Direct and indirect greenhouse gases emissions of two model municipal WWTPs were estimated using carbon footprint analyses. One WWTP was designed with anaerobic digestion, the other one with simultaneous aerobic stabilization of sewage sludge. Emission factors were derived from literature values. For direct nitrous oxide emissions, a new estimation model based on measurements at eight Austrian WWTPs was applied. Results show that the direct nitrous oxide emission from the activated sludge tanks dominates the carbon footprint of WWTPs with a moderate nitrogen removal. Anaerobic digesters and anaerobic sludge storage tanks can also become a relevant source of direct methane emissions

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.

Sugar beet press pulp (SBP) accumulates as a by-product in sugar factories and it is generally silaged or dried to be used as animal food. Rising energy prices and the opening of the European Union sugar market has put pressure on the manufacturers to find alternatives for energy supply. The aim of this project was to develop a technology in the treatment of SBP that would lead to savings in energy consumption and would provide a more competitive sugar production from sugar beets. These goals were met by the anaerobic digestion of SBP for biogas production. Lab-scale experiments confirmed the suitability of SBP as substrate for anaerobic bacteria. Pilot-scale experiments focused on process optimization and procedures for a quick start up and operational control. Both single-stage and two-stage process configurations showed similar removal efficiency. A stable biogas production could be achieved in single-stage at a maximum volumetric loading rate of 10 kgCSB/(m3·d). Degradation efficiency was 75% for VS and 72% for COD. Average specific gas production reached 530 NL/kgCODSBP or 610 NL/kgVSSBP. (CH4: 50 to 53%). The first large-scale biogas plant was put into operation during the sugar processing period 2007 at a Hungarian sugar factory. Digesting approximately 50% of the SBP (800 t/d, 22%TS), the biogas produced could substitute about 40% of the natural gas required for the thermal energy supply within the sugar processing.

The paper will report about the experiences at an Austrian large wastewater treatment plant of 720,000 population equivalents, where anaerobically digested sewage sludge is further stabilised under aerobic conditions. Enhanced stabilisation of the anaerobically digested sludge was required at the plant in order to get a permit for landfill disposal of the dewatered stabilized sludge. By implementing a post-aeration treatment (SRT approximately 6d; 36 degrees C) after anaerobic digestion the organic content of the anaerobically well digested sludge can be decreased by 16%. Investigations on site showed that during digested sludge post-aeration anoxic phases for denitrification are needed to provide stable process conditions. In this way the pH value can be kept in a more favourable range for micro-organisms and concrete structures. Additionally, inhibition of the biological process due to nitrite accumulation can be avoided. By optimising the aeration/pause ratio approximately 45% of total nitrogen in digested sludge can be removed. This significantly improves nitrogen removal efficiency at the wastewater treatment plant. NH(4)-removal occurs mainly through nitritation and denitritation with an efficiency of 98%. The costs/benefit analysis shows that post-aeration of digested sludge results in an increase of total annual costs for wastewater treatment of only 0.84%, corresponding to 0.19 Euro/pe/a. Result of molecular biological analyses (DGGE) indicate that all four ammonium-oxidizing bacteria species present in activated sludge can survive anaerobic digestion, but only two of them can adapt in the digested sludge post-aeration tanks. Additionally, in the post-aerated digested sludge a further ammonium-oxidizing bacteria species was identified.