Cause and effect relationship between foam formation and treated wastewater effluents in a transboundary river

Abstract

The occurrence of foam at weirs in a lowland river in Austria and shortly after the Austrian border with Hungary, as well as, the associated protests from Hungarian locals led to investigations concerning the reasons for foam formation. Three aspects were the main subject of investigation, namely, (i) to assess the dimension of the appearing foam, (ii) to evaluate the reasons for the formation of foam, and (iii) to set abatement-measures. A 1 year monitoring programme included a close network of surface water sampling sites, as well as, the sampling of thirteen municipal and industrial wastewater treatment plants along the river stretch. In addition to classical parameters (physical and chemical) the surface tension and tensides were analysed. Constant observation of foam formation in Hungary was achieved by the installation of an online webcam with combined data recording, which resulted in the development of a seven-stage foam index (0–6) for semi quantitative assessment of foam formation on the river. Also, the effluents of the wastewater treatment plants that were considered were the subject of standardised foaming tests. The basis of the tests was to detect, (i) foam on the sample and, (ii) the dilution of a sample at which no more foam could be observed. The dilution factor was used to calculate the foam potential of an effluent, which is an size for the potential volume of river water that may be foamed by waste water treatment plants’ effluents. The spatial distribution of foam along the river stretch, as well as, the results of the foam tests allowed the identification of three tanneries as the main contributors to foam, although wastewater from these tanneries is treated at wastewater treatment plants by the best available technology (biological treatment with nitrification and denitrification, sludge retention time >20 days, temperature in the activated sludge tank >20 °C). The implementation of an accepted degree of foam formation was desirable to develop measures to reduce the foam index. As no criterion exists for foam in rivers in Austria, as well as in Hungary, the not accepted degree of foam formation was defined as the limit at which population protests from Hungary arose. This approach resulted in a foam index higher than 3.5, which was observed with 40% probability during the investigation period. By developing and performing a simple mathematical regression model the required reduction of foam potential emissions could be calculated in order to minimize the foam index to an accepted standard. By the elimination of 75% of foam potential, a foam index lower than 3.5 would be assured with 95% probability based on long term discharge development.