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The objective of this study was to assess the influence of the hydraulic retention time (HRT) and the pH on the fermentative hydrogen production from a food industry waste (FIW), in a continuous stirred tank bioreactor. Thus, hydrogen production was investigated for HRTs of 12, 8, 6 and 4 h and the results showed a long and stable reactor operation with high hydrogen content in the gas phase and high hydrogen production rates. The optimal HRT was found to be in the range between 6 and 12 h, corresponding to hydrogen yields of 96.27 ± 3.36 and 101.75 ± 3.71 L H 2 /kg FIW for 12 and 6 h, respectively. In the sequel, the effect of the pH (in the range 5–5.9) was investigated, by changing the buffer solution (BS) components, while the HRT value was maintained at 12 h. As anticipated, the results showed that the operating pH had a significant influence on hydrogen production rates and yields. Characterization of the microbial community was performed at various pH values, giving thus a deeper insight to the well-established hydrogen production process from FIW, since the possible biochemical pathways followed by the microbial consortium, under different operational conditions were elucidated. [ABSTRACT FROM AUTHOR]

In this study the effect of nitrogen (N) supplementation in the fungal pretreatment of willow sawdust (WSD) via the white rot fungus Abortiporus biennis (A. biennis) was studied in terms of the fractionation of lignocellulosic biomass and biochemical methane potential (BMP). Thus, different external nitrogen sources (yeast extract (YE), urea (UR), and ammonium nitrate (AN)) at different ratios (N/C of 1/250 and 1/50) were added and the effect of the above parameters on the chemical composition of WSD during solid-state fermentation (SSF) experiments with A. biennis was assessed and compared to the experiment of fungal pretreatment without N supplementation (N/C was 1/500, control experiment). The results indicated that the addition of an external nitrogen source did not facilitate delignification, regardless of the type of nitrogen source and the ratio of N/C used. On the other hand, enhanced cellulose uptake was observed. Samples of the 28th day of cultivation, with and without N supplementation, were used for BMP tests, where a reduction in methane yield was observed, compared to the control experiment. In addition, a combination of fungal with alkali (20% NaOH w/w dry mass) pretreatment was performed in order to assess the effect of combined pretreatment on the lignocellulosic content and the BMP. [ABSTRACT FROM AUTHOR]

Dark fermentation (DF) of several types of wastes is a promising process to alleviate environmental pollution as it leads to the production of valuable hydrogen (H 2) gas and high added value products, such as volatile fatty acids (VFAs). In this study a kinetic model for fermentative H 2 production in an Up-flow column reactor (UFCR) is presented. Τhe model structure includes seven biochemical reactions taking place in a two-phase biofilm-liquid system. The observed difference in the overall stoichiometry of the bioconversion process for different hydraulic retention times (HRTs) is predicted by this model as it is attributed to the difference in the extent of individual bioconversion steps, each of which has a constant stoichiometry but a different rate depending on the HRT. The respective kinetic parameters were estimated through model fitting to the experimental results of the UFCR, which operated at different HRTs (12–2 h) and fed with the soluble fraction of a food industry waste (FIW). A good agreement of the experimental and predicted values of soluble metabolic products and H 2 production was obtained, rendering this model as a useful tool for further investigation and prediction of the characteristics of the DF process in attached-biomass growth systems. The reactions which are considered in the model. [Display omitted] • The model describes H 2 production in attached-biomass growth systems. • 7 reactions take place using fixed stoichiometry and different reaction kinetics. • H 2 production is correlated with butyrate production. • H 2 production may be increased if homoacetogenesis is suppressed. [ABSTRACT FROM AUTHOR]

In this paper a novel data gathering framework for resource-constrained sensor networks is introduced and evaluated. The proposed framework is ideal for application in underwater sensor networking environments, which on top of the conventional sensor technology limitations, present additional constraints such as limited bandwidth and large propagation delays. The newly introduced framework consists mainly of two phases. Initially, due to the lack of infrastructure, a simple bridging technique is used to create a multi-hop tree rooted at the collection center. The corresponding tree is constructed by a simple leveling algorithm and is used to forward packets in the network. In the second phase, due to the fact that the environment under consideration is severely resource constrained, a distributed and probabilistic method is applied by the various sensor nodes to perform data aggregation based on their position (level) on the data gathering tree. The adopted aggregation approach aims at utilizing the available limited resources efficiently and effectively reducing significantly the network traffic, and as a result shortening the delays at the intermediate nodes and reducing the corresponding collisions and energy wastage in data transmission. The performance gains that can be achieved by the proposed data aggregation framework are evaluated via modeling and simulation, under different aggregation scenarios and traffic loads. [ABSTRACT FROM AUTHOR]

In the present study the bioconversion of dried household food waste (FORBI) to energy carriers was investigated aiming to its sustainable management and valorization. FORBI was either directly fermented towards ethanol and hydrogen or was previously subjected to extraction with water resulting to a liquid fraction (extract) rich in sugars and a solid residue, which were then fermented separately. Subsequently, the effluents were assessed as substrates for methane production via anaerobic digestion (AD). Mono-cultures and co-cultures of C5 and C6 yeasts were used for the alcoholic fermentation whereas for the production of hydrogen, mixed acidogenic consortia were used. Taking into account the optimum yields of biofuels, the amount of recoverable energy was estimated based for each different approach. The maximum ethanol yield was 0.16 g ethanol per kg of FORBI and it was achieved for separate fermentation of liquid and solid fractions of the waste. The highest hydrogen yield that was observed was 210.44 L ± 4.02 H 2 /kg TS FORBI for 1% solids loading and supplementation with cellulolytic enzymes. Direct AD of either the whole FORBI or its individual fractions led to lower overall energy recovery, compared to that obtained when fermentation and subsequent AD were applied. The recoverable energy was estimated for the different exploitation approaches of the waste. The maximum achieved recoverable energy was 21.49 ± 0.57 MJ/kg. Unlabelled Image • FORBI (food waste) was bio converted to H 2 , ethanol and methane in 2 stage systems. • Water extraction for separate fermentation of sugars/solids enhanced ethanol yield. • Extraction process did not affect final hydrogen yields. • Energy gain was higher from 2 stage approach than direct AD of FORBI or fractions. [ABSTRACT FROM AUTHOR]

The biotransformation of the pre-dried and shredded organic fraction of kitchen waste to ethanol was investigated, via co-cultures of the yeasts Saccharomyces cerevisiae and Pichia stipitis (Scheffersomyces stipitis ). Preliminary experiments with synthetic media were performed, in order to investigate the effect of different operational parameters on the ethanol production efficiency of the co-culture. The control of the pH and the supplementation with organic nitrogen were shown to be key factors for the optimization of the process. Subsequently, the ethanol production efficiency from the waste was assessed via simultaneous saccharification and fermentation experiments. Different loadings of cellulolytic enzymes and mixtures of cellulolytic with amylolytic enzymatic blends were tested in order to enhance the substrate conversion efficiency. It was further shown that for solids loading up to 40% waste on dry mass basis, corresponding to 170 g.L −1 initial concentration of carbohydrates, no substrate inhibition occurred, and ethanol concentration up to 45 g.L −1 was achieved. [ABSTRACT FROM AUTHOR]