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Today, a quarter of the world's population still does not have access to drinking water and 80% of wastewater is discharged without treatment in nature: 800,000 people die each year, from various diseases. Experts mandated by the UN forecast a shortfall of 40% of fresh water supply by 2030.

Waste, overuse in agriculture and industry, contamination, too rare recycling, poorly preserved freshwater ecosystems... In addition, global warming has also complicated the achievement of the targets set in 2015 for access to water for all by 2030 (SDG No. 6).

The United Nations held a conference on water from 22 to 24 March, the first international summit in almost 50 years. The new action program aims to ensure access to drinking water, sanitation and hygiene for all. In this context, what about recycling wastewater and how can it respond to water stress?

We recall that most of the sources of our drinking water (two thirds in France) come from groundwater, the other from surface water (torrents, rivers, lakes), both driven by precipitation.

In France, only 0.6% of wastewater is recycled, and its reuse is only permitted for agricultural irrigation (far from human transit), the watering of golf or green spaces. While in California or Singapore, once cleaned, wastewater is mixed with ‘virgin’ water for human consumption. While other countries, such as Australia and Namibia, can recycle up to 4% of their wastewater, no country in Europe has yet put in place a system for domestic use. The problem lies partly here.

Before we talk about wastewater treatment, let us look at the treatment of natural water.

First captured in the natural environment, then treated, transported, distributed and consumed, the water is then returned to nature to resume its cycle.

The treatment applied depends on the initial quality of the captured water: While groundwater, which has already benefited from the role of basement filter, receive short treatment of filtration and disinfection, surface waters on their part follow either simple physical chemical treatment with chlorine or ozone disinfection, or advanced physical chemical treatment with sharpening (active sharbons or membranes) and disinfection.

As a reminder, here is a detailed description of the most common steps of treatment (Source: Water Co).

1) Capture
Capture and transport to the Stabilisation Plant.

2) Flashing
Passage through grids to stop floating bodies and large wastes.

3) Screening
Fine film to stop smaller waste, sand, plankton...

4) Flood decantation
Decanting of coagulant to consolidate small packet impurities (flocs) and remove 90% of suspended material.

5) Filtration
Filtration of invisible, but still present, suspended particles on conventional (sand) or absorbent materials (active charcoal in grain or powder).

6) Disinfection - Ozonation
Neutralization of viruses and pathogens, then ozone (gas) injection. Ultraviolet radiation is sometimes used.

7) Special treatment
Absorption on active coal. Refining to eliminate organic matter and improve the organoleptic qualities of water (flavor, smell, limpidity).

8) Chloration
Added a tiny amount of chlorine to preserve water quality throughout its course in pipes to reach the valves.

9)Storage
Once it is made safe to drink, water is then shipped to reservoirs where it is stored and then transferred through a network of underground canalizations in the houses.

Wastewater treatment, for its part, has the sole aim of not polluting the natural environment in which it will be discharged. If these are not considered acceptable to consumption from the outset, they will eventually be acceptable.

After the stages of 1) degrilling/sifting and 2) decantation, the process takes place as follows

3) Organic treatment with activated sludge : The pollution contained is dissolved by bacteria and transformed into biological sludge, carbon dioxide and treated by alternating periods of ventilation and rest in basins.

4) The clarification separates sludge from water, which, when more than 90% cleared, is then released into the environment.

5) Treatment of sludge consists of concentrating organic matter by filtering liquid sludge to make it easier to transport and store solid sludge.

Source : Meuse Grand Sud

In addition to the problem of increasing pollution, there are technical and economic considerations.

It should be noted that in order to obtain drinking water of constant quality, treatments are becoming increasingly expensive given the increased pollution:
• Industrial discharges (chemicals), agricultural discharges (animal manure, pesticides), domestic discharges (toilets, detergents, paints, oils, hydrocarbons, etc.) or accidental discharges.
• Dump of sewage into the sea without treatment due to hydraulic overload: For example, the ageing of the British disposal system has led to many discharges into the Channel and the North Sea, up to 200 per year...

The development of wastewater recycling is a sufficiently attractive solution to water stress to be included in the United Nations Plan of Action. The other benefit would be the reduction of this water pollution.

There is no limit to the imagination in this area: Even the sorting of body waste is being considered. While they represent pollution to aquatic environments, they are a resource for agricultural practices. Nitrogen and the phosphorus they contain feed proliferating algae instead of benefiting agricultural plants. This solution would certainly require separate collection toilets, but the tripling of the price of nitrogen fertilizers makes one think about it...

Text completed on 28 March 2023