No abstract

Several countries have established a number of increased targets for energy production from renewable sources. Biogas production, which will play a key role in future energy systems largely based on renewable sources, is expected to grow significantly in the next few decades. To achieve these ambitious targets, the biogas production chain has to be optimised to obtain economic viability and environmental sustainability while making use of a diversified range of feedstock materials, including agricultural residues, agro-industrial residues and, to some extent, dedicated energy crops. In this study, we integrated energetic, GHG and economic analysis to optimise biogas production from the co-digestion of pig slurry (PS) and sugar beet pulp silage (SB). We found that utilising SB as a co-substrate improves the energy and GHG balances, mostly because of increased energy production. However, utilising SB negatively affects the profitability of biogas production, because of the increased costs involved in feedstock supply. The scale of the processing plant is neutral in terms of profitability when SB is added. The results indicate that medium-to large-sized biogas plants, using low shares of SB co-substrate, may be the preferred solution.

This study investigated biogas production as an alternative treatment of tannery wastewater (TWW) and its use as a pre-treatment medium to increase CH yield from anaerobic digestion (AD) of wheat straw. The TWW had high levels of sulfate and chloride, so biochemical CH potential could be estimated only when the TWW was diluted. Untreated straw yielded 255 NL CH (kg VS), whereas straw that had been pre-treated with TWW yielded 314 NL CH (kg VS) (35% increase). Treatment of TWW by AD with a co-substrate might be possible using a controlled feedstock mixing ratio. Use of TWW as a pre-treatment medium by simple co-storage before AD would be beneficial as an inexpensive treatment of lignocellulosic biomass.

Knowledge about environmental impacts associated with the application of anaerobic digestion residue to agricultural land is of interest owing to the rapid proliferation of biogas plants worldwide. However, virtually no information exists concerning how soil-emitted NO is affected by the feedstock hydraulic retention time (HRT) in the biogas digester. Here, the O planar optode technique was used to visualize soil O dynamics following the surface application of digestates of the codigestion of pig slurry and agro-industrial waste. We also used NO isotopomer analysis of soil-emitted NO to determine the NO production pathways, i.e., nitrification or denitrification. Two-dimensional images of soil O indicated that anoxic and hypoxic conditions developed at 2.0- and 1.5-cm soil depth for soil amended with the digestate produced with 15-d (PO15) and 30-d (PO30) retention time, respectively. Total NO emissions were significantly lower for PO15 than PO30 due to the greater expansion of the anoxic zone, which enhanced NO reduction via complete denitrification. However, cumulative CO emissions were not significantly different between PO15 and PO30 for the entire incubation period. During incubation, NO emissions came from both nitrification and denitrification in amended soils. Increasing the HRT of the biogas digester appears to induce significant NO emissions, but it is unlikely to affect the NO production pathways after application to soil.

Ammonia (NH3) emissions deriving from the management of livestock manure have a significant environmental impact, and therefore it is important to reduce them. Among the available options, the process of NH3 stripping is promising to remove NH3 from manures and digestates recovering it as a mineral fertilizer (e.g., ammonium sulfate) that is more widely adoptable on farms. The traditional stripping process takes place in batches; however, in this study, a continuous process was evaluated using a lab scale plant in which four reactors were used in series with different hydraulic retention times (HRTs) of 12 or 20 days. The NH3 recovery of each reactor was studied for the liquid fraction of pig slurry, dairy cattle slurry and digestate, applying simple headspace aeration. For 20 days of HRT, totals of 92%, 83% and 67% of NH3 were stripped from the digestate, pig slurry and dairy cattle slurry, respectively. For 12 days of HRT, total NH3 recoveries were 83%, 60% and 41% for the digestate, pig slurry and dairy cattle slurry, respectively. The inlet NH3 concentration and inlet total alkalinity had a positive and negative effect, respectively, on the specific NH3 removal rate for each reactor. Stripping NH3 on farm scale can abate NH3 emissions in response to the environmental concerns of European policies.