Water used for human consumption comes from surface waters (streams, lakes, rivers, seas, etc.) or from under ground (aquifers or water tables). Bore holes are used to pump the water or water is abstracted directly from the surface.
Treating Water to Make it Safe to Drink
Once the water has been abstracted, it undergoes several types of treatment processes in order to make it safe to drink.
Screening and Filtering
Water is first filtered through a simple screen in order to retain the largest pieces of waste found in the water (leaves, insects, etc.). It then passes through fine metal meshes which retain the smaller pieces of waste (smaller than 1 mm).
Flocculation and Sedimentation
A coagulant is added to the water to gather together into flocs any waste still present in the water (dust, earth particles, fish eggs, etc.). Heavier than the water, these flocs settle at the bottom of the sedimentation tank, and as a result 90% of suspended matter is removed.
Sand filtration removes matter still visible to the naked eye.
Water is disinfected by using ozone which kills bacteria and viruses. This gas, when mixed with water, also acts on organic matter by breaking it down into pieces. It also improves the color and taste of the water.
Activated Carbon Filtration
Activated carbon filters retain micro pollutants such as pesticides and consume some of the organic matter ”broken down” by the ozone.
Chlorine is added as the water leaves the water treatment plant and at various stages throughout the water supply system. This prevents bacteria from growing as well as maintaining water quality as the water passes through the water mains.
The desalination of seawater is a solution that increases the available freshwater resources, provides a solution in the event of drought, and for coping with shortages and crisis situations.
There are two main desalination technologies:
- thermal desalination (distillation)
- membrane desalination (reverse osmosis)
- The Annet-sur-Marne water treatment plant - France
- Seawater thermal desalination - Bahrain
- Reverse osmosis seawater desalination
Once it is made safe to drink, the water is transported via a network of closed, cast-iron or steel pipes to tanks which are usually placed on high ground. These could be underground tanks on the top of hills or water towers.
These storage points use the principle of communicating vessels. This ensures there is regular and sufficient pressure within the system in accordance with the rate of consumption.
They also provide an emergency backup should there be an incident within the system or an unusually steep rise in consumption.
Video and slideshow available
- The tank of the Cima di Gavi - France
- A water tower seen from the inside and outside
Water is supplied to consumers using a complex system of pipes that are equipped with valves and control instruments.
Tank levels, water flow, water pressure and water quality are constantly monitored at various stages throughout the system.
Video and slideshow available
- The control center of Shanghai Pudong - China
- System maintenance and quality water control
Collecting and Treating Wastewater
Collecting Wastewater and the Sewerage System
Wastewater is collected using a sewerage system. This network consists of a collection of pipes (also known as “drains”) made from cement, concrete or PVC which carry the polluted water to be sent to a wastewater treatment plant.
Regular maintenance is carried out on the sewerage system in order to prevent clogging and corrosion.
Water is filtered through a first screen in order to remove large waste and suspended matter found in the water.
Sand and Oil Removal
The sand and gravel are deposited at the bottom of the basin and are then evacuated (sand removal). Thanks to the injection of air, fats float to the surface where they are collected (oiling).
Suspended solids settle to the bottom of the basin to form the "primary sludge”. These are then recovered by scraping and sent to thickeners.
The primary settling will remove approximately 70% of suspended solids. By adding chemical reagents, up to 90% can be eliminated.
This process uses micro-organisms (aerobic bacteria) to degrade the dissolved organic matter in water. This degradation takes place in an aeration tank where air is injected to accelerate the work of the bacteria. They will consume the pollutants and form, in aggregate, so-called biological sludge.
Videos and slideshow available:
- Collecting rain water in Toulouse - France
- Ginestous-Garonne Wastewater treatment plant, Toulouse - France
- Maintenance and cleaning of pipes of the sewerage system
Sludge Treatment and Recycling
The biological treatment process of wastewater results in sludge being produced that has a high nitrogen and phosphorus content. Veolia Water treats this sludge and reuses it in various treatment systems, for example, for use in agriculture or energy production.
Video about the agricultural reuse of sludge from wastewater treatment
Recycling Treated Water
Recycled water provides a safe alternative to drinking water for all uses which do not require water to be of drinking water quality. It can be used in agriculture, industry, maintaining urban areas (watering lawns, cleaning roads, etc.).
The recycled wastewater can also be used for aquifer recharge.
Water recycling is a key element in sustainable development initiatives.
- Recycling water in Australia: agriculture, industry and municipal use.
- Aquifer storage and recovery, Adelaïde - Australia
Returning Clean Water to the Natural Environment
Once treated in a wastewater treatment plant, water is released into the natural environment. It continues its natural treatment process in rivers or in seas, without adversely affecting the natural balance of the environment.