Conventional Activated Sludge System Research Articles

Microbiome structure and function in parallel full-scale aerobic granular sludge and activated sludge processes

Aerobic granular sludge (AGS) and conventional activated sludge (CAS) are two different biological wastewater treatment processes. AGS consists of self-immobilised microorganisms that are transformed into spherical biofilms, whereas CAS has floccular sludge of lower density. In this study, we investigated the treatment performance and microbiome dynamics of two full-scale AGS reactors and a parallel CAS system at a municipal WWTP in Sweden. Both systems produced low effluent concentrations, with some fluctuations in phosphate and nitrate mainly due to variations in organic substrate availability. The microbial diversity was slightly higher in the AGS, with different dynamics in the microbiome over time. Seasonal periodicity was observed in both sludge types, with a larger shift in the CAS microbiome compared to the AGS. Groups important for reactor function, such as ammonia-oxidising bacteria (AOB), nitrite-oxidising bacteria (NOB), polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs), followed similar trends in both systems, with higher relative abundances of PAOs and GAOs in the AGS. However, microbial composition and dynamics differed between the two systems at the genus level. For instance, among PAOs, Tetrasphaera was more prevalent in the AGS, while Dechloromonas was more common in the CAS. Among NOB, Ca. Nitrotoga had a higher relative abundance in the AGS, while Nitrospira was the main nitrifier in the CAS. Furthermore, network analysis revealed the clustering of the various genera within the guilds to modules with different temporal patterns, suggesting functional redundancy in both AGS and CAS.Key points• Microbial community succession in parallel full-scale aerobic granular sludge (AGS) and conventional activated sludge (CAS) processes.• Higher periodicity in microbial community structure in CAS compared to in AGS.• Similar functional groups between AGS and CAS but different composition and dynamics at genus level.Graphical abstract

Modeling the effects of sidestreams recycling on wastewater treatment plant performance operated by anaerobic-anoxic-oxic (A2/O) processes using GPS-X8 simulation

The water separated by sludge treatment processes is known as the sidestreams, such as the centrate, underflow belt thickener, and supernatant gravity thickener. The sidestreams have significant amounts of contaminants, particularly nutrients, and are returned to the wastewater treatment plant head. Returning the sidestreams to the head of the plant causes both technical and financial problems. This paper models the effect of sidestreams on the treatment of mainstream in full-scale conventional activated sludge systems operated with anaerobic/anoxic/oxic (A2/O). The effects of sidestreams nutrients and inert materials, the pollutants fate in the A2/O system, oxygen consumed, and energy required by sidestreams are modeled using GPS-X8 simulation. The results showed that the discharge ratio of the sidestreams to the mainstream is 4 %. The proportion of the pollutants present in the sidestream to the mainstream were 4.3 %, 4.7 %, 8.8 %, 21 %, 40 %, and 11 % for BOD, COD, TSS, NH4, PO4, and H2S, respectively. The GPS-X model showed that high orthophosphate and hydrogen sulfide concentrations were seen in the anaerobic zone. While all other pollutants begin to decay, phosphates and hydrogen sulfide gas also begin to decrease gradually in anoxic and oxic zones, noting the high concentrations of nitrates and sulfates in the oxic zone. An added 10 % of oxygen is needed for the sidestreams ammonia oxidation, which is more than the amount needed for the oxidation of BOD. The percentage of alkali used to improve the nitrification process due to the ammonia in the sidestreams was 30 % of the specified amount of ammonia in the mainstream. This study proved that the plant's sidestreams adversely increased oxygen, energy, and alkali consumption.

Micropollutant biotransformation under different redox conditions in PhoRedox conventional activated sludge systems

