Mitigation of diffuse CH4 and H2S emissions from the liquid phase of UASB-based sewage treatment plants: challenges, techniques, and perspectives

Direct Methane Removal from Air by Aerobic Methanotrophs

The rapid pace of climate change has created great urgency for short-term mitigation strategies. Appropriately, the long-term target for intervening in global warming is CO2, but experts suggest that methane should be a key short-term target. Methane has a warming impact 34 times greater than CO2 on a 100-year timescale, and 86 times greater on a 20-year timescale, and its short half-life in the atmosphere provides the opportunity for near-term positive climate impacts. One approach to removing methane is the use of bacteria for which methane is their sole carbon and energy source (methanotrophs). Such bacteria convert methane to CO2 and biomass, a potentially value-added product and co-benefit. If air above emissions sites with elevated methane is targeted, technology harnessing the aerobic methanotrophs has the potential to become economically viable and environmentally sound. This article discusses challenges and opportunities for using aerobic methanotrophs for methane removal from air, including the avoidance of increased N2O emissions.

Impact of hydrogen sulfide on anammox and nitrate/nitrite-dependent anaerobic methane oxidation coupled technologies

The coupling between anammox and nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) has been considered a sustainable technology for nitrogen removal from sidestream wastewater and can be implemented in both membrane biofilm reactor (MBfR) and granular bioreactor. However, the potential influence of the accompanying hydrogen sulfide (H2S) in the anaerobic digestion (AD)-related methane-containing mixture on anammox/n-DAMO remains unknown. To fill this gap, this work first constructed a model incorporating the C/N/S-related bioprocesses and evaluated/calibrated/validated the model using experimental data. The model was then used to explore the impact of H2S on the MBfR and granular bioreactor designed to perform anammox/n-DAMO at practical levels (i.e., 0∼5% (v/v) and 0∼40 g/S m3, respectively). The simulation results indicated that H2S in inflow gas did not significantly affect the total nitrogen (TN) removal of the MBfR under all operational conditions studied in this work, thus lifting the concern about applying AD-produced biogas to power up anammox/n-DAMO in the MBfR. However, the presence of H2S in the influent would either compromise the treatment performance of the granular bioreactor at a relatively high influent NH4+-N/NO2--N ratio (e.g., >1.0) or lead to increased energy demand associated with TN removal at a relatively low influent NH4+-N/NO2--N ratio (e.g., <0.7). Such a negative effect of the influent H2S could not be attenuated by regulating the hydraulic residence time and should therefore be avoided when applying the granular bioreactor to perform anammox/n-DAMO in practice.

Carbon emissions, wastewater treatment and aquatic ecosystems

As a nexus of environmental pollution, fossil fuel consumption and the global warming, carbon emissions are critical in China's long-term environmental strategies. In the water cycle, carbon is released during wastewater discharge, wastewater treatment, and subsequent changes in aquatic ecosystems. To gain a comprehensive understanding of this entire process, we investigate the intricate connections using balanced panel data from 261 prefecture-level cities in China spanning the period from 2000 to 2020. Each sample is quantified using 48 features derived from hydrosphere, biosphere, anthroposphere, atmosphere, pedosphere and lithosphere. This paper contributes to the relevant studies in the following ways: Firstly, to analyze the basic interaction within the water cycle, we utilize Structural Equation Modeling (SEM). Our results indicate a weak linear relationship between wastewater treatment and carbon emissions. We also substantiate the crucial role of the aquatic ecosystems in carbon fixation. Secondly, in order to comprehend the intricate interactions within the Earth system, we employ eight machine learning models to predict carbon emissions. We observe that extremely randomized trees (ET) exhibit the highest predictive accuracy among these models. Thirdly, in interpreting the ET model, we utilize Explainable artificial intelligence (XAI) techniques, including Shapley Additive Explanations (SHAP) and Accumulated Local Effects (ALE). Our 3D-SHAP analysis reveals heterogeneity in the emission effects of wastewater treatment across different sub-groups, indicating that emissions are especially sensitive to increased wastewater treatment in agricultural and tourism cities. Furthermore, 3D-SHAP analysis of the aquatic ecosystems exhibits a series of spikes, signifying that aquatic plants will abruptly lose their carbon storage ability once the degradation of the aquatic ecosystems exceeds a certain threshold. Finally, our ALE evaluation, depicting the dispersion tendency of feature importance, identifies the uncertainty of wastewater carbon release in agricultural and tourism cities, while also affirming the vulnerability of the aquatic ecosystems.

