Stefan Heiske

Consequences of field N2O emissions for the environmental sustainability of plant‐based biofuels produced within an organic farming system

One way of reducing the emissions of fossil fuel‐derived carbon dioxide (CO2) 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 (N2O). Previous studies on biofuel production systems showed that emissions of N2O 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 N2O to the reduction in fossil fuel‐derived CO2, 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 CO2, where the N2O emission was subtracted. This value did not account for fossil fuel‐derived CO2 emissions from farm machinery and during conversion processes that turn biomass into biofuel. The greatest net GHG reduction, corresponding to 700–800 g CO2 m−2, 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 CO2 m−2), 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 N2O emissions.

Improving Anaerobic Digestion of Wheat Straw by Plasma-Assisted Pretreatment

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 CH4/gVS.

Effect of anaerobiosis on indigenous microorganisms in blackwater with fish offal as co-substrate.

The aim of this study was to compare the effect of mesophilic anaerobic digestion with aerobic storage on the survival of selected indigenous microorganisms and microbial groups in blackwater, including the effect of addition of Greenlandic Halibut and shrimp offal. The methane yield of the different substrate mixtures was determined in batch experiments to study possible correlation between methanogenic activity in the anaerobic digesters and reduction of indigenous microorganisms in the blackwater. By the end of the experiments a recovery study was conducted to determine possible injury of the microorganisms. In both anaerobic and aerobic samples, survival of Escherichia coli was better in the presence of Greenlandic Halibut offal when compared to samples containing blackwater only and blackwater and shrimp offal, possibly due to more available carbon in the samples containing Greenlandic Halibut offal. Reduction of faecal streptococci was large under both anaerobic and aerobic conditions, and the results indicated a complete removal of faecal streptococci in the anaerobic samples containing blackwater and a mixture of blackwater and shrimp offal after 17 and 31 days, respectively. Amoxicillin resistant bacteria were reduced in the anaerobic samples in the beginning of the study but increased towards the end of it. The opposite pattern was observed in the aerobic samples, with a growth in the beginning followed by a reduction. During the anaerobic digestion tetracycline resistant bacteria showed the least reduction in the mixture of blackwater and shrimp offal, which had the lowest methane yield while the highest reduction was observed in the mixture of blackwater and Greenlandic Halibut, where the highest methane yield was measured Reduction of coliphages was larger under anaerobic conditions. Addition of fish offal had no effect on survival of coliphages. The results of the recovery study indicated that a fraction of the E. coli in the aerobic blackwater sample and of the faecal streptococci in both the anaerobic and aerobic samples containing blackwater and Greenlandic Halibut were injured only, and thus able to resuscitate during recovery. The use of anaerobic digestion in the Arctic is limited to substrate types like those tested in this study because of absence of agriculture. The results indicate that anaerobic digestion of wastewater could benefit from the addition of fish offal, with respect to both microbial reduction and energy production.