Up to now, aerobic granulation of activated sludge is only realized in SBRs, where the discontinuous feed and sedimentation allow the formation of dense granules with excellent settling properties. However, aerobic granulation in continuous-flow plants (CFP) is gaining more and more interest in order to exploit the advantages of these excellent sludge properties to construct compact and efficient WWTP. Within the scope of this project, a SBR and CFP were operated in parallel to investigate the aerobic granulation of activated sludge and to compare the biomass in terms of their structure and settling behavior. CFP operation included two experimental phases with different reactor designs. The use of synthetic wastewater during phase I led to a biomass with a SVI of 42 ml g -1 , whereby the SVI declined only to 85 ml g -1 in the second phase and the use of municipal sewage. After the start-up period, microscopic images of the biomass from CFP comprised small compact granules with a high flocculent fraction. Particle size distribution for phase II confirm, that 72% of the particles had a size over 200 µm. A strong correlation was observed between the appearance of NOx-N in the first reactor and the SVI. The results illustrate, that the anaerobic conditions during feeding are essential to keep stable granules.

BACKGROUND Although a growing number of full‐scale wastewater treatment plants have already been constructed and operated with aerobic granular sludge (AGS), only limited information is available about further post‐treatment, in particular about sludge stabilization and dewaterability. The aim of the present study was to investigate the biodegradation and methane yield of AGS by the use of anaerobic laboratory‐scale reactors operated under mesophilic conditions and hydraulic retention times of 25 and 40 days. RESULTS The methane yield of AGS was ca 260 mL gVSS−1 (volatile suspended solids) and thus slightly increased compared to that of suspended activated sludge (SAS; 240 mL gVSS−1). A clear difference between the methane yield was found for separated pure granules (500 μm), which was ca 50% higher compared to that for SAS. VSS removal of AGS during anaerobic degradation was ca 52%. Dewaterability of AGS after anaerobic digestion was slightly lower compared to SAS. Extracellular polymeric substance (EPS) extraction and fluorescence analysis showed tryptophan contents which were almost twice as high compared to the EPS extracted from SAS. CONCLUSIONS Overall, the anaerobic digestion of AGS was found to be a suitable stabilization strategy with the benefit of recovering energy in the form of methane. Further tests are needed to validate the decreased dewatering behaviour with full‐scale applications. The presented approach for tryptophan measurement allows the transfer of qualitative results from a fluorescence analysis into quantitative values and could be further adapted for identifying relevant EPS constituents. © 2019 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The influence of temperature and solids retention time (SRT) on high-solid digestion of municipal sewage sludge was investigated in laboratory-scale reactors. Digestion with high-solid concentration reduces the required digestion volume and is advantageous for urban areas. The experimental conditions comprised total suspended solids (TSS) in digested sludge between 4.0 and 4.6%, temperatures in a range of 33 to 41 °C and the SRT between 10 and 25 d. High-solid digestion operates with increased NH4-N concentrations released from organic compounds. The anaerobic process can be limited by high NH4-N concentration and toxic NH3. In this study a stable digestion was observed up to 2,000 mg L(-1) NH4-N and 75 mg L(-1) NH3. Volatile suspended solids (VSS) and chemical oxygen demand removal was 53% and 57% respectively. However, digestion with 10 d SRT led to a declined VSS removal of 49%. The removal at 41 and 37 °C showed minor differences, while reduced NH4-N release and reduced methane production were observed at 33 °C. For economic reasons, high-solid digestion at 41 °C is not recommended, but will not impair VSS removal. The outcomes of this study confirm that digestion with up to 7.8% TSS in the feed is feasible for the tested temperatures and SRT down to 15 d.

The emissions of climate-relevant nitrous oxides from wastewater treatment with aerobic granular sludge (AGS) are of special interest due to considerable structural as well as microbiological differences compared with flocculent sludge. Due to the compact and large structures, AGS is characterised by the formation of zones with different dissolved oxygen (DO) and substrate gradients, which allows simultaneous nitrification and denitrification (SND). N2O emissions from AGS were investigated using laboratory-scale SBR fed with municipal wastewater. Special attention was paid to the effects of different organic loading rates (OLR) and aeration strategies. Emission factors (EF) were in a range of 0.54% to 4.8% (gN2O/gNH4-Nox.) under constant aerobic conditions during the aerated phase and different OLR. Higher OLR and SND were found to increase the N2O emissions. A comparative measurement of two similarly operated SBR with AGS showed that the reactor operated under constant aerobic conditions (DO of 2 mg L−1) emitted more N2O than the SBR with an alternating aeration strategy. Total nitrogen (TN) removal was significantly higher with the alternating aeration since non-aerated periods lead to increased anoxic zones inside the granules. The constant aerobic operation was found to promote the accumulation of NO2-N, which could explain the differences in the N2O levels.

This paper describes and evaluates a large-scale SBR with a design capacity of 35,000 p.e. where the activated sludge exhibits excellent settling properties. The sludge volume index (SVI) of all four SBR is mostly below 50 ml g -1 and shows annual fluctuations; the lowest values of 30 ml g -1 are measured during summer. The focus of this study was to identify reasons for this excellent settling behavior. Microscopic images of the sludge showed a compact and dense structure with small granules. Particle size distribution indicates that about 74.4% of the particles had a size above 200 µm, which is a characteristic size of aerobic granules. Approx. 50% of the sludge particles were larger than 320 µm. SV 10 /SV 30 ratio was calculated with 1.21. Based on the existing knowledge of aerobic granular sludge it can be assumed that the long filling during denitrification leads to anaerobic conditions and promotes the formation of aerobic granules. Legal requirements for the effluent quality were met the entire year. The average COD and TN removal amounted to 94.2 and 83.3%.