• Energy Research

Performances of mechanical and low-temperature (<100°C) thermal pre-treatments were investigated to improve the present efficiency of anaerobic digestion (AD) carried out on waste activated sludge (WAS) in the largest Italian wastewater treatment plant (2,300,000p.e.). Thermal pre-treatments returned disintegration rates of one order of magnitude higher than mechanical ones (about 25% vs. 1.5%). The methane specific production increased by 21% and 31%, with respect to untreated samples, for treatment conditions of respectively 70 and 90°C, 3h. Thermal pre-treatments also decreased WAS viscosity. Preliminary energy and economic assessments demonstrated that a WAS final total solid content of 5% was enough to avoid the employment of auxiliary methane for the pre-treatment at 90°C and the subsequent AD process, provided that all the heat generated was transferred to WAS through heat exchangers. Moreover, the total revenues from sale of the electricity produced from biogas increased by 10% with respect to the present scenario.

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%.

The current geopolitical landscape of the European Union has made it clear that the energy sector must be a top priority in EU policy, especially in light of the sudden escalation of Russian–Ukrainian conflicts. Energy efficiency has been used as the first tool of EU policy to tackle energy and climate crises, given the issues surrounding energy vulnerability and the need to limit gas emissions that contribute to climate change. The white certificate mechanism in Italy has played a pivotal role in encouraging measures to achieve the country’s energy-saving goals. Given the high energy requirements of Wastewater Treatment Plants (WWTPs), especially for aeration in the biological section, this paper examines the replacement of the air distribution system for a large WWTP as a viable intervention. In order to provide economic perspective for the plant, both the discounted Payback Period (dPBP) and the Net Present Value (NPV) were calculated for the investment. When viewed through an economic lens, the dPBP metric exhibits values that span from less than 1 year to nearly 4.5 years. Additionally, the investment’s cost-effectiveness was emphasized by the NPV, which, depending on the factors considered, can exceed 17.5 million euros. Finally, given the centrality of the theme of climate change, the avoided greenhouse gas emissions generated by the efficiency intervention were calculated, according to the GHG Protocol, resulting in a quantity of avoided emissions equivalent to over 57,770 tonnes of CO2e. These results highlight important achievements in terms of both the cost-effectiveness of the plant and the reduction of greenhouse gas emissions.

According to the European Union Directive 2009/28/EC, the goals of obtaining 20% of all energy requirements from renewable sources and a 20% reduction in primary energy use must be fulfilled by 2020. In this work, an evaluation was performed, from the environmental and energy point of view, of anaerobic digestion as a valid solution for the treatment of the byproducts obtained from the coffee-roasting process. In particular, thermophilic anaerobic digestion tests were carried out. Output values from the laboratory were used as input for the MCBioCH4 model to evaluate the produced flow of biogas and biomethane and two different biogas valorization alternatives, namely, the traditional exploitation of biogas for heat/energy production and biomethane conversion. The results of the preliminary simulation showed that a full-scale implementation of the coffee waste biogas production process is technically feasible and environmentally sustainable. Furthermore, the performed analysis validates a general methodology for energy production compatibility planning.