Ground food waste discharge to sewer enhances methane gas emission: A lab-scale investigation

Bio-corrosion in concrete sewer systems: Mechanisms and mitigation strategies

The deterioration of concrete sewer structures due to bio-corrosion presents critical and escalating challenges from structural, economic and environmental perspectives. Despite decades of research, this issue remains inadequately addressed, resulting in billions of dollars in maintenance costs and a shortened service life for sewer infrastructure worldwide. This challenge is exacerbated by the absence of standardized test methods and universally accepted mitigation strategies, leaving industries and stakeholders confronting an increasingly pressing problem. This paper aims to bridge this knowledge gap by providing a comprehensive review of the complex mechanisms of bio-corrosion, focusing on the formation and accumulation of hydrogen sulfide, its conversion into sulfuric acid and the subsequent deterioration of concrete materials. The paper also explores various factors affecting bio-corrosion rates, including environmental conditions, concrete properties and wastewater characteristics. The paper further highlights existing corrosion test strategies, such as chemical tests, in-situ tests and microbial simulations tests along with their general analytical parameters. The conversion of hydrogen sulfide into sulfuric acid is a primary cause of concrete decay and its progression is influenced by environmental conditions, inherent concrete characteristics, and the composition of wastewater. Through illustrative case studies, the paper assesses the practical implications and efficacy of prevailing mitigation techniques. Coating materials provide a protective barrier against corrosive agents among the discussed techniques, while optimised concrete mix designs enhance the inherent resistance and durability of the concrete matrix. Finally, this review also outlines the future prospects and challenges in bio-corrosion research with an aim to promote the creation of more resilient and cost-efficient materials for sewer systems.

Land footprint and GHG emissions from global food loss

BACKGROUND

Efficient land use represents a global challenge in the context of high levels of food loss (FL) and waste (FLW) and increasing greenhouse gas (GHG) emissions from global agricultural activities. This study aimed to estimate the land footprint (LF) associated with FL worldwide. It also estimated the GHG emissions from crop residues and their burning, and their relationship with food loss for the main crops worldwide. The study analyzed data from the Food and Agriculture Organization of the United Nations (FAO) regarding land use, FL, and global GHG emissions from crop residues.

RESULTS

The findings suggest that the average LF associated with FL worldwide is about 69 million ha per year, and the main food items responsible for most of the LF associated with FL are maize, wheat, and rice. The annual average emissions derived from burning crop residues of FL are 48.8 kilotons year^-1 of CH₄ and 1.26 kilotons year^-1 of N₂ O, and the emission of N₂ O derived from crop residues of FL is about 24.1 kilotons year^-1 , considering the three crops.

CONCLUSION

Food loss implies high levels of LF and GHG emissions, reinforcing the need for proper public and private initiatives worldwide to reduce FL and waste (FLW). Organizations such as the FAO and the Organisation for Economic Co-operation and Development (OECD) should incorporate indicators regarding FLW reduction to evaluate and monitor countries' performance. An international agreement also needs to be addressed to engage the world's nations in the reduction of FLW levels. © 2023 Society of Chemical Industry.

Reduced sulfide and methane in rising main sewer via calcium peroxide dosing: Insights from microbial physiological characteristics, metabolisms and community traits

Although the biocidal effect of calcium peroxide (CaO2) has attracted increasing attention in wastewater and sludge management, its potential in the reduction of sulfide and methane from sewer is not tapped. This study aims to fill this gap through the long-term operated sewer reactors. Results showed one-time dose of 0.2% (w/v) CaO2 with 12-h exposure decreased the average sulfide and methane production by 80% during one week. The electron paramagnetic resonance and free radical quenching tests indicated free radicals from CaO2 decomposing posed a major contribution on sewer biofilms (•OH>•O2->alkali). Mechanistic analysis revealed extracellular polymeric matrix breakdown (e.g., protein secondary structure) and cell membrane damage were caused by the increased lipid peroxidation of cells and exacerbated intracellular reactive oxygen species under CaO2 stress. Moreover, the intracellular metabolic pathways, such as electrons provision and transfer, as well as pivotal enzymatic activities (e.g., APS reductase, sulfite reductase and coenzymes F420) were significantly impaired. RT-qPCR analysis unveiled the absolute abundances of dsrA and mcrA were decreased by 7.53-40.37% and 67.00-74.85%, respectively. Although this study broadens the application scope of CaO2 and provides in-depth understanding of advanced oxidation-based technology in sewer management, the pipe scale risk due to the release of calcium ions warrants further investigation.

