Peak concentrations with first rainfall
To quantify leaching, facade elements in the laboratory were subjected to artificial rainfall and exposed to ultraviolet light and various temperatures. Tests were also carried out in the open air under real-life weather conditions, using a specially constructed small building (see photo). Finally, researchers also performed experiments on several new buildings in the Zurich region. Biocide concentrations were found to be extremely high especially in the first facade runoff collected from freshly rendered or painted houses. For example, the concentration of diuron (herbicide) measured in the first litre of facade runoff was 7000 μg/L. If released into a stream, this litre would therefore have to be diluted by a factor of 70,000 so as not to exceed the standard specified in the Water Protection Ordinance. This shows the problems that may be associated with the infiltration of facade runoff and drainage from urban areas into small waterbodies. Subsequently, however, concentrations declined rapidly, both during prolonged rainfall, and following further rain events. Leaching rates are dependent not only on the solubility of the agent in question, but also on the photochemical degradation of substances and on the quality of the renders and paints.
Effective in minute concentrations
According to the study, there is no doubt that the biocide concentrations measured in the facade runoff and additionally estimated using a computer model have toxic effects on algae and aquatic plant and animal life. Contaminated facade runoff can enter watercourses directly via drains or stormwater sewers. Substances that inhibit algal growth on facades exert the same effects in waterbodies even when heavily diluted. It is known that even a few nanogrammes per litre of certain substances can produce toxic effects; these include the marine antifouling agent cybutryne (also known as Irgarol®1051). It is thus important not only to bear in mind the general legal quality requirements but also to identify substances that are of particular ecotoxicological concern. In addition, a great deal of research is still required to elucidate the environmental effects of cocktails of substances.
Developing solutions with practitioners
Michael Burkhardt, who coordinated the URBIC project, points to the existence of a regulatory gap in the Biocidal Products Ordinance, since direct inputs of material protection agents from facades into receiving waters are not covered at present. Burkhardt emphasizes that, in general, greater attention needs to be paid to the selection of construction materials and water quality in the disposal of rainwater from urban areas. In his view, it is also legitimate to question the need for the routine use of biocides, since algal and fungal growth is certainly not to be expected everywhere. In addition, many facade-related problems could be avoided: architects could include moisture control as a design element. Likewise, property owners could assume greater responsibility through upkeep measures such as facade cleaning or tree pruning, rather than demanding warranties. As a result of guarantees, in particular, manufacturers feel increasingly obliged to incorporate biocides into synthetic renders and facade paints. However, the research team is also involved in discussions with manufacturers concerning innovative embedding processes for biocides and the use of less problematic agents or biocide-free coatings, and it is promoting a close dialogue between academia, industry and the authorities.