This study explored the volatile fatty acid (VFA) production potential of seven waste streams from urban and agroindustrial sources. For that purpose, batch assays were performed under acidic (pH 5.5) and alkaline (pH 10) conditions at both mesophilic (35 degrees C) and thermophilic (55 degrees C) temperature. Overall, the VFA yield was influenced by temperature, and it was positively affected by pH, ranging between 220 and 677 mg COD g(-1) CODfed for liquid waste streams and between 127 and 611 mg COD g(-1) CODfed for solid waste streams and urban sludge. The highest VFA concentration and highest VFA/sCOD ratio was obtained during the organic fraction of municipal solid waste (OFMSW) fermentation, with 8,320 mg COD L-1 and 94% at alkaline pH and mesophilic temperature. The results of this study suggest that selective VFA production, i.e. via propionic, butyric and acetic acid production, might be feasible for scaling-up purposes with specific waste streams by adjusting the process parameters.

The literature shows a diversity of real-time algorithms for automatic detection of bending-points in batch-operated waste treatment systems. In this study a new methodology is proposed for tuning the parameters of these algorithms when uncertainty specifications are considered at the outset. In this method the effects of slow and fast dynamic responses on the shape of signal trajectories were treated separately in order to cover via simulation all possible operating scenarios for a real situation. Long-term uncertainty and steady-state simulations were combined to derive probability distributions for biomasses. These probability distributions were then merged with short-term uncertainty to run one-cycle random simulations with which to reproduce the entire diversity of signal trajectories. Finally, an optimisation problem was formulated in terms of the algorithm parameters. The methodology was satisfactorily applied to tune an algorithm for detection of bending-points in an Autothermal Thermophilic Aerobic Digestion (ATAD) process. (C) 2015 Elsevier Ltd. All rights reserved.

Seven mixed sewage sludges from different wastewater treatment plants, which have an anaerobic digester in operation, were evaluated in order to clarify the literature uncertainty with regard to the sewage sludge characterisation and biodegradability. Moreover, a methodology is provided to determine the Anaerobic Digestion Model No. 1 parameters, coefficients and initial state variables as well as a discussion about the accuracy of the first order solubilisation constant, which was obtained through biomethane potential test. The results of the biomethane potential tests showed ultimate methane potentials from 188 to 214 mL CH4 g(-1) CODfed, COD removals between 58 and 65% and two homogeneous groups for the first order solubilisation constant: (i) the lowest rate group from 0.23 to 0.35 day(-1) and (ii) the highest rate group from 0.27 to 0.43 day(-1). However, no statistically significant relationship between the ultimate methane potential or the disintegration constant and the sewage sludge characterisation was found. Next, a methodology based on the sludge characterisation before and after the biomethane potential test was developed to calculate the biodegradable fraction, the composite concentration and stoichiometric coefficients and the soluble COD of the sewage sludge; required parameters for the implementation of the Anaerobic Digestion Model No. 1. The comparison of the experimental and the simulation results proved the consistency of the developed methodology. Nevertheless, an underestimation of the first order solubilisation constant was detected when the experimental results were simulated with the solubilisation constant obtained from the linear regression experimental data fitting. The latter phenomenon could be related to the accumulation of intermediary compounds during the biomethane potential assay. (C) 2013 Elsevier Ltd. All rights reserved.

The pilot-scale high-solids anaerobic digestion (HS-AD) of agro-industrial wastes and sewage sludge was analysed in terms of stability by monitoring the most common parameters used to check the performance of anaerobic digesters, i.e. Volatile Fatty Acids (VFA), ammonia nitrogen, pH, alkalinity and methane production. The results reflected similar evolution for the parameters analysed, except for an experiment that presented an unsuccessful start-up. The rest of the experiments ran successfully, although the threshold values proposed in the literature for the detection of an imbalance in wet processes were exceeded, proving the versatility of HS-AD to treat different wastes. The results evidence the need for understanding the dynamics of a high-solids system so as to detect periods of imbalance and to determine inhibitory levels for different compounds formed during anaerobic decomposition. Moreover, the findings presented here could be useful in developing an experimental basis to construct new control strategies for HS-AD. (C) 2013 Elsevier Ltd. All rights reserved.

Dry anaerobic digestion systems work with wastes that have a dry matter content above 20%. This technology has been used above all to treat the organic fraction of municipal solid waste making its application to the agro-food industry waste a novel approach. Co-digestion of vegetable and MBM by means of a batch system shows a good efficiency in spite of the presence of certain compounds at inhibitory levels (N-NH3 and VFA) which are detected in the process leachate.

Laboratory and pilot-scale experiments were carried out in order to evaluate the influence of thermal pre-treatment time on waste-activated sludge properties and anaerobic biodegradability. Six experimental conditions were analyzed from 0 to 30 min of hydrolysis time. Solubilization of macromolecular compounds, changes in the main sludge properties and anaerobic biodegradability of the sewage sludge were evaluated. A similar carbohydrate solubilization degree was achieved, from 53% to 70% and 59% to 75% for lab- and pilot-scale experiments, respectively. In the case of proteins, the values of solubilization were lower in the pilot-scale experiment than in the laboratory, with 31-45% and 47-70%, respectively. Ammonia and volatile fatty acid did not undergo important changes; however the sludge dewaterability enhanced at increased pre-treatment times. All the pre-treatment conditions had a positive effect with regard to anaerobic biodegradability and by fitting experimental data with a simplified mathematical model, it was concluded that the maximum biogas production rate is more influenced by the pre-treatment time than the total biogas production. (C) 2010 Elsevier Ltd. All rights reserved.