A probabilistic approach to the quantification of methane generation in sewer networks

Quantifying greenhouse gas (GHG) emissions from the conveyance of wastewater is an essential part of emissions reduction as it can identify areas of high emissions that can be targeted for mitigative action. In this study, a Monte Carlo algorithm that employs a reach-based methane generation sub-model was developed and applied to a full-scale municipal sewer system in Ontario, Canada. The algorithm employed eight categories of random variables including sewage temperature, slope, and coefficients described in the sewer reach model. Using best estimates for the employed distributions and algorithm design choices, it was estimated that 2.1-3.0 g CH₄/m³ (of total wastewater conveyed) is generated in the sewer system. Gravity reaches contributed 1.3-2.2 g CH₄/m³, and force main reaches contributed 0.6-0.9 g CH₄/m³, or 30% of total sewer-generated methane despite contributing only 4.4% of total network length. The results suggest that addressing force main methane generation (such as employing chemical addition) could reduce a large fraction of sewer-generated methane while only requiring action on a small fraction of sewer reaches which is consistent with literature. Extending the results from this study to all sewage generated in Canada indicates that anthropogenic emissions from the wastewater sector are increased by 28-40% if sewer-generated methane is included in the assessment. After testing alternative distributions and model designs, it was determined that replacing the fullness and temperature distributions with constant (no distribution) average conditions yielded identical results to that of the base case assessment, suggesting that these random variables can be excluded from future modelling exercises. It was also observed that treating model coefficients as random variables resulted in a significant increase in the standard deviation of estimates, indicating that much of the uncertainty in the results is due to the uncertainty associated with the model coefficients. The results were sensitive to the temperature correction coefficient in the methane generation model and the Manning's n used in flow calculations; indicating that dedicating resources to accurately characterize these values will increase model accuracy.

Effects of the aeration mode on nitrogen removal in a compact constructed rapid infiltration system for advanced wastewater treatment

The configuration and the effective operation of constructed rapid infiltration (CRI) systems are of significance for advanced wastewater treatment. In this study, a novel CRI system was developed with a compact structure consisting of two stages, i.e., oxic and anoxic stages. The CRI system was continuously operated for about 140 days under different aeration modes, i.e., tidal flow, continuous aeration, and intermittent aeration. Nitrogen removal was not desirable with tidal flow due to the insufficient oxygen supply in the oxic stage for nitrification, while continuous aeration could achieve good performance for chemical oxygen demand (COD), ammonium, total nitrogen (TN), and total phosphorus (TP) removal. By comparison, the CRI system operated with intermittent aeration was more favorable due to the effective removal ability for pollutants and relatively lower energy demand. The microbial community analysis revealed that Proteobacteria was the dominant phylum in both oxic and anoxic stages of the developed CRI system. Functional microbial groups (Plasticicumulans, Pseudomonas, and Nitrospira in the oxic stage; Thauera, Candidatus_Competibacter, and Dechloromonas in the anoxic stage) were identified for the mediation of carbon, nitrogen, and phosphorus in the system. This study evaluated the feasibility and the optimal aeration mode of the developed CRI system for advanced wastewater treatment, which could satisfy the requirement for the high standard of effluent quality.

Evaluating the oxidation inhibition of sulfide in urban sewers using a novel quantitative method

Sulfide inhibition is a critical task for the secure operation of sewer systems, and oxidation is usually utilised to achieve this purpose. However, the effects and mechanism of oxidation during the transformation of sulfur-associated pollutants in gas-liquid-solid phases of sewers have not been extensively evaluated. In this study, a method for quantifying sulfur-associated pollutant exchange pathways in gas-liquid-solid phases of sewers was established. The results showed that although the concentration of sulfide decreased under different oxidation conditions, the consumption of sulfate in sewers clearly increased. The transformation strength of elemental sulfur was high (18.65 mg/L, 35.52% of sulfate from the influent) and the accumulation of sulfate in sediment was obvious (3.49 mg/L, 6.65% of sulfate from the influent). Higher concentrations of sulfate in the influent promoted the generation of sulfide in sediment (8.98 mg/L, 17.10%). Thus, the oxidation process led to the generation of more absolute sulfide. In addition, promoting the metabolism of sulfate-reducing bacteria enhanced the loss of organic carbon in sewers, which might weaken the efficacy of nitrogen and phosphorus removal in wastewater treatment plants. Based on the evaluation of the exchange pathways of sulfur-associated pollutants in sewers, further studies into sulfide inhibition in sewers should consider the above issues to enhance the safe management of urban sewers.

