Michel Torrijos

Single-phase and two-phase anaerobic digestion of fruit and vegetable waste: comparison of start-up, reactor stability and process performance.

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 3.5 kgVS/m3 d), 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 0.45 m3 CH4/kg VS fed and VS removal of 83%. The TPAR shifted to acidification mode at an OLR of 10.0 kgVS/m3 d and then achieved stable performance at 7.0 kgVS/m3 d and pH 5.5-6.2, with very high substrate solubilization rate and a methane yield of 0.30 m3 CH4/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.

Anaerobic co-digestion of solid waste: Effect of increasing organic loading rates and characterization of the solubilised organic matter

The impact of stepwise increase in OLR (up to 7.5kgVS/m(3)d) 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/m(3)d) was used solely to observe the methane potential of the feed substrate. Specific methane yield was 0.33lCH(4)/gVS at the lowest OLR and dropped by about 20% at the maximum OLR, while volumetric methane production increased from 0.35 to 1.38m(3)CH(4)/m(3)d. 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.

DEFINITION OF OPTIMAL CONDITIONS FOR THE HYDROLYSIS AND ACIDOGENESIS OF A PHARMACEUTICAL MICROBIAL BIOMASS

The effect of operational parameters such as hydraulic retention time (HRT), influent substrate concentration (So), pH, temperature and recycling on the acidogenic digestion of a pharmaceutical biomass was investigated. The total COD content of the pharmaceutical effluent used was 50 g/litre, only 13·6% occurred in soluble form and consisted mostly of protein (22 g/litre). Substrate degradation was quantified in terms of COD, protein and carbohydrate degradation. Variations in COD solubilization were mainly due to protein solubilization. The best results, in terms of COD solubilization and concentrations of produced VFA, were obtained at pH = 8·5, T = 35°C and OLR = 5·4 gCOD/litre/day. Under these conditions 50·6% of the initial COD content was solubilized and 17·1 g/litre of volatile fatty acids (VFA) were produced. Recycling of biomass resulted in greater COD solubilization but in lower acidogenic activity. Moreover, the effluent of the acidogenic reactor showed poor settling properties.

Slaughterhouse fatty waste saponification to increase biogas yield.

A thermochemical pretreatment, i.e. saponification, was optimised in order to improve anaerobic biodegradation of slaughterhouse wastes such as aeroflotation grease and flesh fats from cattle carcass. Anaerobic digestion of raw wastes, as well as of wastes saponified at different temperatures (60 degrees C, 120 degrees C and 150 degrees C) was conducted in fed-batch reactors under mesophilic condition and the effect of different saponification temperatures on anaerobic biodegradation and on the long-chain fatty acids (LCFAs) relative composition was assessed. Even after increasing loads over a long period of time, raw fatty wastes were biodegraded slowly and the biogas potentials were lower than those of theoretical estimations. In contrast, pretreated wastes exhibited improved batch biodegradation, indicating a better initial bio-availability, particularly obvious for carcass wastes. However, LCFA relative composition was not significantly altered by the pretreatment. Consequently, the enhanced biodegradation should be attributed to an increased initial bio-availability of fatty wastes without any modification of their long chain structure which remained slowly biodegradable. Finally, saponification at 120 degrees C achieved best performances during anaerobic digestion of slaughterhouse wastes.

Kinetic modelling of anaerobic hydrolysis of solid wastes, including disintegration processes

A methodology to estimate disintegration and hydrolysis kinetic parameters of solid wastes and validate an ADM1-based anaerobic co-digestion model is presented. Kinetic parameters of the model were calibrated from batch reactor experiments treating individually fruit and vegetable wastes (among other residues) following a new protocol for batch tests. In addition, decoupled disintegration kinetics for readily and slowly biodegradable fractions of solid wastes was considered. Calibrated parameters from batch assays of individual substrates were used to validate the model for a semi-continuous co-digestion operation treating simultaneously 5 fruit and vegetable wastes. The semi-continuous experiment was carried out in a lab-scale CSTR reactor for 15 weeks at organic loading rate ranging between 2.0 and 4.7 gVS/Ld. The model (built in Matlab/Simulink) fit to a large extent the experimental results in both batch and semi-continuous mode and served as a powerful tool to simulate the digestion or co-digestion of solid wastes.

