Anaerobic digestion of spent cow bedding in batch leach-bed reactors (LBRs) was compared in mesophilic and thermophilic conditions for the first time. Results show that the use of thermophilic conditions enhanced only the degradation kinetics of easily-degradable matter during the first days of the digestion, whereas similar methane yields (80% of the Biomethane Potential) were reached after 42days at both temperatures. Therefore, the anaerobic digestion in LBRs of spent cow bedding, a substrate rich in slowly-degradable compounds, was not improved in term of methane production considering the overall digestion time. Moreover, the high initial biogas production rate in thermophilic reactors was found to significantly reduce the energetic performance of the cogeneration unit at industrial scale, leading to a 5.9% decrease in the annual electricity production when compared to a mesophilic one.

In anaerobic leach-bed reactors (LBRs) co-digesting an easily- and a slowly-degradable substrate, the importance of the leachate flush both on extracting volatile fatty acids (VFAs) at the beginning of newly-started batches and on their consumption in mature reactors was tested. Regarding VFA extraction three leachate flush-rate conditions were studied: 0.5, 1 and 2Lkg−1TSd−1. Results showed that increasing the leachate flush-rate during the acidification phase is essential to increase degradation kinetics. After this initial phase, leachate injection is less important and the flush-rate could be reduced. The injection in mature reactors of leachate with an acetic acid concentration of 5 or 10gL−1 showed that for an optimized VFA consumption in LBRs, VFAs should be provided straight after the methane production peak in order to profit from a higher methanogenic activity, and every 6–7h to maintain a high biogas production rate.

Spent animal bedding is a valuable resource for green energy production in rural areas. The properties of six types of spent bedding collected from deep-litter stables, housing either sheeps, goats, horses or cows, were compared and their anaerobic digestion in a batch Leach-Bed Reactor (LBR) was assessed. Spent horse bedding, when compared to all the other types, appeared to differ the most due to a greater amount of straw added to the litter and a more frequent litter change. Total solids content appeared to vary significantly from one bedding type to another, with consequent impact on the methane produced from the raw substrate. However, all the types of spent bedding had similar VS/TS (82.3–88.9)%, a C/N well-suited to anaerobic digestion (20–28, except that of the horse, 42) and their BMPs were in a narrow range (192–239NmLCH4/gVS). The anaerobic digestion in each LBR was stable and the pH always remained higher than 6.6 regardless of the type of bedding. In contrast to all the other substrates, spent goat bedding showed a stronger acidification resulting in a methane production lag phase. Finally, spent bedding of different origins reached, on average, (89±11)% of their BMP after 60days of operation. This means that this waste is well-suited for treatment in LBRs and that this is a promising process to recover energy from dry agricultural waste.

The influence of different proportions of lignocellulosic substrate (cow manure with straw, CM) on the single-phase (conventional reactor) and two-phase (acidification/methanation with solids and liquid recirculation) digestion of a readily biodegradable substrate (fruit and vegetable waste, FVW) was investigated in order to determine the optimum cosubstrate ratio and the process best suited for codigestion. Both processes were fed initially with FVW, followed by FVW and CM at 80%:20% and 60%:40% (on volatile solids, VS basis) during an experiment run over eleven months. For the single-phase process, energy yield and VS destruction decreased by 11% and 9% with the 80%:20% FVW and CM ratio and by 16% and 17% with the 60%:40% feed ratio when compared to 100% FVW feed. For the two-phase process, energy yield and VS destruction decreased by 21% and 14% with 80%:20% feed ratio and by 48% and 33% with 60%:40% feed ratio compared to 100% FVW. Substrate solubilization in the acidification reactor was very efficient for all the feed proportions but it resulted in compounds other than volatile fatty acid (non-VFA COD) which were not easily amenable to methane generation. This led to a lower energy yield per kg of VS fed in the two-phase process compared to the single-phase process for the respective waste combination. For single-phase digestion, both 80%:20% and 60%:40% ratios were effective co-substrate combinations due to their higher energy yield. The two-phase process can be used for these ratios if higher VS reduction and a higher loading rate are the objectives.

Coupling algal biomass production and anaerobic digestion is one of the most promising bioprocesses for economically viable algal production. This study assesses the production rates of some native microalgae growing in media supplemented with algal digestate, urban wastewater or digested sludge. Native microalgal populations isolated from temperate freshwaters (Scenedesmus spp.) and marine ecosystems (Nannochloris spp.) had the highest potential production rates (about 100 mg DW L−1 d−1) with algal digestate at about 20% loading ratio. However, no growth was measured for Nannochloris spp., when the ammonium concentration exceeded 100 mg L−1 although Scenedesmus spp. appeared to be tolerant to higher NH4+ concentrations. Very low production rates, or no growth, were measured when microalgae isolated from high salinity waters (Dunaliella salina, Lyngbya aestuarii) were used, suggesting that populations well adapted to extreme environmental conditions are not suitable candidates for growing on wastewater or anaerobic digestate.

