A full-scale high-rate cascade anaerobic digestion (CAD) system was evaluated for its ability to enhance enzymatic sludge hydrolysis. The system included a newly built digester, innovatively divided into three pie-shaped compartments (500 m³ each), followed by an existing, larger digester (1500 m³). The system treated a mixture of waste activated sludge and primary sludge, achieving a stable total chemical oxygen demand reduction efficiency (56.1 ± 6.8 %), and enhanced sludge hydrolytic enzyme activities at a 14.5-day total solids retention time (SRT). High-throughput sequencing data revealed a consistent microbial community across reactors, dominated by consortia that govern hydrolysis and acidogenesis. Despite relatively short SRTs in the initial reactors of the CAD system, acetoclastic methanogens belonging to Methanosaeta became the most abundant archaea. ‬‬‬‬‬‬‬‬‬‬‬‬‬ This study proves that the CAD system achieves stable sludge reduction, accelerates enzymatic hydrolysis at full-scale, and paves the way for its industrialization in municipal waste sewage sludge treatment.

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Particle-bubble collisions in dissolved air flotation (DAF) systems play a crucial role in the removal of total suspended solids (TSS). DAF particle-bubble collision models incorporate factors such as particle diameters, charge and density, bubble diameters, and collision factors. The challenge lies in accounting for the wide range of particle and bubble sizes and obtaining complex model inputs. To address this, a simplified model for TSS removal in DAF units was established using low-cost laboratory measurements, including particle size distribution and density. Additionally, microbubble diameter profiles were derived from bubble velocities using particle image velocimetry software (PIV). Six independent variables, encompassing influent particle characteristics (such as particle size distribution and density) and DAF running characteristics (temperature, contact zone detention time, inflow and recycle flows), were employed in the simplified model. The model's accuracy was evaluated using a laboratory-scale DAF system with two different influents: Delft canal water and anaerobic sludge. The predicted TSS removal from the simplified model aligned well with the laboratory-scale DAF results, yielding removal efficiencies of 68 ± 1 % and 77 ± 3 % for Delft canal water and anaerobic sludge, respectively. Furthermore, when the simplified model was applied to two full-scale DAF systems, it successfully identified an underperforming system (DAF2) with a TSS removal efficiency of 91 %, contrasting with the theoretical removal model-predicted efficiency of 98 %. This study highlights the utility of combining bubble size distribution measured by PIVlab and particle size distribution obtained using FIJI-ImageJ as an economical and efficient approach to acquiring the necessary inputs for predicting TSS removal in DAF systems.

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Thermal hydrolysis process (THP) is a widely used pre-treatment method in the anaerobic digestion (AD) of waste municipal sewage sludge. A post AD dewatering step of the digestate produces a liquid stream called reject water. THP increases the concentration of humic substances (HSs) and nutrients in the produced reject water. Struvite precipitation is a widely used technique to remove and (potentially) recover PO₄³⁻ -P and the corresponding amount of total ammoniacal nitrogen from reject water. The chemical characteristics of the THP-produced HSs influence reaction yields and morphology of struvite. In our current study, struvite batch precipitation experiments were conducted at different pHs, and different concentrations of HSs, consisting of either melanoidins or humic acids. Our results showed that at pH 6.5 struvite precipitation was severely retarded. However, increased concentrations of melanoidins at pH 6.5 enhanced struvite precipitation. Batch experiments conducted at pH 7.25 and 8 with increased melanoidins concentrations showed PO₄³⁻-P precipitation yields over 86 %. Humic acids negatively impacted struvite precipitation at all analysed pH values, presumably because of Mg²⁺ complexation. Morphological analysis showed that the presence of both HSs affected Feret diameters, aspect ratio, and cleavage pattern of struvite. Also, HSs rendered coloured crystals. Overall, our results showed that struvite precipitation is affected by HSs intrinsic characteristics, affecting yield, morphology, and colour of the formed precipitates.