The dissolved methane recovery from treated sewage in upflow anaerobic sludge blanket (UASB) reactors: The energy demand, carbon footprint and financial cost

The goal of this research was to quantify the energy demand and carbon footprint over the life cycle, along with the financial cost, of sewage treatment with the recovery of dissolved methane (d-CH₄). The sewage treatment is composed of pre-treatment, followed by treatment in upflow anaerobic sludge blanket (UASB) reactors, trickling filter and secondary decanter, post-treatment with disinfection, and biogas recovery in the three-phase separator of the UASB reactor. The methods used in this study were attributional life cycle assessment and techno-economic analysis - LCA and TEA, respectively. The energy demand, carbon footprint and financial cost for 1 m³ sewage treatment in the evaluated scenario without d-CH₄ recovery (S1) were 3.4 MJ, 1.7 kg CO₂eq and 0.17 USD respectively, while those with d-CH₄ recovery (S2) varied by 12%, -16% and 2.3% compared to S1. The produced biogas for lower heating value in S2 (2.6 MJ) was 27% higher than that in S1 (2.0 MJ) and this varied from 1.3 MJ to 4.6 MJ in the scenarios for different influent chemical oxygen demand (COD) in the sewage treatment plant (STP) and COD removal efficiency in the UASB reactor. The highest eco-efficiency for 1 MJ heat production from the STP biogas was achieved in the scenario with d-CH₄ recovery, higher influent COD, higher COD removal efficiency in the UASB reactor, d-CH₄ saturation, photovoltaic electricity supply, and a higher energy efficiency in d-CH₄ recovery combined (S2,COD+,R+,S,PV,EE+), which reduced the energy demand by 55%, carbon footprint by 66% and financial cost by 63% compared to S1. Furthermore, the STP functionality change from a single-product (biogas) to a multi-product (biogas, water for reuse and biosolid fertilizer) approach (S1,WR, BF and S2,WR,BF) made the biogas a competitive product compared to those from fossil sources. Therefore, resource recovery from the sewage treatment in higher influent COD, higher COD removal efficiency, the use of a more efficient, clean and economical electricity source and higher energy efficiency in d-CH₄ recovery in a multi-product STP contribute to achieving the energy self-sufficiency over the life cycle while reducing the carbon footprint and financial cost of its products.

Fouling characterisation in PVDF membrane contactors for dissolved methane recovery from anaerobic effluents: effect of surface organofluorosilanisation

Characterisation of the fouling attached to PVDF membranes treating an anaerobic effluent for dissolved CH₄ recovery was carried out. A commercial flat-sheet PVDF membrane and a PVDF functionalised by grafting of organofluorosilanes (mPVDF) that increased its hydrophobicity were subjected to a continuous flux of an anaerobic reactor effluent in long-term operation tests (> 800 h). The fouling cakes were studied by the membrane autopsy after these tests, combining a staining technique, FTIR, and FESEM-EDX, and the fouling extraction with water and NaOH solutions. Both organic and inorganic fouling were observed, and the main foulants were proteins, polysaccharides, and different calcium and phosphate salts. Also, a significant amount of live cells was detected on the fouling cake (especially on the non-modified PVDF). Although the fouling cake composition was quite heterogeneous, a stratification was observed, with the inorganic fouling mainly in the bulk centre of the cake and the organic fouling mainly located in the lower and upper surfaces of the cake. The mPVDF suffered a more severe fouling, likely owing to a stronger hydrophobic-hydrophobic interaction with the foulants. Irreversible fouling remained on both membranes after the extraction, although a higher irreversible fouling was detected in the mPVDF; however, a complete polysaccharide removal was observed. Regarding the operation performance, PVDF showed a lower stability and suffered a severe degradation, resulting in a lower thickness and perforations. Finally, the decrease in the methane recovery performance of both membranes was associated with the fouling depositions.

