The greenhouse gas nitrous oxide (N₂O) is produced in activated sludge tanks as a byproduct of nitrification and heterotrophic denitrification. Insufficient knowledge on how microbiological N₂O generation and degradation pathways impact N₂O emissions in activated sludge tanks still hampers the development of effective mitigation strategies. Our research contributes to overcome this gap by quantifying N₂O emissions through extensive measurement campaigns at ten full-scale wastewater treatment plants and correlating them to relevant operating parameters by multivariate regression analysis. Measurements revealed that N₂O production depends mainly on the activity of nitrifying bacteria and is triggered by high ammonium concentrations. In contrast, well-performing heterotrophic denitrification plays a key role as a sink of N₂O in activated sludge tanks. Following these patterns, low loaded plants achieving high nitrogen removal (83-92%) exhibited the lowest N₂O emission intensity (0.0012 ± 0.001 kg N₂O-N emitted per kg TKN in the influent wastewater). The regression analysis corroborated these results by revealing a negative linear correlation between the N₂O emission factor and the total nitrogen removal degree of the plants. The regression model represents a novel estimation method that links N₂O emissions with plant performance and provides a significant improvement over approaches applying fixed N₂O emission factors.

In this research, sources of methane emissions of an anaerobic digester (AD) system at a municipal wastewater treatment plant (WWTP) with 260,000 population equivalent (PE) capacity were detected by a non-dispersive infrared (NDIR) camera. The located emissions were evaluated qualitatively and were documented with photos and video films. Subsequently, the emission sources were quantified individually using different methods like the Flux-Chamber method and sampling from digesters circulation pipe. The dissolved methane in the sludge digester was measured via GC-MS and 6.8% oversaturation compared to the equilibrium after Henry's law was found. Additionally the residual gas potential of the digestate was measured using batch tests with ten days additional stabilisation time. The PE-specific residual gas production of the full-scale AD was calculated to 12.4 g CH₄/(PE · y). An extended COD balance including methane emissions for the whole digester system was calculated. Also the measured methane loads were calculated and summed up. The total methane loss of the AD was calculated to 24.6 g CH₄/(PE · y), which corresponds to 0.4% of the produced biogas (4,913 g CH₄/(PE · y)). PE-specific methane emission factors are presented for each investigated (point) source like the sludge outlet at the digesters head, a leaking manhole sealing and cracks in the concrete structure.

Biological methanation of carbon dioxide using hydrogen makes it possible to improve the methane and energy content of biogas produced from sewage sludge and organic residuals and to reach the requirements for injection into the natural gas network. Biofilm reactors, so-called trickling bed reactors, offer a relatively simple, energy-efficient, and reliable technique for upgrading biogas via ex-situ methanation. A mesophilic lab-scale biofilm reactor was operated continuously for nine months to upgrade biogas from anaerobic sewage sludge digestion to a methane content >98%. To supply essential trace elements to the biomass, a stock solution was fed to the trickling liquid. Besides standard parameters and gas quality, concentrations of Na, K, Ca, Mg, Ni, and Fe were measured in the liquid and the biofilm using ICP-OES (inductively coupled plasma optical emission spectrometry) to examine the biofilms load-dependent uptake rate and to calculate quantities required for a stable operation. Additionally, microbial community dynamics were monitored by amplicon sequencing (16S rRNA gene). It was found that all investigated (trace) elements are taken up by the biomass. Some are absorbed depending on the load, others independently of it. For example, a biomass-specific uptake of 0.13 mg·g⁻¹·d⁻¹ for Ni and up to 50 mg·g⁻¹·d⁻¹ for Mg were measured.

Biological methanation as a method of sector coupling between electric and gas grids is expected to be an integral part of the green energy change. Wastewater treatment plants (WWTPs) involving anaerobic digestion (AD) allow existing infrastructure to operate as energy conversion plants, to close carbon cycles and to generate long-term storable energy in the form of biomethane. Therefore, municipal raw sludge and additional organic residuals (co-substrates) are converted into biogas. Hydrogen is added to convert the carbon dioxide in the biogas into methane via biological methanation (BM). In this study, the energy amount that is convertible via BM in municipal digesters in Austria was calculated. The amount of energy, which can be transformed from electric surplus energy into biomethane, was assessed. Operational data from lab-scale digesters were combined with data from 28 Austrian full-scale wastewater treatment plants with AD. They represent 9.2 Mio population equivalents (PE), or 68% of Austria’s municipal AD capacity for WWTPs > 50,000 PE (in sum, 13.6 Mio PE). Energy flows for BM including water electrolysis and anaerobic digestion were created on a countrywide basis. It was found that 2.9–4.4% (220–327 GWh·y⁻¹) of Austria’s yearly renewable electricity production (7470 GWh·y⁻¹) can be transformed into biomethane via BM in municipal digesters.

Operational data over 2 years from three large Austrian wastewater treatment plants (WWTPs) with design capacities of 4 million, 950,000 and 110,000 population equivalent (PE) were examined. Salt peaks, due to thawing road salt were detected and quantified by electrical conductivity, temperature and chloride measurement in the inflow of the WWTPs. Daily NaCl inflow loads up to 1,147 t/d and PE-specific loads of 0.26–0.5 kg NaCl/(PE · y) were found. To mimic the plants' behaviour in a controlled environment, NaCl was dosed into the inflow of a laboratory-scale activated sludge plant. The influence of salt peaks on important activated sludge parameters such as sludge volume index, settling velocity and floc size were investigated. Influent and effluent were sampled extensively to calculate removal rates. Respiration measurements were performed to quantify activated sludge activity. Particle size distributions of the activated sludge floc sizes were measured using laser diffraction particle sizing and showed a decrease of the floc size by approximately two-thirds. The floc structure was examined and documented using light microscopy. At salt concentrations below 1 g/L, increased respiration was found for autotrophic biomass, and between 1 and 3 g NaCl/L respiration was inhibited by up to 30%.

Fluorescence spectroscopy has numerous applications to characterize natural and human-influenced water bodies regarding dissolved organic matter (DOM) and contamination. Analyzing samples in a timely manner is crucial to gaining valid and reproducible excitation-emission matrices (EEM) but often difficult, specifically in transnational projects with long transport distances. In this study, eight samples of different water sources (tap water, differently polluted rivers, and wastewater treatment plant (WWTP) effluents) were stored under standardized conditions for 59 days and analyzed regularly. With this data set, the sample and fluorescence spectra stability was evaluated. Established analysis methods such as peak picking and fluorescence metrics were compared over time and benchmarked against dissolved organic carbon (DOC) and a maximal change of 10% in terms of their variability. Additional high-performance liquid chromatography (HPLC) data to identify single organic compounds provides insights into these DOM alterations and allows for conclusions about the underlying biological processes. Our results corroborate in a systematic way that the higher the organic or microbial load, the faster the sample must be processed. For all water sources, considerable changes were found between days zero and one, indicating a potential systematic bias between in-situ and laboratory measurements. The absolute signals of individual peaks vary substantially after only a few days. In contrast, relative metrics are robust for a much longer time. For specific metrics, when filtered and stored under cool and dark conditions, tap water may be stored for up to 59 days, non-polluted river water for up to 31-59 days, and WWTP effluents for up to 14-59 days. The storability thus depends both on the specific water source and the analytical plan. By systematizing our understanding of how the specific water source and DOM concentration determine the stability of samples during storage, these conclusions facilitate efforts to establish a standardized protocol.