The ecotoxicological safety of the water bodies relies on the reduction of micropollutant emissions from wastewater treatment plants (WWTP). The ecotoxicological safety of the water bodies relies on the reduction of micropollutant emissions from wastewater treatment plants (WWTP). Quantification of micropollutant removal at full-scale WWTP is scarce. To our knowledge, the anaerobic conversion rates determined at conventional activated sludge processes are, so far, scarcely available in the literature for most of the micropollutants. In this research, we quantified the biotransformation rate constants and the removal efficiencies of 16 micropollutants (4,5-methylbenzotriazole, azithromycin, benzotriazole, candesartan, carbamazepine, clarithromycin, diclofenac, gabapentin, hydrochlorothiazide, irbesartan, metoprolol, propranolol, sotalol, sulfamethoxazole, trimethoprim, and venlafaxine), under aerobic, anoxic, and anaerobic redox conditions; using as inoculum wastewater and biomass from a full-scale conventional activated sludge (CAS) system in the Netherlands. Clarithromycin was the compound that exhibited the highest aerobic (76%) and anaerobic (78%) removal efficiencies, while gabapentin showed the highest removal under anoxic conditions (91%). A preference for cometabolic biotransformation of the targeted micropollutants was observed. The highest biotransformation rate constants obtained were: at aerobic conditions clarithromycin with 1.46 L.gSS−1.d−1; at anoxic conditions, gabapentin with 2.36 L.gSS−1.d−1; and at anaerobic redox conditions clarithromycin with 1.87 L.gSS−1.d−1. The obtained results of biotransformation rates will allow further modelling of micropollutant removal in CAS systems, under various redox conditions. These biotransformation rates can be added to extended ASM models to predict effluent concentration and optimize targeted advanced oxidation processes allowing savings in the operational costs and increasing the process viability.

Treatment and Handling of Hydraulic Shock Load of Urea Fertilizer Wastewater in Sequencing Batch Reactor

The production process in the urea fertilizer industry produces wastewater with a very high ammonia content, which exceeds the quality standards for fertilizer wastewater. Therefore, it is necessary to treat urea fertilizer wastewater, which has a high ammonia content. One of the technologies that can be used to treat this type of wastewater is the Sequencing Batch Reactor (SBR) technology. The SBR technology was chosen because it only requires one reactor for the entire process, in which in conventional activated sludge systems it occurs in several reactors. Shock loading often occurs in wastewater treatment plants, including both organic shock loads and hydraulic shock loads. The waste used in the SBR operation in this research is urea fertilizer wastewater originating from a urea fertilizer industry in West Java, Indonesia. The parameters to be tested were COD, MLVSS, DO, pH, temperature, turbidity, and ammonia concentration. The results showed that the efficiency of reducing ammonia levels under normal loading with a flow rate of 300 mL/day was 99.5%, whereas when given a shock load of 600 mL/day, an efficiency of 98% was obtained. This proves that SBR can handle shock loads even though its efficiency slightly decreases.

Pilot-scale investigation of performance and microbial community in a novel system combining fixed and suspended activated sludge

The conventional activated sludge (CAS) process is a widely used method for wastewater treatment due to its effectiveness and affordability. However, it can be prone to sludge abnormalities such as sludge bulking/foaming and sludge loss, which can lead to a decrease in treatment efficiency. To address these issues, a novel bag-based fixed activated sludge (BBFAS) system utilizing mesh bags to contain the sludge was developed for low carbon/nitrogen ratio wastewater treatment. Pilot-scale experiments demonstrated that the BBFAS system could successfully avoid the sludge abnormalities. Moreover, it was not affected by mass transfer resistance and exhibited significantly higher nitrogen removal efficiency, surpassing that of the CAS system by up to 78%. Additionally, the BBFAS system demonstrated comparable organic matter removal efficiency to CAS system. 16S rRNA gene high-throughput sequencing revealed that the bacterial community structure within the BBFAS system was significantly different from that of the CAS system. The bacteria associated with ammonium removal were more abundant in the BBFAS system than in the CAS system. The abundance of Nitrospira in the BBFAS could reach up to 6% and significantly higher than that in the CAS system, and they were likely responsible for both ammonia-oxidizing and nitrite-oxidizing functions. Clear stratification of microbial communities was observed from the outer to inner layers of the bag components due to the gradients of dissolved oxygen and other substrates. Overall, this study presents a promising approach for avoiding activated sludge abnormalities while maintaining high pollutant removal performance.

