Biological swimming ponds of public use

Public Biological swimming pools

A public-use biological swimming pool (Piscinas Biológicas®) follows the international rules for public biological swimming pools and must be designed by an accredited specialist in the area who shares the responsibility for ensuring bathing water quality with the public biological swimming pool.

Designing a public biological swimming pool is the task of an experienced team, able to bring together various disciplines from the natural sciences, such as freshwater biology and landscape architecture.

The design of a public biological swimming pool can vary widely, from the look of a lake with a waistline of plants and water lilies, to a biologically treated pool with no visible plants. Everything is possible.

The configuration of each project depends on the wishes of the promoter, the maximum number of users expected and the constraints of the site of deployment. In order to decide on the construction and the respective viability of a public biological swimming pool , it is necessary to carry out a previous exhaustive analysis of the conditioning factors – a Feasibility Study. This document forms the basis for the design, the participation of the population in the project and the organization of the financing.

In the initial phase, the Feasibility Study seeks to clearly define the criteria and priorities for implementing a biological swimming pool (Piscina Biológica®). This work allows to explain in a simple and intuitive way the advantages and disadvantages of the realization of the project, through an appropriate weighting. The study analyzes the “pros and cons” related to the construction of a biological swimming pool, also presenting different technical solutions for its execution. The necessary technical and financial resources are evaluated. And will also be presented, a first schedule of the realization of the biological swimming pool.

In essence, a feasibility study takes into account economic viability (eg construction costs, operational costs), technical feasibility (groundwater groundwater level, construction of support facilities and infrastructure). Plus the resources and existing structures to take advantage of (possibly support facilities) and organization and management (life guard, etc.).

If, in this analysis, there are no results that make construction unfeasible, that is, if the feasibility study is positive, then the planning phase of the public biological swimming pool can be started.

A biological swimming pool must be impermeable against the subsoil, in the majority of cases with a high quality plastic screen specially designed for swimming ponds.

The biological swimming pool allows bathing in naturalized, bath-quality, clear and transparent water. Water that is not harmful to human health or the environment, since these pools produce bathing water, through natural processes. No chemical additives, no salt, no chlorine are required.

The plants, in a biological swimming pool, play three important roles: oxygenation of water, assimilation of nutrients and shading of the water surface. Through plant bioactivity, clean, transparent and bathing water is, so to speak, poor in nutrients and rich in oxygen. It then provides an antiseptic function for organisms harmful to human health.

The zooplankton (microscopic living beings) present in the water is a living filter and removes the bacteria and algae in excess, becoming a permanent and permanent filter of the water of this bathing system and without requiring the presence of any technical equipment. It can be said that the pool water itself is “equipped” with a “live” filter in which to bathe. Plankton’s energy source is the oxygen produced by the bioactivity of aquatic plants.

Sometimes biological pools also have external filters that remove particles suspended in water. In this filter lives a community of other microorganisms, which ensures the decomposition of filtered impurities up to their mineralization.

For biological swimming pools  for public use, Directive 2006/7/ EC of the European Parliament and of the Council of 15 February 2006 on the management of bathing water quality shall not be applied. This Directive excludes application to “confined water bodies created artificially and separate from surface waters and groundwater”.

The CNQ Directive No. 23/93 on quality in swimming pools for public use is also not applicable in large part, especially with regard to the treatment system.

This situation, of a certain absence of legal basis, is not a peculiarity of Portugal. The same situation exists or existed in most EU countries. There are, however, several examples of how to overcome this situation and there are already about 250 public swimming pools with biological treatment, many with hundreds of users per day in several EU countries such as Austria, France, Belgium, Sweden, Denmark, Poland, the Czech Republic, Italy and Switzerland.

Based on a proposal for a guideline directive, drawn up by the International Organization for Natural Bathing Waters IOB (, a proposal for a regulation was drawn up by the Iberian Group of Naturalized Bathing Waters GIABN (