Single-phase and two-phase digestion of fruit and vegetable waste were studied to compare reactor start-up, reactor stability and performance (methane yield, volatile solids reduction and energy yield). The single-phase reactor (SPR) was a conventional reactor operated at a low loading rate (maximum of 3.5kgVS/m3d), while the two-phase system consisted of an acidification reactor (TPAR) and a methanogenic reactor (TPMR). The TPAR was inoculated with methanogenic sludge similar to the SPR, but was operated with step-wise increase in the loading rate and with total recirculation of reactor solids to convert it into acidification sludge. Before each feeding, part of the sludge from TPAR was centrifuged, the centrifuge liquid (solubilized products) was fed to the TPMR and centrifuged solids were recycled back to the reactor. Single-phase digestion produced a methane yield of 0.45m3CH4/kg VS fed and VS removal of 83%. The TPAR shifted to acidification mode at an OLR of 10.0kgVS/m3d and then achieved stable performance at 7.0kgVS/m3d and pH 5.5–6.2, with very high substrate solubilization rate and a methane yield of 0.30m3CH4/kg COD fed. The two-phase process was capable of high VS reduction, but material and energy balance showed that the single-phase process was superior in terms of volumetric methane production and energy yield by 33%. The lower energy yield of the two-phase system was due to the loss of energy during hydrolysis in the TPAR and the deficit in methane production in the TPMR attributed to COD loss due to biomass synthesis and adsorption of hard COD onto the flocs. These results including the complicated operational procedure of the two-phase process and the economic factors suggested that the single-phase process could be the preferred system for FVW.

The rapid development of anaerobic digestion brought with it the problem of biomass resources and supply. Marine biomass is emerging as an advantageous substrate. Such macroalgae as Palmaria palmata are promising substrates for anaerobic digestion as they possess a high methane potential (308±9mLgVS−1). The aim of this paper was to study the efficiency of the anaerobic digestion of P. palmata after a range of thermal and chemical pretreatment. The anaerobic digestion of raw and pretreated macroalgae was carried out in batch mesophilic biomethane potential tests (BMP). Thermal (between 20 and 200°C) and thermo-chemical (addition of NaOH and HCl) pretreatment were performed on P. palmata. Thermal pretreatments at 20, 70, 85 and 120°C and acid or soda pretreatments at 160°C had no significant effect on P. palmata's methane potential. After high temperature pretreatment (180–200°C), the BMP decreased with the temperature which can be explained by the formation of refractory compounds in the liquid fraction. In contrast, the addition of 0.04gNaOHgTS−1 at 20°C led to a release of proteins and induced an increase in the BMP from 308±9mLgVS−1 (untreated) to 365±9mLgVS−1. Thus, P. palmata can be used advantageously as a substrate for anaerobic digestion and its methane production enhanced by the addition of NaOH.

The impact of stepwise increase in OLR (up to 7.5kgVS/m3d) on methane production, reactor performance and solubilised organic matter production in a high-loading reactor were investigated. A reference reactor operated at low OLR (<2.0kgVS/m3d) was used solely to observe the methane potential of the feed substrate. Specific methane yield was 0.33lCH4/gVS at the lowest OLR and dropped by about 20% at the maximum OLR, while volumetric methane production increased from 0.35 to 1.38m3CH4/m3d. At higher loadings, solids hydrolysis was affected, with consequent transfer of poorly-degraded organic material into the drain solids. Biodegradability and size-fractionation of the solubilised COD were characterized to evaluate the possibility of a second stage liquid reactor. Only 18% of the organics were truly soluble (<1kD). The rest were in colloidal and very fine particulate form which originated from grass and cow manure and were non-biodegradable.

Biogas production is one of the means to produce a biofuel from microalgae. Biomass consisting mainly of Scenedesmus sp. was thermally pretreated and optimum pretreatment length (1h) and temperature (90°C) was selected. Different chemical composition among batches stored at 4°C for different lengths of time resulted in organic matter hydrolysis percentages ranging from 3% to 7%. The lower percentages were attributed to cell wall thickening observed during storage for 45days. The different hydrolysis percentages did not cause differences in anaerobic digestion. Pretreatment of Scenedesmus sp. at 90°C for 1h increased methane production 2.9 and 3.4-fold at organic loading rates (OLR) of 1 and 2.5kgCODm−3day−1, respectively. Regardless the OLR, inhibition caused by organic overloading or ammonia toxicity were not detected. Despite enhanced methane production, anaerobic biodegradability of this biomass remained low (32%). Therefore, this microalga is not a suitable feedstock for biogas production unless a more suitable pretreatment can be found.

Association of microalgae culture and anaerobic digestion seems a promising technology for sustainable algal biomass and biogas production. The use of digestates for sustaining the growth of microalgae reduces the costs and the environmental impacts associated with the substantial algal nutrient requirements. A natural marine algae–bacteria consortium was selected by growing on a medium containing macro nutrients (ammonia, phosphate and acetate) specific of a digestate, and was submitted to a factorial experimental design with different levels of temperature, light and pH. The microalgal consortium reached a maximum C conversion efficiency (i.e. ratio between carbon content produced and carbon supplied through light photosynthetic C conversion and acetate) of 3.6%. The presence of bacteria increased this maximum C conversion efficiency up to 6.3%. The associated bacterial community was considered beneficial to the total biomass production by recycling the carbon lost during photosynthesis and assimilating organic by-products from anaerobic digestion.