What are the overflows of the sanitation system during rainy episodes (VDSS)?
Discharges into the Public Hydraulic Domain (DPH) during rain events that come from sanitation systems, whether combined or separate, not treated in wastewater treatment plants.
What are PIGSS?
The Comprehensive Sanitation System Management Plans (PIGSS) are documents that allow you to understand the condition of the sanitation networks, identifying their discharge points, both in dry periods and during rain events, and they define the structural or other measures that should be implemented within a certain timeframe to reduce the pollution caused by sanitation system overflows during rain episodes.
Who should prepare the PIGSS?
The holders of the discharge permits for urban agglomerations included in the inventory approved by each River Basin Authority.
What's the content of the PIGSS?
Its minimum content includes:
- Detailed description and characterization of the sanitation system.
- Goals for reducing pollution from system overflows during rainfall events.
- Measures that should be adopted, and studies of alternatives that justify them, to achieve the reduction goals, identification of the parties involved and their responsibilities.
In Annex XI of the Public Hydraulic Domain Regulation, after its amendment by RD 665/2023, the "Basic technical standard for controlling discharges from sanitation system overflows during rainfall events" is established, which specifies the criteria for designing works and facilities for the management of runoff water.
The following document can be consulted for the preparation of the PIGSS:
You can also access the recording of the "Technical Conference on Urban Agglomerations That Must Develop Comprehensive Management Plans for the Sanitation System and Update of the Urban Wastewater Treatment Directive" organized by MITERD in June 2024 at the following link:
Data of Pd,80% in the Segura Hydrographic Demarcation to prepare the PIGSS.
To assess whether the measures implemented in the sanitation system associated with each urban area are effective in reducing pollution from overflows of sanitation systems (VDSS) during rainfall events, the indicator "hydraulic performance of the sanitation system" will be used, both in combined systems and in separate stormwater systems. This indicator measures the amount of rain that can be properly managed across all elements of the urban area, also taking into account the volume of domestic wastewater in dry conditions, all defined for a design or calculation precipitation.
Since most of the pollution in VDSS is linked to typical rainfall that exceeds the primary treatment flow of the WWTP or the maximum capacity of combined sewers, or that generates urban runoff in separated networks, for analyzing the hydraulic performance of the sanitation system, the indicative design rainfall is taken as the daily precipitation not exceeded on 80% of days with precipitation over 1 mm (Pd,80%) during the study period.
Below are the values of Pd,80% that should be used as a reference for the preparation of PIGSS in the Segura River Basin, obtained from the processing and analysis of daily precipitation series provided by the State Meteorological Agency (AEMET-Ministry for the Ecological Transition and the Demographic Challenge), as well as other information from the Automatic Hydrological Information System (SAIH), the Murcia Agricultural Information System (SIAM), and the Agro-Climatic Information System for Irrigation (SIAR):
In addition, the "Instruction of the Water Commissioner's Office establishing correction factors for climate change in the calculation of Pd80% in the design projects of regulation infrastructures for sewage system overflows" approved on 12-16-2025 should also be taken into account.