Comprehensive characterization of the liquid fraction of digestates from full-scale anaerobic co-digestion

Waste management by anaerobic digestion generates a final byproduct, the digestate, which is usually separated into solid and liquid fractions to reduce the volume for transportation. The composition of the solid fraction has been recently studied to allow its valorization. However, full composition of liquid fraction of digestate and its size fractionation are less considered in the literature for efficient post treatment and valorization purposes. Therefore, here we characterized in detail liquid fraction of digestate obtained after solid-liquid separation from 11 full-scale co-digestion plants. The liquid fraction has a high concentration in organic matter with Chemical Oxygen Demand (COD) from 9.2 to 78g/L with 60-96% of COD in suspended particles (>1.2μm), 2-27% in colloids (1.2μm to 1kDa) and 2-18% in dissolved matter (<1kDa). Besides, it contained from 1.5 to 6.5g/L total nitrogen and high ions concentrations (0.5-3.1g/L NH4+, 1.05-5.48g/L K+, 0-2.13g/L PO43-). In addition, liquid fraction of digestate has poor biodegradability due to presence of humic substances making aerobic treatment inefficient. Only physico-chemical post treatment can be proposed for organic matter removal.

Mesophilic anaerobic digestion of several types of spent livestock bedding in a batch leach-bed reactor: substrate characterization and process performance.

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.

Methane Potential of Waste Activated Sludge and Fatty Residues: Impact of Codigestion and Alkaline Pretreatments

The aim of this study was to maximise methane production from waste activated sludge (WAS) originating from extended aeration process and presenting a low methane potential (190 mL CH4.g -1 OM). WAS co-digestion with fatty residues (FR, 560 mL CH4.g -1 OM) produced during pretreatments of the effluents from wastewater treatment plants in the Lille area and fatty wastewaters (FW, around 700 mL CH4.g -1 OM) collected from restaurants was assessed by batch experiments. Moreover saponification/alkali pretreatments improved kinetics of anaerobic digestion but had a low impact on methane potential (+ 6-7%) of the mixed waste composed of 66.6% of FW, 33.3% of WAS and 0.1% of FR. As results did not depend on pH ranging from 8 to 10 (addition of 0.12 to 0.21 gKOH gOM -1 ) nor temperature ranging from 80 to 120 °C, the least severe studied pretreatment conditions (80°C and pH=8) may be selected for further studies on con- tinuous anaerobic reactors.

Anaerobic biodegradability of water separated from extracted crude oil

Abstract A study of the anaerobic biodegradability of the three categories of water separated from extracted crude oil (Venezuelan oilfields) ‐ light, medium or heavy crude ‐ was carried out at laboratory scale using UASB reactors working at mesophilic conditions. Chemical oxygen demand (COD) removal in a low loaded UASB reactor fed with water separated from extracted light crude was high, with an average 87 % purification efficiency. The remaining COD was made up of the non‐biodegradable and the very slowly biodegradable fractions of the organic matter in the water. During a second period, the hydraulic retention time was reduced in stages, thus increasing the loading rate. In the experimental conditions used, COD concentration at the outlet remained below the Venezuelan standard limit for discharge into the environment (350 mg COD 1‐1) when the hydraulic retention time (HRT) was above 10 hours and the OLR under 3 g COD l‐1 d‐1. For HRT less than 10 hours, or organic loading rate (OLR) greater than 3 g COD I‐1 d‐1, COD at the outlet of the reactor rose as a consequence both of increased volatile fatty acids (VFA) concentrations (indicating an overloading of the methanogenic population) and, also, of the increase in the non‐VFA COD (indicating a decrease in the acidification efficiency). On the other hand, results with UASB reactors operated at a low loading rate and fed with water separated from extracted medium and heavy crude oil showed that purification efficiency was low, with only 20 % and 37 % COD removal respectively. Continuing the operation of the UASB reactor fed with water separated from medium oil over a prolonged period did not bring any improvement, indicating that no adaptation of the sludge occurred.

Characterisation of the biodegradability of post-treated digestates via the chemical accessibility and complexity of organic matter

The stability of digestate organic matter is a key parameter for its use in agriculture. Here, the organic matter stability was compared between 14 post-treated digestates and the relationship between organic matter complexity and biodegradability was highlighted. Respirometric activity and CH4 yields in batch tests showed a positive linear correlation between both types of biodegradability (R2=0.8). The accessibility and complexity of organic matter were assessed using chemical extractions combined with fluorescence spectroscopy, and biodegradability was mostly anti-correlated with complexity of organic matter. Post-treatments presented a significant effect on the biodegradability and complexity of organic matter. Biodegradability was low for composted digestates which comprised slowly accessible complex molecules. Inversely, solid fractions obtained after phase separation contained a substantial part of remaining biodegradable organic matter with a significant easily accessible fraction comprising simpler molecules. Understanding the effect of post-treatment on the biodegradability of digestates should help to optimize their valorization.