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Investigating the interaction between influent particles and biomass is basic and important for the biological wastewater treatment. The micro-level methods allow for this, such as the microscope image analysis method with the conventional ImageJ processing software. However, these methods are cost and time-consuming, and require a large amount of work on manual parameter tuning. To deal with this problem, we proposed a deep learning (DL) method to automatically detect and quantify microparticles free from biomass and entrapped in biomass from microscope images. Firstly, we introduced a “TU Delft-Interaction between Particles and Biomass” dataset containing labeled microscope images. Then, we built DL models using this dataset with seven state-of-the-art model architectures for a instance segmentation task, such as Mask R-CNN, Cascade Mask R-CNN, Yolact and YOLOv8. The results show that the Cascade Mask R-CNN with ResNet50 backbone achieves promising detection accuracy, with a mAP50_box and mAP50_mask of 90.6 % on the test set. Then, we benchmarked our results against the conventional ImageJ processing method. The results show that the DL method significantly outperforms the ImageJ processing method in terms of detection accuracy and processing cost. The DL method shows a 13.8 % improvement in micro-average precision, and a 21.7 % improvement in micro-average recall, compared to the ImageJ method. Moreover, the DL method can process 70 images within 1 min, while the ImageJ method costs at least 6 h. The promising performance of our method allows it to offer a potential alternative to examine the interaction between microparticles and biomass in biological wastewater treatment process in an affordable manner. This approach offers more useful insights into the treatment process, enabling further reveal the microparticles transfer in biological treatment systems.

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Next to the challenges of paramount importance represented by water scarcity, food security, energy transition, and environmental protection issues, the obstacles faced on the matter of water, sanitation, and hygiene (WASH) are immense. WASH interventions are essential to support human health, prosperity, and dignity, as they provide the base for an adequate standard of living. In many low- and middle- income countries, especially in rural and low-income areas, decentralized wastewater treatment systems (DEWATS) can offer a solution to convey, treat, and dispose of or reuse wastewater closer to the source and through smaller conveyance networks. In Indonesia, and as such in the Brantas basin on East Java, focus area of this study, the government has recognized DEWATS as their best available option for improving sanitation in dense low-income urban settings. Although the percentage of households with access to proper sanitation in the province of East Java has been increasing steadily, service coverage and the quality of sanitation systems still need to be increased to reach the desired coverage by 2024. Similar to other fields of application, within WASH and concerning DEWATS, stakeholders engagement, ethics and gender dimension are key topics to develop and strengthen integrated approaches. It is challenging to formulate targeted interventions in the watershed since they depend on the willing support of various stakeholders who may have different priorities (even within their own institutions), having diverse (and sometimes conflicting) viewpoints. This may result in stakeholders strongly contesting the appropriateness of various solutions. An exploration of stakeholder priorities is therefore needed to facilitate the application of wastewater treatment technologies. Due to its participatory approach and the type of interpretation that the method allows, Q-methodology was selected to explore this situation. Q-methodology is a set of techniques which allow for the study of ‘subjectivity’, combining statistics with the depth provided by qualitative data. It is composed of the data collection technique (called Q-sorting) and a data analysis step via correlation and factor analysis. In this contribution, we explore the perspectives and priorities of various stakeholders regarding decentralized wastewater treatment solutions to assess the applicability and acceptability of DEWATS in the Brantas river basin. This allows us to identify context-based criteria and challenges to the implementation of DEWATS in the Brantas watershed. As such, we propose the Q-methodology as a strong methodology to further develop the required transdisciplinary scientific efforts to promote relevant insights and solutions through meaningful, pertinent, and effective stakeholder engagement.@en

This study assessed the evolution of wastewater systems during the rapid urbanization of Beijing, with special focuses on the carbon footprints and growing underground WWTPs (u-WWTPs). Specifically, the Bishui plant (in situ constructed u-WWTP) was assessed in detail regarding eco-environmental benefits. Our results showed that, the direct emission intensity of 65 WWTPs decreased from 0.47 to 0.24 kg CO_2eq/m³, when the electricity intensity increased from 0.22 to 0.39 kWh/m³ from 2010 to 2020. Bishui u-WWTP emitted 36.6 kt CO_2eq/year (0.09 kg CO_2eq/m³), with electricity intensity of 0.43 kg CO_2eq/m³. Additionally, compare to the hypothetical relocating scenario, it saved 6.67 × 10⁴ m² land and 33.0 kt CO_2eq/year, and the created urban river carries 6.5 × 10¹³ J/year heat outside town. The evaluation and balance of choice for conventional or underground WWTP should be made case by case. However, this study demonstrated that u-WWTP is not only a construction manner, but a sustainable management model with positive eco-environment effects, algin with future city expansion, and circular economy visions.

