Anaerobic digestion of rice straw and food waste designed for hydrogen production ended up producing more nitrogen, and less hydrogen. The inoculum is a seed sludge, (the source/details are confidential) however, the methanogens was inactivated by heating at 90-100 ℃. It has also been cultivated with sugar and some other enrichment medium prior to the batch test. I am more experienced on methane production and new on hydrogen production. Do you have any idea as to why this occured and how to prevent it?
Anaerobic digestion is a very complex process which firstly depend on the feedstock. If the feedstock is dominated by nitrogen-rich substrates then there is the probability that it's likely to generate a medium with high pH value.
Please ignore the part with probability as a form of duplication in the sentence. Sorry for that
Gas production from mixed denitrifying and methanogenic cultures is dependent on the quantity of different species, type of substrates, COD/N ratio, operational conditions (temperature, pH) as discussed in an excellent paper by Andalib et al. 2016 Article Simultaneous denitrification and methanogenesis (SDM): Revie...
"Denitrifying one mole of NO3 - consumes 5e- . Based on the estimation by Gujer and Zehnder (1983), producing one mole of CH4 would consume 5.2e- (0.3×8+0.7×(8/2)= 5.2). Therefore, fractions of electron flow to methanogenesis and denitrification can be estimated from the amount of nitrate denitrified and methane produced. In the strict anaerobic degradation of organic matter, 30% of the carbon source for methanogenesis was from bicarbonate (Reaction 1 Table 2) and 70% from acetate (Reaction 5 Table 2) [63]. Since stoichiometrically, complete nitrate reduction to nitrogen gas requires 5 electron equivalents per mol of nitrate, coexistence of fermenters/methanogens and denitrifiers may cause channeling of electrons away from methanogenesis, which may result in decrease in the overall methane production [10]. Ruiz et al. (2006) showed that COD/N ratio has a strong influence on biomass activity, and therefore on the metabolic pathways of nitrate and organic matter utilization. Low COD/N values generated high denitrifying activities, and high COD/N value elevated methanogenic activities. Fang and Zhou demonstrated that electron flows to methanogenesis and denitrification were dependent on the COD/NO3 - -N ratio. At the COD/NO3 - -N ratio of 3.34, all electrons were utilized by denitrification, as evidenced by the cessation of methane production. The fraction of electron flow to methanogenesis increased, with the COD/NO3 - -N ratio, from nil at the ratio of 3.34 to 21.2% at 5.23." Therefore, obviously the COD/NO3 ratio in your substrate was < 3.34. I would not use seed sludge as inoculum; instead I would start fermentation with indigenous community of rice straw and food waste.
Thank you so much, Dr. Gilbert Selormey and Dr. Anne Menert . I greatly appreciate your valuable suggestions and recommendation.
Take a look Dr. Delmaria Richards
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