What is and how does the integrated water cycle work?

Water capture

Water capture is the starting point of the water cycle and involves its withdrawal from the environment thanks to intake works, which are characterised by the particular management and protection of the water resource.

Capturing takes place in different ways, which differ according to the source from which the water is taken: 

• Capturing from the source: in natural springs, rainwater accumulates after being slowly filtered underground. Through some small cavities, it reaches an impermeable layer of soil, on which it begins to accumulate, generating an ‘aquifer’. Subsequently, the groundwater flows naturally to the surface, settling in springs from which water can be drawn and drawn;
• Capturing from a well: a well is an intake work that takes water from one or more aquifers and directs it to the aqueduct system through pumping. The nature of the aquifer from which the water is drawn characterises the type of well, which may be artesian or groundwater;
• Capturing from rivers, lakes and reservoirs: by means of pumping systems or water conveyance on certain channels, the resource is drawn and brought towards the adduction network;
• Capturing from non-conventional sources: non-conventional sources may be desalinated sea water or sewage or industrial wastewater, suitably treated to make it drinkable.
  

The potabilisation

After it has been captured, water must be potabilised, in order to be safely and without waste directed to end users.

In Italy, for example, current legislation regulates fresh surface water intended for the production of drinking water. In this sense, surface waters are classified into three categories, according to their physical, chemical and microbiological characteristics described in the legislation itself. For each of these categories, the drinking water treatments to be carried out are described in detail:

• A1 - simple physical treatment and disinfection;
• A2 - normal physical and chemical treatment and disinfection;
• A3 - heavy physical and chemical treatment, refining treatment and disinfection. 

The physical and chemical treatments dealt with in the first two points are divided into several stages. They aim to eliminate non-settleable suspended solids and correct the chemical characteristics of raw water by eliminating substances that are incompatible with the civil uses the water is intended for. Specifically, the treatments are as follows: 

• Pre-ozonation: the oxidation of dissolved substances that must then be eliminated;
• Clariflocculation: substances are aggregated and made to settle;
• Sand filtration: even the finest parts are removed. From this point, the water already meets the requirements for potability;
• Sterilisation: through the use of ozone, the water is made even purer;
• Refining with activated carbon: for the removal of tastes and odours;
• Final disinfection: through the use of sodium hypochlorite or chlorine dioxide, bacteria are destroyed and the water is protected on its way to the taps of the end users.

Once the water has undergone the above-mentioned treatments, it can be sent to the reservoirs.

The adduction

After potabilisation, the water is sent to reservoirs for collection and storage. In this way, it can be distributed to users without having to keep pumps constantly running and to generate the pressure needed to send water to the highest floors of houses, making up for water shortages caused, for example, by maintenance to capture and drinking water systems. 

The distribution

Distribution is the phase in which water arrives in users' homes for civil use or in commercial establishments. The water is fed into the aqueduct network, which is made up of pipes of different sizes and diameters that branch off along all the streets and roads of the towns and the countryside.

From the primary network then branch off into the connections (secondary network), pipes through which water is delivered to each user, in the same way as in the gas sector. Finally, the terminal point of the connection is the meter, which measures the water consumption of the user to which it refers.

The wastewater collection

After use, water is collected by sewage systems, which direct it to treatment plants. The quantities of water in the sewers are called ‘wastewater’. 

Wastewater can be collected through collective or individual networks: the former consists of filter systems used mostly for domestic wastewater; the latter, on the other hand, is a special system implemented according to existing constraints, such as ground conditions, isolated domestic dwellings, particular distance from a treatment plant.

The wastewater purification

Non-stormwater is sent to the purification plant, which purifies it and releases it back into the environment. Purification also consists of several stages: 

• Lifting: the starting phase in which water treatment begins;
• Screening: removal of materials such as pieces of plastic or wood, hygiene products, etc.;
• Desanding: separation of settled sand and oils and grease;
• Biological process: this in turn is divided into two phases, oxidation (degradation of organic matter in wastewater in an aerobic environment) and denitrification (nitrogen removal). Both processes take place in the primary sedimentation tank and, during them, so-called ‘good sludge’ is formed;
• Separation from the sludge: in the secondary sedimentation tank, the water is separated from the good sludge;
• Sludge treatment: in order to be disposed of properly, the sludge undergoes anaerobic or aerobic digestion and subsequent mechanical dewatering treatment, which reduces its volume.

The waste water exiting from the final sedimentation and separation from the sludge can be considered clean and free of slag and is therefore returned to the environment, from which it will then resume its cycle through a new capture.