Reuse water and use of rainwater: what are the differences?

Understand the differences between reuse water and rainwater harvesting systems. Choose the ideal type for you

The problems of water scarcity are faced by countries around the world as a result of the disorderly development of cities, pollution of water resources, population, and industrial growth, among others. These factors generate an increase in the demand for water, causing the depletion of this resource. Two options to avoid the excessive expenditure of this very valuable resource are reuse water and the use of rainwater. But do you know the difference between them?

In many cities or places where water is not available, solutions are needed that deal with the context, specificity, and characteristics of the area. Another important factor in the water issue is the availability of these resources in the regions of the world and even in Brazil because even though we have 13.7% of all surface fresh water on Earth, 70% of this total is located in the Amazon region and only 30% is distributed throughout the rest of the country. In addition, while there is industrialization, there is also a great potential for contamination, which further restricts freshwater sources. And more and more people try to use new techniques to reduce water consumption and expenditure.

The reuse of water and the use of rainwater for non-potable purposes, in the case of rural areas, can be solutions to deal with scarcity. With the correct treatment, rainwater can be used for drinking purposes as well. Find out more in the article: “How to treat rainwater?”

However, there is a difference between reused water and the use of rainwater, as each type has a different need for treatment, management and depends on the location (whether rural or urban, house or apartment). Let’s understand the differences between these types:


Also called wastewater, are all discarded water that results from the use of various processes. Article 2 of Resolution nº 54 of November 28, 2005, of the National Water Resources Council – CNRH, classifies these waters as: “sewage, discarded water, liquid effluents from buildings, industries, agro-industries and agriculture, treated or not”. The Environmental Technology and Sanitation Company of the State of São Paulo (Cetesb) exemplifies that domestic wastewater comes from bathrooms, kitchens, washing of domestic floors; while industrial wastewater comes from industrial processes.

Reuse water

In the aforementioned article from the CNRH, reuse water is considered to be wastewater found within the standards required for its use in the intended modalities, that is, water reuse consists of the reuse of certain water that has already been used for the development of the human activity. This reuse occurs from the transformation of wastewater generated in some activity into reuse water. This transformation takes place through treatment. According to scientific bases, reuse can be direct or indirect, resulting from planned or unplanned actions:

Unplanned indirect water reuse

It occurs when water, used in some human activity, is discharged into the environment and used again downstream, in its diluted form, in an unintentional and uncontrolled way.

Planned indirect water reuse

It occurs when the effluents, after being treated, are discharged in a planned way into bodies of surface or underground water, to be used downstream, in a controlled manner, in the fulfillment of some beneficial use. There is control over any new effluent discharges on the way, thus ensuring that the treated effluent will only be mixed with other effluents that also meet the quality requirement of the intended reuse.

Planned direct water reuse

It happens when the effluents, after being treated, are sent directly from their point of discharge to the place of reuse, not being discharged into the environment. This is the case with the highest occurrence, intended for use in industry or irrigation.

Reusing gray water produced in homes also falls into this category – gray water is a type of reused water from baths, washing machines, and bathroom sinks – these are waters that have not come into contact with black water (those mixed with feces and urine). This water can be collected through domestic cisterns and reused for flushing, cleaning floors or yards, and even washing the car, depending on the type of residue that the gray water contains. Learn more about gray water in the articles: “Grey water: how to take advantage of reused water” and “The colors of effluents: understand the differences between gray water and black water“.

Rain water

Rainwater is often considered to be sewage, as it most commonly passes through roofs and floors, going straight to storm drains. From there, since it acts as a “universal solvent”, this water carries with it all kinds of dissolved impurity or just mechanically takes the waste to a stream and, later, to rivers – this is the main way in which the garbage reaches the oceans. Read more in the articles: “90% of the plastic in the oceans comes from just 10 rivers” and “What is the origin of the plastic that pollutes the oceans?”

However, if rainwater harvesting is carried out in areas of restricted access before this route, it can be used for non-potable purposes without the need for more complex treatment. For this, it is recommended to discard the first 1 mm of water or up to 2 mm in urbanized areas, as studies have shown that this initial discharge (first flush) carries impurities suspended in the air and on the roof, which may contain animal feces and organic matter.

These first millimeters are a result of the project’s calculation, for example, when capturing water from a roof, its size, and how much it rains in the region (which can be found here). These will be determining factors in the design of the initial disposal and the size of the storage tank. Usually, 1 mm of rain is adopted on 1 m² of the roof, which is equivalent to 1 liter of water – that is, if your roof is 50 m², the first 1 mm of rain would be 50 liters, a volume that must be discarded initially and led to the rainwater drainage system. Never connect the discharge of this first rainwater to sewage collection systems.

However, the system designer must follow the ABNT NBR 15527 standard of 2007, which establishes guidelines for projects regarding water parameters, as this type of water is not considered potable in its original state and can bring risks when ingested. and when in contact with mucous membranes, which requires a dosage of chlorine in the tank. But, as we said above, it is possible to treat rainwater at home for drinking, if that is a need in your region.

Water applications

  • According to Cetesb, it is possible to use reuse water in some situations:
    Landscape irrigation: parks, cemeteries, golf courses, freeway domain lanes, university campuses, greenbelts, residential lawns and green roofs;
  • Irrigation of fields for crops: planting of forage, fibrous and grain plants, food plants, nurseries of ornamental plants, protection against frost;
  • Industrial uses: refrigeration, boiler feed, process water;
  • Recharge of aquifers: recharge of potable aquifers, control of marine intrusion, control of subsoil settlements;
    Non-potable urban uses: landscape irrigation, fire fighting, flushing toilets, air conditioning systems, washing vehicles, washing streets and bus stops, etc.;
  • Environmental purposes: flow increase in water courses, application in swamps, wetlands, fishing industries;
    Various uses: aquaculture, construction, dust control, animal watering.

Be careful to observe which types of residues are present in the water that you want to reuse: if there are residues of detergent, cleaning products, bacteria, and body secretions, such as sweat and skin oil. Environmental awareness and the appreciation of our water resource is of paramount importance and the idea of ​​using rainwater and reusing domestic water must be increasingly disseminated. But remember: there are professionals on the market trained to design and build these systems within the established parameters, so, if in doubt, look for them.

It is also necessary to be careful when storing these waters for later consumption. The ideal is to use domestic cisterns, which are already equipped with systems for capturing and filtering water. You can also make your own residential cistern.

Source: Ecycle


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