Quick answer
- The DUCA (Water Use and Pollution Declaration) is the tax return the Catalan Water Agency (Agència Catalana de l'Aigua, ACA) uses to calculate the water levy (cànon de l'aigua) payable by industrial and equivalent users.
- As a general rule, it is compulsory for establishments in sections B, C and D of the Catalan activity classification (CCAE) and in groups A032, E360, E383 and J581 that use more than 1,000 m³ a year, and for anyone who receives a request from the ACA.
- The basic DUCA expires after 4 years and must be renewed between 1 October and 31 December of its final year of validity.
- Since 8 October 2025, the simplified DUCA no longer expires with time: it only has to be filed again if any data relevant to the levy changes or if the ACA requests it.
- The declaration is prepared with the e-DUCA application and submitted electronically with a digital certificate.
What the DUCA is and what it is for
The Water Use and Pollution Declaration (Declaració de l'ús i la contaminació de l'aigua, DUCA) records the volume of water an establishment uses and the quality of the wastewater it discharges. From that data the ACA sets the rate of the water levy, which for industrial uses is the sum of a general rate, linked to use, and a specific rate, linked to pollution.
It is therefore not a mere informative formality: what is declared, or left undeclared, has a direct effect on what the company pays on every water bill.
Basic DUCA and simplified DUCA
- Simplified DUCA (DUCA abreviada): the simplified version. It applies to establishments which, because of their activity, volume, pollutant load and treatment system, do not have to declare their pollution individually. It places the establishment under the ordinary regime of the levy rate.
- Basic DUCA (DUCA bàsica): the detailed version. It is filed by establishments that must declare the pollutant load of their discharges individually. It places the establishment under the special regime, with a specific rate calculated from its own pollution data.
Which companies are required to file
Whether a declaration is required, and which type, depends on the activity code (CCAE), the annual volume of water supplied and, in addition, the water not discharged, the pollutant load and the treatment system. According to the criteria published by the ACA:
- Activities in sections B, C and D and in groups A032, E360, E383 and J581 using more than 1,000 m³/year: must file a DUCA. It will be a basic DUCA if they meet any of the specific requirements and a simplified DUCA if they do not.
- The same activities using less than 1,000 m³/year: only file a simplified DUCA, for registration purposes, if requested to do so.
- All other activities: not required to file unless requested; above 7,000 m³/year and subject to certain requirements, they may voluntarily opt into the special regime with a basic DUCA.
Deadlines worth keeping under control
- Initial declaration
within three months of the establishment starting operations, whether basic or simplified. - Renewal of the basic DUCA
every four years, between 1 October and 31 December of the final year of validity. - Simplified DUCA
does not expire with time (ACA Instruction 1/2025). It is filed again when any data needed to determine the levy changes or when the ACA requests it. - Significant changes
if discharges, treatment or the establishment's data change significantly, it is worth considering a new or supplementary declaration without waiting for the renewal.
What data has to be declared
The simplified DUCA mainly covers identification details of the establishment and its water supply. The basic DUCA is considerably longer (forms B1 to B5) and includes, among other things, the volumes of water supplied and discharged, the discharge points, the treatment system and the values of the pollution parameters used in the specific rate: suspended solids, oxidisable matter, soluble salts, inhibitory substances, nitrogen and phosphorus.
Before renewing, it is worth checking whether the data is still representative. A change of process, an upgrade to the treatment plant or water reuse can alter the applicable rate, and there are reduction coefficients, such as the one for reuse, that only apply if they are declared.
How it is filed
The ACA provides the e-DUCA application, which is installed on a Windows computer. It allows the declaration to be completed, gives a simulation of the estimated amount of the levy and generates a .zip file that is sent to the ACA through the Generalitat's online procedures portal, using a digital certificate or another accepted means of identification. Always work with the latest version of the application, because the simulation uses the tariffs and coefficients in force when it was installed.
Companies, as legal persons, are required to deal with the public authorities electronically, so in practice the online route is the only sensible one.
What happens if it is not filed
The regulations on the taxes managed by the ACA (Decree 103/2000) classify failure to file the DUCA, or filing it incomplete or with false data, as a tax infringement. The ACA may also demand the declaration and, if the demand is not met, determine the pollution data on its own initiative, by measuring the discharge or, where that is not possible, by indirect estimation.
The risk is ending up paying a levy calculated on data that does not reflect the reality of the establishment, and then having to dispute it. Filing the declaration on time and with verified data avoids reaching that point.
Not sure whether it affects you?
If your basic DUCA expires this year, send us the last declaration you filed and we will tell you which data should be updated before the October renewal.