Effect of dissolved oxygen on N2O release in the sewer system during controlling hydrogen sulfide by nitrate dosing

Nitrate dosing is commonly used for controlling hydrogen sulfide in sewer systems. However, it may potentially facilitate N₂O emission due to the denitrification process promoted by nitrate addition. In this study, lab-scale sewer reactors were operated to investigate the impact of nitrate addition on N₂O production in sewer systems. Results showed that the N₂O flux even increased by six times with the addition of nitrate when dissolved oxygen (DO) in the wastewater exceeded 0.4 mg/L. Principal component analysis showed that the N₂O concentration was notably affected by DO and oxidation-reduction potential (ORP) in the wastewater. Furthermore, it was founded that N₂O flux had a strong linear relationship with the DO concentration in the batch test. The microbial analysis found that the nosZ possessing organisms decreased significantly in the micro-aerobic condition and the copy numbers of nosZ gene declined consequently. It indicated that the inhibition of N₂O reduced to N₂ was responsible for significant accumulation and emission of N₂O in the micro-aerobic condition. Given the gravity sewers are not completely anaerobic, the DO concentration is ranged from 0.1 to 2.4 mg/L in gravity sewers with the partially filled flow. Therefore, more attention should be paid to the N₂O production when nitrate dosing for hydrogen sulfide controlling in gravity sewers.

"Food waste-wastewater-energy/resource" nexus: Integrating food waste management with wastewater treatment towards urban sustainability

Sustainable food waste management is a global issue with high priority for improving food security and conserving natural resources and ecosystems. Diverting food waste from the solid waste stream to the wastewater stream is a promising way for food waste source separation, collection, treatment, and disposal. Given the advances in wastewater treatment, this integrated system has great potential for the concurrent recovery of water, resource, and energy. To this end, many efforts from lab-scale to full-scale studies have been devoted to evaluating the feasibility and associated impacts on both solid waste and wastewater systems. This paper summarizes the current status of food waste diversion from the aspects of principle and application. The impacts of food waste diversion on solid waste treatment, sewer system, wastewater treatment, and environmental benefits have been comprehensively reviewed and analysed. In the context of the critical review, this paper further identified the challenges of food waste diversion in unified definitions of the field, sewer network assessment, emerging wastewater treatment technologies, scale-up studies, and policy drivers. Perspectives on the contribution of food waste diversion to a food waste management hierarchy were discussed for initiating the nexus of "food waste-wastewater-energy/resource". We conclude that food waste diversion could facilitate sustainable urban development, but the area-specific factors (e.g., household practices, water resource, sewerage system condition, and treatment techniques) require adequate evaluations to determine the implementation. The outcomes of this study could contribute to the practice and policy-making of food waste management towards urban sustainability.

Combination of nitrate and sodium nitroprusside dosing for sulfide control with low carbon source loss in sewer biofilm reactors

Nitrate has been widely used in sewer systems for sulfide control. However, significant chemical consumption and the loss of carbon source were observed in previous studies. To find a feasible and cost-effective control strategy of the sulfide control, the effect of nitrate combined with sodium nitroprusside (SNP) dosage strategy was tested in lab-scale sewer biofilm reactors. Results showed that nitrate and SNP were strongly synergistic, with 30 mg N/L nitrate and 20 mg/L SNP being sufficient for sulfide control in this study. While large amount of nitrate alone (100 mg N/L) is required to achieve the same sulfide control effectiveness. Meanwhile, the nitrate combined with SNP could reduce the organic carbon source loss by 80%. Additionally, the high-throughput sequencing results showed that the relative abundance of autotrophic, nitrate reducing-sulfide oxidizing bacteria genera (a-NR-SOB) such as Arcobacter and Sulfurimonas was increased by around 18%, while the heterotrophic, nitrate-reducing bacteria (hNRB) such as Thauera was substantially reduced. It demonstrated that the sulfide control was mainly due to the a-NR-SOB activity under the nitrate and SNP dosing strategy. The microbial functional prediction further revealed that nitrate and SNP promoted the dissimilatory nitrate reduction process which utilizes sulfide as an effective electron donor. Moreover, economic assessment indicated that using the combination of nitrate and SNP for sulfide control in sewers would lower the chemical costs by approximately 35% compared with only nitrate addition.

