Silver-based nanoparticles (Ag-b-NPs) emitted by wastewater treatment plants (WWTPs) are considered to be widely present in the natural environment. However, there is much that is unknown about the effect of WWTP effluent on the occurrence of Ag-b-NPs in surface waters. On the basis of field analysis of representative WWTPs in Germany, we demonstrate that more than 96.4% of Ag-b-NPs from wastewater influent are removed through WWTPs, even though influent contains Ag-b-NP concentrations of tens to hundreds ng L(-1), resulting in effluent Ag-b-NP concentrations of 0.7-11.1 ng L(-1) over the seasons. The estimated flux of Ag-b-NPs associated with WWTPs effluent discharge is ∼33 kg y(-1) in Germany. WWTPs effluent increases Ag-b-NP levels of the River Isar to 2.0-8.6 ng L(-1), while remarkable decreases are observed at sites ∼1.5 km downstream of each discharge point, and Ag-b-NP levels then keep stable (0.9-2.3 ng L(-1)) until the next discharge point, showing subtle differences in Ag-b-NP levels between the river and reference lakes without industrial sources and WWTPs effluent discharge. Our results demonstrate that WWTPs effluent can exert a clear influence on the occurrence of Ag-b-NPs in surface waters.

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Physical, chemical and biological characteristics were investigated for aerobic granules and sludge flocs from three laboratory-scale sequencing batch reactors (SBRs). One reactor was operated as normal SBR (N-SBR) and two reactors were operated as granular SBRs (G-SBR1 and G-SBR2). G-SBR1 was inoculated with activated sludge and G-SBR2 with granules from the municipal wastewater plant in Garching (Germany). The following major parameters and functions were measured and compared between the three reactors: morphology, settling velocity, specific gravity (SG), sludge volume index (SVI), specific oxygen uptake rate (SOUR), distribution of the volume fraction of extracellular polymeric substances (EPS) and bacteria, organic carbon and nitrogen removal. Compared with sludge flocs, granular sludge had excellent settling properties, good solid-liquid separation, high biomass concentration, simultaneous nitrification and denitrification. Aerobic granular sludge does not have a higher microbial activity and there are some problems including higher effluent suspended solids, lower ratio of VSS/SS and no nitrification at the beginning of cultivation. Measurement with CLSM and additional image analysis showed that EPS glycoconjugates build one main fraction inside the granules. The aerobic granules from G-SBR1 prove to be heavier, smaller and have a higher microbial activity compared with G-SBR2. Furthermore, the granules were more compact, with lower SVI and less filamentous bacteria.

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