Linking the use of reclaimed water to indicators of crop stress by metabolomic and transcriptomic analyses. A tool to compare water irrigation quality

The occurrence of contaminants of emerging concern (CECs) or heavy metals in reclaimed water used for agricultural irrigation may affect crop morphology and physiology. Here, we analyzed lettuce (Lactuca sativa) grown in outdoor lysimeters and irrigated with either tap water, used as a control, or reclaimed water: CAS-reclaimed water, an effluent from a conventional activated sludge system (CAS) followed by chlorination and sand filtration, or MBR-reclaimed water, an effluent from a membrane biological reactor (MBR). Chemical analyses identified seven CECs in the reclaimed waters, but only two of them were detected in lettuce (carbamazepine and azithromycin). Metabolomic and transcriptomic analyses revealed that irrigation with reclaimed water increased the concentrations of several crop metabolites (5-oxoproline, leucine, isoleucine, and fumarate) and of transcripts codifying for the plant stress-related genes Heat-Shock Protein 70 (HSP70) and Manganese Superoxide Dismutase (MnSOD). In both cases, MBR-water elicited the strongest response in lettuce, perhaps related to its comparatively high sodium adsorption ratio (4.5), rather than to its content in CECs or heavy metals. Our study indicates that crop metabolomic and transcriptomic profiles depend on the composition of irrigating water and that they could be used for testing the impact of water quality in agriculture.

Comparative assessment of different scenarios for upgrading activated sludge wastewater treatment plants in developing countries

Improvement of current wastewater treatment plants (WWTPs) to accommodate the growing influent flow rate and cope with the increasingly stringent regulations is hindered by the allowed space and the difficulty of changing the design parameters. Mathematical modeling is a useful tool for assessing the performance of WWTPs in light of broadening objectives. We herein explore the utilization of mathematical modeling to improve effluent quality in conventional activated sludge systems. BioWin was used to model Mansoura WWTP, one of the largest WWTPs in Egypt. Lab records, design reports, and additional analyses were conducted through site visits and a comprehensive sampling campaign. The wastewater was characterized, and the plant-wide model was calibrated following the protocol of the Dutch Foundation for Applied Water Research STOWA. Important kinetic and stoichiometric parameters were identified and adjusted during the calibration process. The model validity was assessed using different validation periods considering average relative deviation (ARD) values below 20 % as acceptable. The optimized nitrification and denitrification processes involved 16 scenarios with different operational conditions. By changing some zones in the aeration basin from aerobic to anoxic and increasing the return activated sludge, the average ammonia and nitrate concentrations were significantly reduced from 23.06 and 0.5 mg/L to 4.64 and 0.07 mg/L respectively. Furthermore, phosphorus removal optimization was carried out through biological and chemical processes. Chemical phosphorus removal was 85.76 % by using a coagulant dosage of 25 mg/L, resulting in an effluent concentration of 1.04 mg P/L. Biological phosphorus removal was increased to 85.43 % by modifying the volume of anaerobic and aerobic zones with lower power consumption. Overall, this study demonstrates the effectiveness of mathematical modeling in enhancing effluent quality and reducing energy consumption to meet stringent wastewater treatment regulations.

Prevention of water pollution through combined sewer overflow using high-speed biofiltration in urban watershed

Over recent decades, the rural areas of Korea, including containing source waters, have become extensively developed. Combined sewer systems (CSSs) have been constructed to collect the sewage generated from these areas. Presently, it is common to experience localized heavy rain events, likely due to climate change, and sometimes, the volume of rainfall can become too high for a CSS to handle. In these cases, combined sewer overflow (CSOs) occurs, contaminating surface water, which may have been used as source water. Therefore, the Korean government will begin regulating the biochemical oxygen demand (BOD) of CSOs occurring during first flushes of less than 40 mg/L from 2024 onwards. In this study, a biofiltration-based technology was proposed to treat CSOs. Thus, this study aimed to remove BOD from CSO using this biofiltration system to contain cube-type media. For this, two different loading rates were tested on a pilot scale. Although the proposed system resulted in a footprint of only 10 % of that of a conventional activated sludge system, it was still capable of achieving efficient BOD removal. At a high loading rate, BOD removal efficiency was observed at approximately 53 %, and when this loading rate was reduced by 50 %, the removal efficiency was slightly improved to about 58 %. In both cases, the BODs of treated CSOs could be maintained below the proposed future standard: 31 mg/L and 28 mg/L for high and low loading rates, respectively. Therefore, the proposed system could potentially be preventing environmental contamination from COSs.

