The continuous city expansion and use of various impermeable pavement materials greatly hinder the rainwater surface runoff by conventional drainage systems, especially during heavy and extreme precipitation, which is occurring more frequently. Also, mostly as a rule, natural watercourse discontinuity and/or their canalization results in more frequent floods in cities. An additional problem is rainwater pollution, which, with conventional drainage systems, ends up in natural watercourses, rivers, lakes and seas often without any or even minimal treatment. These are some of the main reasons why rainwater management is becoming an increasingly more important and indispensable segment of contemporary sustainable city development. Among many other approaches, it also includes conventional rainwater drainage systems replacement and the application of biological rainwater treatment systems as much as possible. The positive impact of sustainable and environmentally friendly rainwater treatment systems has already been widely recognized. However, their application is still slightly limited compared to the problems that occur in modern cities. The aim of this paper is to present biological rainwater treatment systems (basic categories and characteristics) and to analyze their implementation possibilities in urban green areas. Their ecological and aesthetic values will be considered, and how users generally perceive structures of ‘natural’ characteristics within designed landscapes will be discussed. The analysis of fourteen recently constructed parks will be carried out to determine the ways of integrating biological purifier systems in the park, the possibilities of their use, and the degree of biological purification systems influence on the overall park design concept. Criteria or guidelines that should be taken into account when designing biological rainwater purification systems will be proposed in order to simultaneously meet their basic function and other roles of such structures within the city park.