Centralized systems covey wastewater in large networks, consisting of hundreds of kilometres of pipelines, conveying wastewater at long distances. They treat it in a wastewater treatment plant, but large population centres may have several treatment facilities. The vast majority of existing systems are centralized. Most often, the disposal of the treated effluent is in the form of discharge to a close-by water body, but also, it could be reused at a large scale. The system shown on Fig. 1 is a centralized one. In most cases when we speak about "the sewerage", a centralized system is considered in mind.
Decentralized systems consist of numerous separate systems within the same population centre, each one having its own short conveyance network and treatment facility. The disposal or reuse of the treated wastewater may be a problem and should be given careful consideration. The total or partial reuse of the effluent could alleviate the solution of this problem. Decentralization of wastewater systems may be applied at different scales, at on-site level or covering a satellite township.