SARS-CoV-2 removal in municipal wastewater treatment plants: Focus on conventional activated sludge, membrane bioreactor and anaerobic digestion

This work focuses on the removal of SARS-CoV-2 RNA in the various stages of a full-scale municipal WWTP characterised by two biological processes in parallel: (i) conventional activated sludge (CAS) and (ii) membrane bioreactor (MBR). The monitoring was carried out during the Omicron wave in 2022, a period characterised by a high concentration of SARS-CoV-2 in influent wastewater. The average concentration of SARS-CoV-2 in influent wastewater was 3.7 × 104 GU/L. In the primary sedimentation, the removal of SARS-CoV-2 was not appreciable. The largest log removal value of SARs-CoV-2 occurred in the biological stages, with 1.8 ± 0.9 and 2.2 ± 0.7 logs in CAS and MBR systems. The mean concentrations of SARS-CoV-2 in the CAS and MBR effluents were 6.8 × 102 GU/L and 6.4 × 102 GU/L, respectively. The MBR effluent showed more negative samples, because small particles are retained by membrane and cake layer.The analysis of the different types of sludge confirmed the accumulation of SARS-CoV-2 in primary (5.2 × 104 GU/L) and secondary sludge (3.5 × 104 GU/L), due to the affinity of enveloped viruses towards biosolids. A SARS-CoV-2 concentration in the digested sludge equal to 4.8 × 104 GU/L denotes a negligible reduction in the mesophilic anaerobic digester at temperature of 31–33 °C.

In-depth insight on microbial electrochemical systems coupled with membrane bioreactors for performance enhancement: a review.

A conventional activated sludge (CAS) system has traditionally been used for secondary treatment in wastewater treatment plants. Due to the high cost of aeration and the problem of sludge treatment, researchers are developing alternatives to the CAS system. A membrane bioreactor (MBR) is a technology with higher solid-liquid separation efficiency. However, the use of MBR is limited due to inevitable membrane fouling and high energy consumption. Membrane fouling requires frequent cleaning, and MBR components must be replaced, which reduces membrane lifetime and operating costs. To overcome the limitations of the MBR system, a microbial fuel cell-membrane bioreactor (MFC-MBR) coupling system has attracted the interest of researchers. The design of the novel bioelectrochemical membrane reactor (BEMR) can effectively couple microbial degradation in the microbial electrochemical system (MES) and generate a microelectric field to reduce and alleviate membrane fouling in the MBR system. In addition, the coupling system combining an MES and an MBR can improve the efficiency of COD and ammonium removal while generating electricity to balance the energy consumption of the system. However, several obstacles must be overcome before the MFC-MBR coupling system can be commercialised. The aim of this study is to provide critical studies of the MBR, MES and MFC-MBR coupling system for wastewater treatment. This paper begins with a critical discussion of the unresolved MBR fouling problem. There are detailed past and current studies of the MES-MBR coupling system with comparison of performances of the system. Finally, the challenges faced in developing the coupling system on a large scale were discussed.

Start-up and maintenance of indigenous microalgae-bacteria consortium treating toilet wastewater through partial nitrification and nitrite-type denitrification

Microalgae-bacteria consortium (MBC) provides an alternative to sustainable treatment of human toilet wastewater (TWW) and resource recovery. This study compared the conventional activated sludge system and wastewater indigenous MBC system (IMBC) for nitrogen removal in TWW through the coupled partial nitrification (PN) and nitrite-type denitrification process. PN was firstly established by alternating FA and FNA. Subsequently, the successful PN maintenance with the nitrite accumulation rate ranging between 90.1–95.3% was achieved using two strategies: light irradiation with the appropriate specific light energy density at 0.0188–0.0598 kJ/mg VSS and the timely nitrite-type denitrification with the algae-secreted organics as the carbon source, eventually resulting in the nitrite accumulation rate ranging between 90.1–95.3%. In the IMBC-PN system, bacterial metabolism contributed to 91.5% of nitrogen removal and the rest was through microalgal assimilation. This study offers a sustainable hybrid IMBC-PN process for high NH4+-N strength wastewater treatment (e.g., TWW), which theoretically saves 23.5% aeration and 34.2% carbon source as well as reduces 17.0% sludge production.

