• Energy Research

Even if the demand for energy continues to increase, the European Union Directive 2009/28/EC has set a goal of obtaining 20% of all energy from renewable sources by 2020. In this context, this study investigated two different scenarios: direct combustion and anaerobic digestion. Direct combustion was evaluated using the mass and energy balance method in order to obtain indications of the energy revenue, the emissions, and consequently the necessary flue gas depuration equipment. Anaerobic digestion was evaluated by performing experimental tests in the laboratory, in order to obtain indications of the energy revenue. The results show that both the processes present some advantages: from the energy viewpoint the more suitable process is direct combustion (energy revenue equal to 1.16 MWh/tinput instead to the 0.48 MWh/tinput obtained for the anaerobic digestion), while from the viewpoint of greenhouse gas emissions (GHGs, in particular the CO2 emissions) the results are rather similar. (We evaluated that the emission is equal to 1.18 kg CO2/kginput for the direct combustion and to 1.06 kg CO2/kginput for the anaerobic digestion.) The anaerobic digestion presents more advantages from the viewpoint of features a simpler plant layout; the combustion scenario requires a more complex technology, in particular for dealing with NOx emissions. (The concentration coming from the process is equal to 900 mg/Nm3.) For this specific study, based on the obtained results, the anaerobic digestion proved to be the more suitable treatment process. Furthermore, the analysis considered here demonstrates a general methodology useful for energy production compatibility planning.

AbstractWaste activated sludge (WAS) is commonly treated by using anaerobic digestion (AD), which has the advantage of reducing sludge volumes and generating methane. However, the sludge structure ...

Abstract This study presents an integrated feasibility analysis approach to reduce the carbon footprint in the largest Italian wastewater treatment plant (WWTP). Firstly, a model-based feasibility analysis was carried out to assess the applicability of upgrading scenarios, for an ongoing anaerobic sludge digestion process. Application of dynamic sludge thickener, as well as hybrid thermo-alkali pre-treatment of waste activated sludge, were assessed to enhance the biogas production in the WWTP. Further, an implementation of the selective membranes was proposed and studied to upgrade the produced biogas in sludge treatment units to biomethane with an average efficiency of 98.6%. Model-based sludge pre-treatment and biogas upgrading strategies were developed and evaluated in terms of mass, energy, and greenhouse gas emission balance. The obtained results prove that practicing the proposed upgrading scenario can lead to an 18% improvement in biogas production and a significant reduction of thermal energy auto-consumption and total greenhouse gas emissions. In the second phase, the laboratory-based feasibility analysis was performed about the integration of microalgae technology into the current process of the WWTP. A planar photobioreactor was built to estimate the volumetric mass transfer coefficient (KLa) and CO2 consumption of the reactor. By the use of 44 and 76 μmol/m2/s light intensities, the results show 80% and 70% reductions in total CO2, respectively. The tested configuration guaranteed 11.763 and 27.943 mg/l/h CO2 consumptions, as well as 0.5775 h−1 and 17.7 h−1 KLa values. Overall, the results prove that applications of the technologies proposed in this study can significantly reduce the carbon footprint of the WWTP.

Abstract This study was carried out with the principal aim of obtaining reliable outcomes for the future implementation of a temperature-phased anaerobic digestion (TPAD) process in a large (2 M population equivalent, p.e.) WWTP. With the aid of pilot-scale (10 L) reactors fed by pure primary sludge (PS), a TPAD process, where the first and the second reactor were operated at 50 °C and 38 °C, respectively, was compared with a conventional mesophilic (38 °C) anaerobic digestion (AD) process. The initial hydraulic retention time (HRT) of the first, acidogenic, reactor of the TPAD was reduced from 3 to 2 days in the second part of the test. The results demonstrated that the TPAD system had been stable for all the duration of the test (approx. 100 days), as testified by the steady values of pH and tVFAs/TA ratio, notwithstanding the decrease in the HRT. The TPAD proved to be more efficient in volatile solid (VS) reduction and methane generation, compared to the conventional mesophilic AD process. In fact, the VS reduction increased from 42% to approx. 55% and the specific methane potential (SMP) from 280 to 332 NL/kg VS added. An excellent phase separation was observed between the two acidogenic and methanogenic reactors, as demonstrated by the low SMP (only 3% of the overall production) recorded from the first reactor of the TPAD system. However, the energy analysis demonstrated that the higher SMP obtained in the TPAD was not sufficient to compensate the higher amounts of heat required for sludge heating and heat loss compensation. Only a process of heat recovery could make the TPAD system really profitable, thus increasing the aliquot of energy in the form of methane, available for users external to the WWTP, by 20%. This result represents a step in the evolution of traditional WWTPs towards more energy efficient and sustainable facilities.

