In the present study we tested four macroalgae species--harvested in Denmark--for their suitability of bioconversion to methane. In batch experiments (53 degrees C) methane yields varied from 132 ml g volatile solids(-1) (VS) for Gracillaria vermiculophylla, 152 mi gVS(-1) for Ulva lactuca, 166 ml g VS(-1) for Chaetomorpha linum and 340 ml g VS(-1) for Saccharina latissima following 34 days of incubation. With an organic content of 21.1% (1.5-2.8 times higher than the other algae) S. latissima seems very suitable for anaerobic digestion. However, the methane yields of U. lactuca, G. vermiculophylla and C. linum could be increased with 68%, 11% and 17%, respectively, by pretreatment with maceration. U. lactuca is often observed during 'green tides' in Europe and has a high cultivation potential at Nordic conditions. Therefore, U. lactuca was selected for further investigation and co-digested with cattle manure in a lab-scale continuously stirred tank reactor. A 48% increase in methane production rate of the reactor was observed when the concentration of U. lactuca in the feedstock was 40% (VS basis). Increasing the concentration to 50% had no further effect on the methane production, which limits the application of this algae at Danish centralized biogas plant.

Plasma-assisted pretreatment (PAP) of lignocellulosic biomass has been shown to be an efficient method to decompose lignin and consequently facilitate microbial access to cellulose and hemicellulose. In the present study, PAP was tested for its suitability to enhance bioconversion of wheat straw to methane. In thermophilic batch experiments, methane yields of up to 366 mL/g volatile solids (VSs) were achieved, accounting for a yield increase of 45%. Common lignin-derived inhibitors like 5-hydroxymethylfurfural (5-HMF) and furfural were not detected after PAP, but toxicity test resulted in lower methane yields at higher substrate concentrations, indicating the presence of other unidentified inhibitors. However, in a continuous lab-scale biogas reactor experiment, stable codigestion of cattle manure with 20% PAP wheat straw was demonstrated, while no signs of adverse effects on the anaerobic digestion process were observed. After the introduction of the pretreated wheat straw to the reactor, volatile fatty acid concentrations remained low and stable, while gas production increased. In co-digestion, the PAP wheat straw was converted at an average yield of 343 mL CH 4 /gVS.

BackgroundUtilization of lignocellulosic feedstocks for bioenergy production in developing countries demands competitive but low-tech conversion routes. White-rot fungi (WRF) inoculation and ensiling are two methods previously investigated for low-tech pretreatment of biomasses such as wheat straw (WS). This study was undertaken to assess whether a combination of forced ensiling with Lactobacillus buchneri and WRF treatment using a low cellulase fungus, Ceriporiopsissubvermispora, could produce a relevant pretreatment effect on WS for bioethanol and biogas production.ResultsA combination of the ensiling and WRF treatment induced efficient pretreatment of WS by reducing lignin content and increasing enzymatic sugar release, thereby enabling an ethanol yield of 66 % of the theoretical max on the WS glucan, i.e. a yield comparable to yields obtained with high-tech, large-scale pretreatment methods. The pretreatment effect was reached with only a minor total solids loss of 5 % by weight mainly caused by the fungal metabolism. The combination of the biopretreatments did not improve the methane potential of the WS, but improved the initial biogas production rate significantly.ConclusionThe combination of the L. buchneri ensiling and C. subvermispora WRF treatment provided a significant improvement in the pretreatment effect on WS. This combined biopretreatment produced particularly promising results for ethanol production.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0437-x) contains supplementary material, which is available to authorized users.

One way of reducing the emissions of fossil fuel-derived carbon dioxide (CO 2 ) is to replace fossil fuels with biofuels produced from agricultural biomasses or residuals. However, cultivation of soils results in emission of other greenhouse gases (GHGs), especially nitrous oxide (N 2 O). Previous studies on biofuel production systems showed that emissions of N 2 O may counterbalance a substantial part of the global warming reduction, which is achieved by fossil fuel displacement. In this study, we related measured field emissions of N 2 O to the reduction in fossil fuel-derived CO 2 , which was obtained when agricultural biomasses were used for biofuel production. The analysis included five organically managed feedstocks (viz. dried straw of sole cropped rye, sole cropped vetch and intercropped rye-vetch, as well as fresh grass-clover and whole crop maize) and three scenarios for conversion of biomass into biofuel. The scenarios were (i) bioethanol, (ii) biogas and (iii) coproduction of bioethanol and biogas. In the last scenario, the biomass was first used for bioethanol fermentation and subsequently the effluent from this process was utilized for biogas production. The net GHG reduction was calculated as the avoided fossil fuel-derived CO 2 , where the N 2 O emission was subtracted. This value did not account for fossil fuel-derived CO 2 emissions from farm machinery and during conversion processes that turn biomass into biofuel. The greatest net GHG reduction, corresponding to 700-800 g CO 2 m À2 , was obtained by biogas production or coproduction of bioethanol and biogas on either fresh grass-clover or whole crop maize. In contrast, biofuel production based on lignocellulosic crop residues (i.e. rye and vetch straw) provided considerably lower net GHG reductions ( 215 g CO 2 m À2 ), and even negative numbers sometimes. No GHG benefit was achieved by fertilizing the maize crop because the extra crop yield, and thereby increased biofuel production, was offset by enhanced N 2 O emissions.

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