Nitrous oxide (NO) is an important pollutant which is emitted during the biological nutrient removal (BNR) processes of wastewater treatment. Since it has a greenhouse effect which is 265 times higher than carbon dioxide, even relatively small amounts can result in a significant carbon footprint. Biological nitrogen (N) removal conventionally occurs with nitrification/denitrification, yet also through advanced processes such as nitritation/denitritation and completely autotrophic N-removal. The microbial pathways leading to the NO emission include hydroxylamine oxidation and nitrifier denitrification, both activated by ammonia oxidizing bacteria, and heterotrophic denitrification. In this work, a critical review of the existing literature on NO emissions during BNR is presented focusing on the most contributing parameters. Various factors increasing the NO emissions either per se or combined are identified: low dissolved oxygen, high nitrite accumulation, low chemical oxygen demand to nitrogen ratio, slow growth of denitrifying bacteria, uncontrolled pH and temperature. However, there is no common pattern in reporting the NO generation amongst the cited studies, a fact that complicates its evaluation. When simulating NO emissions, all microbial pathways along with the potential contribution of abiotic NO production during wastewater treatment at different dissolved oxygen/nitrite levels should be considered. The undeniable validation of the robustness of such models calls for reliable quantification techniques which simultaneously describe dissolved and gaseous NO dynamics. Thus, the choice of the N-removal process, the optimal selection of operational parameters and the establishment of validated dynamic models combining multiple NO pathways are essential for studying the emissions mitigation.

Direct nitrous oxide (N 2 O) emissions during the biological nitrogen removal (BNR) processes can significantly increase the carbon footprint of wastewater treatment plant (WWTP) operations. Recent onsite measurement of N 2 O emissions at WWTPs have been used as an alternative to the controversial theoretical methods for the N 2 O calculation. The full-scale N 2 O monitoring campaigns help to expand our knowledge on the N 2 O production pathways and the triggering operational conditions of processes. The accurate N 2 O monitoring could help to find better process control solutions to mitigate N 2 O emissions of wastewater treatment systems. However, quantifying the emissions and understanding the long-term behaviour of N 2 O fluxes in WWTPs remains challenging and costly. A review of the recent full-scale N 2 O monitoring campaigns is conducted. The analysis covers the quantification and mitigation of emissions for different process groups, focusing on techniques that have been applied for the identification of dominant N 2 O pathways and triggering operational conditions, techniques using operational data and N 2 O data to identify mitigation measures and mechanistic modelling. The analysis of various studies showed that there are still difficulties in the comparison of N 2 O emissions and the development of emission factor (EF) databases; the N 2 O fluxes reported in literature vary significantly even among groups of similar processes. The results indicated that the duration of the monitoring campaigns can impact the EF range. Most N 2 O monitoring campaigns lasting less than one month, have reported N 2 O EFs less than 0.3% of the N-load, whereas studies lasting over a year have a median EF equal to 1.7% of the N-load. The findings of the current study indicate that complex feature extraction and multivariate data mining methods can efficiently convert wastewater operational and N 2 O data into information, determine complex relationships within the available datasets and boost the long-term understanding of the N 2 O fluxes behaviour. The acquisition of reliable full-scale N 2 O monitoring data is significant for the calibration and validation of the mechanistic models of-describing the N 2 O emission generation in WWTPs. They can be combined with the multivariate tools to further enhance the interpretation of the complicated full-scale N 2 O emission patterns. Finally, a gap between the identification of effective N 2 O mitigation strategies and their actual implementation within the monitoring and control of WWTPs has been identified. This study concludes that there is a further need for i) long-term N 2 O monitoring studies, ii) development of data-driven methodological approaches for the analysis of WWTP operational and N 2 O data, and iii) better understanding of the trade-offs among N 2 O emissions, energy consumption and system performance to support the optimization of the WWTPs operation.

Significant growth of the human population is expected in the future. Hence, the pressure on the already scarce natural water resources is continuously increasing. This work is an overview of membrane and filtration methods for the removal of pollutants such as bacteria, viruses and heavy metals from surface water. Microfiltration/Ultrafiltration (MF/UF) can be highly effective in eliminating bacteria and/or act as pre-treatment before Nanofiltration/Reverse Osmosis (NF/RO) to reduce the possibility of fouling. However, MF/UF membranes are produced through relatively intensive procedures. Moreover, they can be modified with chemical additives to improve their performance. Therefore, MF/UF applicability in less developed countries can be limited. NF shows high removal capability of certain contaminants (e.g. pharmaceutically active compounds and ionic compounds). RO is necessary for desalination purposes in areas where sea water is used for drinking/sanitation. Nevertheless, NF/RO systems require pre-treatment of the influent, increased electrical supply and high level of technical expertise. Thus, they are often a highly costly addition for countries under development. Slow Sand Filtration (SSF) is a simple and easy-to-operate process for the retention of solids, microorganisms and heavy metals; land use is a limiting factor, though. Rapid Sand Filtration (RSF) is an alternative responding to the need for optimized land use. However, it requires prior and post treatment stages to prevent fouling. Especially after coating with metal-based additives, sand filtration can constitute an efficient and sustainable treatment option for developing countries. Granular activated carbon (GAC) adsorbs organic compounds that were not filtered in previous treatment stages. It can be used in conjunction with other methods (e.g. MF and SSF) to face pollution that results from potentially outdated water network (especially in less developed areas) and, hence, produce water of acceptable drinking quality. Future research can focus on the potential of GAC production from alternative sources (e.g. municipal waste). Given the high production/operation/maintenance cost of the NF/RO systems, more cost-effective but equally effective alternatives can be implemented: e.g. (electro)coagulation/flocculation followed by MF/UF, SSF before/after MF/UF, MF/UF before GAC.

