Research progress on biogas fermentation improvements by bio-available trace elements

Abstract: The addition of exogenous trace elements to enhance the enzyme activity is one of the research hotspots to further enhance the bio-methane transformation potential in anaerobic digestion system. However, the disunity of the trace elements dosage strategy as well as the irregularity of trace element dosing effects restricts the development of this technology. In order to achieve the exogenous trace element dosing controllability, and to promote the research and development of the high and stable yield of biogas production technology from various feedstocks, this paper explored the following research points from the published literatures: the background value of trace element in different feedstocks, the trace element concentrations in different anaerobic digestion(AD) systems, the physiological functions of trace element for the anaerobic methanogens, the promoting effect brought by trace element addition, and the impact factors in the anaerobic digestion system on the chemical form migration and bioavailability of trace elements. Based on the aspects above, the reasons and limiting factors for the improvement of anaerobic digestion performance by trace element addition strategy were discussed. It was generally believed that trace elements could exist and transform interactively in the following forms: dissolution, adsorption, carbonate binding, phosphate binding, sulfide precipitation and residual state, with the bioavailability decreasing successively. The possible reasons found for the inconsistent research results of the optimal dosage amount of trace elements(ETs) were as follows: 1) different background value; 2) different AD operation conditions; 3) different methanogens with different TEs uptake ability; and 4) different levels of actual bioavailable TEs. When feedstocks and digestive conditions were changed, it was not applicable to take the optimal dosage obtained under certain research conditions as a reference standard. Beside, since the anaerobic digestion process was a complex biochemical reaction system, in the engineering application process, not only the feedstock types, but also the basic technological operating parameters such as the temperature, pH value, redox potential and hydraulic retention time exerted influence on the AD performance. This was because the basic technological operating parameters could directly exert influence on the existence chemical form of inorganic and organic ligands, including the carbonates, phosphates, sulfides, cell wall, extracellular polymers, soluble microbial products, organic sulfur compounds, organic acids, organic chelators, etc.. It also impacted the ecological change of the micro-organisms population, which determined the biochemical environment in the anaerobic digestate in return. These ligands were derived from the macromolecule in feedstock and generated during the anaerobic digestion degradation process, which all could exert influence on the chemical distribution and thus the bioavailability of trace elements. Besides, the possible approach to acquire the chemical distribution of the trace elements, namely, the multi-step chemical extraction approach was recommended in this paper. With the approach, which was a relatively mature approach to investigate the migration and transformation rule of metal elements in nature, the elemental speciation in anaerobic digestion could be divided into valence state, chemical state, binding state and structural state. This could be beneficial to gain a panoramic view of the dynamic migration and transformation of trace elements in biogas production system. In summary, it was believed that the difference of background value of trace elements in raw material and the internal environment of fermentation system may be the major reason for the difference of the trace elements chemical distribution and dynamic migration and transformation pattern in different anaerobic digestion system. It was also considered that the promotion of bio-availability or bio-utilization rate of trace elements was the key step to exploit the precise trace elements addition technology for anaerobic digestion. The reveal of the dynamic distribution of trace elements in different chemical form was an important aspect to realize the effective utilization of trace elements in biogas fermentation system.