Methane Emissions from Municipal Wastewater Collection and Treatment Systems

Municipal wastewater collection and treatment systems are critical infrastructures, and they are also identified as major sources of anthropogenic CH₄ emissions that contribute to climate change. The actual CH₄ emissions at the plant- or regional level vary greatly due to site-specific conditions as well as high seasonal and diurnal variations. Here, we conducted the first quantitative analysis of CH₄ emissions from different types of sewers and water resource recovery facilities (WRRFs). We examined variations in CH₄ emissions associated with methods applied in different monitoring campaigns, and identified main CH₄ sources and sinks to facilitate carbon emission reduction efforts in the wastewater sector. We found plant-wide CH₄ emissions vary by orders of magnitude, from 0.01 to 110 g CH₄/m³ with high emissions associated with plants equipped with anaerobic digestion or stabilization ponds. Rising mains show higher dissolved CH₄ concentrations than gravity sewers when transporting similar raw sewage under similar environmental conditions, but the latter dominates most collection systems around the world. Using the updated data sets, we estimated annual CH₄ emission from the U.S. centralized, municipal wastewater treatment to be approximately 10.9 ± 7.0 MMT CO₂-eq/year, which is about twice as the IPCC (2019) Tier 2 estimates (4.3-6.1 MMT CO₂-eq/year). Given CH₄ emission control will play a crucial role in achieving net zero carbon goals by the midcentury, more studies are needed to profile and mitigate CH₄ emissions from the wastewater sector.

Artificial neural network based models for predicting the effluent quality of a combined upflow anaerobic sludge blanket and facultative pond: Performance evaluation and comparison of different algorithms

The main objective of this work was to test different artificial neural network (ANN) based models, i.e. the ANN feed forward back propagation (ANN-FFBP), deep feed forward backpropagation (DFFBP), and deep cascade forward back propagation (DCFBP) models, for predicting the effluent quality of an upflow anaerobic sludge blanket-facultative pond (UASB-FP) system. The overall removal efficiency in the UASB-FP was >84% at organic loading rates of ∼26 kg d^-1. The chemical oxygen demand (COD), ammonical nitrogen (AN), total suspended solids (TSS), biochemical oxygen demand (BOD), total Kjeldahl nitrogen (TKN), and total phosphorus (TP) were inputs to each model, while the water quality characteristics of the UASB-FP effluent was used as the output. The dataset of 180 samples, collected over a one-year period, was utilized to train, test, and validate the developed models. Compared to ANN-FFBP and DFFBP, the DCFBP network demonstrated the strongest capacity for prediction. The correlation coefficient R_Train and the root-mean-squared error (RMSE) for the selected DCFBP model (3 hidden layers and 11 neurons/layer) in the training data set were 0.997 and 6.018, respectively. The sensitivity analysis of the DCFBP model shows that the model's performance is very sensitive to BOD followed by AN, COD, TP, TSS and TKN, respectively. The results of this study will be helpful to wastewater treatment (WWTP) plant managers in their pursuit of data-driven UASB-FP based WWTP management.

Characterisation and perspectives of energetic use of dissolved gas recovered from anaerobic effluent with membrane contactor

Biogas is a source of renewable energy, and its production and use has been validated in anaerobic-based sewage treatment plants (STPs). However, in these systems, a large amount of methane is lost as dissolved methane (D-CH₄) in the liquid effluent. In this study, the characteristics and potential energetic uses of the gas recovered during the desorption of D-CH₄ from anaerobic effluents with hollow fibre membrane contactors were investigated. A pilot-scale experiment was performed using sewage and two types of membrane contactors. The recovered gas contained considerable amounts of CH₄, CO₂, H₂S, N₂, and O₂; therefore, a gas upgrade is required prior to its use as a biofuel. The recovery process should be energetically self-sustainable, and induce a considerable decrease in the STP carbon footprint. Recovering D-CH₄ with membrane contactors could increase the energetic potential of anaerobic-based STPs up to 50 % and allow for more sustainable systems.