Nitrous oxide (N 2 O), a significant contributor to the greenhouse effect, is generated during the biological nutrient removal in wastewater treatment plants (WWTPs). Developing mathematical models estimating the N 2 O dynamics under changing operational conditions (e.g. dissolved oxygen, DO) is essential to design mitigation strategies. Based on the activated sludge models (ASM) structure, this work presents an ASM2d-N 2 O model including all the biological N 2 O production pathways for a municipal WWTP under an anaerobic/anoxic/oxic (A 2 /O) configuration with biological removal of organic matter, nitrogen and phosphorus, and its application in different dynamic scenarios. Three microbial N 2 O production pathways were considered: nitrifier denitrification, hydroxylamine oxidation, and heterotrophic denitrification, with the first two being activated by ammonia oxidizing bacteria (AOB). A stripping effectivity (SE) coefficient was added to reflect the non-ideality of the stripping modeling. With the DO in the aerobic compartment ranging from 1.8 to 2.5 mg L −1 , partial nitrification and high N 2 O production via nitrifier denitrification were noted, indicating that low aeration strategies lead to a low overall carbon footprint only if complete nitrification is not hindered. High N 2 O emissions were predicted as a combination of low DO (∼1.1 mg L −1) with high ammonium concentration. With the AOB prevailing over the nitrite oxidizing bacteria (NOB), nitrite was accumulated, thus activating the nitrifier denitrification pathway. After suddenly increasing the influent ammonium load, the AOB had a greater growth compared to the NOB and the same pathway was considered as N 2 O hotspot. Especially under conditions promoting partial nitrification (i.e. low DO) and raising the stripping effect importance (i.e. high SEs), the highest N 2 O emission factors were predicted.

Industrial wastewater contains complex and slowly biodegradable compounds often ineffectively treated by conventional activated sludge (CAS) systems. Alternatively, advanced anaerobic technologies are implemented. The current study reviews different potential anaerobic schemes, factors influencing their final performance and optimum combinations of operational/design parameters. Anaerobic membrane bioreactors, upflow anaerobic sludge blanket reactors, expanded granular sludge beds, anaerobic hybrid reactors and inverse fluidized bed reactors are discussed. Their major advantages include: low energy requirements, energy recovery through biogas generation and high organic load removal. pH~7, operation in a mesophilic environment and a hydraulic retention time long enough to enable anaerobic digestion in economically accepted reactor volumes are conditions that optimize the performance of anaerobic configurations. The evaluation additionally considers environmental aspects. The life cycle assessment of anaerobic industrial wastewater treatment reveals its positive environmental effect in terms of greenhouse gases emissions. Methane (a greenhouse gas) primarily contained in the biogas, despite being produced during anaerobic digestion, is utilized for energy production (heating, electricity) instead of being emitted to the atmosphere. Finally, anaerobic wastewater treatment is analyzed as part of the European Commission Innovation Deal that aims at converting conventional wastewater treatment plants to water resource recovery facilities able to combine sustainable wastewater treatment and water reuse. Response to Reviewers: Reviewer #1: The topic and the results of the current study are generally very

In this study, factors influencing boron removal via the continuous electrocoagulation process were investigated at lab-scale. Different influent pH values (4, 5, 6, 7.45 and 9) and contact times (10, 25, 50 and 100 min) were examined as variable parameters. Plate-type aluminium electrodes with 5 mm distance between them were used. All the experiments were conducted in continuous mode and the current density was kept constant at 5 A throughout the whole experimental period. The initial boron concentration was selected to be 1000 mg L-1. The first set of experiments concerning the influence of the influent pH showed that the highest boron removal (67%) was obtained at pH=6 since it was the optimal pH for boron precipitation through aluminium borate formation. Under the constant current density of the study and with the initial pH adjusted to 6, increasing the duration of the electrocoagulation process from 10 to 100 min resulted in raising the boron removal from 45 to 79% during the second set of experiments. The greater duration of the electrocagulation process enabled higher aluminium dissolution, thus allowing the existence of a higher number of coagulants within the reactor. Moreover, it enhanced boron precipitation because of the longer contact time between the boron ions and the coagulants. After optimizing significant parameters such as the influent pH and the electrocagulation duration, the continuous electrocoagulation process was found to constitute an effective alternative for boron removal.

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In this study, the removal of selected heavy metals (nickel (Ni), lead (Pb), arsenic (As) and zinc (Zn)) by a lab-scale submerged membrane bioreactor (MBR) combined with a granular activated carbon (GAC) system was investigated.Different membrane fluxes (16, 20 and 24 L m -2 h -1 ) and hydraulic retention times (HRTs: 12.8 h, 10.4 h and 9.2 h) were applied as variable parameters to examine how they influence removal efficiency. Synthetically prepared wastewater was pre-treated in the MBR. Then, the efficiency of GAC adsorption applied as post-treatment was additionally examined in the current study. Chemical oxygen demand (COD), ammonium (NH 4 -N) and phosphates (PO 4 -P) were measured in two phases: first, after the MBR treatment step and, secondly, after the GAC posttreatment. The highest heavy metal removal efficiency after the 1 st treatment stage (i.e. MBR effluent) was obtained at the lowest flux value (i.e. 16 L m -2 h -1 ). Ni, Pb, Zn and As removals were measured as equal to 96.9, 98.3, 98 and 8.5%, respectively. Under the lowest applied flux value, COD, NH 4 -N and PO 4 -P removals were also found to be the highest (96.8, 98.9 and 46%, respectively) for the MBR effluent due to minimized heavy metal toxicity. More importantly, heavy metal concentration was under the limit of detection after the GAC post-treatment; over 99% removal was achieved for all heavy metals.