Analysing water-energy-GHG nexus in a wastewater treatment plant of Mumbai Metropolitan Region, India

Optimizing refuse-derived fuel production from scheduled wastes through Aspen plus simulation

This study aims to improve the quality of fuel with high calorific value namely Sfuel - a commercial high-quality refuse-derived fuel (RDF) from hazardous waste via modifying the process design and operating parameters of thermal conversion process. The study analyses key parameters of RDF quality, such as calorific value and heavy metal content, before and after process modifications based on the combination of experimental and simulation using Aspen Plus. In this study, the temperature and pressure of the simulation system are varied from 100 to 700 °C and from 1 to 5 bar, respectively. Findings indicate that there are a total of eleven heavy metals and 179 volatile compounds in the "Sfuels". The quality of the targeted product is greatly improved by the metal evaporation at high temperatures and pressures. However, the calorific value of RDF significantly decreases at 700 °C due to a large amount of the carbon content being evaporated. Although the carbon content at high temperatures is significantly lost, the heat from the vapour stream reactor outlet, which is reused to preheat the nitrogen gas stream supplied to the system, reduces energy consumption while improving the thermal conversion efficiency of the system. Besides, low pressure along with high temperature are not the optimal conditions for quality Sfuels improvement by thermal conversion. Results also indicate that electric heating is more economically efficient than natural gas heating.

Anaerobic Co-digestion of sewage sludge and organic fraction of municipal solid waste: Focus on mix ratio optimization and synergistic effects

The utilization of municipal solid waste (MSW) and sewage sludge (SS) as a source of renewable energy is crucial in achieving sustainable and integrated MSW management. SS and organic fraction of municipal solid waste (OFMSW) can be anaerobically digested to produce methane for energy. However, anaerobic digestion of specific substrates is challenging with respect to substrate characteristics. The problem of mono-digestion can be mitigated by co-digestion of these two major organic wastes because of their complementary characteristics. Moreover, there is a lack of studies on optimization of different mix ratios of organic fraction of municipal solid waste (OFMSW) and sewage sludge (SS) based on total solids (TS). The study aims to optimize the mix ratio for anaerobic co-digestion of OFMSW and SS. The study further elucidates synergistic effects associated with the co-digestion process. Different mix ratios of SS and OFMSW (0:100, 20:80, 40:60, 60:40, 80:20, 100:0) at 5, 7.5 and 10% TS were assessed for biomethane potential assessment. The results showed that with an increase in SS in the mix ratio feed the methane yield increased by 91% and 50% as compared to mono digestion of sewage sludge and OFMSW respectively at TS 7.5%. Based on the kinetic analysis, it was observed that the lag phase reduced for 60:40 mix ratio leading to higher rate of biodegradation. Positive synergistic effects were observed for 40:60, 60:40 and 80:20 mix ratio of SS:OFMSW based on co-digestion impact factor value. Response surface modelling was used to get the optimized mix ratio and TS %. The optimum mix ratio with the highest methane yield (388 ml/gVS added) was 70:30 (SS: OFMSW) at TS 7.5% with a desirability value of 0.98. These findings demonstrate that co-digesting SS and OFMSW is a preferable alternative for harnessing renewable energy and managing organic waste in a sustainable manner.

Greenhouse gas emission potential of sewage treatment plants in Himachal Pradesh

In recent times, waste management has emerged as a significant environmental challenge, and sewage is among the major contributors due to the rapidly increasing population. Despite sewage treatment plants (STPs) being the solution for the treatment of sewage, they have been identified as sources of greenhouse gas (GHG) emissions. This study aimed to estimate the contribution of STPs to GHG emissions in the state. This was achieved by visiting the sites, filling scientifically designed questionnaires, sample collection as well as computational methods by Intergovernmental Panel on Climate Change. The assessment of direct and indirect emissions from the STPs revealed that emissions were caused by the activated sludge process, electricity consumption, transportation, and sludge storage. Electricity consumption by STPs was responsible for the highest emissions, accounting for 43% of the total emissions, equivalent to 20,823 tCO₂ eq. The activated sludge process contributed 31% (14,934 tCO₂ eq) of the emissions, while storage of sludge in landfills accounted for 24% (11,359 tCO₂ eq). Additionally, transportation contributed 2% (1121 tCO₂ eq) of the emissions. In total, the STPs in Himachal Pradesh had the potential to contribute 48,237 tCO₂ eq GHG emissions annually. Thus, the study suggests process-level modifications in STPs of Himachal Pradesh to mitigate GHG emissions. This research provides insight into the GHG emissions from STPs and highlights the need for their management to reduce environmental impacts.

Carbon footprint analysis and comprehensive evaluation of municipal wastewater treatment plants under different typical upgrading and reconstruction modes

The issue of greenhouse gas (GHG) emissions resulting from the upgrading and reconstruction of municipal wastewater treatment plants (MWWTPs) along with improved water quality is receiving attention and research. There is an urgent need to explore the impact of upgrading and reconstruction on carbon footprint (CF) in order to address concerns that the upgrading and reconstruction will increase GHG emissions while improving water quality. Here we accounted for the CF of five MWWTPs in Zhejiang Province, China, before and after three different upgrading and reconstruction models - "Improving quality and efficiency" ("Mode I"), "Upgrading and renovation" ("Mode U") and "Improving quality and efficiency plus Upgrading and renovation" ("Mode I plus U"). The upgrading and reconstruction was found to not necessarily result in more GHG emissions. In contrast, the "Mode I" had a more significant advantage in terms of CF reduction (1.82-12.6 % reduction in CF). Overall, the ratio of indirect emissions to direct emissions (indirect emissions/direct emissions) and the amount of GHG emitted per unit of pollutant removed (CF_COD、CF_TN、CF_TP) decreased, while both the carbon and energy neutral rates increased significantly (up to 33.29 % and 79.36 % respectively) after all three upgrading and reconstruction modes. In addition, the wastewater treatment efficiency and capacity are the main factors that affect the level of carbon emission. The results of this study can provide a calculation model that can be used for other similar MWWTPs during the upgrading and reconstruction processes. More importantly, it can provide a new research perspective as well as valuable information to revisit the impact of upgrading and reconstruction in MWWTPs on GHG emissions.