Integrating sulfur, iron(II), and fixed organic carbon for mixotrophic denitrification in a composite filter bed reactor for decentralized wastewater treatment: Performance and microbial community

Decentralized wastewater treatment in rural areas is an imperative challenge around the world, particularly in developing countries. The composite filter bed reactor is viable for decentralized wastewater treatment, but its performance on nitrogen removal often fluctuates with the unstable influent characteristics and loadings. Here, a composite filter bed reactor integrating sulfur, iron(II), and fixed organic carbon (shaddock peel) was developed and continuously operated under different conditions. The fixed organic carbon source promoted nitrogen removal with an efficiency higher than 90% and reduced effluent sulfate level by 40%, indicating that the integrated electron donors could improve the resistance and stability of the reactor. Moreover, sulfur-oxidizing bacteria (Thiomonas, Sulfuriferula, and Acidithiobacillus), iron-oxidizing bacteria (Ferritrophicum), and denitrifiers (Simplicispira and Hydrogenophaga) were identified in the anoxic/anaerobic layer of the reactor, suggesting that mixotrophic denitrification was stimulated by sulfur, iron(II), and fixed organic carbon. The findings of this study indicate that the developed reactor with the integrated electron donors could be reliable for carbon, nitrogen, and phosphorus removal and promising for the application of decentralized wastewater treatment.

Diversion of food waste into the sulfate-laden sewer: Interaction and electron flow of sulfidogenesis and methanogenesis

Diverting food waste (FW) into the sulfate-laden sewer may pose a significant influence on the production of methane and sulfide in sewers. Identifying microbial electron utilization is essential to understanding the interaction of sulfidogenesis and methanogenesis in depth. Here, we reported sulfide and methane production from the sewer bioreactors receiving sulfate-laden wastewater (160 mg S/L), with and without FW addition. Long-term monitoring showed that the addition of FW (1 g/L) could boost both sulfide (by 39%) and methane (by 44%) production. As for the electrons used for sulfidogenesis and methanogenesis, about 98% flowed to sulfidogenesis. Cryosection-fluorescence in situ hybridization showed that high sulfate content suppressed the accumulation of methanogens in biofilm outer layer, whereas methanogens in the inner layer were enriched with FW addition. Moreover, the FW addition fostered the diversity of the fermentative bacteria and changed the type of methanogens in biofilms, and up-regulated the key enzymes expressions for sulfidogenesis and methanogenesis. A model-based investigation suggests that increased FW-to-sewage ratios would exert a significant impact on methane production than on sulfide production. The microbial electron flows were highly dependent on sulfate concentration and FW-to-sewage ratios. The findings of this study suggest that sulfate and substrate levels play a key role in microbial electron utilization for sulfide and methane production, and diverting FW into the sulfate-laden sewer may exert negative impacts on sewer management and the environment.

The role of pH on sewer corrosion processes and control methods: A review

The production and emission of hydrogen sulfide (H₂S) in sewer systems is associated with the corrosion of sewer structures and harmful odour. Numerous studies have been conducted to find the best solution to overcome this issue. The pH plays a critical role not only on microbial and chemical processes that are responsible for all processes of corrosion but also on the efficiency of several control methods. This paper first critically reviews the literature on the interplay between pH and various chemical and microbial in-sewer processes, followed by a review of the control methods that depend on pH or indirectly alter pH. The paper argues that proper evaluation of each method should include the impact the control method has on downstream processes. This paper concludes the raising of pH has several benefits but is operationally difficult to implement. It also emphasises single control method may not be as efficient as combination of one or two methods in controlling the production and emission of H₂S. Finally, the research requirements and future directions in relation to emerging and potential methods that are not heavily reliant on pH control are discussed.

Biofilm development in a pilot-scale gravity sewer: Physical characteristics, microstructure, and microbial communities

