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Biogas production from anaerobic digestion (AD) of biowastes is restricted by the recalcitrant nature of many substrates, and this may also reduce the fertiliser value of the produced digestate. The degradability of substrates can potentially be enhanced by physico-chemical pre-treatments before AD, and/or the degradation can be increased by a longer digestion time. In this study, we evaluated the effects of electrokinetic (high voltage) and ultrasonication pre-treatments of biowastes in a two-step AD process on nitrogen fertiliser replacement value (NFRV) of digestates obtained from two biogas plants with contrasting hydraulic retention time (HRT) in the primary AD step. The fertiliser value was tested by direct injection to spring barley and surface-banding to winter wheat, and the ammonium N was 15N-labelled to evaluate ammonia losses. The electrokinetic pre-treatment step significantly (p < 0.05) increased the NH 4 +-N/total N in the digestates before the second AD step but had an insignificant effect on the fertiliser value in winter wheat and spring barley. Ultrasonication pre-treatment had also no significant effect on the fertiliser value. The two-step AD significantly (p < 0.001) increased 15N recoveries and mineral fertiliser equivalence of labelled ammonium-N in winter wheat and reduced ammonia losses, with a significant effect (p < 0.001) observed in digestates sourced from a shorter HRT biogas reactor. The fertiliser equivalence of labelled ammonium-N in the digestates was 80–88% after injection, indicating relatively low N immobilisation with all the digestates. NFRV in the crops was mainly explained by the NH 4 +-N/total N ratio, C/N ratio and dry matter content of the digestates. The findings suggest that electrokinetic and ultrasonication pre-treatments combined with a second AD step have no considerable impact on the fertiliser value of digestates, whereas a second AD step significantly reduced ammonia losses after application by surface-banding in winter wheat. [Display omitted] • Two-step AD of digestates increased recoveries of labelled NH 4 +-N in winter wheat. • Two-step AD reduced ammonia losses after surface-banding to winter wheat. • Electrokinetic pre-treatment increased the NH 4 +-N/total N prior to the second AD step. • Electrokinetic and ultrasonication pre-treatments had no influence on the N fertiliser value. [ABSTRACT FROM AUTHOR]

Abstract This study investigated the temperature dependency of CH 4 emission from pre-storage of animal manure prior to anaerobic digestion at 15, 20, 25 and 30 °C using lab-scale anaerobic digesters. The manure was added and removed daily to simulate the pre-storage process at biogas plants. CH 4 emission accounted for 1–46% of total CH 4 potential from pig manure (PM) and 1–2% of that from cattle manure (CM) at the investigated temperatures, with significant increases above 25 °C. Addition of food waste (FW) reduced the CH 4 emission when storage temperature was 20 °C or lower for PM and 25 °C or lower for CM due to volatile fatty acid accumulation and lower pH (<5.5) but emissions increased with higher storage temperatures. Graphical abstract Image 1 Highlights • The temperature dependency of CH 4 emissions during storage of manure was evaluated. • The effect of co-storage of manure with food waste was investigated. • Food waste addition inhibited CH 4 emission at lower temperatures. • Emission at higher temperatures increased with addition of food waste. [ABSTRACT FROM AUTHOR]

Two strategies to include acidified cattle manure (AcCM) in co-digestion with normal cattle manure (CM) are presented in this work. The strategies are a single thermophilic (50 °C) continuous stirred tank reactor (CSTR) anaerobic digestion and a two-step (65 °C + 50 °C) CSTR process. In both strategies, two different inclusion levels of H 2 SO 4 -acidified CM (10% and 20%) in co-digestion with normal CM were tested and compared with a control CSTR fed only CM. Important enhancement of methane (CH 4 ) yield and solid reductions were observed in the thermophilic one-step CSTR working with 10% AcCM. However, a higher inclusion level of AcCM (20%) caused volatile fatty acid accumulation in the reactor and a more than 30% reduction in CH 4 production. In terms of CH 4 production, when 10% of AcCM was co-digested with 90% of CM, the two-step anaerobic co-digestion yielded less than the single step. During the first step of the two-step CSTR process, acidogenesis and a partial sulfate reduction were achieved. However, sulfide stripping between the first and the second step must be promoted in order to advance this technology. [ABSTRACT FROM AUTHOR]

The perennial C 4 grass miscanthus was evaluated for use as an energy crop for methane production when harvested green in the autumn. Miscanthus × giganteus ( M. × giganteus ) and Miscanthus sinensis ( M. sinensis ) were harvested on five occasions, from August to November 2012. Methane yields from stems and leaves were analysed using batch assay after 90 d digestion. Estimated dry matter yields were highest on 1st October for M. × giganteus and 13th September for M. sinensis. Cellulose and lignin contents were greater with M. × giganteus than M. sinensis and low lignin content in leaves led to rapid degradation during the early periods of anaerobic batch assay. After 90 d of anaerobic digestion, cumulative specific methane yields for M. × giganteus varied for stem and leaf from 285 to 333 and 286 to 314 Nl (normalised litre) kg −1 [VS] and 291 to 312 and 298 to 320 Nl kg −1 [VS] for M. sinensis . Estimated methane yields per ha were positively correlated with the dry matter yields of miscanthus ( r = 0.92) and the optimal harvesting time was between September–October. Methane yield at optimal harvest time was estimated as 3.824 × 10 6 Nl ha −1 (stem) and 1.605 × 10 6 Nl ha −1 (leaf) for M. × giganteus and 3.507 × 10 6 Nl ha −1 (stem) and 2.957 × 10 6 Nl ha −1 (leaf) for M. sinensis . There was a discrepancy between the estimating dry matter yield by sampling single shoots and whole plot harvesting. This needs to be further investigated. [ABSTRACT FROM AUTHOR]