Insights into the microbial community structure in the biodegradation process of high-strength ammonia digestate liquid fraction in conventional activated sludge system

Biodegradation of digestate liquid fraction was performed in the activated sludge system with acetic acid, flume water, and molasses as external carbon sources. High-throughput sequencing was used to gain in-depth insight into the activated sludge microbial community. The type and amount of carbon source in influent (COD/TN ratio) significantly influenced microbial community structure, especially at the genus level, and thus the biodegradation performance of digestate liquid fraction. The highest total nitrogen and chemical oxygen demand removal efficiencies averaging 85.3% and 88.3%, respectively, were achieved in series with acetic acid and flume water and COD/TN ratio of 10.7 and 11.2, respectively. The microbial diversity in these series averaged at 3.08 and 3.65. The dominant bacteria at the phylum level in series with acetic acid were Proteobacteria and Bacteroidota, and at the genus level Azospira, while in series with flume water they were Bacteroidota and Firmicutes, and Macellibacteroides, respectively.

Impact of the anaerobic feeding mode on substrate distribution in aerobic granular sludge

There is a growing interest to implement aerobic granular sludge (AGS) in existing conventional activated sludge (CAS) systems with a continuous flow-through configuration. The mode of anaerobic contact of raw sewage with the sludge is an important aspect in the adaptation of CAS systems to accommodate AGS. It remains unclear how the distribution of substrate over the sludge by a conventional anaerobic selector compares to the distribution via bottom-feeding applied in sequencing batch reactors (SBRs). This study investigated the effect of the anaerobic contact mode on the substrate (and storage) distribution by operating two lab-scale SBRs; one with the traditional bottom-feeding through a settled sludge bed similar to full-scale AGS systems, and one where the synthetic wastewater was fed as a pulse at the start of the anaerobic phase while the reactor was mixed through sparging of nitrogen gas (mimicking a plug-flow anaerobic selector in continuous flow-through systems). The distribution of the substrate over the sludge particle population was quantified via PHA analysis, combined with the obtained granule size distribution. Bottom-feeding was found to primarily direct substrate towards the large granular size classes (i.e. large volume and close to the bottom), while completely mixed pulse-feeding gives a more equal distribution of substrate over all granule sizes (i.e. surface area dependant). The anaerobic contact mode directly controls the substrate distribution over the different granule sizes, irrespective of the solids retention time of a granule as an entity. Preferential feeding of the larger granules will enhance and stabilise the granulation compared to pulse-feeding, certainly under less advantageous conditions imposed by real sewage.

Selection of indicator contaminants of emerging concern when reusing reclaimed water for irrigation — A proposed methodology

Organic and microbial contaminants of emerging concern (CECs), even though not yet regulated, are of great concern in reclaimed water reuse projects. Due to the large number of CECs and their different characteristics, it is useful to include only a limited number of them in monitoring programs. The selection of the most representative CECs is still a current and open question. This study presents a new methodology for this scope, in particular for the evaluation of the performance of a polishing treatment and the assessment of the risk for the environment and the irrigated crops. As to organic CECs, the methodology is based on four criteria (occurrence, persistence, bioaccumulation and toxicity) expressed in terms of surrogates (respectively, concentrations in the secondary effluent, removal achieved in conventional activated sludge systems, Log Kow and predicted-no-effect concentration). It consists of: (i) development of a dataset including the CECs found in the secondary effluent, together with the corresponding values of surrogates found in the literature or by in-field investigations; (ii) normalization step with the assignment of a score between 1 (low environmental impact) and 5 (high environmental impact) to the different criteria based on threshold values set according to the literature and experts' judgement; (iii) CEC ranking according to their final score obtained as the sum of the specific scores; and (iv) selection of the representative CECs for the different needs.Regarding microbial CECs, the selection is based on their occurrence and their highest detection frequency in the secondary effluent and in the receiving water, the antibiotic consumption patterns, and recommendations by national and international organisations.The methodology was applied within the ongoing reuse project SERPIC resulting in a list of 30 indicator CECs, including amoxicillin, bisphenol A, ciprofloxacin, diclofenac, erythromycin, ibuprofen, iopromide, perfluorooctane sulfonate (PFOS), sulfamethoxazole, tetracycline, Escherichia coli, faecal coliform, 16S rRNA, sul1, and sul2.