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Organic micropollutants (OMPs) enter the aquatic environment via municipal wastewater treatment plants (WWTPs). As conventional WWTPs have limited capacity for the removal of OMPs, additional processes are required, like ozone - granular activated carbon (GAC) filtration. A specific lay-out of this process is the O3-STEP® process, in which the removal of suspended solids, OMPs, phosphate and nitrate is combined. However, ozonation may result in formation of bromate, a compound with a strict water quality standard of 1 μg/L for surface waters in The Netherlands. This limits the applicability of ozonation in wastewater treatment. This study examined biological bromate removal associated with denitrification processes in the GAC filter of the O3-STEP® process. In this GAC filter methanol is dosed for nitrate removal by biological denitrification. In column experiments, bromate and nitrate were removed simultaneously under both anoxic and oxic conditions. Depletion of oxygen within the biofilm surrounding the GAC granules most probably is the reason for denitrification under oxic bulk conditions, although aerobic denitrification cannot be excluded. In batch experiments, the presence of nitrate did not affect bromate removal, whereas the presence of dissolved oxygen had a slight inhibitory effect on bromate removal and nitrate removal. Addition of methanol increased both nitrate and bromate removal, which is hypothesized to occur through an increased availability of electron donors in the water. The results show that a denitrifying GAC filter in the ozone - GAC filtration process mitigates the bromate formation, which broadens the applicability of this process for OMP removal from wastewater.@en

Full-scale thermal hydrolysis processes (THP) showed an increase in nutrients release and formation of melanoidins, which are considered to negatively impact methanogenesis during mesophilic anaerobic digestion (AD). In this research, fractionation of THP-sludge was performed to elucidate the distribution of nutrients and the formed melanoidins over the liquid and solid sludge matrix. Degradation of the different fractions in subsequent AD was assessed, and the results were compared with non-pre-treated waste activated sludge (WAS). Results showed that the THP-formed soluble melanoidins were partially biodegradable under AD, especially the fraction with molecular weight under 1.1 kDa, which was related to protein-like substances. The use of THP in WAS increased the non-biodegradable soluble chemical oxygen demand (sCOD) after AD, from 1.1% to 4.9% of the total COD. The total ammoniacal nitrogen (TAN) concentration only slightly increased during THP without AD. However, after AD, TAN released was 34% higher in the THP-treated WAS compared to non-treated WAS, i.e., 36.7 ± 0.7 compared to 27.4 ± 0.4 mgTANreleased/gCODsubstrate, respectively. Results from modified specific methanogenic activities (mSMAs) tests showed that the organics solubilised during THP, were not inhibitory for acetotrophic methanogens. However, after AD of THP-treated sludge and WAS, the mSMA showed that all analysed samples presented strong inhibition on methanogenesis due to the presence of TAN and associated free ammonia nitrogen (FAN). In specific methanogenic activities (SMAs) tests with incremental concentration of TAN/FAN and melanoidins, TAN/FAN induced strong inhibition on methanogens, halving the SMA at around 2.5 gTAN/L and 100 mgFAN/L. Conversely, melanoidins did not show inhibition on the methanogens. Our present results revealed that when applying THP-AD in full-scale, the increase in TAN/FAN remarkably had a greater impact on AD than the formation of melanoidins.

