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Water is very abundant on our planet. However, it is very unevenly spread over the surface of the globe and only a limited portion of all this water is actually directly available for our consumption. The fresh water on the planet, i.e. the water with a salt concentration of less than 3 grams per litre, only represents 3% per cent of the volume of water stored in the hydrosphere. This fresh water is used by households, agriculture and industry. Over the last 50 years, though, given the deficit of the water resource and the increase in demand, an alternative source of fresh water has been developed: recycled water.
Fresh water or “raw water” reservoirs
High in the mountains, streams are formed from glacier meltwater: this is the glacial regime. In fact, the thick layer of ice acts as a thermal insulating layer against the very cold outside air and the temperature at ground level is, therefore, generally above zero. This is why the ice melts, producing small streams or raging torrents, which flow underneath the glaciers to emerge at the glacier foot.
In Canada, Siberia and the north of Europe, rivers are fed by water from the melting snow: this is the snow regime.
Other streams of water are produced by stormwater run-off and/or groundwater emerging in the form of springs: this is the rain regime (ground water comes from the infiltration of rainwater into the soil).
All these streams flow downhill, joining together to form large and majestic rivers, which end up flowing into an ocean or inland sea.
The notion of drinking water
Raw water intended for human consumption is drawn from a river or groundwater reserves. It is then brought to a drinking water?production plant, where it undergoes various physical, chemical and biological treatment processes.
The treatment processes. The standard full water treatment process is carried out in several stages, some of which are not necessary for the cleanest water.
Oxidation: if the water to be treated contains a lot of organic matter, or indeed ammonia, iron or manganese, a preliminary oxidation stage is necessary. This enables these substances to be more easily eliminated during the following stage known as clarification. For this, an oxidizer such as chlorine or ozone is used.
Clarification: clarification allows for the elimination of suspended particles. After passing through screens, which filter out the largest materials, the water is then delivered into ponds known as settling basins. The settled water is then filtered through one or several layers of course substrate, containing granular material such as sand, which retains the residual finest particles.
Disinfection: at the end of the treatment process, disinfection enables pathogenic micro-organisms (bacteria and viruses) to be eliminated. For this, either a chemical disinfectant such as chlorine or ozone, or ultraviolet radiation is used.
Now made drinkable, the water can then be distributed to the consumers.
Mineral water or spring water.
Originating from unpolluted groundwater reserves, deep-down or protected from the debris produced by human activities, so-called spring water is water that is naturally fit for human consumption. The only treatment processes that can be applied to it, in order to eliminate unstable elements such as gases, iron and manganese, are aeration, sedimentation and filtration. These spring waters are generally consumed at regional level, because the transport would make them too expensive. There are about a hundred such springs in France.
Mineral waters are spring waters with special properties: they contain minerals and trace elements that are likely to give them therapeutic properties and their composition is stable over time. Like spring waters, they cannot be treated. Once bottled, these waters are transported far and wide and are even exported.
Green water – blue water.
The water that falls as precipitation can be divided into two categories: the blue water flow that rapidly enters the streams, lakes, groundwater aquifers and seas; it represents about 35% of the total precipitation mass. The second category is the green water flow, which, stored in the soil, is evaporated or consumed and transpired by plants and returned directly into the atmosphere; it is by far the largest quantity, since it represents 65% of the total precipitation mass.
The blue water flow used by industry and households is largely returned to the system after treatment; on the other hand, the bluewater consumed by agriculture (irrigation) or stored is not put back into circulation: it remains linked to the agricultural products or, just like green water, it is transpired and returned to the atmosphere.
Wastewater is water that has been adversely affected by human activities following domestic, manufacturing, industrial, agricultural or other uses. It is considered to be polluted and has to be treated. There are several types of wastewater (adapted from Julien Deborde’s thesis: “Experimental and numerical study of a self-regulating emitter for irrigation”):
• Domestic wastewater from our homes. It is mainly polluted by organic matter. It includes household water discharged from kitchens and bathrooms, polluted by detergents, washing powders, fats… It also includes the water flushed from toilets. The waste matter present in this contaminated water is made up of degradable organic and mineral matter. Domestic wastewater is filtered, treated and returned to the rivers and/or broken down by phytoremediation.
• Sewage. This is the waste material extracted via the operation and maintenance of individual wastewater treatment systems. It constitutes a concentrated source of domestic pollution, its removal and collection forming an integral part of the sewerage and sanitation system. It includes the sludge taken from on-site (non-collective) sanitation and sewage installations (septic tanks, cesspools, cesspits). Sewage is treated and spread on the land (in accordance with the water regulations).
• Agricultural wastewater is produced by animal waste and is mainly polluted by animal faeces and urine. It consists of slurry and liquid manure. It is filtered, treated and broken down by phytoremediation.
• Industrial wastewater. Its characteristics are extremely variable and are directly related to the type of industry: food-processing, chemicals, mining or other industries. After the first specific treatment processes have been carried out, industrial wastewater can be mixed with domestic wastewater, which allows it to be discharged into the public sewerage system. It includes winery effluents and industrial resins. Industrial wastewater is filtered, treated and broken down by phytoremediation.
• Rainwater. It can be highly polluted especially when the rains begin, because it washes the smog out of the atmosphere, cleans the greasy roads and roofs laden with dust, leaches chemicals from agricultural areas (run off of liquid manure, fertilisers and weed-killer products) and washes the terraced areas (mud running off the surfaces). The pollutants present in this water can be biodegradable or non-biodegradable organic matter, in suspension or dissolved, including toxins. It includes rainwater and surface run-off water. Rain water is recovered for domestic use (WCs, washing machines).
Black water – grey water.
Wastewater can sometimes be described as grey water when it is water that is not heavily laden with pollutants, for example, water of domestic origin, resulting from washing dishes, washing the hands, baths and showers. We speak of black water when it contains a number of substances that are more polluting or harder to eliminate, such as faecal matter, cosmetic products or any type of industrial by-product mixed with the water. In most countries and particularly in urbanised environments, wastewater is collected and channelled via the sewers (or sewerage system), either to a wastewater treatment station or an independent sewerage treatment site.
The level of treatment is defined according to the quality required by the type of recycled waste water use.
There is a wide range of water treatment processes, thus offering the best technical choice according to the local context: clarification, biological treatment, filtration, membrane technology (microfiltration, ultrafiltration, nanofiltration, reverse osmosis), ultraviolet disinfection, treatment by granular activated carbon treatment, ozone disinfection, or even industrial wastewater recovery plants.
After decontamination or purification, this recycled water constitutes a “second hand resource”, which is found to be mainly useful for the industrial washing and conveyance of materials, industrial cooling, irrigation, local authority uses (street cleaning, fire fighting…)
In view of the reduction in available water resources, it is considered that more and more “wastewater” will be used for irrigation. This needs to be well thought out and researched with regard to the sanitary (human health) and toxicological (toxic contamination of the plants) effects and the technical constraints (appropriate distribution equipment, particularly in micro-irrigation and sprinkler irrigation). It is important in these studies to specify what type of water will be used, in particular, its level of treatment: settled/ decanted, clarified, purified water… This will also determine its type of use in irrigation: for irrigating green spaces or forests, or irrigating food crops; as well as the type of equipment used: there is a greater risk of harm being caused by undesirable substances with sprinklers than with drip irrigation, especially subsurface drip irrigation systems.