Wastewater treatment based on activated sludge processes faces a major challenge due to an increased number of people contributing to wastewater and changes in wastewater characteristics in regions with seasonal winter tourism. The combination of significant increases in carbon (COD) and nitrogen (N) loads by a factor of four to five within a short periode of time and low wastewater temperatures (<12 °C), represents a bottleneck in meeting essential effluent requirements. According to literature research, however, little attention has been paid to these special framework conditions for wastewater treatment. Based on this fact, a model-based simulation study of the wastewater treatment plant (WWTP) in Montafon (Austria) evaluates operational optimization approaches for the above-mentioned framework conditions and is intended to provide new insights for nitrification-optimized and energy-optimized wastewater treatment. The WWTP Montafon is designed for a COD load of 6250 kg d−1, a daily dry weather water discharge of 12,700 m3 d−1 and is a typical plant in a winter tourism region. The 5 studied smart operation approaches shown are a combination of two different aeration control strategies based on a time-controlled (i) or ammonia-based feedback aeration control (ABAC) (ii) for intermittent nitrification-denitrification and sludge removal control strategies based on a constant sludge concentration to 3 g L−1 (iii), a constant sludge retention time (SRT) to 20 days (iv) and a constant sludge retention time to 20 days combined with a higher level sludge concentration control to max. 3 g L−1 (v). The simulation results show that a combination of a time-controlled aeration strategy and controlling the sludge concentration to a constant 3 g L−1 is not recommended in order to comply with the emission limits. Under the mentioned influent conditions, ABAC control alone does not inevitably lead to a nitrification-optimized process and reduction in energy consumption. Sufficient nitrifying bacteria in the activated sludge are the key to a nitrification optimized process and reduction of energy consumption for aeration. Hence, a combination of an intermittent aeration control based on ammonium measurement and a constant sludge retention time to 20 days is preferable for nitrification capacity and is the most energy-efficient control strategy to obtain required COD and N effluent quality. An average energy consumption of 57–63 Wh PE120−1 d−1 for the aeration is to be expected for combinations of (i,iv), (ii, iii) (ii,iv) and (ii,v) which is low in relation to the influent challenges and to plants with higher wastewater temperatures (>12 °C) between 27 Wh PE120−1 d−1 and 121 Wh PE120−1 d−1 according to literature. The approach with a time-controlled intermittent aeration and a SRT to constant 20 days was implemented at the WWTP Montafon and confirms the results of the associated simulation. According to this simulation study, a 35 % increase in nitrification capacity in existing WWTPs designed as activated sludge processes can be achieved through optimized operating conditions without additional high investments.

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.

The operational costs of wastewater treatment plants (WWTPs) are mainly driven by electric power consumption, making the energy-efficient operation an all-time present target for engineers and operators. A well known approach to reduce the demand for purchased electricity is the operation of an anaerobic sludge stabilisation process. Although anaerobic digesters make it possible to recover large quantities of energy-rich methane gas, additional strategies are required to handle the increased internal return flow of nitrogen, which arises with the sludge dewatering effluent (SDE). SDE treatment increases the oxygen demand and in turn the energy required for aeration. In this study, different SDE treatment processes were compared with regard to the treatment in mainstream, sidestream nitritation, as well as nitritation combined with anammox for two-stage and single-stage WWTPs. Although SDE treatment in sidestream nitritation was found to have no effect on the energy demand of single-stage WWTPs, this concept allows the treatment capacity in the activated sludge tank to be raised, while contributing to a high nitrogen removal under carbon limitation. In contrast, SDE sidestream treatment showed great potential for saving energy at two-stage WWTPs, whereby sidestream nitritation and the further treatment in the first stage was found to be the most efficient concept, with a savings of approx. 11% of the aeration energy.

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%.

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.

BACKGROUND The construction of small digesters will become more important for regions with demographic increase and limited areas. Savings of digester volume and a subsequently smaller footprint can be realized by the treatment of a more thickened raw sludge (high-solid digestion). However, this operation can lead to instabilities of the anaerobic process due to rising ammonium and ammonia levels. The aim of the study was to identify the limit of saving digester volume by increasing the total suspended solids (TSS) of the treated raw sludge. RESULTS The effects of high TSS and ammonia levels in the digested sludge were investigated in lab-scale reactors. Ammonium nitrogen (NH4-N) levels were set by dosing a urea solution as well as by feeding a higher concentrated raw sludge. Chemical oxygen demand (COD) removal declined from 64 to 54% with NH4-N levels rising from 2000 to 3200 mg L-1. However, the anaerobic biodegradation was not completely interrupted, which indicates the possible adaptation of anaerobic bacteria on high NH4-N levels. CONCLUSION From the results, it can be concluded that dewatering the raw sludge up to 7% TSS leads to an optimum benefit with maximum savings in digester volume. The benefits of the operation with high solids are a lower energy demand for heating up the raw sludge and reduced construction costs (-20%) due to smaller required digester volumes. (c) 2019 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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.

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 mu m, which is a characteristic size of aerobic granules. Approx. 50% of the sludge particles were larger than 320 mu m. SV10/SV30 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%.

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 mu m), 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. (c) 2019 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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.

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 photographs and video films. Subsequently, the emission sources were quantified individually using different methods like the Flux-Chamber method and sampling from the digester's circulation pipe. The dissolved methane in the sludge digester was measured via gas chromatography-mass spectrometry (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 10 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 chemical oxygen demand (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 at 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 digester's head, a leaking manhole sealing and cracks in the concrete structure.

High total suspended solids (TSS) digestion of municipal sewage sludge reduces the required space and volume for digestion plants. However, an important consequence of high TSS is the major influence on sludge rheology. The present case study investigates the rheology of sludge from a 130 m3 high solids digestion pilot plant at Vienna's main wastewater treatment plant (4 M PE120). Raw sludge ranged from 6 to 8% TSS and digested sludge from 3.2 to 4.6%. TSS show an exponential impact on rheological parameters. Increasing raw sludge TSS from 6 to 8% at least doubles the shear stress and increases friction loss by a factor of three. However, under real operating conditions simulated at the pilot plant, there are additional impact factors. The mixing ratio between waste activated and primary sludge influences raw sludge rheology, while solids retention time and loss on ignition affects digested sludge rheology. Nevertheless, friction loss calculations based on a simple power law relationship between shear rate and shear stress proved to be applicable and sufficiently accurate for both raw and digested sludge with high TSS. Altogether, this case study underlines the relevance of comprehensive rheological considerations, measurements and calculations when designing high TSS sludge digestion.

Up to now, aerobic granulation of activated sludge is only realised 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.

Depending on design capacity, agitators consume about 5 to 20% of the total energy consumption of a wastewater treatment plant. Based on inhabitant-specific energy consumption (kWh PE120-1 a-1; PE120 is population equivalent, assuming 120 g chemical oxygen demand per PE per day), power density (W m-3) and volume-specific energy consumption (Wh m-3 d-1) as evaluation indicators, this paper provides a sound contribution to understanding energy consumption and energy optimization potentials of agitators. Basically, there are two ways to optimize agitator operation: the reduction of the power density and the reduction of the daily operating time. Energy saving options range from continuous mixing with low power densities of 1 W m-3 to mixing by means of short, intense energy pulses (impulse aeration, impulse stirring). However, the following correlation applies: the shorter the duration of energy input, the higher the power density on the respective volume-specific energy consumption isoline. Under favourable conditions with respect to tank volume, tank geometry, aeration and agitator position, mixing energy can be reduced to 24 Wh m-3 d-1 and below. Additionally, it could be verified that power density of agitators stands in inverse relation to tank volume.

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 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.