Recovery of methane dissolved in the effluent of a novel upflow anaerobic hybrid reactor (UAHB) submitted to temperature variation

Recent studies point out losses of 30-40% of the produced methane in the effluent of anaerobic reactors treating sewage, reducing the renewable energy potential and the environmental footprint. A novel bench-scale upflow anaerobic hybrid (UAHB) reactor combining a sludge blanket at the bottom and a filter media at the top, both with three-phase separators, was proposed to evaluate the recovery of dissolved methane. UAHB was operated with volumetric organic loading rate of 1.24 kg COD m^-3 d^-1 and hydraulic retention time of 8 h for 218 days to evaluate the influence of temperature (18°C, 23°C, and 28°C) in the methane dissolved in the effluent and collected from three-phase separators. Chemical oxygen demand (COD) and total suspended solids (TSS) removals efficiencies remained constant during the operation and equal to 90 and 95%, respectively, related to the activity of biomass retained in the filter media. Temperature increase influenced more the methane production in the sludge blanket rather than in the upper bed. The volume of recovered methane increased about 20% with the installation of the support media and the upper three-phase separator (3PHS). The loss of methane dissolved in the effluent was strongly influenced by the temperature, and higher with the decrease of this parameter. Non-statistically significant correlations were observed between the temperature and the methane production in the upper bed (p-value = 0.0943) and total (p-value = 0.0930). Thus, it can be concluded that the evaluated temperatures did not influence the global efficiency and the total methane yield of the UAHB reactor.

Simultaneous removal of dissolved sulphide and dissolved methane from anaerobic effluents with hollow fibre membrane contactors

Dissolved gases in the effluent of anaerobic reactors, specifically dissolved methane (D-CH₄) and sulphide (S^2-), are a drawback for anaerobic-based sewage treatment plants (STPs). This article studied the simultaneous desorption/removal of both gases from anaerobic effluents with hollow fibre membrane contactors (HFMCs), evaluating two types of membrane materials (e.g. microporous and dense) at different operating conditions (atmospheric air as sweeping gas or vacuum, and different gas/liquid flows and vacuum pressures). The transfer of other gases, such as O₂ and CO₂, was studied as well. Desorption/removal efficiencies up to 99% for D-CH₄ and 100% for S^2- were obtained, with the higher efficiencies reported for the dense HFMC and with air as sweeping gas. It was found that the removal mechanism for S^2- was oxidation with O₂ from the air. In addition, the use of air as sweeping gas allowed the obtention of a nearly O₂ saturated effluent, with more elevated dissolved oxygen concentrations in the microporous HFMC. Finally, it was found that the higher mass-transfer resistance in the dense membrane was compensated by a better performance in the liquid phase (lower mass-transfer resistance) in this unit, which allowed better D-CH₄ desorption efficiencies.

Performance evaluation of a novel upflow anaerobic sludge blanket reactor with built-in downflow hanging sponge module for the treatment of municipal sewage

The present study investigates the potential of a novel upflow anaerobic sludge blanket (UASB) reactor with a built-in polishing facility for the treatment of municipal sewage. A laboratory-scale reactor, which combined an inner-centric hybrid UASB module and an outer concentric downflow hanging sponge (DHS) unit, was designed and fabricated as a part of this research work. The reactor had been continuously operated for 210 days under ambient conditions. With anaerobically digested sewage sludge (seed sludge blended with Locust bean gum polymer) as inoculum, the reactor attained a successful startup in 40 days. Ensuring startup, the reactor was employed under varying organic loading rates (OLR) from 0.87 to 5.09 kg COD/m³/d, with corresponding hydraulic retention times (HRT) of 24-4 h. The novel UASB reactor made its mark with a high COD and TSS removal of 93.2% and 93%, respectively, under the optimum conditions of 6-h HRT. In addition, biogas rich in methane content (68%), with a maximum specific yield of 0.42± 0.02 m³ biogas /kg COD destroyed, was generated. The VSS/TSS ratio observed a steady rise (0.51 to 0.75) with increasing sludge age, bearing witness to the active growth of biomass. Hence, in brief, the novel UASB reactor can be coined as an energy economic system with improved effluent quality.

Performance evaluation of a full-scale upflow anaerobic sludge blanket reactor coupled with trickling filters for municipal wastewater treatment in a developing country