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This study reports the effects of microaeration on a laboratory-scale AnMBR (MA-AnMBR) fed with synthetic concentrated domestic sewage. The imposed oxygen load mimics the oxygen load coming from a dissolved air flotation (DAF) unit, establishing an anaerobic digester-DAF (AD-DAF) combination with sludge recycling. Results showed a reduced COD concentration in the MA-AnMBR permeate compared with the AnMBR permeate, from 90 to 74 mgCOD L–1, and a concomitant 27% decrease in biogas production. The MA-AnMBR permeate ammonium (NH4+) concentration increased by 35%, to 740 mgNH4+-N L–1, indicating a rise in the hydrolytic capacity. Furthermore, the MA-AnMBR biomass seemingly adapted to an increased oxygen load, which corresponded to 1% of the influent COD load (approximately 55 mLO2 d–1). Concomitantly, an increase in the superoxide dismutase activity (SOD) of biomass was detected. Meanwhile, negligible changes were observed in the specific methanogenic activity (SMA) of the microaerated biomass that was subjected to an oxygen load equivalent to 3% of the influent COD load in batch tests. The obtained results showed that an AD-DAF system could be a promising technology for treating concentrated domestic wastewater, improving sewage sludge hydrolysis, and overall organic matter removal when compared to an AnMBR.@en

This study investigates the effects, conversions, and resistance induction, following the addition of 150 μg·L⁻¹ of two antibiotics, sulfamethoxazole (SMX) and trimethoprim (TMP), in a laboratory-scale micro-aerated anaerobic membrane bioreactor (MA-AnMBR). TMP and SMX were removed at 97 and 86%, indicating that micro-aeration did not hamper their removal. These antibiotics only affected the pH and biogas composition of the process, with a significant change in pH from 7.8 to 7.5, and a decrease in biogas methane content from 84 to 78%. TMP was rapidly adsorbed onto the sludge and subsequently degraded during the long solids retention time of 27 days. SMX adsorption was minimal, but the applied hydraulic retention time of 2.6 days was sufficiently long to biodegrade SMX. The levels of three antibiotic-resistant genes (ARGs) (sul1, sul2, and dfrA1) and one mobile genetic element biomarker (intI1) were analyzed by qPCR. Additions of the antibiotics increased the relative abundances of all ARGs and intI1 in the MA-AnMBR sludge, with the sul2 gene folding 15 times after 310 days of operation. The MA-AnMBR was able to reduce the concentration of antibiotic-resistant bacteria (ARB) in the permeate by 3 log.@en

Gas-mixing is commonly applied in anaerobic digesters, yet the resulting flow and hydraulic mixing are difficult to evaluate because of limited full-scale experimental data and uncertainties in integrating sludge rheological data. This study used computational fluid dynamics (CFD) to assess the impact of treated sludge rheology on flow and mixing characterisation in a full-scale biogas-mixed digester. The CFD model, which was firstly validated using a lab-scale setup, showed that flow and mixing predictions depended on the rheological properties, especially at low shear rates. The predicted dominant shear rate was out of the effective shear-rate range of the Ostwald model, leading to flow and mixing performance overestimation. The results indicated that there are limitations in applying the Ostwald model and the conventional approaches for determining dead-zone. The Herschel-Bulkley model was more appropriate for the prevailing low shear rates and predicted large viscosity gradients in the digester, indicating two distinct compartments with different flow and mixing behaviour based on the gas-sparging height: a plug-flow compartment with dominant vertical convection above, and a dead-zone compartment with considerable segregation below. The results showed that the applied gas-sparging induced insufficient flow and mixing, but contributed to the well-functioning of the digester. To correctly assess flow and mixing, the applied rheological data should be in agreement with the type of sludge that is treated in the digester. Our results indicate that the shear rate in the digester must be increased and various options for achieving this are proposed.

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Thermal hydrolysis process (THP) is a well-established anaerobic digestion (AD) pre-treatment technology. Despite the THP benefits the pre-treatment increases the concentrations of nutrients and melanoidins in the digestate reject water after dewatering. The increased concentrations of nutrients and melanoidins formed during THP-AD can impact downstream processes, such as struvite precipitation and partial nitritation/anammox (PN/A). In our present work, six full-scale PN/A influents and effluents were sampled in The Netherlands (4 with THP and 2 without THP). Full-scale samples were characterised and the stoichiometric O₂ consumption and melanoidins chelated to trace elements were analysed. The results showed that THP increased the concentration of total ammoniacal nitrogen (TAN), chemical oxygen demand (COD), total organic carbon (TOC), UVA 254 and colour, which are indicators of melanoidins occurrence. THP furthermore decreased the stoichiometric NO₃⁻-N production from the PN/A reaction in effluents. The disparity between stoichiometric and measured NO₃⁻ -N in the THP-using plants was explained by the proliferation of denitrifiers. Moreover, denitrification improved the N removal efficiency due to the consumption of the stoichiometrically-produced NO₃⁻ -N. Also, the stoichiometric O₂ consumption increased in the plants using THP, reaching up to 56% of the O₂ used for partial oxidation of TAN. Trace elements analysis revealed that the plants with elevated concentrations of melanoidins in the effluent showed a high percentage of chelated multivalent cations, particularly transition metals such as Fe. Kendall correlation coefficient analysis showed that the chelation of multivalent cations was correlated mainly with colour occurrence in the reject waters. Overall, the results indicated that in PN/A systems using THP-AD increased O₂ consumption and trace elements availability should be considered during the process design.

