Historically and in the latest developments, on-site sanitation is the most widely applied scale of decentralization. In this case the treatment, and in most cases the reuse or disposal of the effluent, is located very close to the source of generation of the wastewater. The conveying network consists of the plumbing arrangements in the building and a short pipeline to the treatment facility. For this reason, the focus is on the choice of appropriate treatment and disposal of the effluent.
The whole system is located within the private premises and correspondingly, the cost and responsibility of the design, construction, operation and maintenance (O&M) is with the owner. The local authorities may provide support for the O&M in the form of collecting wastes, issuing certificates/licences for standardized treatment equipment or for selected qualified private companies to do the job. From regulatory point of view, the control of the quality of treated effluent for reuse, discharge or disposal is entirely the responsibility of local or governmental authorities. However, it is in the owner's interest to operate and maintain properly, especially in the case of reuse of the treated effluent.
Treatment facilities, which are not well designed, constructed and operated may be a serious hazard to the health of the owners and the neighbours, but also to the Environment, and more specifically, could be a serious source of ground water pollution.
There is a large variety of possible treatment/ disposal facilities but only the most widely applied are presented briefly. All the methods are applicable for on-site solutions; wastewater treatment plants can be applied at larger scale of decentralization and in small centralized communities. The treatment and disposal facilities described briefly are: