The link between urban planning and the water or the wastewater system is well-known. But, in many practical cases, urban planners consider a centralized solution only, no matter what are the specific conditions in the area under consideration. It would be highly recommendable to assess different alternatives for decentralization, in terms of scale and beneficial reuse. The choice of the optimal one should be based not only on cost estimations but on social and environmental criteria for sustainability as well.
The following recommendations could be considered:
1. The size of the plot of residential housing developments would be an important factor to determine what type of system to apply
High-density areas with small plots (area < 500 m2) or with blocks of flats are best served by a sewer system and are not suitable for on-site solutions requiring infiltration. But if there is no other possibility a small wastewater treatment plant would be the only solution
In medium size plots (area from 500 to 1500 m2 ) all described methods for on-site sanitation are possible, but also, semi-centralized solutions or decentralization at different scales should be considered
In large plots (area > 1500 m2) centralized solutions require very long pipelines network and on-site solutions are recommendable
2. High water tables and clay or rocky grounds are not appropriate for applications requiring infiltration. In such cases, a possible application is a vacuum sewer system with decentralization at neighbourhood level.
3. The specifics of the area (geology, topography and the population density) would determine the optimum choice of a wastewater system. Decentralized applications at district level may depend heavily on the topography to determine the number of the small wastewater systems. In such cases, the administrative boundaries might not coincide with the boundaries of the system. The water basin principle should be applied.
4. If an on-site decentralized wastewater system is chosen to be implemented, or when it actually exists, it is important to consider and provide solutions for the storm water collection and disposal. With on-site applications, the storm water on the premisses could be collected and reused together with gray water. However, excess storm water from the dwellings and the one from the streets/roads must be collected and disposed properly. Usually, the drainage system should be designed and implemented during the building or renovation of the streets/roads. This is an obligatory requirement in order to provide safe and decent urban environment, and also to protect the transportation system from erosion.
There is well-pronounced public perception that decentralized solutions, and more specifically on-site sanitation, belongs to the past and are typical for under-developed areas with poor or not existing infrastructure. On the other hand, many environmentalists see decentralized wastewater systems as the only sustainable and correct way forward. Actually, a careful and competent consideration of all factors, the cost estimation of different alternatives, and the opinion of the community would decide which alternative to choose. It would be recommendable to include such a procedure in the waste management plans and the practice of urban/rural water management. Decentralization is not a "must" but a choice. If a centralized solution is more cost effective and the community accepts it, then it would be the best choice.
In areas with an existing centralized sewer system the vast majority of the inhabitants would choose to use it and to avoid the burden of maintaining additional facilities. However, in such cases it would be advisable to think in terms of sustainability. A gray water reuse system could reduce considerably the fresh water consumption; especially in areas with water shortages, this is a serious incentive. Then the black water could be directed to the existing sewer system. We all know that the fresh water resources are not unlimited and may be in future the cost of drinking water would rise considerably. Why waste it for activities, which do not require high level of treatment, such as toilet flushing and irrigation? Public awareness and consciousness could be an additional drive for the implementation of decentralized solutions.