Machine learning reveals the complex ecological interplay of microbiome in a full-scale membrane bioreactor wastewater treatment plant

Membrane bioreactor (MBR) systems are one of the most widely used wastewater treatment processes for various municipal and industrial waste streams. The present study aimed to advance the understanding of ecologically important keystone taxa that play an important role in full-scale MBR systems. A machine-learning (ML) modeling framework based on microbiome data was developed to successfully predict, with an average accuracy of >91.6%, the operational characteristics of three representative full-scale wastewater systems: an MBR, a conventional activated sludge system, and a sequencing batch reactor. ML-based feature-importance analysis identified Ferruginibacter as a keystone organism in the MBR system. The phylogeny and known ecophysiology of members of Ferruginibacter supported their role in metabolizing complex organic polymers (e.g., extracellular polymeric substances) in MBR systems characterized by high concentrations of mixed liquor suspended solids and a high solid retention time. ML regression modeling also revealed temporal patterns of Ferruginibacter in response to water temperature. ML modeling was thus successfully employed in the present study to investigate complex/non-linear relationships between keystone taxa and environmental conditions that cannot be detected using conventional approaches. Overall, our microbiome-data-enabled ML modeling approach represents a methodological advance for identifying keystone taxa and their complex ecological interactions, which has implications for the sustainable and predictive management of MBR systems.

Microaerophilic activated sludge system for ammonia retention from high-strength nitrogenous wastewater: Biokinetics and mathematical modeling

It is crucial to shift from conventional ammonia removal to recovery to solve environmental problems and resource scarcity. Recovering ammonia from high-strength nitrogenous wastewater can simultaneously save the energy and cost involved in ammonia oxidation, produce ammonia without CO2 emissions, and prevent N2O emissions. A technology for concentrating and recovering ammonia by retrofitting a conventional activated sludge (CAS) system is promising but faces significant challenges in oxidizing organic carbon but not ammonia in wastewater. This study employed biokinetic analysis and mathematical modeling to comprehensively assess the operation and effectiveness of a microaerophilic activated sludge (MAS) system for simultaneously removing chemical oxygen demand (COD) and retaining ammonia. We show that heterotrophic bacteria (HB) in the MAS system had higher maximum specific growth rate (µHB), biomass yield coefficient (YHB), and decay coefficient (bHB) but lower affinities for oxygen and COD than HB in CAS. Simulations using the developed model demonstrated that COD removal efficiencies of > 90%, ammonia retention efficiencies of > 90%, and low N2O emission factors (N2OEF) of < 0.01% can be achieved at solids retention times (SRTs) of 3–50 d, hydraulic retention times (HRTs) of 0.1–1 d, and a volumetric oxygen transfer coefficient (KLa) of (24.58 ± 0.30)HRT−1. The combination of the biokinetics and mathematical modeling revealed the favorable MAS operating conditions allowing efficient ammonia retention and COD removal, which paves the way for turning a CAS system into a MAS system by retrofitting the configuration without high capital expenditures, requiring future intensive study.

Energy efficiency and nutrient removal performance: comparison between several types of activated sludge process

The operation of a wastewater treatment plant entails a huge amount of electricity. The majority of energy inputs are consumed by aeration systems to support biological processes for treated wastewater. Urban wastewater treatment plants are energy-intensive facilities that consume significant amounts of energy. For conventional activated sludge systems, 25% to 60% of the operating costs are associated with energy use. Malaysia’s wastewater treatment plants have fallen short in terms of technological advancement in the sewerage industry. The goal of this research is to analyse and make a comparison of the capabilities of the sequencing batch reactor (SBR) and other activated sludge (AS) system treatment plants in Selangor, Malaysia. High energy electricity consumption was an important issue that affected the operational cost and development of wastewater treatment plants (WWTP). The result and discussion for the analysis had been presented into comparison result for each objective of this research studied which was to determine lowest energy efficiency, to assess highest nutrient removal efficiency and compliance rate for each WWTP process plant. This paper presents best practices that can be implemented and adapted by operators in their pursuit of energy and reduce cost or expenses in the sewage treatment plant.

