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INTRODUCTION
Activated sludge parameters, like sludge volume index (SVI) and settling velocity (vs), are crucial for the design of activated sludge plants. The total suspended solids (TSS) concentration in the effluent of the secondary clarifier and thus the retention of mixed liquor suspended solids (MLSS) in the aeration tank are important performance indicators (DWA 2016). The oxygen uptake rate (OUR) of the activated sludge gives important information about the process performance and the plant's capacity (Cech et al. 1985; Hagman & Jansen 2007).
In the winter, road salt, mainly NaCl, is spread on the streets. Hoffman et al. (2011) reported that in Austria 117,000–377,000 t/y (271,000 t/y on average) salt is used for street de-icing, with NaCl being most common salt, while <10% other salts, mostly CaCl2, are used. With precipitation and snowmelt, high loads of road salt are flushed into the sewer systems (combined sewers) and transported to the wastewater treatment plants (WWTPs). At knowledge-exchange events for operators, Austrian WWTP operators have reported negatively affected effluent quality caused by bulking sludge after road-salt events, but there has been no proven evidence for the correlation.
The influence of highly saline wastewater on the activity of activated sludge has been studied by different authors worldwide. Aslan & Simsek (2012) described the inhibitory effects on nitrite oxidizing bacteria (NOB) due to NaCl peaks and that ammonium oxidizing bacteria (AOBp) activity was more affected than NOB activity. Wang et al. (2005) performed inhibition tests with NaCl concentrations up to 20 g/L and reported reduced oxygen uptake rate and total organic carbon (TOC) removal for salinity peaks with concentrations greater than 0.5 g NaCl/L. Salvadó et al. (2001) reported a reduction of the protozoa biomass in activated sludge at NaCl concentrations between 3 and 10 g NaCl/L and a total abundance of protozoa between 20 and 40 g/L. Pernetti & Di Palma (2005) reported that after a few weeks of adaption (depending on the operating conditions) activated sludge was able to adapt to 30–50 g NaCl/L. In this adaption phase, the settling properties of the activated sludge were reduced and the volatile suspended solids (VSS) specific oxygen uptake rate (sOUR) was inhibited by up to 80%. It should be noted that this study...