M.J. Cuetos

Anaerobic digestion and co-digestion of slaughterhouse waste (SHW): influence of heat and pressure pre-treatment in biogas yield.

Mesophilic anaerobic digestion (34+/-1 degrees C) of pre-treated (for 20 min at 133 degrees C, >3 bar) slaughterhouse waste and its co-digestion with the organic fraction of municipal solid waste (OFMSW) have been assessed. Semi-continuously-fed digesters worked with a hydraulic retention time (HRT) of 36 d and organic loading rates (OLR) of 1.2 and 2.6 kg VS(feed)/m(3)d for digestion and co-digestion, respectively, with a previous acclimatization period in all cases. It was not possible to carry out an efficient treatment of hygienized waste, even less so when OFMSW was added as co-substrate. These digesters presented volatile fatty acids (VFA), long chain fatty acids (LCFA) and fats accumulation, leading to instability and inhibition of the degradation process. The aim of applying a heat and pressure pre-treatment to promote splitting of complex lipids and nitrogen-rich waste into simpler and more biodegradable constituents and to enhance biogas production was not successful. These results indicate that the temperature and the high pressure of the pre-treatment applied favoured the formation of compounds that are refractory to anaerobic digestion. The pre-treated slaughterhouse wastes and the final products of these systems were analyzed by FTIR and TGA. These tools verified the existence of complex nitrogen-containing polymers in the final effluents, confirming the formation of refractory compounds during pre-treatment.

Anaerobic co-digestion of livestock wastes with vegetable processing wastes: A statistical analysis

Anaerobic digestion of livestock wastes with carbon rich residues was studied. Swine manure and poultry litter were selected as livestock waste, and vegetable processing waste was selected as the rich carbon source. A Central Composite Design (CCD) and Response Surface Methodology (RSM) were employed in designing experiments and determine individual and interactive effects over methane production and removal of volatile solids. In the case of swine manure co-digestion, an increase in vegetable processing waste resulted in higher volatile solids removal. However, without a proper substrate/biomass ratio, buffer capacity of swine manure was not able to avoid inhibitory effects associated with TVFA accumulation. Regarding co-digestion with poultry litter, substrate concentration determined VS removal achieved, above 80 g VSL(-1), NH(3) inhibition was detected. Statistical analysis allowed us to set initial conditions and parameters to achieve best outputs for real-scale plant operation and/or co-digestion mixtures design.

Anaerobic co‐digestion of poultry blood with OFMSW: FTIR and TG–DTG study of process stabilization

The potential of anaerobic digestion for the treatment of poultry blood has been evaluated in a co‐digestion process. The organic fraction of municipal solid waste (OFMSW) was employed as the co‐substrate to avoid digestion inhibition by dilution of nitrogen content and improvement of biodegradability. A semi‐continuous mesophilic anaerobic digester was studied with a hydraulic retention time (HRT) of 36 days and an organic loading (OLR1) of 1.5 kg VSSfeed m−3 d−1. The normal operational conditions of the reactor were altered with the application of an OLR2 of 2.0 kg VSSfeed m−3 d−1 for a short period causing an imbalance in the process. The reduction of the OLR to initial conditions allowed the recovery of the system. The digestion process reached a final specific gas production (SGP) and a methane yield of 0.33 and 0.20 m3 kg−1 VSSfeed, respectively, maintaining low total and free ammonia concentrations. Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were used to gain an insight into transformations experimented by the organic matter at the end of the stabilization process. Furthermore, these analytical techniques were used for evaluating the transformations undergone by the nitrogen‐rich protein components of blood after digestion. It was proved that a reduction in volatile content and aliphatic structures of biowastes along with an increase in the degree of aromaticity occurred during the digestion process.

Anaerobic digestion of solid slaughterhouse waste: study of biological stabilization by Fourier Transform infrared spectroscopy and thermogravimetry combined with mass spectrometry

In this paper, Fourier Transform infrared spectroscopy (FTIR) along with thermogravimetric analysis together with mass spectrometry (TG–MS analysis) were employed to study the organic matter transformation attained under anaerobic digestion of slaughterhouse waste and to establish the stability of the digestates obtained when compared with fresh wastes. Digestate samples studied were obtained from successful digestion and failed systems treating slaughterhouse waste and the organic fraction of municipal solid wastes. The FTIR spectra and TG profiles from well stabilized products (from successful digestion systems) showed an increase in the aromaticity degree and the reduction of volatile content and aliphatic structures as stabilization proceeded. On the other hand, the FTIR spectra of non-stable reactors showed a high aliphaticity degree and fat content. When comparing differential thermogravimetry (DTG) profiles of the feed and digestate samples obtained from all successful anaerobic systems, a reduction in the intensity of the low-temperature range (≈300°C) peak was observed, while the weight loss experienced at high-temperature (450–550°C) was variable for the different systems. Compared to the original waste, the intensity of the weight loss peak in the high-temperature range decreased in the reactors with higher hydraulic retention time (HRT) whereas its intensity increased and the peak was displaced to higher temperatures for the digesters with lower HRT.

