Bioaugmentation as a solution for solving ammonia toxicity in Biogas Plants.

The addition of beneficial microorganisms to anaerobic digesters for improved performance (ie Bioaugmentation) has been shown to reduce recovery time after organic overload or toxicity disorder. AUTH research, showed that inoculation of bioreactor with adapted mixed culture of methanogenic populations at high levels of ammonia contributed to a significant increase in methane production by the anaerobic digestion of poultry waste.

Therefore, the use of tolerant methanogenic microorganisms at high ammonia concentrations improves the anaerobic digestion of nitrogen-rich substrates. Previous research, showed that it is possible to adapt mixed hydrogenotrophic populations under gradually increasing NH3 concentrations to the high level of 7 g NH4 + -N / L. It is known that NH3 mainly inhibits the action of methanogenic archaea, while the metabolic pathway of acetic acid oxidation followed by hydrogenotrophic methanogenesis is more tolerant to high concentrations of NH3. Therefore, the anaerobic process is possible even under high concentrations of ammonia through hydrogenotrophic methanogenesis.

Indeed, it has recently been shown in the laboratory that bioaugmentation (using adapted hydrogenotropic populations) is possible in batch anaerobic reactors with high concentrations of NH3, but not possible in continuous stirred tank reactors (CSTRs) due to microorganisms’ washout. Therefore, bioaugmentation is a very promising solution to ammonia toxicity. However, in order to be applicable on industrial scale, the following challenges must be addressed: a) the selection and development of suitable mixed hydrogenotropic methanogenic population, b) the finding of the “critical biomass” (minimum amount) required to inoculate the bioreactor in order to achieve bioaugmentation, c) the determination of optimal operating conditions (organic charge rate, retention time, pH, etc.) to maintain the bioaugmentation population in the continuous stirred tank reactors and d) the design and operatation of a prototype continuous stirred tank reactor with real operating and supply conditions using different substrates to ensure system reliability under different conditions.

Facing the above challenges, NH3end will develop an innovative method of bioaugmentation and maintenance of mixed methanogenic populations in anaerobic bioreactors capable of fully meeting the current levels of NH3 concentrations in Greek biogas plants. Enhancing the effectiveness of anaerobic digestion will improve the environmental performance of biogas plants as the degree of decomposition of organic matter (waste) will increase. In addition, the operation of the units will be achieved at their maximum efficiency by using smaller quantities of organic matter, which implies lower costs of transporting both the organic matter to the unit and the processed organic matter to the final recipient. In addition to the economic benefit, the environmental footprint of the units will be reduced as CO2 emissions will be reduced due to the reduction of transport.

Furthermore, in order to improve the environmental footprint of the biogas plants, a combined modified method will be developed where the dissolved NH3 in the effluents of the biogas plant will be recovered in the form of biomass (algae) contributing to the reduction of nitrogen and phosphorus in the treated substrates. Finally, NH3end will enhance the automated operation of the biogas plants by developing a predictive controller to control the operating parameters of the bioreactor by optimizing the process. The combination of innovative actions that will be developed will lead to an increase in methane production by 30%. This means that the annual profit will increase by 45% (revenue will increase by 30%, while costs remain the same) so for 1 MWe biogas plant installation, the repayment period will be reduced by 1.2 years making the investment more attractive (1 MWe is a common situation in Greece).

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