Department Process Engineering

During water reuse, wastewater is treated centrally or decentrally and can then be used as an alternative water resource for various applications:

• Agricultural irrrigation
• Irrigation of urban green infrastructure
• Industry
• Household applications
• Cleaning purposes e.g. roads or sewers

Water reuse is already an established practice in countries and regions with low water availability. The European Union has also set itself the goal of promoting water reuse in the member states. As of 26.06.2023, the corresponding Regulation 2020/741 on minimum requirements for water reuse in agriculture has entered into force, but only for those Member States that have opted for the national implementation of the Regulation.

In Switzerland, an increase in heat waves and dry periods is expected for the future due to climate change. This will be accompanied by an increase in water demand for irrigation, which in turn will also be amplified by the increased implementation of the sponge city concept in urban areas. The demand for cooling water is expected to increase as well, due to the rising temperatures.

By using recycled water, withdrawals of water from surface waters and groundwater can be substituted, which would contribute to the protection of these waters bodies. The ecological quality of surface waters can also be improved by reducing the discharge of wastewater treatment plant effluents.

In order to reduce the emission of micropollutants into the environment, Switzerland is advancing with the implementation of an additional fourth treatment stage at wastewater treatment plants. As a result, there are already good technological

Regarding the subject of water reuse, quantitative requirements (ecological minimum flows) as well as qualitative aspects (biological and chemical water quality) have to be considered. The environmental compartments (soil, groundwater, surface water, plants, humans) that can potentially be affected by water reuse have to be protected and must not be adversely affected by the practice. Furthermore, it is important to consider social aspects such as the acceptance and institutional legitimacy of water reuse projects.

In the literature on water reuse, certain terms are regularly utilized. A selection is defined below, to create a common understanding of the subject.              

Wastewater

Water that has been altered by domestic, industrial, commercial, agricultural or other uses, as well as water that flows which such altered water continuously through the sewer system, or rainwater that runs off fortified or paved areas. [1] - Art. 4 (e)

Service water

Water that is not intended for direct human consumption. As a result, the water quality does not usually have to meet drinking water standards. Service water can be obtained from conventional water bodies such as surface waters or groundwater, but also from wastewater treatment plant effluent. It must be distributed in its own pipe system, separated from drinking water. [2]

Brownwater

With separation toilets, urine is collected separately. The remaining wastewater stream contains flushing water, feces and toilet paper and is called brown water. [3]

Decentralized wastewater management

Wastewater generated on a building level is collected locally and treated on site. This option can be applied in cases where no connection to a central sewage system is possible/reasonable or existing infrastructure should not be further burdened. The treated wastewater is available for reuse on site. Decentralized wastewater treatment approaches can be based on material flow separation. In this case, wastewater with different pollution loads (grey water, black/brown water, yellow water) is treated appropriately. [4]

Yellowwater

Wastewater from separation toilets containing only urine and optionally flushing water. [5]

Greywater

Wastewater originating from bathrooms (sinks, showers, bathtubs, washing machines) and kitchens (kitchen sinks, dishwashers), excluding wastewater from toilets. [6]

Indirect water reuse

Treated wastewater from wastewater treatment plants is usually discharged into a receiving water body, commonly a river or lake. During indirect water reuse, water is taken from such a receiving water body and utilized e.g. for drinking water production, agricultural irrigation or other purposes. Although the share of treated wastewater in different surface waters can be significant, especially during dry periods, this fact is usually not explicitly considered in water quality regulations or risk management plans.

Further utilized terms for this practice are "unplanned reuse" or "de facto reuse". [7]

Blackwater

Wastewater containing flushing water, toilet paper, feces and urine. [4]

Drought stress in plants

Drought stress or water stress in plants occurs when the water available in the soil and the actual evapotranspiration of a plant that depends on it is less than the potential evapotranspiration. [8]

Water stress

The water use index describes the ratio of anthropogenic water withdrawals to available renewable water resources. A water use index greater than or equal to 20% is internationally considered as water stress. [8]

Water reuse

Intended and planned utilization of treated wastewater. The treatment can go beyond conventional wastewater treatment, depending on the required quality of the reused water. [9]

The term «water reclamation» is used as a synonym for «water reuse». [7]

Central wastewater management

All the produced wastewater (grey water, black water, toilet flush water, and in the case of combined sewer systems also storm water) is discharged via the sewer system and conveyed to a central wastewater treatment plant. There, the wastewater is treated and the cleaned wastewater treatment plant effluent is usually discharged into a receiving water body. [10]