Water-Energy-Carbon Nexus: Greenhouse Gas Emissions from Integrated Urban Drainage Systems in China

Greenhouse gas (GHG) emissions from integrated urban drainage systems (IUDSs), including sewer, wastewater treatment plants (WWTPs), and receiving water systems, have not yet been integrated due to the lack of modeling tools. Here, we updated the computable general equilibrium-based System Dynamics and Water Environmental Model (CGE-SyDWEM), a recently developed model simulating the water-energy-carbon nexus at the watershed level, to calculate the direct and indirect (electricity use and external) GHG emissions from IUDSs considering carbon mitigation strategies and water engineering practices. The updated CGE-SyDWEM was applied to an estuary watershed in Shenzhen, the fourth largest city in China. With increasing socio-economic development and water infrastructure systems upgrading, GHG emissions are projected to increase from 129.2 (95% CI: 95.9-162.5) kt in 2007 to 190.7 (144.8-236.6) kt in 2025, with 89% from WWTPs (direct: 17%; electricity use: 65%; and external: 7%), 10% from the sewer (direct: 1% and electricity use: 9%) and 1% from receiving waters (direct). Carbon mitigation can reduce GHG emissions by 7% and emission intensity by 6% by 2025, with 63% contributed by external emission reduction from chemical uses. The integrated model can aid water, energy, and carbon decision-makers in finding cost-effective solutions for water and energy security in the future.

Thermophilic biological fluidized bed reactor in sludge line reduces greenhouse gas emissions in wastewater treatment system

Greenhouse gas (GHG) emissions represent one of the main drawbacks of wastewater (WW) treatment. However, results of a detailed estimation of the emissions can be a valid tool to define optimal solutions for minimizing impact of WW treatment system on the environment. Thermophilic biological fluidized bed reactor (TBFBR) has been recently proposed as an alternative solution for biological sludge minimization in wastewater treatment plant (WWTPs). In this work, 5 diverse scenarios of sludge line composition were studied and combined with 5 diverse sludge disposal options. GHG emissions in 25 combinations were fully investigated to define optimal sludge treatment and disposal option. Results suggested that TBFBR help to reduce net emitted GHGs with respect to scenario with conventional stabilization treatment in sludge line (anaerobic digestion) (-32.3 ± 3.55 %) thanks to (i) the reuse in water line of the aqueous residue of TBFBR as alternative carbon source, (ii) the significant minimization of sludge production, and (iii) the contained impact of gross GHG emissions due to the energy consumption of this process. The strong minimization of sludge also led to a decisive reduction in GHG emissions in the subsequent phases of transport, additional treatments, and final disposal making the choice of the disposal option indifferent on the overall GHG emission estimation. Moreover, the coupling of processes for the simultaneous and preventive maximization of energy recovery (TCH, and AnaD) before sludge minimization in TBFBR determined a limited reduction of GHG emission compared to scenario with TBFBR alone (-3.71 ± 1.47 %).

Nutrient removal and recovery from wastewater by microbial fuel cell-based systems - A review

Microbial fuel cell (MFC) is a green innovative technology that can be employed for nutrient removal/recovery as well as for energy production from wastewater. This paper summarizes the recent advances in the use of MFCs for nutrient removal/recovery. Different configurations of MFCs used for nutrient removal are first described. Different types of nutrient removal/recovery mechanisms such as precipitation, biological uptake by microalgae, nitrification, denitrification and ammonia stripping occurring in MFCs are discussed. Recovery of nutrients as struvite or cattiite by precipitation, as microalgal biomass and as ammonium salts are common. This review shows that while higher nutrient removal/recovery is possible with MFCs and their modifications compared to other techniques as indicated by many laboratory studies, field-scale studies and optimization of operational parameters are needed to develop efficient MFCs for nutrient removal and recovery and electricity generation from different types of wastewaters.

Energy Consumption and Carbon Footprint of Greek Wastewater Treatment Plants

Wastewater treatment plants (WWTPs) exhibit significant energy consumption and produce large amounts of Greenhouse Gas emissions (GHG emissions). Energy efficiency and reduction in GHG emissions in WWTPs have become important issues, especially in view of the climate crisis. The core objective of this work is to assess the energy and carbon footprint of Greek WWTPs and to propose methods to improve energy efficiency and reduce GHG emissions. Data were collected from 31 Greek WWTPs with an average treatment capacity between 250 and 3,650,000 population equivalents (PE). The total population served by the 31 WWTPs was over 6,000,000, which is more than half of the population in Greece with access to WWTPs. Based on the results, the annual average energy consumption for small, medium and large WWTPs equals 137 kWh/PE, 48 kWh/PE and 32 kWh/PE, respectively. Accordingly, annual average GHG emissions, both biogenic and non-biogenic in small, medium and large WWTPs are equal to 207 kgCO2e/PE, 144 kgCO2e/PE and 89 kgCO2e/PE, respectively. Annual average on-site GHG emissions are equal to 56.5 kgCO2e/PE, while the average off-site GHG emissions account for 16.9 kgCO2e/PE. Based on the results, acceptable and attainable targets for WWTPs energy consumption and GHG emissions are proposed.