Digestion of cattle manure: thermogravimetric kinetic analysis for the evaluation of organic matter conversion.

Anaerobic digestion of cattle manure was studied under thermophilic and mesophilic conditions with the purpose of evaluating the effect of temperature on the quality of the final digestate. Non-isothermal thermogravimetric kinetic analysis was applied for assessing organic matter conversion of biological stabilization. The mathematical approximation proves to be a useful tool for evaluating the differences attained during biological degradation. The anaerobic digestion of the organic substrate resulted in a reduction of the activation energy value obtained from the different applied kinetic models. Results obtained from thermal kinetic analysis were in accordance with those from the monitoring of the anaerobic digestion process. The higher values of methane yield reported for the mesophilic digestion in comparison to that of the thermophilic indicated a greater capability of the former process in the utilization of substrate and thus a higher conversion of organic matter which can be quantified by the activation energy value.

Feasibility of anaerobic co-digestion of poultry blood with maize residues.

The potential of anaerobic digestion for the treatment of poultry blood was evaluated in batch assays at laboratory scale and in a mesophilic semi-continuously fed digester. The biodegradability test performed on poultry blood waste showed a strong inhibition. Maize residues were used as co-substrate to overcome inhibition thanks to nitrogen dilution. Under batch operation, increasing the maize concentration from 15% to 70% (volatile solids (VS) basis) provided an increase of biogas from 130±31 to 188±21 L CH4/kg VS. In the semi-continuous mesophilic anaerobic digester, the biogas yield was 165±17 L CH4/kg VS fed, as a result of strong volatile fatty acid (VFA) accumulation. Although physical modifications of maize particles were observed by Scanning Electron Microscopy (SEM), an incomplete degradation was confirmed from analysis of digestates. Furthermore, Fourier Transform Infrared (FTIR) spectroscopy analysis demonstrated that along with VFA build-up, an accumulation of non-degraded materials took place.

Study of thermal pre-treatment on anaerobic digestion of slaughterhouse waste by TGA-MS and FTIR spectroscopy

Thermogravimetric analysis coupled to mass spectrometry (TGA-MS) and Fourier-transform infrared spectroscopy (FTIR) were used to describe the effect of pasteurization as a hygienic pre-treatment of animal by-products over biogas production. Piggery and poultry meat wastes were used as substrates for assessing the anaerobic digestion under batch conditions at mesophilic range. Poultry waste was characterized by high protein and carbohydrate content, while piggery waste presented a major fraction of fat and lower carbohydrate content. Results from anaerobic digestion tests showed a lower methane yield for the pre-treated poultry sample. TGA-MS and FTIR spectroscopy allowed the qualitative identification of recalcitrant nitrogen-containing compounds in the pre-treated poultry sample, produced by Maillard reactions. In the case of piggery waste, the recalcitrant compounds were not detected and its biodegradability test reported higher methane yield and production rates. TGA-MS and FTIR spectroscopy were demonstrated to be useful tools for explaining results obtained by anaerobic biodegradability test and in describing the presence of inhibitory problems.

A comparison of analytical techniques for evaluating food waste degradation by anaerobic digestion.

Organic matter contained in food waste was degraded by anaerobic digestion under mesophilic and thermophilic conditions at two hydraulic retention times. Evolution of the digestion process was followed by thermogravimetry analysis, fluorescence spectroscopy and 1H nuclear magnetic resonance. All analytical methods suggested that longer retention times might be required for food waste stabilization under mesophilic conditions as compared to thermophilic stabilization. All the analytical methods showed that the stabilization process consisted of two steps, where complex organic molecules were formed during initial stabilization and then digested providing sufficient hydraulic retention time. Longer hydraulic retention times were required for food waste stabilization under mesophilic conditions. Overall, thermal and 1H NMR analyses of the digestate samples might be recommended if more detailed analysis is required, while fluorescence measurements can be used as a fast screening technique, which provides qualitative assessment of the stabilization process.

Enhancing anaerobic digestion of poultry blood using activated carbon

Graphical abstract

Improvement of biogas production by co-digestion of swine manure and residual glycerine

Anaerobic digestion of swine manure supplemented with residual glycerine from a biodiesel production plant was investigated as a means for increasing biogas production on livestock farms treating residues by the anaerobic digestion process. Two different manures were tested proceeding from two local facilities, being evaluated under batch digestion. Semi-continuous co-digestion was studied using single-stage completely mixed anaerobic digesters. Mixtures were prepared at 2, 5 and 8% (v/v) of glycerine. Improvement of biogas production was observed with the increment in the content of residual glycerine fed to the digester, increasing the biogas production from 2.1 to 5.4 l/day. Inhibition by ammonium was presented in all systems evaluated (with free ammonia concentrations above 300 mg/l), resulting in volatile fatty acids build-up, being higher as the content of glycerine was increased in the feeding mixture. Although systems were submitted to a 30-day adaptation period and the time of study was performed until completing three-times the hydraulic retention time, volatile fatty acid measurements presented great variability being indicative that a time greater than three volume turnovers is needed if conditions of steady state are to be set.