Continuous biomethanization of agrifood industry waste: A case study in Spain

Biochemical Μethane potential of most promising agricultural residues in Northern and Southern Greece

Greece produces significant amounts of residual biomass due to its intense agricultural and agro-industrial sector. The anaerobic digestion process has been frequently considered as the best environmental and economic solution for energy recovery from different biodegradable waste such as agricultural waste, livestock manure, agro-industrial waste, as well as for their co-digestion. The aim of this study was the assessment of biochemical methane potential (BMP) of biomass feedstocks representative of Northern and Southern Greece, which are available during the fall/winter and spring/summer seasons, through the implementation of BMP assays. The raw residues evaluated in the current work included: (a) crop residues (corn silage and unsuitable for human consumption watermelon), (b) agro-industrial residues (malt, tomato processing residues, orange peels and olive pomace) and (c) livestock (cattle) manure. Tests of both single substrates and various mixtures were conducted for the evaluation of their methane yields. The results of the mono-substrates are in accordance with other studies in the literature, with watermelon presenting the highest methane potential (421.0 ± 3.4 ml CH₄/g VS_added). After the evaluation of the mixtures and mono-substrates results, the most promising mixtures seemed to be the following: a) for Northern Greece, 10% corn silage-80% cattle manure-10% malt, b) for Southern Greece spring/summer season, 10% corn silage-14% cattle manure-66% watermelon-10% tomato processing residues, and c) for Southern Greece fall/winter season, 10% corn silage-57% cattle manure-23% orange peels-10% olive pomace.

Valuable Compound Extraction, Anaerobic Digestion, and Composting: A Leading Biorefinery Approach for Agricultural Wastes

In a society where the environmental conscience is gaining attention, it is necessary to evaluate the potential valorization options for agricultural biomass to create a change in the perception of the waste agricultural biomass from waste to resource. In that sense, the biorefinery approach has been proposed as the roadway to increase profit of the agricultural sector and, at the same time, ensure environmental sustainability. The biorefinery approach integrates biomass conversion processes to produce fuels, power, and chemicals from biomass. The present review is focused on the extraction of value-added compounds, anaerobic digestion, and composting of agricultural waste as the biorefinery approach. This biorefinery approach is, nevertheless, seen as a less innovative configuration compared to other biorefinery configurations, such as bioethanol production or white biotechnology. However, any of these processes has been widely proposed as a single operation unit for agricultural waste valorization, and a thoughtful review on possible single or joint application has not been available in the literature up to now. The aim is to review the previous and current literature about the potential valorization of agricultural waste biomass, focusing on valuable compound extraction, anaerobic digestion, and composting of agricultural waste, whether they are not, partially, or fully integrated.

Phenols recovery after steam explosion of Olive Mill Solid Waste and its influence on a subsequent biomethanization process

A promising source of high added value compounds is the Olive Mill Solid Waste (OMSW). The aim of this research was to evaluate the viability of a biorefinery approach to valorize OMSW through the combination of steam explosion, phenols extraction, and anaerobic digestion. Steam explosion treatment increased the total phenol content in the steam exploited OMSW, which was twice than that the total phenol content in raw OMSW, although some undesirable compounds were also formed. Phenol extraction allowed the recovery of 2098mg hydroxytyrosol per kg of OMSW. Anaerobic digestion allowed the partial stabilization of the different substrates, although it was not improved by the steam explosion treatment. The economic suitability of the proposed biorefinery approach is favorable up to a phenol extract price 90.7% lower than the referenced actual price of 520€/kg.

Anaerobic co-digestion of agro-food waste mixtures in a fed-batch basis

The agro-food industry (including livestock) generates millions of tonnes of waste products. A solution to this sector's waste disposal challenges was explored by a joint treatment model of organic waste products from several industries. An inventory of agro-food industry organic waste streams with high potential for biogas production was carried out in a logistically viable area (Cider Region, Asturias, Spain). Three industries were selected as those with the higher potential for this study: livestock, dairy and beverage. The kinetics of anaerobic degradation and methane production of four mixtures of selected waste streams were investigated. The specific methane production at five different substrate-to-inoculum ratios (0.50, 0.75, 1.00, 1.50 and 2.00) showed a slightly decreasing trend at the higher ratios. Some hints of a synergistic effect have been observed in mixtures with higher content in milled apple waste, while antagonistic symptoms were noted in mixtures mainly composed of dairy wastes. The estimation of fluxes of waste and methane potentials in the Cider Region suggests centralised anaerobic digestion as a sustainable solution for the valorisation of livestock and agro-food wastes generated in this area. Sector-specific waste streams (livestock and agro-food industry) could cover up to 12% of regional total energy demand.

Anaerobic digestion of tomato processing waste: Effect of alkaline pretreatment

The objective of the work was to assess the effect of mild alkaline pretreatment on the anaerobic biodegradability of tomato processing waste (TPW). Experiments were carried out in duplicate BMP bottles using a pretreatment contact time of 4 and 24 h and a 1% and 5% NaOH dosage. The cumulative methane production during a 30 d period was recorded and modelled. The alkaline pretreatment did not significantly affect methane production in any of the treatments in comparison to the control. The average methane production for all runs was 320 NmL/gVS. Based on first order kinetic modelling, the alkaline pretreatment was found to slow down the rate of methanogenesis, mainly in the two reactors with the highest NaOH dosage. The biodegradability of the substrates ranged from 0.75 to 0.82 and from 0.66 to 0.72 based on two different approaches.

Batch and continuous biogas production arising from feed varying in rice straw volumes following pre-treatment with extrusion

This paper studies the synergistic effects on biogas production obtained when different feedstocks are co-digested with varying proportions of rice straw and explores their behavior at the laboratory scale in continuously stirred digesters. Evaluative measures included methane production, volatile solids degradation, ash accumulation, and extrusion effectiveness. The effect of extrusion on the production of energy was also investigated. Results indicated that continuous stirred digesters fed with substrates composed of 10% or 30% of ensiled rice straw (on total FM) produced 146.1 and 140.0lNCH4kgDM(-1)day(-1), respectively. When extrusion was employed, organic matter degradation was promoted and methane production was significantly raised-by as much as 16%. For the feeds containing 10% rice straw, the increase in obtained energy was higher than the energy needed for the extrusion, but the energy balance was close to zero when the percentage of rice straw was the 30% of the feed.