• Energy Research

The aim of this study consists in the individuation of the most appropriate configuration for the thermophilic anaerobic digestion of Waste Activated Sludge (WAS); to this purpose, the Anaerobic Digestion Model n degrees 1 (ADM1) was adopted. To enhance the energy production, the sludge underwent a thermal pre-treatment. Semi-continuous tests were performed for 281 days, both with raw and pre-treated WAS. The disintegration constant (k_dis) and the maximum acetate uptake rate (km_ac), were calibrated; the latter parameter was increased up to 2.2 KgCOD Kg(-1)COD d(-1) by the pre-treatment. To evaluate the performances of different plant configurations, the specific methane yield and digester productivity were estimated for varying Hydraulic Retention Times (HRTs) and substrate concentrations. The pre-treatment impact on the substrate resulted in the 20% enhancement of the specific methane production for plants run with HRT <10 days; the most proper input concentration consisted in 30 g of Volatile Solids (VS) d(-1). This procedure enabled to estimate the most appropriate HRT range for a) preventing the biomass washout (6/7 days for lower and 8 days for higher input concentration) and b) maximizing the methane and energy production for unit of digester volume (10/12 days). (C) 2017 Elsevier Ltd. All rights reserved.

Olive husks, typical solid by-products from the olive oil industry, were selected to carry out anaerobic digestion tests. Before digestion, olive husks were subjected to ultrasonic or thermal pretreatments in order to release the organic matter into solution. Both sonication and thermal pretreatment allowed to solubilize the particulate matter with 22% and 72% increase in soluble organics of olive husks, respectively. Nevertheless, such pretreatments caused the release of unwanted molecules in solution, with the related risks of inhibition of the methanogenic process. Biochemical Methane Potential (BMP) tests on olive husks mixed with olive-mill wastewater and dairy wastewater, either pretreated or not, showed that ultrasound pretreatment resulted in 15% increase in volatile solids reduction and a 13% increase in biogas production, while after thermal pretreatment no benefits were observed.

Sewage sludge processing is a key issue in wastewater treatment plants due to the inefficacy of conventional treatments to produce high quality sludge to be safely used in agriculture. Under this framework, the performances of several enhanced stabilization processes, namely ultrasound-pretreated Mesophilic Anaerobic Digestion (US-MAD), thermophilic anaerobic digestion (TAD), thermal pretreated TAD (TH-TAD) and ultrasound-pretreated inverse Temperature Phased Anaerobic Digestion (iTPAD) have been investigated. The enhanced anaerobic processes resulted in higher specific biogas production and higher destruction of E.coli and Somatic coliphages with respect to conventional MAD process, thus confirming the feasibility of these innovative processes in full-scale implementation perspectives.

High-frequency ultrasounds have recently gained interest as oxidative technique for sonochemical degradation of organic contaminants in water. In this study an innovative approach applying 200 kHz ultrasounds to improve both sludge anaerobic biodegradability and decontamination is proposed. Digestion tests were performed on batch reactors fed either with untreated or sonicated sludge, at different food/inoculum (F/I) ratio, in the range 0.3-0.9. First order kinetic highlighted a decreasing trend of the hydrolysis rate by increasing F/I, both for untreated and sonicated sludge. Positive effect of ultrasounds on specific biogas production was evident, but the conversion rate for pretreated sludge was strongly affected by F/I, and decreased by increasing F/I. Anionic surfactants anaerobic removal occurred in all tests, but the effect of ultrasounds was significant only at F/I=0.3. By pretreating sludge with high frequency ultrasounds, low F/I was the ideal ratio improving both sludge anaerobic digestion and decontamination.

The biochemical valorization potential of food waste (FW) could be exploited by extracting decreasing added-value bio-based products and converting the final residues into energy. In this context, multi-purpose and versatile schemes integrating thermal and biochemical conversion processes will play a key role. An upstream thermal pretreatment + solid-liquid separation unit was here proposed to optimize the conversion of the liquid fraction of FW into valuable chemicals through semi-continuous fermentation process, and the conversion of the residual solid fraction into biomethane through anaerobic digestion. The solid residues obtained after thermal pretreatment presented a higher soluble COD fraction, which resulted in higher methane production with respect to the raw residues (0.33 vs. 0.29 Nm3CH4 kg-1VSfed) and higher risk of acidification and failure of methanogenesis when operating at lower HRT (20d). On the contrary, at HRT = 40 d, the pretreatment did not affect the methane conversion rates and both tests evidenced similar methane productions of 0.33 Nm3CH4 kg-1VSfed. In the reactor fed with pretreated residue, the association of hydrogenotrophic methanogens with syntrophic bacteria prevented the acidification of the system. Modelling proved the eligibility of the FW solid residues as substrates for anaerobic digestion, given their small inert fractions that ranged between 0% and 30% of the total COD content.

