Ángela Rodríguez-Abalde

Effects of thermal pre-treatments on solid slaughterhouse waste methane potential

The effects of thermal pre-treatments on the biogas production potential of two solid slaughterhouse waste types (poultry and piggery slaughterhouse by-products) were assessed by means of batch experiments. Both animal by-products were characterized in terms of fat, protein and carbohydrate concentrations. The selected thermal pre-treatments, pasteurization (70 °C for 60 min) and sterilization (133 °C and 3 bars for 20 min), are included in the current European regulations for the disposal or use of animal by-products. The pre-treatments produced notable improvements in organic matter solubilization, but had different effects on the anaerobic bioavailability of the treated substrates. The methane yield of the initial volatile solids did not increase significantly after pre-treatment when carbohydrate concentration was high, reaching a maximum of 0.48 m(CH4)(3) kg(VS)(-1) for the pasteurized poultry waste. However, this yield increased by up to 52.7% after pasteurization and 66.1% after sterilization for the lower carbohydrate concentration sample (piggery waste), reaching maxima of 0.88 and 0.96 m(CH4)(3) kg(VS)(-1), respectively. The maximum methane production rates, measured as the maximum slope of the accumulated methane production curve, per unit of initial biomass content, were also different. While this rate increased by 52.6% and 211.6% for piggery waste after pasteurization and sterilization, respectively, it decreased by 43.8% for poultry waste after pasteurization with respect to untreated waste. Compounds with low biodegradability that are produced by Maillard reactions during thermal pre-treatment could explain the low bioavailability observed for waste with a high carbohydrate concentration.

Determination of Chemical Oxygen Demand in Heterogeneous Solid or Semisolid Samples Using a Novel Method Combining Solid Dilutions as a Preparation Step Followed by Optimized Closed Reflux and Colorimetric Measurement

This paper reports the development of an innovative sample preparation method for the determination of the chemical oxygen demand (COD) in heterogeneous solid or semisolid samples, with high suspended solids and COD concentrations, using an optimized closed reflux colorimetric method. The novel method, named solid dilution (SD), is based on a different technique of sample preparation, diluting the sample with magnesium sulfate (MgSO(4)) previous to COD determination. With this, it is possible to obtain a solid homogeneous mixture much more easily analyzable. Besides, a modification of concentration and ratio of reagents was optimized to make the closed reflux colorimetric method suitable for complex substrates with COD levels ranging from 5 to 2500 g O(2) kg(-1) TS. The optimized method has been tested with potassium hydrogen phthalate (KHP) as primary solid standard and using different solid or semiliquid substrates like pig slaughterhouse waste and sewage sludge, among others. Finally, the optimized method (SD/SM-CRC) was intensively tested in comparison to the standard titrimetric method (SM-ORT) using different certified reference materials (CRM). The developed method was found to give higher accuracy, 1.4% relative standard deviation (RSD) vs 10.4%, and bias of 2.8% vs 8.0%, in comparison to the standard open reflux titrimetric method.

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.

Optimization of the anaerobic co-digestion of pasteurized slaughterhouse waste, pig slurry and glycerine

The feasibility of co-digestion of blends of two different animal by-products (pig manure and pasteurized slaughterhouse waste) and recovered glycerine was studied in mesophilic conditions. Experiments were performed in a lab-scale CSTR along 490days, with a hydraulic retention time of 21-33days and with a step-wise increased organic loading rate, by adding and/or changing the wastes ratio, from 0.8 to 3.2kgCODm-3d-1. The best methane production rate (0.64Nm3CH4m-3d-1) represented an increment of 2.9-fold the initial one (0.22Nm3CH4m-3d-1 with pig manure solely). It was attained with a ternary mixture composed, in terms of inlet volatile solids, by 35% pig slurry, 47% pasteurized slaughterhouse waste and 18% glycerine. This blend was obtained through a stepwise C/N adjustment: this strategy led to a more balanced biodegradation due to unstressed bacterial populations through the performance, showed by the VFA-related indicators. Besides this, an improved methane yield (+153%) and an organic matter removal efficiency (+83%), regarding the digestion of solely pig slurry, were attained when the C/N ratio was adjusted to 10.3.