The level of decentralization would influence the number of systems within a population centre. From this perspective, we can have decentralization at different scales:

1. At the level of a suburb/district or satellite township – at this level the system could also be refereed as centralized but when applied to small towns or rural areas. If it is applied only to selected suburbs or districts in medium or large population centres, with existing centralized system, the whole system could be defined as a “hybrid” system.

2. At the level of a neighbourhood – in this category we can include clusters of homes, gated communities, small districts and areas, which are served by gravity or vacuum sewers. An illustration is shown on Figure 5.

3. At on-site level – within property boundaries of individual buildings

Completely decentralized systems are not a common practice, especially in cases with good infrastructure and contemporary housing development. On the other hand, it is a typical case in rural areas of developing countries, which lack basic sanitation facilities. In such conditions, the use of a properly build and maintained pit latrine or septic tank facility for each household can be considered as good sanitation practice. It is an example of completely decentralized system.

In locations with developed infrastructure, decentralized wastewater systems could be a viable alternative of the conventional centralized system, especially in cases of upgrading or retrofitting existing systems. Many different combinations and variations of “hybrid” systems are possible. The development of new treatment technologies allows decentralized solutions, which are technically and aesthetically sound and acceptable.

Decentralized applications are a necessity in cases of new housing or commercial developments, where the construction of the infrastructure is not ready or will be executed in future. In many countries and locations, the infrastructure development (roads, water supply and especially wastewater/drainage systems) is executed years after the housing development. Usually, this is due to lack of funding and poor planning practices. In such cases decentralized applications are obligatory in order to prevent public health and ecological problems.

Figure 5 Decentralization at neighbourhood level ( WTP – wastewater treatment plant)