Anaerobic digestion is a worldwide technology for the treatment of organic waste streams with clear environmental benefits including generation of methane as renewable energy. However, the need to improve process feasibility of existing applications as well as to expand anaerobic digestion to a range of new substrates has raised interest on several intensifications techniques. Among them, the supplementation of inorganic and biological additives has shown good results at improving digesters performance. This manuscript presents a comprehensive review about recent advances in the utilization of inorganic and biological additives. On the one hand, reviewed inorganic additives comprise: (i) macro- (e.g. P, N and S) and micro- (e.g. Fe, Ni, Mo, Co, W and Se) nutrients supplements, (ii) ashes from waste incineration, (iii) compounds able to mitigate ammonia inhibition, and (iv) substances with high biomass immobilization capacity. Among them, iron (Fe 0 and Fe(III)) has shown particularly promising results, which have been mainly related to their action as electron donor/acceptor and cofactor of key enzymatic activities. On the other hand, reviewed biological additives include: (i) the dosage of microbial inocula with high hydrolytic or methanogenic activity (bioaugmentation), and (ii) the addition of enzymes able to facilitate particulate organic matter solubilization.

The role of additives on anaerobic digestion: a review

The availability of trace metals as micro-nutrients plays a very significant role on the performance and stability of agricultural biogas digesters, which are operated with energy crops, animal excreta, crop residues, organic fraction of municipal solid wastes or any other type of organic waste. The unavailability of these elements in biogas digesters is probably the first reason of poor process efficiency without any other obvious reason, despite proper management and control of other operational and environmental parameters. However, trace metal requirements of biogas digesters operated with solid biomass are not often reported in literature. Therefore, the aim of this article is to review the previous and current literature about the trace metal requirements of anaerobic biogas digesters operated with solid organic substrates for production of methane.

Trace element requirements of agricultural biogas digesters during biological conversion of renewable biomass to methane

It has been suggested that a decrease in atmospheric methane levels triggered the progressive rise of atmospheric oxygen, the so-called Great Oxidation Event, about 24 Gyr ago(1) Oxidative weathering of terrestrial sulphides, increased oceanic sulphate, and the ecological success of sulphate-reducing microorganisms over methanogens has been proposed as a possible cause for the methane collapse(1), but this explanation is difficult to reconcile with the rock record(2,3) Banded iron formations preserve a history of Precambrian oceanic elemental abundance and can provide insights into our understanding of early microbial life and its influence on the evolution of the Earth system(4,5) Here we report a decline in the molar nickel to iron ratio recorded in banded iron formations about 27 Gyr ago, which we attribute to a reduced flux of nickel to the oceans, a consequence of cooling upper-mantle temperatures and decreased eruption of nickel-rich ultramafic rocks at the time We measured nickel partition coefficients between simulated Precambrian sea water and diverse iron hydroxides, and subsequently determined that dissolved nickel concentrations may have reached similar to 400nM throughout much of the Archaean eon, but dropped below similar to 200nM by 25 Gyr ago and to modern day values(6) (similar to 9 nM) by similar to 550 Myr ago Nickel is a key metal cofactor in several enzymes of methanogens(7) and we propose that its decline would have stifled their activity in the ancient oceans and disrupted the supply of biogenic methane A decline in biogenic methane production therefore could have occurred before increasing environmental oxygenation and not necessarily be related to it The enzymatic reliance of methanogens on a diminishing supply of volcanic nickel links mantle evolution to the redox state of the atmosphere

Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event

https://zenodo.org/record/983/files/Banks%20et%20al%20TE%20and%20ammonia%202012%20-%20scholar%20text.pdf

Trace element requirements for stable food waste digestion at elevated ammonia concentrations

It has been suggested that a decrease in atmospheric methane levels triggered the progressive rise of atmospheric oxygen, the so-called Great Oxidation Event, about 2.4 Gyr ago(1). Oxidative weathering of terrestrial sulphides, increased oceanic sulphate, and the ecological success of sulphate-reducing microorganisms over methanogens has been proposed as a possible cause for the methane collapse(1), but this explanation is difficult to reconcile with the rock record(2,3). Banded iron formations preserve a history of Precambrian oceanic elemental abundance and can provide insights into our understanding of early microbial life and its influence on the evolution of the Earth system(4,5). Here we report a decline in the molar nickel to iron ratio recorded in banded iron formations about 2.7 Gyr ago, which we attribute to a reduced flux of nickel to the oceans, a consequence of cooling upper-mantle temperatures and decreased eruption of nickel-rich ultramafic rocks at the time. We measured nickel partition coefficients between simulated Precambrian sea water and diverse iron hydroxides, and subsequently determined that dissolved nickel concentrations may have reached similar to 400nM throughout much of the Archaean eon, but dropped below similar to 200nM by 2.5 Gyr ago and to modern day values(6) (similar to 9 nM) by similar to 550 Myr ago. Nickel is a key metal cofactor in several enzymes of methanogens(7) and we propose that its decline would have stifled their activity in the ancient oceans and disrupted the supply of biogenic methane. A decline in biogenic methane production therefore could have occurred before increasing environmental oxygenation and not necessarily be related to it. The enzymatic reliance of methanogens on a diminishing supply of volcanic nickel links mantle evolution to the redox state of the atmosphere.

Masami Numaguchi

Papers

Influence of Ni2+ and Co2+ on methanogenic activity and the amounts of coenzymes involved in methanogenesis