Modified batch anaerobic digestion assay for testing efficiencies of trace metal additives to enhance methane production of energy crops

Abstract

Batch biochemical methane potential (BMP) assays to evaluate the methane yield of biogas substrates such as energy crops are usually carried out with undiluted inoculum. A BMP assay was performed on two energy crops (green cuttings and grass silage). Anaerobic digestion was performed both with and without supplementation of three commercial additives containing trace metals in liquid, solid or adsorbed form (on clay particles). In order to reveal positive effects of trace metal supplementation on the methane yield, besides undiluted inoculum, 3-fold and 10-fold dilutions of the inoculum were applied for substrate digestion. Diluted inoculum variants were supplemented with both mineral nutrients and pH-buffering substances to prevent a collapse of the digestion process. As expected, commercial additives had no effect on the digestion process performed with undiluted inoculum, while significant increases of methane production through trace element supplementation could be observed on the diluted variants. The effect of inoculum dilution may be twofold: (1) decrease in trace metal supplementation from the inoculum and (2) reduction in the initial number of bacterial cells. Bacteria require higher growth rates for substrate degradation and hence have higher trace element consumption. According to common knowledge of the biogas process, periods with volatile fatty acids accumulation and decreased pH may have occurred in the course ofanaerobic digestion. These effects may have led to inhibition, not only ofmethanogenes and acetogenes involved in the final phases of methane production, but also offibre-degrading bacterial strains involved in polymer hydrolysis. Further research is required to confirm this hypothesis.

Anaerobic treatability of liquid residue from wet oxidation of sewage sludge

“…4). This result did not confirm the outcomes of previous works (Karlsson et al 2012;Brulé et al 2013) in which trace elements were pointed out to influence anaerobic degradation. Actually, in this experimentation, the tested substrates in the vessels were highly diluted; thus, it can be supposed that trace element content in the inoculum was enough for enabling anaerobic degradation process.…”

Effects of substrate concentration on methane potential and degradation kinetics in batch anaerobic digestion

“…For the two setups including dilution, after the addition of inoculum and substrate, either distilled water or nutrient/buffer solution was added to reach 400 mL; the corresponding blank samples contained inoculum diluted with distilled water or nutrient/buffer solution respectively. The nutrient/buffer solution was prepared according to Brulé et al (2013). For each series, five different substrate loads at a fixed ISR of 2 (based on VS) were studied.…”

“…growth (Angelidaki and Sanders, 2004;Raposo et al, 2012). It has also been shown that additions of these types of medium improve the methane potential of diluted inocula while undiluted ones are not affected (Brulé et al, 2013). Nonetheless, there are few guidelines recommending what substrate concentration to use in a BMP test to reach optimal results.…”

The impact of Ni, Co and Mo supplementation on methane yield from anaerobic mono-digestion of maize silage

“…[27] Thus, lack of trace elements could result in reduced biogas production in mono-digestion plants without manure addition, depending on the type of the energy crop used, and furthermore, too high amounts could also pose adverse impacts and cause severe problems as well. [6,8,[28][29][30] Some energy crops may be lacking these trace metals, such as maize silage used in this study for Co and Ni, or alfalfa silage for Co. [30] Therefore, both macro-and micronutrient supplementation for monodigestion of maize and sugar beet silage [31,32] or for energy crop mixtures (sugar beet, maize and triticale) [33] is strongly recommended to attain high methane yields at short hydraulic retention times along with low volatile fatty acid production. In this work, in addition to the adequate C/N ratio already available in the substrate (Table 1), [34] the amount of trace metals supplemented seemed to provide higher methane yields and a stable digestion period as indicated by steady biogas and methane production values accompanied by neutral pH range in batch digesters.…”