Wastewater treatment plants (WWTPs) are known to be one of the most energy-intensive industrial sectors. In this work, demand response was applied to the biological phase of wastewater treatment to reduce plant electricity cost, considering that the daily peak in flowrate typically coincides with the maximum electricity price. Compressed air storage system, composed of a compressor and an air storage tank, was proposed to allow energy cost reduction. A multi-objective modelling approach was applied by analyzing different scenarios (with and without anaerobic digestion, AD), considering both plant characteristics (in terms of treated flowrate and influent chemical oxygen demand, COD, concentration) and storage system properties (volume, air pressure), together with the current Italian market economic conditions. The results highlight that air tank volume has a strong positive influence on the obtainable economic savings, with a less significant impact held by air pressure, COD concentration and flowrate. In addition, biogas exploitation from AD led to an improvement in economic indices. The developed model is highly flexible and can be applied to different WWTPs and market conditions.

Nowadays, sustainable approaches to waste management are becoming critical, due to increased generation and complex physicochemical composition. Waste electric and electronic equipment (WEEE) management, in particular, is being given increasing attention due to the continuous augment in electronic equipment usage and the limited recycling rates. In this work, a multi-objective engineering optimization approach using a decision support system (DSS) was used to analyze the feasibility of installing a WEEE treatment plant in the Friuli-Venezia Giulia region (Northeastern Italy), considering that most of the produced WEEE is currently exported outside the region. Meaningful economic and environmental parameters were considered in the assessment, together with current WEEE production and composition. Plant investment cost was in the range of EUR 7–35 M for a potentiality of 8000–40,000 ton of treated WEEE/yr, the lower bound corresponding to the WEEE produced in the region. Payback time was 4.3–10 yr, strongly depending on the market’s economic conditions as well as on plant potentiality. Proper public subsidies should be provided for a plant treating only the locally produced WEEE, establishing a circular economy. The fraction of recovered materials was 78–83%, fulfilling the current EU legislative requirements of 80% and stabilizing around values of 80% for a higher washing machine fraction. An increase in personal computers may allow to augment the economic revenues, due to the high conferral fees, while it reduces the amounts of recovered materials, due to their complex composition. CO₂ emission reduction thanks to material recovery was in the range of 8000–38,000 ton CO₂/yr, linearly depending on the plant potentiality. The developed DSS system could be used both by public authorities and private companies to preliminarily evaluate the most important technical, financial and environmental aspects to assess overall plant sustainability. The proposed approach can be exported to different locations and integrated with energy recovery (i.e., incineration of the non-recoverable fractions), analyzing both environmental and economic aspects flexibly.