Water Cycle


Water users all over the world need safe water. They also need their wastewater to be collected safely. People and the environment need to be protected from polluted waters. Public authorities have to match users’ expectations at an optimised cost. Private operators exist to respond to all these demands as instructed, regulated and controlled by public authorities.

Source. Wikipedia

The term “human water cycle”, otherwise known as the “small water cycle”, is the way communities organise the flow of water from the point where they take it from nature, prepare it for use, use it for many purposes, and then clean it and return it to the natural environment. Sometimes referred to “from source to tap”, and “from tap to source”.


  1. Collection

The first step in the human water cycle is the abstraction of raw water, either from surface water or from groundwater, using gravity flow or pumps. Water sometimes needs to be stored in reservoirs to make sure enough is available when it is needed.

  1. Treatment 

Water is treated at water treatment plants to meet water quality standards / requirements and to be safe for all uses.

Processes may vary depending on location, technology and quality of water in the catchment. Water treatment is always based on the same principles to eliminate various elements: solid matter, biological matter (virus and microbes), chemical contaminants, contained in water through a range of successive steps.

  • Screening (Coarse & Fine) = “straining the lumps out”
  • Filtering = removing small particles, organic substances, some chemicals
  • Coagulation & Clarification = removing small particles
  • Adsorption = removing organic substances, some dissolved matter and chemicals
  • Biochemical reactors = removing complex chemicals and substances
  • Polishing = treatments that “tidy up” at the end of the treatment process. This can be done using a variety of physical, chemical or natural processes that include things like use of reed beds.
  • Membrane treatment = an alternative way of achieving several of the processes above
  • Buffering = adjusting the quality of water to meet requirements such as acidity/hardness
  • Disinfection = sterilising the water and ensuring safety in the distribution systems (usually chlorine or ozone)
  1. Storage and distribution

Once water has been collected and treated, it flows to smaller service reservoirs and is ready to be distributed to customers - populations and industrial clients - through pumps and pipelines. Water operators use service reservoirs to store water and to be able to face variations in the demand for water and in water stocks depending on season and precipitation.

  1. Use

Water operators aim to provide a regular and reliable water supply through the distribution network. They work night and day all year long to provide constant supplies of safe water at all times.

Note: personal reservoirs that are never cured in Africa (clapet anti-retour, not cleaned, pollution of the network if loss of pressure) 


Water-users (industry, households, institutions, agriculture…) generate wastewater streams that contain rubbish, suspended solids, dissolved chemicals, biodegradable organics, pathogenic bacteria and  nutrients. Wastewater operators aim to eliminate these pollutants before returning clean water to nature safely. As far as possible they use the materials recovered in beneficial ways.

  1. Wastewater collection

Private operators collect used water in drains and convey it to a wastewater treatment facility via large pipes (sewers). Drains and sewers have to be managed properly and cleaned regularly to ensure that the water flows through them easily.

Ideally wastewater flows through the collection system by gravity, but it is often necessary to pump the wastewater several times before it reaches the treatment plant. Sewage pumping stations are expensive, can be complicated to operate and if they break down, the consequences are serious. For example, the breakdown of a sewage pumping station can quickly lead to public spaces and private property being flooded thus causing real danger to public health, inconvenience and damage to property.

  1. Septic tanks

In some locations, domestic wastewater is not collected in the public system, but in private septic tanks or pits. These need to be emptied periodically and the septage and faecal sludge treated at a treatment plant.

  1. Wastewater treatment
  • Pre-treatment: When the wastewater arrives at the treatment plant it first passes through a pre-treatment stage. As much as possible of the rubbish, grease, large particles, stones, sand and grit are removed by screens and grit and grease removal tanks.
  • Primary treatment: The wastewater is slowed down by passing through large tanks to let particles settle out. 
  • Secondary treatment (or biological and biochemical treatment): The secondary treatment removes nutrients and remaining solids through bacterial decomposition. This treatment uses carefully enhanced naturally occurring biological and chemical processes to remove nutrients, certain other chemicals and remaining solids. A properly designed and operated treatment plant with secondary treatment can reduce the quantities of organic material by more than 90% and reduce the presence of faecal germs by 99%.
  • Tertiary treatment: It removes bacteria, viruses and parasites, which are harmful to public health and the environment. Then water can be reused, recycled or discharged to the environment. In some cases, the final effluent is disinfected before the water is discharged or reused.

Via treatment processes water operators also have to take account of the smells that come from wastewater and control the risks of pest, notably flies.

  1. Wastewater reuse and recycling

Water, that has been used, is almost always contaminated in some way. It is often called wastewater. This gives the impression that it is a waste and therefore useless. In a growing number of situations this is not the case. Used water can be cleaned so that it can be recycled or reused to other purposes it can thus increase water security and reduce water stress.

Recycling is most often done within a large industrial premises. Reuse can be for irrigation, agriculture, urban landscaping or in industry. Private operators have made rapid progress in developing these approaches. Good examples include São Paulo Brazil, Shanghai China, Mexico and California 


  1. Treatment and recovery of gas and materials removed in water and wastewater treatment

The materials that have been removed from water and from wastewater have to be managed in the most effective, sustainable and safe way.

Much of the material is recovered in the form of sludge, which is a mixture of organic and inorganic material, chemicals, minerals, and metals. The volumes of materials and sludge recovered from wastewater are more significant than those from water treatment.

The sludge from wastewater treatment is rich in nutrients and organic matter which makes it potentially valuable for use in agriculture, landscaping and forestry. Modern sludge treatment processes employed by private operators aim to turn the sludge into a safe and convenient material that can be transported to farms and forests. Alternatively, it can be used as a fuel for furnaces to generate heat or electricity.


The collection and treatment of wastewater can produce significant volumes of gas. Some of these gases are dangerous and need to be managed accordingly, others notably methane (a “clean energy”) are useful sources of energy that can be used in the treatment process itself or exported for other purposes. This contributes to reducing costs and carbon footprint.



  1. Quality control, monitoring, data management, records and reporting



The quality of the water and the work carried out at all the stages in the management of the water cycle is of great importance to private water operators, public authorities, the people they serve and the environment.

All contracts contain a number of quality control and performance criteria that must be met. Private water operators usually have obligations to report to their clients and to regulatory authorities on these indicators.

In addition, most operators have their own additional criteria that they use to manage their operations and to ensure efficiency and compliance.


The information needed to ensure that the criteria are being met, to make adjustments and to warn of difficulties or breakdowns, is created by a range of different processes. These include, periodic sampling, laboratory tests, and the use of online real-time sensors, gauges and alarms. In addition, prescribed protocols for inspection and testing and random checks are used.

Many of these techniques are now completely integrated into the operational control systems that are used to control the infrastructure and processes by actuating pumps valves and other equipment and to show the actual status and performance of the service as it occurs.


Today many control systems are highly computerised and are also linked to Geographical Information Systems (GIS) and to the Customer Relationship Management Systems (CRMS).

These systems create a great deal of data, which needs to be stored and managed carefully. Data management involves the creation of regular compliance reports for the public authority and regulators. It also requires analysis to show up areas where reliability or performance can be improved. This is a way to support the continuous search for savings in energy, chemicals and other consumables. Data analysis also helps in forward planning of maintenance, replacements and extensions to the infrastructure.

Accurate and safe records of both raw and analysed data and reports need to be kept.

All of these steps are necessary to convey the appropriate information to management, regulators, planners, public authorities and the end water users.