Decentralised Supply

Centralised water supply is often considered the optimal water supply system, since it provides the most convenient service. However, in 2008, only 57% of the global population got its drinking water from a large-scale piped connection in the user’s dwelling, plot, or yard. In developing regions, this percentage was only 49%, with a large disparity between urban (73% having access) and rural communities (31% having access) (UNICEF WHO 2011).

Moreover, large distribution networks have high maintenance costs and are often prone to failures because of poor operation and maintenance. Failure of centralised systems to provide clean, adequate drinking water depends on a number of technical (see also intermittent supply and leakage control), economic, and legal factors. Because of the large amount of infrastructure (e.g. treatment plants, pipes, etc.) needed, there are many situations where it may not be possible to connect the whole population to the centralised supply system, such as in rural areas where populations are dispersed over large surface areas. Especially in developing or transition countries, high population growth in urban areas often leads to the establishment of informal settlements which remain disconnected from supply lines, as providing centralised supply is not technically or economically feasible. Moreover, as these settlements are frequently illegal, the government has no obligation to provide water and sanitation services. Furthermore, centralised water treatment and distribution facilities are often poorly maintained and fall into disrepair, so that even when users are connected to the centralised supply network, the quality and quantity of water may be unreliable (see also intermittent water distribution).

Decentralised supply offers the possibility to provide clean, reliable drinking water to rural or informal settlements where centralised systems are not economically or technically possible. Even when centralised drinking water is provided, decentralised supply can supplement other water uses in agriculture (e.g. precipitation water, optimised irrigation), or at home (see also rainwater harvesting rural for toilet flushing, or water-saving appliances).

Decentralised supply systems are classified based on the quantity of water they can supply. There are three main categories of decentralised supply systems:

• Point-of-use (POU) supply: treats approximately 25 l/day for one household.
• Point of entry (POE) supply: treats all water entering into a household, usually as an additional purification of water from a centralised supply.
• Small-scale systems (SSS): provides water to communities in quantities of 1,000-10,000 l/day. SSS are often also employed as emergency camp water supply systems.

Because they provide water to one household, both POU and POE water supply can also be referred to as household water treatment systems (see also HWTS), and use methods.

While surface water sources (e.g. lakes, rivers, man-made reservoirs, etc.) may be collected and purified, groundwater or rainwater are more favourable due to the lower risk of contamination with pathogens.

Groundwater can be collected with dug wells or drilled wells (see well development and rehabilitation). Rainwater may also be a valuable water source in both rural and urban settings.

Once a water source has been identified, it needs to be purified before being safe to consume. POU and POE systems use purification methods that can be divided into three main categories:

Heat or radiation

Using heat or radiation can effectively destroy pathogens. This includes techniques such as boiling, solar radiation, SODIS, and UV tubes. However, while pathogens may be killed, these methods have the disadvantage of providing no protection against recontamination.

Chemical treatment

Chemicals are widely used for purification and disinfection purposes. Methods include coagulation flocculation, precipitation (e.g. arsenic removal technologies), adsorption, ion exchange, and chemical disinfection (e.g. chlorination, WATASOL).

Physical removal processes

Physical removal processes separate contaminants from water using sedimentation or filtration techniques. Technologies include sedimentation or settling, filtration (e.g. membranes, ceramic and fibre filters, or colloidal silver filters), granular filter media (e.g. biosand filters, slow sand filtration, rapid sand filtration), or aeration.

The technologies employed in SSS are generally the same as in POU and POE systems, but scaled up to provide drinking water for communities in quantities of 1,000-10,000 l/day.  They can also include technologies usually applied on a large scale: for example, liquid chlorine or chlorine oxide dosage may be replaced by chlorine tablets (see point of use chlorination and centralised chlorination) or coagulant flocculant mixing conducted in pipes. Slow sand filtration is often used for community water treatment in developing countries in combination with roughing filters, when maintenance or transport of chemicals is limited or not possible. SSS are also the systems most often employed to provide emergency water supply.

The suitability of a decentralised water supply and purification system depends on local needs and context. Before choosing a decentralised system, factors such as water quantity, water quality, and financial resources need to be closely regarded (see also decentralisation in water supply).

Point-of-use systems

Whether used in a traditional way (e.g. boiling), introduced by NGOs or the market, point-of-use systems are currently widely applied by households with different financial resources in developing, transitional and sometimes even industrialised countries.

Point-of-entry systems

Point-of-entry systems are mostly used in industrialised countries as a supplementary treatment of tap or good quality well water, or in homes of wealthy people, hotels, childcare, and medical institutions of developing and transition countries.

Small-scale systems

The most typical application of small-scale systems is for community water supply. However, they have also had important applications in emergency water supply. Because emergency situations necessitate provision of clean water with limited time and resources, SSS are one of the best options to quickly provide safe water for community and camp water supply.

SSS are also frequently employed to purify water for informal vendors selling water in water kiosks. This type of water supply can have the benefit of not only providing widespread clean water, but also for the kiosk attendants to earn commission from water sales. Learn more about water kiosks and informal vendors by reading about water vendors.

As previously mentioned, selection of an appropriate decentralised supply system depends on the local context. Generally, the following factors should be considered (see table below):

• Performance
• Ease of use
• Maintenance needs
• Independence from utilities (e.g. electricity and fuel)
• Costs

There are also ways to combine decentralised technologies with centralised supply systems if water quality from the centralised system is unreliable. This type of combined system is called a dual system, of which there are two main types:

• Dual central treatment: water of two different qualities is produced and distributed, one for drinking and one for general use (agriculture, washing, etc.).
• Central treatment for household purposes, decentralised treatment for drinking water quality.

Decentralised supply can lead to large improvements in public health by making water both available and safe to drink in areas where centralised supply fails to provide adequate, safe drinking water. However, regardless of the decentralised system used, safe storage of drinking water should always be practised in order to prevent recontamination.

The cost of decentralised supply depends on the technology chosen. While some can be prohibitively expensive, others are affordable even in poor communities. Smaller systems often require less operation and maintenance making them, in some contexts, more sustainable.

See the table below for more information about specific technologies.

Compared to centralised supply, decentralised supply requires time-consuming daily operation and maintenance for users. However, centralised water supply networks also require expensive, intensive maintenance, with the difference that it is out of the control of users. Therefore, decentralised supply has the benefit of putting users in control of their system maintenance. See the table below for more information about specific technologies.

Table: Comparison of some different technologies for POU, POE, and SSS water treatment. For POU, costs are estimated for one family of four for one year ($USD), and SSS for 50 m3 of water per year. For performance, “++” means water is microbiologically safe under WHO standards, while “+” means water is safe only if the treatment is done correctly. For ease of use, “++” means daily operation is limited to filling in raw water and collecting treated water, while “+” means additional operations are necessary but may be done without training. Source: PETER-VARBANETS et al (2009).