Poor wastewater management remains a critical health and environmental challenge in most developing countries in Sub-Saharan Africa due to the lack of adequate infrastructure for collection and treatment. This study evaluated the performance and methane production of a full-scale upflow anaerobic sludge blanket (UASB) reactor of capacity 18000 m3/d, with post-treatment unit: trickling filters followed by final settling tanks for municipal wastewater treatment in Ghana. Data was collected on operational conditions and physicochemical parameters of wastewater (influent and effluent) over a period of 35 weeks in 2021 (from January to August). The influent biochemical oxygen demand to chemical oxygen demand (BOD:COD) ratio was 0.58 ± 0.16, indicating the presence of highly biodegradable compounds in the sewage. Operational conditions for the UASB reactors were observed to be within the optimal range for anaerobic systems, with an applied organic loading rate of 1.30 ± 0.79 kgCOD/m3/d. Generally, Plant performance was satisfactory with carbon removal at 93% for COD and 98% for BOD. Biogas yield was 0.2 m3/kgCOD removed, culminating in an average biogas production rate of 831.6 ± 292.7 m3/d. Average methane composition was 64.7 ± 11.9% of the biogas output, whilst an estimated 35% of the methane generated remained dissolved in the UASB effluent. The UASB reactor presents an efficient technology that can be implemented in developing countries for effective and sustainable wastewater management.

Use of spray nozzles to recover dissolved methane from an Upflow Anaerobic Sludge Blanket (UASB) reactor effluent

Methane is a powerful greenhouse gas and a source of energy. Recovering this gas means lower greenhouse gas emission and potential reduction of energetic costs. The lack of full-scale results, the use of different methodologies to detect dissolved methane (d-CH₄) and the fact that no process to remove d-CH₄ from anaerobic effluents is energetically or economically viable at full-scale urged a different approach to the problem. To avoid methodological interference and facilitate comparison of results the Standard Test Method number D8028-17 published by ASTM International can be used to determine d-CH₄. The use of real anaerobic reactor effluent also helps results to be compared. In this study, 80 samples from a full-scale anaerobic reactor showed an average concentration of dissolved methane of 14.9 mg·L^-1, meaning an emission of 229 kg of CO₂ eq·h^-1 and an average of 113.5 kW wasted. Using spray nozzles, an alternative to the methods being researched, the average methane recovery was 11.5 mg·L^-1 of CH₄, an efficiency of 81.6%, meaning 177 kg of CO₂ eq·h^-1 emissions avoided and 87.9 kW of recoverable energy.

CFD modeling of different mass transfer coefficients on hydrogen sulfide emission in a flux chamber

Hydrogen sulfide (H₂S) is commonly used as an indicator for odorous gas emission monitoring in wastewater treatment plants. The H₂S emission estimations can be performed using algebraic mathematical models or carrying out measurements at the source, with the dynamic flux chamber, for example. This work brings together these two methodologies in a computational fluid dynamics analysis. Fifteen liquid-phase mass transfer coefficient ([Formula: see text]) models were initially evaluated in establishing, at the liquid-gas interface in a flux chamber, an H₂S emission flux based on the friction velocity field from three different inlet flows (2, 5, and 10 L min^-1). Ten [Formula: see text] models were fully simulated, and the numerical results were compared with available experimental data. The higher the inlet flow, the higher the friction velocity at the interface, and the higher the H₂S emission. The H₂S emission was also strongly dependent on the constant coefficients of the existing [Formula: see text] models. Small variability on those coefficients generates considerable changes in emissions at the interface. Few and different models performed well in describing the available concentration data at the outlet sampling probe for different inlet flows, which shows there is still no single model capable of representing all simulated friction velocity ranges (0.005 to 0.017 m s^-1).

The comprehensive measurement method of energy conservation and emission reduction in the whole process of urban sewage treatment based on carbon emission

It is of great significance to establish a carbon emission management system and carbon emission reduction target to put forward emission reduction measures for each subunit of a sewage treatment plant. In this paper, a mathematical model was constructed for calculating carbon emission in the whole sewage treatment system process. Meanwhile, the model calculated the carbon emission changes after upgrading three sewage treatment plants and identified the critical controlling unit. The results showed that the CO₂ produced from electric energy consumption and chemical application was the primary source of carbon emission of wastewater treatment. Raising sewage discharge standards appropriately could effectively reduce the carbon emission generated by each link of the wastewater treatment plant. Further improvement of effluent standards could adversely affect sewage treatment plants in terms of energy, resources, and greenhouse gas emissions. In addition, raising the standard of total phosphorus concentration in the effluent may lead to a corresponding increase in the amount of phosphorus removal agents, as well as an increase in indirect carbon emission, material consumption, and chemical sludge. Therefore, it is necessary to develop sewage treatment technologies that are economical, applicable, energy-saving, and environmental friendly to realize the environmental benefits of carbon emission reduction in sewage treatment and sustainable utilization of energy and resource from wastewater.