A key prerequisite for sustainable development is to maintain water quality in groundwaters and watercourses. Wastewater collection and reuse or disposal directly on site is a characteristic of decentralized systems which provide alternative solutions to traditional centralized large systems. The concept of a decentralised approach is to treat and reuse wastewater where it is most appropriate, as close as possible to the place where it was generated i order to avoid the construction of expensive pipeline networks that only move pollution from one place to another. In Croatia, almost 40% of the population lives in towns and cities of 1-20000 inhabitants, and on the islands this ratio is even more pronounced, so decentralized sewage systems are in fact optimal solution for our islands. Cluster systems are not significantly more expensive, and in particular come to the fore if the soil is unsuitable for processing systems at the site of origin. Water resource management must be a priority for a country that wants to develop as a top tourist destination and wastewater treatment systems must be designed in such a way that in addition to meeting all environmental requirements, they must withstand a peak burden in the tourist season when people in multifold numbers reside on the islands compareds to winter months.