A high demand for agricultural biomass production in Germany was attributed to the increasing number of biogas plants every year. The value of a crop as a substrate for biogas production via anaerobic digestion depends on its biomass yield capacity compared to the effort for cultivation and on its ability to produce biogas with high methane content.After the EEG 2012 amendment which determined the amount of maize that should be used in biogas production farmers searching for alternative substrates for biogas production. These alternatives can be cereal crops, vegetables, house hold wastes, grasses or farm residuals. Few farmers grow spices as marginal crops. The rests and residues of spices after harvesting can be used in the biogas plants. These spices contain secondary metabolites such as terpenes, flavenoids and other phenolic compounds. In the literature not enough studies can be found that focused on the effect of plant secondary metabolites on biogas and methane production through anaerobic digestion.The influence of plant secondary metabolites mainly terpenes in oregano leaves, fennel seeds and hop cones on biogas and methane production was investigated in this study.Two types of experiments were conducted. First two in vitro experiments were carried out using Methanosarcina barker as model for methanogens. It was cultivated in artificial bicarbonate buffered and sulfide reduced mineral media supplied with vitamins and trace minerals in serum bottles at mesophilic conditions. Oregano methanolic extract, carvacrol and thymol as main components of its essential oil were added at doses of 1.5 ml, 2.0 ml and 3.0 ml/50 ml of media. Carvacrol, thymol and their mixture were added at 0.1 and 0.3 ml/50 ml of media.Essential oil of oregano contains mainly carvacrol and thymol, p-cymene, limonene and α- and β-pinene in small quantities. The essential oil of fennel contains mainly trans-anethol, fenchone and estragol. The essential oil of hops contains mono and sesquiterpenes in addition to resins, flavonoids, glycosides and phenolic acids.It was found that the extracts of oregano at the concentration of 2.0 ml and 3.0 ml have clear negative effect on the activity of Methanocarcina barkeri to producemethane. Nevertheless the dose of 1.5 ml of oregano extract has no negative effect. All doses of carvacrol, thymol and their mixture showed negative effect on the methane. An increase in concentration of each of tested material was associated with a decrease in the methane production.In second type of experiments 46 parallel running 20 liter biogas digesters were fed with 15 kg cattle liquid manure 3-5% dry matter. The cattle liquid manure was brought from a second phase running biogas plant. Oregano was supplied at 62.5 g, 125 g, 250 g and 500 g/digester. Fennel seeds were added at 25 g, 50 g, 100 g, 200 g, and 400 g/digester. Hop cones were applied at 25 g, 50 g, 100 g, 200 g and 400 g/digester. In case of the inhibition test standard cellulose at 100 g/digester was mixed with the cattle liquid manure. Maize and standard cellulose were used as reference materials at concentration of 450 g and 100 g/digester respectively.Oregano in batch digesters at 250 g (16.7 g/kg manure) and 500 g (33.3 g/kg of manure) doses showed negative effect on the biogas and methane production and no methane was produced by the 500 g (33.3 g/kg manure) dose. Contrary the doses 62.5 g and 125 g showed positive effect. Fennel seeds at doses 200 g (13.3 g/kg manure) and 400 g (26.7 g/kg manure) have clear negative effect on biogas and methane production. Neither methane nor biogas was produced by all hop cones concentrations.The findings of the conducted study added valuable information about the inhibition of biogas by plant secondary metabolites found in oregano, fennel and hop cones.
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