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The Barapullah drain crosses through New Delhi, India, and transports millions of cubic meters of stormwater, municipal sewage and industrial sewage to the Yamuna River. Seasonal variations and ambiguous annual discharges cause 20-fold fluctuations in hydraulic flows, pollutants type and concentration. Furthermore, New Delhi is among the most densely populated areas on the planet, with limited surface area and high water stress. Dissolved Air Flotation (DAF) units are known to be highly compact, robust, and an efficient suspended solids separation technology that enables further water recovery in a treatment train. Thus, a down-scaled column DAF was designed and used to determine the total suspended solids removal efficiencies, under different influent conditions. Three influents that resemble the Barapullah drain seasonal variations in composition, and a fourth that imitates the feed of DAF when located after an anaerobic bioreactor were tested. A total of 60 batch DAF experiments were completed and used to assess seven independent control variables for DAF operation, which are influent Total Suspended Solids (TSS), pH, temperature, DAF particles residence time, white water pressure, coagulants and flocculants concentration, and coagulation and flocculation time. Results showed that the down-scaled DAF could be steered from low to high removal efficiencies, comparable to full-scale systems. Maximum TSS removal varied between 92 and 96%. The effect and statistical relevance of the different performance variables on the measured separation efficiencies depended on the influent type. All variables, except temperature and pH, had a significant performance effect with a p-value below 0.1, for at least one influent. Pressure had a positive effect on separation efficiency, due to its importance in bubble formation. Moreover, the down-scaled DAF system had low removal efficiency for particles with spherical shapes, and diameters below 10 µm. Based on the high TSS removal for all tested influents, and the effect of the studied control variables, a full-scale DAF could efficiently remove the suspended solids of the Barapullah drain. The unit robustness for different flows and pollutant concentrations, and small footprint, show DAF suitability as part of a treatment train for water recovery, in densely populated areas.

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Aerobic granular sludge (AGS) is an advanced biofilm-based technology for wastewater treatment. Diffusion of substrates into the granules is a key aspect of this technology. Domestic wastewater contains soluble organic substrates of different sizes that could potentially diffuse into the granules. In this study, the relation between the molecular weight of a substrate and its diffusion coefficient within the granule was studied with model substrates (polyethylene glycols (PEGs) with a molecular weight between 62 and 10 000 Da). The diffusion coefficients of the model substrates within granules from a full-scale installation were measured with the ‘transient uptake of a non-reactive solute’ method. The diffusion coefficients in the granules were not significantly different from the diffusion coefficients in water, at least up to 4000 Da molecular weight. This indicates that these PEGs were not obstructed by the granule matrix. The 10 kDa PEG behaved differently from the lighter PEGs, as it could not penetrate the entire granule. Furthermore, the granule structure was characterized with Environmental Scanning Electron Microscopy (ESEM). The granules displayed an open structure with large macropores and semi-solid regions, which contained microbial cells. The diffusion results suggest that most diffusing molecules were unobstructed in the macropores and barely obstructed in the semi-solid regions. Only the diffusion of the 10 kDa PEG seemed to be hindered by the semi-solid regions, but not by the macropores. Lastly, the apparent molecular weight distribution of domestic wastewater soluble COD was determined with ultrafiltration membranes of 100, 10, and 1 kDa molecular weight cut-off. The influent fractionation revealed that a large part (61-69%) of the influent soluble COD was lighter than 1 kDa. As molecules lighter than 1 kDa diffuse easily, the majority of the influent soluble COD can be considered as diffusible COD. These findings provide new insight into the availability of influent COD for granular sludge.@en