The existence of abundant biofilms on sewer pipeline walls can lead to negative environmental impacts, such as poisonous gas release and pipe corrosions through transforming various pollutants. Investigating the formation process of sewer biofilms is of importance in advancing knowledge of sewer operation and maintenance. In this study, the changes in physical characteristics, microstructure, and microbial communities of sewer biofilm were analyzed in-depth in a pilot-scale gravity sewer during a 45-day operation. The results show that a high specific surface area at the early stage could channel the substrates for stimulating the primary colonizers (e.g., Cytophagia, Sphingobacteriia, Alpha-, and Betaproteobacteria), which could excrete an extracellular matrix to facilitate biofilm growth. The sewer biofilms were gradually formed with 62 g VS/m² organic content, 1.2 mm biofilm thickness, and 89 mg/cm³ dry density after 45 days operation. Moreover, the biofilm growth promoted the emergence of facultative bacteria and anaerobes (affiliated with Flavobacteriia, Gemmatimonadetes, Deltaproteobacteria, and Epsilonproteobacteria). Microelectrode analysis further verified that an anaerobic zone existed in mature biofilm with a negative oxidation-reduction potential (-105 mV), where approximately 0.1 μmol/L of sulfide was produced. Our results suggest that the migration of the microbial community correlated with the changes in the evolved physical characteristics and microstructure of sewer biofilm, and this can contribute to the strategies for sulfide control for improving sewer maintenance.

Integrated food waste management with wastewater treatment in Hong Kong: Transformation, energy balance and economic analysis

Diversion of food waste (FW) away from the solid waste stream into the wastewater stream is proved viable through the use of food waste disposers (FWDs). However, this may cause unwanted influences on the wastewater treatment system. In this context, this study has comprehensively evaluated integrated food waste and wastewater management on a city scale for the first time. A plant-wide COD-based transformation model was first established to assess the impacts of the use of FWDs on the networks of biological wastewater treatment plants (WWTPs) in Hong Kong. The biological WWTPs can remove about 78% of solids and 58% of chemical oxygen demand (COD) in FW. Moreover, the diversion of FW poses limited impacts on treatment capacity and effluent quality in WWTPs with the FWDs penetration rate up to 30%. The increases in energy consumption and operational cost are highly dependent on the treatment processes and the FWDs penetration rates, while municipal solid waste treatment can benefit from the diversion of FW. This study suggests that upgrading treatment processes (e.g., with less aeration) and optimizing the operation of WWTPs (e.g., reduce sludge retention time) may be required with the use of FWDs to achieve an energy-efficient and cost-effective goal. More importantly, this study not only provides a methodology for effectively evaluating the impacts of diverting FW into wastewater treatment in Hong Kong but also facilitates FW management in similar metropolises.

A novel approach for rapidly measuring volatile fatty acids in anaerobic process

Volatile fatty acids (VFAs), the intermediate of the anaerobic process, are considered to be the critical, high-sensitive and reliable indicators of the process stability. Close monitoring and control of VFAs are paramount for the efficient operation of the anaerobic reactors. In this study, a buffer intensity-based mathematical model was developed, and the least square method was integrated into the model to solve the issue of non-linear fitting of the titration curve. An automatic analyzer embedded with the developed model was designed and implemented for measuring VFAs and alkalinity. Through model optimization, the pH range of 3.5-5.6 was found to be suitable for VFAs analysis. The developed approach was validated by different VFAs (up to 500 mg/L as acetic acid) and carbonate alkalinity concentrations (up to 1500 mg/L as CaCO₃) with high recovery rates (>0.9). Optimal ratios of carbonate alkalinity to VFAs are identified in the range of 2.4-7.5 for accuracy. Owing to the non-linear fitting of the titration curve, the impact of other weak acid subsystems (e.g., phosphate, ammonium and sulfide) can be negligible. The one-year real-time monitoring of environmental samples by using the automatic analyzer indicates a high consistency and stability compared with the 5 pH point titration. This approach proves to be rapid (<3 min/sample), accurate, reliable and can be applied for real-time automatic monitoring of the anaerobic process.

Test of transformation mechanism of food waste and its impacts on sulfide and methane production in the sewer system

Food waste (FW) management has become an important issue worldwide. Diverting FW into the sewer system is considered promising to tackle the FW issue. However, the transformation of FW in sewers and its impact on the sewer process have not received adequate attention due to the overlooked sewer networks. In this study, a laboratory-scale sewer reactor system was established to investigate the transformation of FW and the production of sulfide and methane under anaerobic conditions. The transformation of FW in the sewer reactor could result in an increase in the substrate level through hydrolyzing and converting biodegradable substances into preferred substrates. Moreover, the generated substrates from the addition of FW were preferable for the metabolism of key microbes in sewer biofilms. As a result, methane production from the sewer reactor could be enhanced from the addition of FW, whereas sulfide production was not affected at a low sulfate concentration. The findings of this study suggest that the diversion of FW may exert an adverse impact on sewers and the environment in terms of greenhouse gas emission. Hence, more research is necessary to clarify the detailed impacts on FW management and wastewater treatment.