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Giorgio Bertanza, Matteo Canato, Giuseppe Laera

RECUPERO DI RISORSE NEGLI IMPIANTI DI DEPURAZIONE. Analisi tecnico-economica e ambientale di scenari di intervento

RESOURCE RECOVERY IN WWTPs. Techno-Economic Analysis of Alternative Scenarios

Wastewater treatment plants are being considered as facilities aimed at recovering value-added products from wastewater. Although this may be seen as a chance for the next decades, at present several alternatives are already commercially available for addressing energy recovery (towards the self-sufficiency condition) and/or material recovery (the water itself and the sludge representing at the moment the most interesting and feasible recovery options). In principle, it is clear that while pushing towards material recovery (say water reuse), an increased power consumption derives (e.g. for water tertiary refining): this highlights the interaction (some times a conflict) between the two tasks of material recovery and energy saving (or production).
This also explains why it is necessary, whenever studying a recovery option, to take simultaneously into consideration both aspects (material and energy balance). Thus, an objective evaluation tool is required for comparing upgrading alternatives, based on a multi criteria analysis. Actually, such a kind of assessment is markedly site-specific (peculiarities of each plant being highly affecting the final result) and should be based on a multitude of aspects: economic, environmental, technical, social etc.. The WFD (Water Framework Directive 2000/60/EC) itself states that environmental technologies have to be assessed based on an integrated approach. It is then clear the need of developing simple tools which can support the decision maker in (i) performing the cost-benefit analysis overcoming the critical (if not impossible) economic quantification of those factors that are not typically valuable because they are not marketable (externalities) and (ii) highlighting all critical aspects related to the implementation of a new environmental technology or management
strategy. 
The aim of this work is to highlight advantages and criticisms of recovery options by simulating their application to two reference activated sludge WWTPs of different nominal size (50,000 and 500,000 p.e., respectively) and different configuration, discharging into sensitive area. The main features of the small plant are: no primary sedimentation, no sludge separate digestion, sand filtration of the effluent. Differently, the large plant is equipped with primary sedimentation and sludge anaerobic digestion.
For assessing the different scenarios, a specific procedure was adopted, which was previously developed by the authors within the EU project ROUTES (http://cordis.europa.eu/project/rcn/98727_it.html) in order to evaluate the techno-economic sustainability of different sewage sludge minimization systems (details in Svanström et al., 2014 and Bertanza et al., 2015a). This protocol was modified so as to include also environmental aspects. 
The starting point of the assessment process is the mass balance calculation that was performed for all the configurations/systems under analysis. Based on that, detailed energy balances (heat and power) were calculated. For the technical assessment, a work-sheet was built (Excel® environment) adopting a hierarchical modelling approach. At this scope nine categories are considered crucial: reliability, complexity and integration with existing structures, flexibility/modularity, social & authorization aspects, residues & recovered materials, consumption of raw materials & reagents, electric energy consumption, thermal energy consumption, energy available to feed into the grid or a district heating system. The economic assessment was performed by calculating, for each analyzed scenario, both capital expenditures (thus depreciation) and operation expenditures. In order to highlight the most sensitive cost items for the analyzed scenarios, two opposite situations were considered: 1) income maximization (and costs minimization), named the most favourable conditions, and 2) cost maximization (and incomes minimization), called the worst conditions. As for the environmental evaluation, the following items were considered: residues to be disposed of, recovered materials, recovered energy, impact of transportation, consumption of reagents, gaseous emissions.

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