The pre-treatment of waste activated sludge (WAS) has become more common since it often results in improved bioconversion to methane, in both rate and extent. However, thorough insights on the possible effects and mechanisms of mild pre-treatment techniques, such as temperatures <100 °C combined with the addition of H₂O₂, are still limited. This study reports the effects of the addition of 5–30 mgH₂O₂/g TS and its interaction with thermal pre-treatment at 70 °C on methane production, using WAS as the substrate. It was found that the addition of H₂O₂ increased the methane production rate, coinciding with a decrease in apparent viscosity of WAS, which probably improved mass transfer under non-ideal mixing conditions. While H₂O₂ solubilized proteins and carbohydrates and mineralized a small fraction of the humic substances in WAS, these biochemical transformations did not suffice to explain the observed extent and rate of methane production. A decreased particle size, the presence of Fenton's reagent, and the presence of cationic polymers in the WAS were discarded as the reasons for the observed decrease in apparent viscosity. It was concluded that the pre-treatment conditions applied in the present study might be a strategy to enhance mixing conditions in full-scale anaerobic digesters.

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At wastewater treatment plants (WWTPs), additional steps are introduced for removal of organic micropollutants (OMPs) from the treated effluents, especially pharmaceutical residues. At the same time, a new concern is emerging: antibiotic resistance (AR). This research studied the effect of ozonation, coagulation and granular activated carbon (GAC) filtration applied as tertiary treatment for the removal of OMPs and nutrients, on AR removal. Bacterial culture methods in selective media were used to screen for four different microorganisms: two faecal indicators (Escherichia coli and Enterococci) as antibiotic sensitive bacteria (ASB), and a resistant strain of each of these bacteria, namely Extended-Spectrum Beta-lactamase producing E. coli (ESBL-E.coli) and Vancomycin Resistant Enterococci (VRE) as antibiotic resistant bacteria (ARB). At laboratory scale, ozonation experiments (ozone dose 0.4–0.6 g O ₃/g DOC) and coagulation experiments using Polyaluminum chloride (PAX-214) and FeCl ₃ (coagulant dose 0.004–1 mM/L) were performed using secondary effluent from two municipal WWTPs. In addition in a pilot plant and full-scale plant ozonation (ozone dose 0.4 g O ₃/g DOC) and GAC filtration (empty bed contact time 15 min) were studied for AR removal. No significant differences were found between ARB and ASB removal for coagulation and ozonation which could indicate that ASB can be used as an initial proxy for ARB removal for these technologies. In the laboratory experiments, ozonation and coagulation showed a good removal of both ARB and ASB. However, the doses needed to reach 2–3 log removal were a factor 2.5–4 (ozonation) and 250 (coagulation) higher than applied for OMP removal (by ozonation) and phosphorus (P) removal (by coagulation). In the GAC filters, the risk of ARB enhancement occurred, especially in filters with a matured biology. Although these bacteria are not necessarily directly harmful, they can pass down their resistance to pathogenic bacteria via horizontal gene transfer.

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In aerobic granular sludge (AGS) reactors, granules of different sizes coexist in a single reactor. Their differences in settling behaviour cause stratification in the settled granule bed. In combination with substrate concentration gradients over the reactor height during the anaerobic plug-flow feeding regime, this can result in functional differences between granule sizes. In this study, we compared the hydrolytic activity in granules of 4 size ranges (between 0.5 and 4.8 mm diameter) collected from a full-scale AGS installation. Protease and amylase activities were quantified through fluorescent activity assays. To visualise where the hydrolytic active zones were located within the granules, the hydrolysis sites were visualized microscopically after incubating intact and sliced granules with fluorescent casein and starch. The microbial community was studied using fluorescent in situ hybridization (FISH) and sequencing. The results of these assays indicated that hydrolytic capacity was present throughout the granules, but the hydrolysis of bulk substrates was restricted to the outer 100 µm, approximately. Many of the microorganisms studied by FISH, such as polyphosphate and glycogen accumulating organisms (PAO and GAO), were abundant in the vicinity of the hydrolytically active sites. The biomass-specific hydrolysis rate depended mainly on the available granule surface area, suggesting that different sized granules are not differentiated in terms of hydrolytic capacity. Thus, the substrate concentration gradients that are present during the anaerobic feeding in AGS reactors do not seem to affect hydrolytic activity at the granule surfaces. In this paper, we discuss the possible reasons for this and reflect about the implications for AGS technology.

