Energy recovery from fermentative biohydrogen production of biowaste: a case study based analysis

Centralization of wastewater treatment in a tourist area: A comparative LCA considering the impact of seasonal changes

Nowadays, environmental protection has become a topic of primary importance, and the interest in wastewater treatment plants (WWTPs) has increased due to the need for a paradigm shift from linear to circular economy. The centralization level of wastewater infrastructure is the basis for a successful system. The aim of this study was to investigate the environmental impacts generated from the centralized treatment of wastewater in a tourist area in central Italy. The combined use of BioWin 6.2 simulation software and life cycle assessment (LCA) methodology was implemented to evaluate the potential connection of a small decentralized WWTP to a medium-size centralized facility. Two different scenarios (decentralized system, corresponding to the current situation, and centralized) were evaluated in two separate periods: high season (HS), corresponding to the main tourist season, and low season (LS), which is the period before the main tourist season. Two sensitivity analyses were conducted, assuming different N2O emission factors, and considering the period at the end of tourist season, respectively. Although with modest advantages (up to -6 % in pollutant emissions), WWTP connection was the best management option in 10 out of 11 indicators in HS, and 6 out of 11 categories in LS. The study showed that wastewater centralization was promoted by scale factors in HS, as the most impactful consumptions decreased as the degree of centralization increased; on the other hand, the decentralized system was less penalized in LS, as small WWTP was less stressed and energy consuming in this period. Sensitivity analysis confirmed the results obtained. Site-specific conditions can lead to conflicting circumstances, as key parameters may have different behaviors depending on seasonal variations, and the degree of centralization in tourist areas should be addressed by distinguishing separate periods, based on changes in tourist flows and pollution loads.

Biogas from Anaerobic Digestion: Power Generation or Biomethane Production?

Biogas is a fuel obtained from organic waste fermentation and can be an interesting solution for producing electric energy, heat and fuel. Recently, many European countries have incentivized the production of biomethane to be injected into natural gas grids or compressed and used as biofuel in vehicles. The introduction of an upgrading unit into an existing anaerobic digestion plant to convert biogas to biomethane may have a strong impact on the overall energy balance of the systems. The amount of biomethane produced may be optimized from several points of view (i.e., energy, environmental and economic). In this paper, the mass and energy fluxes of an anaerobic digestion plant were analyzed as a function of the biogas percentage sent to the upgrading system and the amount of biomethane produced. A numerical model of an anaerobic digestion plant was developed by considering an existing case study. The mass and energy balance of the digesters, cogeneration unit, upgrading system and auxiliary boiler were estimated when the amount of produced biomethane was varied. An internal combustion engine was adopted as the cogeneration unit and a CO2 absorption system was assumed for biogas upgrading. Results demonstrated that the energy balance of the plant is strictly dependent on the biomethane production and that an excess of biomethane production makes the plant totally dependent on external energy sources. As for the environmental impact, an optimal level of biomethane production exists that minimizes the emissions of equivalent CO2. However, high biomethane subsides can encourage plant managers to increase biomethane production and thus reduce CO2 savings.

Improvement of Digestate Stability Using Dark Fermentation and Anaerobic Digestion Processes

This paper assessed the effect of dark fermentation, the fermentative phase in a two-stage anaerobic digestion system, in terms of digestate biostabilization efficiency. The digestates analyzed in this study were obtained from a pilot-scale system in which two different substrates were used in order to simulate both the digestion and co-digestion process. Biostabilization performances were evaluated by measuring the specific oxygen uptake rate (SOUR) of the outgoing digestates. This index allowed us to define the degree of effectiveness in terms of stabilization of organic matter, between the traditional anaerobic digestion process and the two-stage configuration. Considering the traditional process as a reference scenario, the results highlighted an increase in biological stability for the two-stage co-digestion process, consisting of a dark fermentation stage, followed by an anaerobic digestion one. Digestates biostabilization efficiency increased up from 6.5% to 40.6% from the traditional one-stage configuration to the two-stage one by improving the anaerobic digestion process through a preliminary fermentative stage. The advantages of the two-stage process were due to the role of dark fermentation as a biological pre-treatment. Considering the partial stability results related to the second stage, biological stability was improved in comparison to a single-stage process, reaching an efficiency of 42.2% and 55.8% for the digestion and co-digestion scenario respectively. The dark fermentation phase allowed for a higher hydrolysis of the substrate, making it more easily degradable in the second phase. Results demonstrated better biostabilization performances of the outgoing digestates with the introduction of dark fermentation, resulting in more stable digestates for both the digestion and co-digestion process.

Feasibility Analysis of Bio-Methane Production in a Biogas Plant: A Case Study

A feasibility analysis, to assess the suitability of converting the biogas produced in an existing anaerobic digestion plant to bio-methane, was carried out. The case study plant was equipped with a micro-gas turbine co-generator. Several upgrading systems of different sizes were considered, to determine the most suitable configuration from a thermodynamic and economic point of view. For this purpose, a model of the whole plant that included digesters, a micro-gas turbine, a sludge line, heat transfer loops, and heat exchangers was developed. A steady-state simulation was performed by using the daily average conditions for the one-year long operation of the plant. The results highlighted that the feasibility depended on the amount of bio-methane produced, as this affected the performance of the cogeneration system and the balance between the costs and revenues. When large amounts of biogas are upgraded to bio-methane, the heat provided by the micro-gas turbine during the winter season is not sufficient to keep the digesters at the desired temperature and, therefore, natural gas integration is necessary. In addition, by increasing the upgrading unit size, the amount of electric energy purchased by the grid increases accordingly. An economic analysis showed that the optimal upgrading system size was strongly dependent on the bio-methane selling price.

Biohydrogen Production from Food Waste: Influence of the Inoculum-To-Substrate Ratio

In this study, the influence of the inoculum-to-substrate ratio (ISR) on dark fermentative hydrogen production from food waste (FW) was evaluated. ISR values ranging from 0.05 to 0.25 g VSinoculum/g VSsubstrate were investigated by performing batch tests at T = 39 °C and pH = 6.5, the latter being the optimal value identified based on a previous study. The ISR was found to affect the fermentation process, clearly showing that an adequate ISR is essential in order to optimise the process kinetics and the H2 yield. An ISR of 0.14 proved to optimum, leading to a maximum H2 yield of 88.8 L H2/kg VSFW and a maximum production rate of 10.8 L H2/kg VSFW∙h. The analysis of the fermentation products indicated that the observed highest H2 production mostly derived from the typical acetate/butyrate-type fermentation.