Abstract Sewage sludge produced in WWTPs are currently digested in mesophilic anaerobic digestion (AD) processes with the aim of recovering heat and electricity. However, often, the low biodegradability of waste activated sludge (WAS) limits the complete thermal self-sustainability of the process. This study presents the results of AD tests carried out on WAS in semi-continuous reactors (44 L and 240 L) in mesophilic (38 °C) and thermophilic (55 °C) thermal regimes. The hydraulic retention time (HRT) was 20 days and the organic loading rate (OLR) of 1 kg VS/m3∙d in all tests. The tests returned a specific methane production (SMP) of 0.120 Nm3/kg VS added for the mesophilic process (240 L reactor) and SMPs of 0.188 and 0.176 Nm3/kg VS added for the tests carried out under the thermophilic regime in 44 L and 240 L, respectively. Experimental data were modelled with a first-order rate reaction, where B0, that is the SMP after an infinite HRT, and k, the hydrolysis constant, were the key parameters. B0 and k were found equal to 0.147 Nm3/kg VS and 0.08 d−1 respectively, for the mesophilic process, and to 0.218 Nm3/kg VS and 0.350 d−1 for the thermophilic process. For the thermophilic process, the model was calibrated with the data from the 44 L reactor and validated with those from the 240 L reactor. An error of only 1% resulted. Finally, it was demonstrated that a full-scale digestion scheme, where primary and secondary sludge were digested separately, in mesophilic and thermophilic conditions respectively, and the heat of the digestates was used to heat the cold sludge, allowed to carry out the process with a complete thermal self-sustainability already at a sludge TS content of 3%.

This study is part of a multi-objective, integrated approach to analyze various possibilities for increasing energy efficiency of the largest Italian wastewater treatment plant (WWTP) at Castiglione Torinese, NW Italy. The final goal of this study was evaluating the optimization interventions on the sludge treatment process in terms of mass, energy and greenhouse gas (GHG) emission balance. An optimization scenario of sludge digestion was simulated and compared the present operating situation. In the optimized scenario, a hybrid thermo-chemical pre-treatment of the waste activated sludge (WAS) entering the digestion process was considered. The biogas produced was upgraded to biomethane with a process working with selective membranes. Full scale simulation of the whole sewage sludge treatment line was performed with the screening model MCBioCH4, developed by the Authors. The results showed that the optimization interventions would provide two important positive impacts. Firstly, a reduction of the sludge volume entering into the digestion process. Secondly, biomethane production would be around 20% higher than the methane fraction contained in the biogas actually produced. The energy saving and the increased specific biomethane production would improve the overall GHG balance of the system. Proceedings of the 28th European Biomass Conference and Exhibition, 6-9 July 2020, Virtual, pp. 311-316

Abstract Reduced biodegradability of waste activated sludge (WAS) limits the production of methane and the consequent energy recovery in an anaerobic digestion (AD) process. Pre-treatments are a solution to increase the biodegradability of bacteria cell biomass, but a large part of poorly degradable organic matter is left after digestion. The utilization of intermediate hydrolysis treatments (IHTs) may help in converting even the most recalcitrant parts of organic matter in methane. This study employed a three-phase experimentation to assess the effect of the hydraulic retention time (HRT) of the digestion first stage, on the overall performance of a two-stage digestion process, with an in-between treatment, carried out on WAS. The three phases of the experimentation included a first-stage digestion (with HRTs = 5, 10 and 15 days), performed in a semi-continuous 10L-reactor, followed by a thermal (90 °C) or a hybrid (thermal 90 °C + chemical, 4% NaOH) IHT, completed by a second-stage digestion carried out in a batch mode. Both the digestion processes were performed in mesophilic conditions (38 °C). The obtained results revealed that, in the presence of a thermal IHT and by fixing the duration of the second stage to 20 days, the overall specific methane production (SMP) tended to a constant value, in the order of 0.205 Nm3/kg VS added, irrespective of the duration of the first stage. Conversely, when a hybrid treatment was applied, the difference between a short (5 days) and a medium (10–15 days) duration of the digestion first stage became evident, with SMPs in the order of 0.247 and 0.230 Nm3/kg VS added, respectively. Energy and economic sustainability of the application of IHTs at a full scale plant required an adequate thickening of sludge/digestate matrices and an efficient heat exchange between donor (sludge after treatment) and acceptor (cold sludge before digestion) agents. It was demonstrated that for separated or joined digestion processes of primary sludge (7.0% TS) and treated digestates, with heat recovery and different combinations of the duration of the first and second stage of AD, TS contents in the order of 4%, 6% and 8–9% were required to make the thermal balance neutral for thermal exchanges efficiencies of 100%, 70% and 50%, respectively.

The problem of acid gas exhaust emissions treatment has not been fully resolved at present. Dry adsorption of acid gases with alkaline sorbents is currently being investigated, to improve solid sorbents. In this study, 5 types of hydrated lime were characterised and tested. The sorption capacities were measured by means of a system consisting of a feed line (HCl/N2), a thermostatic reactor and a water absorber. The physical characteristics of sorbent samples were also compared. Analyses conducted with scanning electronic microscopy revealed that sample C1 showed uniform particle distribution. Samples C2 and C3 showed the co-presence of fine and coarse particles. Sample C4 showed very fine particles with agglomeration phenomena. In sample C5, fibrous elements were found. Energy dispersive spectrometry (EDS) analyses showed a similar composition of the samples, with the exception of the presence of Mg in some of them. After 30 min of testing, the following differences in sorption capacities with respect to C1 (3.59 mg g−1) were found: C2, −20%; C3, −13%; C4, −17%; C5, −3%. Higher sorption capacities were associated with more uniform particle size distributions. Conversely, agglomeration of fine particles may have adversely affected the performance of sorbents.