Evaluation of Ecotoxicity of Wastewater from the Full-Scale Treatment Plants

In this work, the influence of wastewater from full-scale wastewater treatment plants (WWTPs) on aquatic and soil biota was reviewed and presented. Moreover, the methods and model organisms used in testing the ecotoxicity of wastewater were shown. It was found that wastewater usually affected the biochemical activity and growth of organisms such as bacteria, algae and protozoa. They contributed to the immobilization and death of inter alia crustaceans and fishes. The values of degree of inhibition or lethality widely varied dependent on the type of wastewater, the sampling point (influent or effluent) and the model organisms applied in the biotests. Thus, a battery of ecotoxicity tests using model organisms of different sensitivities should be employed. So far, bacteria (e.g., Vibrio fischeri), green microalgae (e.g., Raphidocelis subcapitata) and crustaceans (Daphnia magna) have been frequently used organisms in the biological assessment of wastewater. They were applied in almost half (bacteria) or more than half (microalgae, crustaceans) of papers analyzed in this study. In almost all studies, the reduction of wastewater toxicity after treatment processes was found. It was proven that the conventional activated sludge systems were efficient in the removal of wastewater toxicity from both municipal and industrial wastewater, while the tertiary stage of treatment, in particular chlorination or ozonation, contributed to the increase in wastewater toxicity.

Oxygen transfer efficiency in an aerobic granular sludge reactor: Dynamics and influencing factors of alpha

In the pursuit of reducing carbon footprint and in view of increasing energy prices, energy efficiency is more important than ever before. Batch-wise operated aerobic granular sludge reactors consume up to 50% less energy compared to conventional activated sludge systems because pumping energy is reduced and mixing equipment is not needed. Further energy reduction efforts should therefore target aeration energy requirements. The alpha factor is an important factor influencing the oxygen transfer efficiency, however the dynamic behaviour of alpha has hardly been investigated in general and never for an aerobic granular sludge reactor. This study showed that alpha increases during the aeration phase of a cycle due to the influence of different process parameters. Through a data analysis study of 175 batch cycles of the Prototype Nereda® installation in Utrecht over the summer and winter period of 2020–2021, the exchange ratio and temperature were identified as the main influencing factors on the rate of increase of alpha in a batch cycle. A higher exchange ratio was related to a slower increase in alpha over the aeration phase, while a higher temperature was related to a faster increase in alpha. Moreover, alpha was characterized by a same minimal value at the beginning of every aeration phase, which could be explained by the adsorption of soluble biodegradable organic carbon described by a Langmuir adsorption model. Two mathematical models, a decreasing exponential and a first order model, were set up to unravel the dynamic behaviour of alpha. Both models were discussed in view of their practical implications for the design and performance optimization of aerobic granular sludge reactors and other batch-wise operated aerobic wastewater treatment systems.

Removal of organic micropollutants from municipal wastewater by aerobic granular sludge and conventional activated sludge

Removal performances of organic micropollutants by conventional activated sludge (CAS) and aerobic granular sludge (AGS) were investigated at a full-scale wastewater treatment plant. Lab-scale kinetic experiments were performed to assess the micropollutant transformation rates under oxic and anoxic conditions. Transformation rates were used to model the micropollutant removal in the full-scale processes. Metagenomic sequencing was used to compare the microbial communities and antimicrobial resistance genes of the CAS and AGS systems. Higher transformation ability was observed for CAS compared to AGS for most compounds, both at the full-scale plant and in the complementary batch experiments. Oxic conditions supported the transformation of several micropollutants with faster and/or comparable rates compared to anoxic conditions. The estimated transformation rates from batch experiments adequately predicted the removal for most micropollutants in the full-scale processes. While the compositions in microbial communities differed between AGS and CAS, the full-scale biological reactors shared similar resistome profiles. Even though granular biomass showed lower potential for micropollutant transformation, AGS systems had somewhat higher gene cluster diversity compared to CAS, which could be related to a higher functional diversity. Micropollutant exposure to biomass or mass transfer limitations, therefore played more important roles in the observed differences in OMP removal.