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Granular sludge processes are frequently used in domestic and industrial wastewater treatment. The granule buoyant density and biomass density are important parameters for the design and operation of granular sludge reactors. Different methods to measure the granule density include the pycnometer method, the Percoll density gradient method, the dextran blue method, and the settling velocity method. In this study, a comparison was made between these four methods to measure granule density for granules from a full-scale granular sludge plant treating domestic sewage. The effect of salinity on granule density was assessed as well. Three out of the four evaluated methods yielded comparable results, with granule buoyant densities between 1025.7 and 1028.1 kg/m³ and granule biomass densities between 71.1 and 71.5 g/L (based on volatile suspended solids (VSS)). The settling velocity method clearly underestimated the granule density, due to the complex relation between granule properties and settling velocity. The pycnometer method was the most precise method, but it was also quite susceptible to bias. The granule buoyant density increased proportionally with salinity, to 1049.2 kg/m³ at 36 g/L salinity. However, the granule biomass density, based on VSS, remained constant. This showed that the granule volume was not affected by salinity and that the buoyant density increase was the result of diffusion of salts into the granule pores. Overall, the granule density can be measured reliably with most methods, as long as the effect of salinity is considered. The results are discussed in light of operational aspects for full-scale granular sludge plants.

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High concentrations of particulate COD (pCOD) in the influent of aerobic granular sludge (AGS) systems are often associated to small granule diameter and a large fraction of flocculent sludge. At high particulate concentrations even granule stability and process performance might be compromised. However, pilot-or full-scale studies focusing on the effect of real wastewater partic-ulates on AGS are scarce. This study describes a 3-month period of increased particulate loading at a municipal AGS wastewater treatment plant. The pCOD concentration of the influent increased from 0.5 g COD/L to 1.3 g COD/L, by adding an untreated slaughterhouse wastewater source to the influent. Sludge concentration, waste sludge production and COD and nutrient removal performance were monitored. Furthermore, to investigate how the sludge acclimatises to a higher influent particulate content, lipase and protease hydrolytic activities were studied, as well as the microbial community composition of the sludge. The composition of the granule bed and nutrient removal efficiency did not change considerably by the increased pCOD. Interestingly, the biomass-specific hydrolytic activities of the sludge did not increase during the test period either. However, already during normal operation the aerobic granules and flocs exhibited a hydrolytic potential that ex-ceeded the influent concentrations of proteins and lipids. Microbial community analysis also revealed a high proportion of putative hydrolysing and fermenting organisms in the sludge, both during normal operation and during the test period. The results of this study highlight the robust-ness of the full-scale AGS process, which can bear a substantial increase in the influent pCOD concentration during an extended period.

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Accurate rheological characterisation of sewage waste activated sludge (WAS) is of high importance for downstream processing related to optimised sludge pumping and mixing, assessment of energy demands and overall process design. However, to elaborate rheological behaviour is often challenging under dynamic operational conditions in practice. In this study, two practical influencing factors were investigated: long-term shearing and temperature. Compared to anaerobic digestate (DGT), concentrated WAS had more complex and stronger thixotropic behaviour. Under the long-term shearing conditions, the sludge thixotropic behaviour was well characterised by two quantified limitation states. Temperature had a striking impact on the rheological properties, which was strongly correlated to solids content and digestion process. The impact discrepancy between the long-term shearing and temperature, implied different mechanisms to shift the equilibrium of hydrodynamic and non-hydrodynamic interactions for structure deformation and recovery. The distinct rheological properties between the two determined states were clearly reflected in pipe flow behaviour, revealing a concrete link between lab-measured sludge rheology and its practical flow performance. The pipe flows were well assessed using the developed Computational Fluid Dynamics model with effective rheological data integration, which is promising for practical design and optimisation of sewage sludge systems.

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