the current range of market prices existing for high‐quality protein. The case of producing biomethane for grid injection and upgrading the CO2 with electrolytic hydrogen to microbial protein by means of hydrogen‐oxidizing bacteria was also examined but found less attractive at the current production prices of renewable hydrogen. Our calculations show that this route is only of commercial interest if the protein value equals the value of high‐value protein additives like fishmeal and if the avoided costs for nutrient removal from the digestate are taken into consideration.
Highlights
Anaerobic digestion (AD) is a mature and energy-efficient technology, able to convert a broad variety of organic streams into biogas, a renewable source of methane (CH4), and digestate, a nutrient-rich organic residue (Appels et al, 2011)
Costs associated with the production/recovery of the building blocks for MOB growth represent 71% of the total base case microbial protein (MP) production cost, with 46% for biogas methane, 20% for recovered ammonia and 5% for O2, while 19 % can be attributed to capital expenditure (CAPEX) and Operational expenditure (OPEX) of the MP production unit (293 € ton MP–1) and 10 % to dewatering and drying of the wet product (160 € ton MPÀ1)
The amino acid profile and overall nutritive value of a bacterial meal obtained from MOB growth appeared to be comparable to fishmeal and overall better than soybean meal (Øverland et al, 2010) and, it is likely that the produced microbial protein has a market value higher than or at least equal to fishmeal
Summary
A mixture of predominantly CH4 and CO2, which is typically incinerated to recover electrical and/or thermal energy. In a context of circular economy, the CH4 and CO2 could be used as chemical feedstock in combination with ammonium from the digestate Their combination into protein-rich bacterial, used as animal feed additive, could contribute to the ever growing global demand for nutritive protein sources and improve the overall nitrogen efficiency of the current agrofeed/food chain. In this concept, renewable CH4 and H2 can serve as carbon-neutral energy sources for the production of protein-rich cellular biomass, while assimilating and upgrading recovered ammonia from the digestate. Our calculations show that this route is only of commercial interest if the protein value equals the value of highvalue protein additives like fishmeal and if the avoided costs for nutrient removal from the digestate are taken into consideration
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