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In this study, an experimental setup for the evaluation of a two-stage anaerobic digestion has been developed: a laboratory-scale apparatus was assembled employing solid potatoes as energy crops. Two coupled 5-litre batch-fed stirred reactors, one for the hydrolytic-acidogenic step and one for the acetogenic-methanogenic step, kept at mesophilic temperature (308.1 K), were adopted. Evaluated in the first acidogenic reactor was the influence of fermentative yeast (Saccharomyces cerevisiae) on the degree of hydrolysation and on the acidification rate of the vegetable substrate. All runs were performed without the addition of chemicals. Samples of hydrolysed substrate were sent to the second methanogenic bioreactor, fed with an industrial anaerobic sludge and selected lyophilized anaerobic bacteria, in order to evaluate the methane yield of the produced biogas and the specific methanogenic activity (SMA). The whole procedure was repeated simulating an anaerobic sequencing batch reactor (ASBR) process.

Nowadays, the anaerobic digestion of organic wastes to produce renewable energy is a reality in many countries. Many feedstocks can be processed by anaerobic digestion to produce biogas, however, anaerobic digestion of single substrates can have drawbacks that could be eliminated by the anaerobic co-digestion of their mixtures (two or more). In this paper, the anaerobic co-digestion of maize silage and red chicory (radicchio) waste is presented. Several batch anaerobic biodegradability tests were performed in order to compare the methane production of the blend with those of the two substrates digested separately. The methane production was modelled by a first order kinetic model, focusing on the initial substrate solubilization. The specific methane productions of maize silage and red chicory as single substrates were 0.346 L CH4 g–1 ODM and 0.326 L CH4 g–1 ODM. The first solubilization rate constants (ksol) were 0.231 d–1 for the digestion of maize silage, and 0.389 d–1 for the radicchio waste. Two parameters representing the relative changes in specific methane production and solubilization rate were calculated in order to evaluate the synergistic effects due to co-digestion of studied substrates. Results showed that blending the two substrates enhanced the performances of the AD process, mainly with respect to biogas production kinetics with low increments in the ultimate methane potential. This work is licensed under a Creative Commons Attribution 4.0 International License.

Oxic-settling-anaerobic (OSA) process has been introduced into the treatment line of wastewater in order to upgrade activated sludge processes and to reduce the production of excess sludge. The aim of the present study was to simulate the performance of an OSA pilot plant by implementing the Activated Sludge Model No.2d (ASM2d) into a mathematical modelling software (BioWin). The stepwise calibration, performed both by off-line experiments and software dynamic calibration, was carried out in a heuristic way, adjusting the parameters values that showed a major influence to the effluent and internal concentrations. All the reduction factors introduced into ASM2d to simulate the processes occurring in anoxic and anaerobic conditions were lowered in order to reproduce the concentrations of interest. In addition, the values of parameters of the PAOs (polyphosphate accumulating organisms)-related process (namely qPHA and YPO4) were found lower than those usually adopted. In general, theoretical results were in good agreement with the experimental data obtained from plant’s operation, showing an accurate predictive capacity of the model. Good performance was achieved considering the phosphorus removal related process, while some failures were detected in COD and ammonia simulations.

An OSA (oxic settling anaerobic) pilot plant, preliminarly ran as a conventional activated sludge (CAS) system, was managed under several operating conditions for 16 months. The aim was to evaluate the reduction of excess sludge production due to the recirculation of activated sludge trough the sludge holding tank (SHT). The sludge retention time varied from 37 to 406 days according to the excess sludge wastage. The oxidation/reduction potential (ORP) inside the sludge holding tank was of – 248 ± 133 mV. The observed sludge yield, obtained by feeding the system with real wastewater, varied from 0.112 to 0.465 gTSS/gCOD, with a reduction of the excess sludge production of 49.6 ± 20.7% compared to the CAS system. The recirculation of the sludge through the SHT favoured the COD and ammonia nitrogen solubilization connected to the sludge decay. The release of soluble phosphorus due to the phosphorus accumulating organisms metabolism was also noticed.