Enhancement of microalgae anaerobic digestion by thermo-alkaline pretreatment with lime (CaO)

The aim of this study was to evaluate for the first time the effect of a thermo-alkaline pretreatment with lime (CaO) on microalgae anaerobic digestion. The pretreatment was carried out by adding different CaO doses (4 and 10%) at different temperatures (room temperature (25 °C), 55 and 72 °C). The exposure time was 4 days for pretreatments at 25 °C, and 24 h for pretreatments at 55 and 72 °C. Following, a biochemical methane potential test was conducted with pretreated and untreated microalgae. According to the results, the pretreatment enhanced proteins solubilisation by 32.4% and carbohydrates solubilisation by 31.4% with the highest lime dose and temperature (10% CaO and 72 °C). Furthermore, anaerobic digestion kinetics were improved in all cases (from 0.08 to 0.14 day- 1 for untreated and pretreated microalgae, respectively). The maximum biochemical methane potential increase (25%) was achieved with 10% CaO at 72 °C, in accordance with the highest biomass solubilisation. Thus, lime pretreatment appears as a potential strategy to improve microalgae anaerobic digestion.Peer ReviewedPostprint (author's final draft

Dry fractionation of plant material powders using an electrostatic corona separator: a model study

In a dry bioreffinery scheme, the separation of plant materials into fractions of interest compound is a crucial step. In recent years, electrostatic separation of agri-resources has sparked a growing interest for its potential but optimization efforts remain to be done especially in case of fine powders. The present work is an experimental investigation of factors, which influence the separation process of plant biomass powders in a custom built corona electrostatic separator with view to optimization. Three particle size classes of cork, semolina and wood powders have been characterized in term of charge decay curves and their behavior on the device have been studied. Separation tests of a blend constituted of 1g of wood powder and 1g of semolina have also been conducted with very promising results. Please click Additional Files below to see the full abstract

Biogas from agricultural waste : Turning unavailable residues into accessible resources

Moving from a fossil dependent to a fossil free economy requires increased energy production from renewable resources. This thesis discusses the utilization of agricultural waste streams, such as straw and manure, for biogas production. The first part of the research presented focuses on pretreatment of straw with the aim to reduce the handling issues concerning straw and improve the degradation of the material during anaerobic digestion. The second part concerns process design of agro-based biogas production plants with the aim to find process configurations and feedstocks that lead to high bioenergy yields. The effect of co-digesting a manure-rich stream with a carbohydrate-rich stream and the role of the degree of carbohydrate accessibility on the methane production is also discussed.Wheat straw is a problematic material to digest due to its high porosity which causes it to float and makes it hard to pump/feed. Mechanical pretreatments that applied higher shearing to the straw, such as pelletization and extrusion, led to reduced floating layers. Particle size reduction of wheat straw impacted the methane production rate below 3 mm but did not have an impact on the methane yield. The particle size was, however, not the only factor affecting the methane production rate. Hammer milled straw and extruded straw had a similar particle size but the degradation of extruded straw was faster. To increase the methane production rate, a shearing effect of the pretreatment may be more important. To solve only the handling issues of the straw, it may not be worth the high energy demand of those pretreatments.Wheat straw cannot be digested without the addition of nutrients. Co-digestion with manure or animal bedding is thereby a promising solution. By washing the animal bedding, it was possible to separate out the fibers and subject them to pretreatment with similar yields as pretreatment of wheat straw. Such a process design opened up for parallel production of biogas from the manure-fraction and fiber hydrolysate, and bioethanol from the steam pretreated fibers. Co-digestion of manure and readily available hydrolysate led to an increased initial lag phase and additional studies presented in this thesis showed that a too high degree of carbohydrate accessibility will increase the risk of process instability due to volatile fatty acids accumulation. Further, because part of the carbohydrates was diverted for yeast fermentation, the C/N ratio in the anaerobic digestion step became low. To solve this, cow manure and additional wheat straw were added to the production process. Energy balances and estimated energy demands over the process, in comparisons with other designs, showed that biofuel production was more energy efficient without co-production of ethanol. However, because of the recovery of lignin, there is a great potential of covering most of the energy demand by on-site steam production. Like so, the energy efficiency would much improve

Mechanical pretreatment of lignocelluloses for enhanced biogas production: Methane yield prediction from biomass structural components

In this study, mechanical pretreatment was applied to six different lignocelluloses in two different treatment phases and the prediction of their methane yield was done from biomass chemical composition. Physicochemical, proximate and microbial analyses were carried out on both pretreated and untreated biomass using standard methods. Mechanical pretreatments caused the breakdown of structural materials in all the used biomass which was characterized by reduction of the lagging time during anaerobic digestion and the subsequent increase in methane yield up to 22%. The different loading rate of biomass had no effect on the overall methane yield increase. Both single and multiple linear regressions models were used in order to correlate the chemical composition of the biomass with their methane potentials and a fairly high correlation (R2 = 0.63) was obtained. The study also showed that the pretreatments are economically feasible. Therefore, its further application to other biomass is encouraged

Effects of thermal and mechanical pretreatments of secondary sludge on biogas production under thermophilic conditions

Slow degradation of sewage sludge is a disadvantage of anaerobic digestion leading to high sludge retention times in conventional digesters. Hydrolysis has been pointed as the rate-limiting step in this process. Thermophilic anaerobic digestion has been proved effectively to reduce the retention time needed for sludge stabilization. Sludge pretreatment has been also proposed as a strategy to accelerate the hydrolytic step. The effectiveness of high and low temperature thermal pretreatment, ultrasonic and microwave pretreatments in secondary sludge disintegration has been studied by means of the increment in filterable volatile solids to total volatile solids ratio (FVS/TVS) respect to untreated sludge. Increments in this parameter ranging from three-fold for microwave treatment to nine-fold for high temperature thermal treatment have been obtained. Biogas production under thermophilic (55 °C) conditions for treated and untreated secondary sludge has also been evaluated. In spite of the values of FVS/TVS ratio obtained for all the treatments studied, no differences in biogas production were observed when high temperature thermal treatment, ultrasonic and microwave treatments are compared with untreated sludge. A 50% increment in biogas production was observed for low temperature (70 °C) thermally treated sludge. This type of treatment has been pointed as a predigestion step enhancing biological activity of some thermophilic hydrolytic bacteria

Main directions to increas biogas production for organic wastes management

Main analysis of efficiency of biogas production from organic waste in the frame of pretreatment are presented in the article

Pretreatment techniques used in biogas production from grass

Grass is being considered as a potential feedstock for biogas production, due to its low water consumption compared to other crops, and the fact that it can be cultivated in non-arable lands, avoiding the direct competition with food crops. However, biogas production is limited by the characteristics of the feedstock; in particular its complex lignocellulosic structure. Hence, different pretreatment methods are being investigated for grass structure disruption before undergoing the anaerobic digestion process. The aim of this paper is to review current knowledge on pretreatment techniques used for grassland biomass. Pretreatment techniques were categorized into mechanical, microwave, thermal, chemical and biological groups. The effect of the application of each studied methods on the biogas yield and on the energy balance is discussed. A further comparison between the covered techniques was revealed