In the framework of the waste circular economy, anaerobic digestion (AD) is a promising treatment option, due to both renewable energy and fertilizer production. Nevertheless, in mesophilic conditions a part of the organic carbon fed is not degraded, reducing the possibility to fully exploit the waste energy potential, and opening the research to advanced processes that can increase AD efficiency. In this study, AD of food waste was investigated in thermophilic conditions. Scope of this work was to evaluate the efficiency of a mild thermal pre-treatment on the solubilisation of complex organics and the digestion enhancement potential in terms of H2 and CH4 conversion rates. Thermal pre-treatment promoted complex organics solubilisation (soluble COD up to +40%) in particular with reference to starch and hemi-cellulose fraction. The high amount of released sugars was rapidly transformed into H2 in the first hours of AD, with high yields (up to 2.6 mol H2/mol glucose) and significant gain with respect to untreated waste. Ultimate methane yield was not affected by the substrate pre-treatment, but the positive impact was shown by the increase in anaerobic biodegradability, and kinetics. Proceedings of the 25th European Biomass Conference and Exhibition, 12-15 June 2017, Stockholm, Sweden, pp. 926-931

This study investigates for the first time, on laboratory scale, the possible application of an innovative enhanced stabilization process based on sequential mesophilic/thermophilic anaerobic digestion of waste-activated sludge, with low-energy sonication pretreatment. The first mesophilic digestion step was conducted at short hydraulic retention time (3-5 days), in order to favor volatile fatty acid production, followed by a longer thermophilic step of 10 days to enhance the bioconversion kinetics, assuring a complete pathogen removal. The high volatile solid removals, up to 55%, noticeably higher compared to the performances of a single-stage process carried out in same conditions, can guarantee the stability of the final digestate for land application. The ultrasonic pretreatment influenced significantly the fatty acid formation and composition during the first mesophilic step, improving consequently the thermophilic conversion of these compounds into methane. Methane yield from sonicated sludge digestion reached values up to 0.2 Nm(3)/kgVSfed. Positive energy balances highlighted the possible exploitation of this innovative two-stage digestion in place of conventional single-stage processes.

Thermal hydrolysis pretreatment coupled with Thermophilic Anaerobic Digestion (TAD) for Waste Activated Sludge (WAS) treatment is a promising combination to improve biodegradation kinetics during stabilization. However, to date there is a limited knowledge of the anaerobic biomass composition and its impact on TAD process performances. In this study, the structure and dynamics of the microbial communities selected in two semi-continuous anaerobic digesters, fed with untreated and thermal pretreated sludge, were investigated. The systems were operated for 250 days at different organic loading rate. 16S rRNA gene clonal analysis and Fluorescence In Situ Hybridization (FISH) analyses allowed us to identify the majority of bacterial and archaeal populations. Proteolytic Coprothermobacter spp. and hydrogenotrophic Methanothermobacter spp. living in strict syntrophic association were found to dominate in TAD process. The establishment of a syntrophic proteolytic pathway was favoured by the high temperature of the process and enhanced by the thermal pretreatment of the feeding sludge. Proteolytic activity, alone or with thermal pretreatment, occurred during TAD as proven by increasing concentration of soluble ammonia and soluble COD (sCOD) during the process. However, the availability of a readily biodegradable substrate due to pretreatment allowed to significant sCOD removals (more than 55%) corresponding to higher biogas production in the reactor fed with thermal pretreated sludge. Microbial population dynamics analysed by FISH showed that Coprothermobacter and Methanothermobacter immediately established a stable syntrophic association in the reactor fed with pretreated sludge in line with the overall improved TAD performances observed under these conditions.