Environmental green light for anaerobic digestion at the Montcada i Reixac WWTP

The Department of Territory, Housing and Ecological Transition has published in the Official Gazette of the Generalitat de Catalunya the Resolution TER/2965/2026, of June 23, which issues the environmental impact report for the construction project for anaerobic digestion at the Montcada i Reixac WWTP. The report determines that the project does not have to be submitted to an ordinary environmental impact assessment, and it is published with the official reference CVE-DOGC-A-26226015-2026.

The initiative, processed by the Environmental Authority Services of the Barcelona Metropolitan Area (file OTAABA20250247), aims to define the facilities necessary for the anaerobic digestion treatment of sludge and cogeneration at the Montcada i Reixac WWTP. The plant, built in two phases in 1991 and 1997, has a design capacity of 72,600 m³/day and serves the municipalities of Badia del Vallès, Barberà del Vallès, Cerdanyola del Vallès, Montcada i Reixac (90%), Ripollet, and Sant Cugat del Vallès (80%), with a total population of 423,500 population equivalents.

A project to recover sludge and generate biogas

The execution of the project aims to achieve greater utilization of sewage sludge, reducing its production and volatile matter content, and generating biogas, which can be energetically recovered. In this way, it will no longer be necessary to continue transporting the sludge generated at the Montcada i Reixac WWTP to the Besòs WWTP for treatment.

The alternatives study (BAC, 2023) considered six options, from no expansion (alternative 0) to anaerobic digestion with hydrolysis (alternative 5). Alternative 4 was considered the most environmentally favorable, as it allows for the treatment of more sludge and the generation of more energy. The project will be developed in two phases:

• Phase 1 (alternative 1): anaerobic digestion of the Montcada sludge, including pumping, screening, thickening, digestion, biogas line, motogenerators, and deodorization.
• Phase 2 (alternative 4): transporting sludge from the Besòs WWTP to the plant, with pre-dewatering and thermal hydrolysis.

Three digesters and a deodorization system

The new facility provides for the construction of three anaerobic digesters of 4,000 m³, with a diameter of 18 m and a height of 21.91 m, along with three buildings for sludge, digestion, and cogeneration. Two gasometers of 1,800 m³, a storage tank for digested sludge, two 120 m³ silos, and a biological treatment tower (biotrickling) for deodorization, which must minimize odors in the surrounding area, will also be installed.

Regarding the environmental impact, the document estimates that emissions during the construction phase will be in the order of 3,800 t of CO₂. In the operational phase, emissions will be lower than those of alternative 0 thanks to the generation of energy from biogas, and in phase 2, the plant’s emissions will be neutralized, generating a surplus of low-carbon footprint energy that can be supplied to the grid.

Pending conditions and procedures

The environmental impact report incorporates several additional conditions, among which the following stand out:

• Address the considerations of the reports from the Directorate General for Climate Change and Environmental Quality, the Department of Culture, the Risk Management and Planning Service, and the Territorial Urbanism Commission of the Barcelona Metropolitan Area.
• Complete the documentation with a visibility study of the proposal from the main observation points.
• Schedule the works during periods that do not interfere with the biological cycle of the fauna and provide for the revegetation of the slopes.

Furthermore, authorization must be requested from the Catalan Water Agency, given that the lands are located in the flood zone for the 500-year return period. The Program of measures of the third cycle of the River Basin District Management Plan of Catalonia (2022-2027) already includes the anaerobic digestion of the Montcada WWTP (C1.305).

A commitment to renewable energy and odor reduction

The resolution, signed by the Director General of Environmental Policies and Natural Environment, Marc Vilà Chiaraviglio, represents a step forward so that the treatment plant serving more than 400,000 inhabitants of the metropolitan area can transform its sludge into biogas, reducing emissions, operational costs, and odor nuisances. Once the two phases are completed, the plant will be able to generate a surplus of low-carbon footprint energy, contributing to the decarbonization of the metropolitan sanitation system.

Source: Official Gazette of the Generalitat de Catalunya, no. 9735, August 21, 2026, Announcements section, pp. 1-9 (official reference: CVE-2026-26226015-2026).