Gray water is a type of wastewater that comes from domestic, industrial or commercial activities, but is not yet contaminated to levels dangerous to human health and the environment. It is called “gray water” because it generally has a slightly grayish color due to the presence of organic waste and cleaning products.

Gray water is mainly generated by hand washing, showers, washing machines, dishwashers and kitchen sinks. Although it is not suitable for human consumption without additional treatment, gray water can be treated and reused for non-potable purposes, such as watering gardens, flushing toilets or cleaning floors. Reusing gray water can help conserve water resources and reduce the demand for drinking water in activities that do not require high-quality water.

The Blue Water Footprint is a part of the total water footprint and refers to the volume of freshwater extracted from surface and underground sources for direct use in human activities. This water is mainly used in activities such as irrigating crops, providing drinking water for the population, industrial use and generating hydroelectric energy.

The Blue Water Footprint is important because it represents the direct impact of human activities on local and regional water resources. Excessive or unsustainable use of blue water can lead to overexploitation of aquifers, reduced flow of rivers and streams, degradation of aquatic ecosystems, and scarcity of water for future uses.

The Gray Water Footprint is a part of the total water footprint and refers to the volume of water necessary to dilute the pollutants generated during the production of goods and services, until the water once again meets certain environmental quality standards. These contaminants can include organic waste, chemicals, or any other type of substance that affects water quality.

The Gray Water Footprint is calculated based on the amount of water necessary to dilute contaminants until concentration levels are considered safe for the environment and human health. This calculation can be complex and depends on various factors, such as the toxicity of the contaminants, the self-purification capacity of the receiving water body, and local environmental regulations.

Managing the Gray Water Footprint is essential to prevent water pollution and protect the quality of water resources. Strategies to reduce the gray water footprint include the implementation of more efficient wastewater treatment technologies, reducing the generation of waste and pollutants in production processes, and strict compliance with environmental regulations related to the discharge of effluents.

Global and European Context
European Union

The European Union is implementing various policies and regulations to reduce greenhouse gas emissions as part of the European Green Deal. Within these policies, companies in certain sectors are already required to report their emissions.
EU Emissions Trading System (EU ETS): Industrial facilities and power plants have been required to monitor and report their emissions since 2005.
Non-Financial Disclosure Directive (NFRD): Since 2018, large companies (more than 500 employees) must report on their environmental impacts, including GHG emissions.
Corporate Sustainability Reporting Directive (CSRD): This directive, which will expand non-financial reporting requirements to more companies, is expected to come into force progressively from 2024 for large companies and later for listed SMEs.
Spain

In Spain, the calculation and registration of the carbon footprint is voluntary but encouraged through the Carbon Footprint Registry of the Ministry for the Ecological Transition and the Demographic Challenge.
Climate Change and Energy Transition Law: Approved in May 2021, this law establishes objectives and measures to reduce emissions, although it does not impose a universal obligation to calculate the carbon footprint, it encourages transparency and voluntary reporting.