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Improvement of methane yield using bioaugmentation technique in a thermophilic anaerobic digestion process.
1 , 2 , 2 , 3 , 4 , * 5
1  Instituto Politécnico Nacional, Centro Mexicano para la Producción más Limpia, Av. Acueducto s/n, 07340 Ciudad de México, México. Tel: + 52 (55) 3719 1741.
2  Departamento de Genética y Biología Molecular, Cinvestav Unidad Zacatenco. Av. Instituto Politécnico Nacional 2508, Col. San Pedro Zacatenco, CDMX Código Postal 07360. Ciudad de México Tel: +52 (55) 5747 3800.
3  Instituto Politécnico Nacional, Unidad Profesional Interdisciplinaria de Biotecnología, Departamento de Ciencias Básicas, Av. Acueducto s/n, 07340, Ciudad de México, México. Tel: + 52 (55) 3899 6758.
4  Instituto Politécnico Nacional, Centro Interdisciplinario de Investigaciones y Estudios sobre Medio Ambiente y Desarrollo, Calle 30 de junio de 1520 s/n, 07340 Ciudad de México, México; +52 (55) 1341 4590.
5  Instituto Politécnico Nacional, Centro Interdisciplinario de Investigaciones y Estudios sobre Medio Ambiente y Desarrollo, Calle 30 de junio de 1520 s/n, 07340 Ciudad de México, México; +52 1 55 3654 9092.


Introduction. Anaerobic digestion (AD) is considered one of the most beneficial technologies for the treatment of organic wastes, from which energy in form of biogas and digestate as a soil amendment can be obtained. However, AD often suffers from process instability due to high levels of inorganic toxic substances such as ammonium, phosphates, sulfates or metal ions, variations in operating parameters such as temperature and pH, as well as a lack of proper balance between microbial groups. One of the alternatives to combat these imbalances is bioaugmentation, which is defined as the addition of specific microorganisms (either pure cultures or microbial consortia) into the system to accomplish a certain function. In this work, hydrogenotrophic methanogens were used to improve the methane yield of a thermophilic AD process of the organic fraction of municipal solid wastes (OFMSW) in Mexico City. Methods. The OFMSW (substrate) and leachate (inoculum) were collected from a composting plant in Mexico City. The consortium containing the hydrogenotrophic methanogens was isolated from the leachate. A biochemical methane potential (BMP) test was performed to know if the consortium influenced the methane yield. A 1:1 volatile solids (VS) ratio substrate/inoculum was operated in 125 mL serum bottles with a working volume of 60 ml. The consortium was added to different volume ratios. The process lasted 32 days at 55° C and 60 rpm. The identification of the microbial communities of all treatment groups and consortium were done by high throughput DNA sequencing of bacterial and archaeal 16S rDNA libraries. Results. The highest yield >600 ml CH4/g VSadded was achieved when 25% of the consortium was added followed by 50%; however, between these two treatments, there was not a statistically significant difference. The rest of the treatments were under 600 ml CH4/g VSadded. Nevertheless, it should be noted that the treatment at 25% reached its methane peak after 10 days, meanwhile, the treatment at 50% reached it on approximately 8 days, suggesting in this last treatment, the generation of methane was more accelerated. The methane yield increased by approximately 7% and 5% compared to the control (0%), while using 25% and 50% of the consortium, respectively. Regarding the bacterial communities, it was found that the consortium was mainly composed of members of the phylum Synergistetes. Members of this phylum are known for their acetate oxidizing properties. In all the treatments the phyla Firmicutes, Thermotogae, and Synergistetes were dominant and as the concentration of the consortium in the treatments increased, the presence of the family Thermotogaceae also increased. Conclusions. In this research, we detected that the bioaugmentation using hydrogenotrophic methanogens improved the methane yield of the AD process by more than 5%. The bacterial communities present in the consortium have capacities to degrade cellulolytic compounds and act as acetate oxidants, which play a role in syntrophy with hydrogenotrophic methanogens, however, it is still necessary to know the dynamics of the methanogens. Acknowledgments. The project was supported by CIIEMAD IPN SIP 20172277, 20180942, 20195607, and CONACyT-163235 INFR-2011-01. AKGB was granted a CONACyT-700337 doctorate scholarship.

Keywords: bioaugmentation; hydrogenotrophic methanogens; organic fraction of municipal solid waste; thermophilic anaerobic digestion.