Integrated urban water management by coupling iron salt production and application with biogas upgrading

Integrated urban water management (IUWM) is crucial for sustainable urban development. Efficient technological solutions are needed to optimize urban water systems holistically. Current methods for iron salt production, widely used in water treatment for coagulation, sulfide control, phosphate removal, and sludge dewatering, involve energy-intensive metallurgical processes and long-distance transport, leading to high costs and environmental impacts. Supply chain disruptions highlight the need for localized and sustainable alternatives. Simultaneously, there's a growing emphasis on resource recovery from wastewater, particularly bioenergy in the form of biogas. However, biogas upgrading, particularly CO2 removal, is often energy-intensive and generates waste. This research proposes a novel IUWM strategy that addresses both challenges: producing iron salts (FeCO3) electrochemically during biogas upgrading, utilizing the generated FeCO3 in wastewater treatment, thus creating a closed-loop system within the urban water cycle. This approach promises a more sustainable, cost-effective, and resilient water management system.

Iron salts (FeCl2, FeCl3, FeSO4, Fe2(SO4)3) are extensively used in various stages of urban water management. They are essential coagulants in drinking water treatment, combating hydrogen sulfide (H2S) corrosion in sewer networks (a significant cost and environmental problem), removing phosphate in wastewater treatment plants (WWTPs), and improving sludge dewaterability. The current supply relies on by-products from metallurgical processes (steel pickling or titanium dioxide production), involving hazardous chemicals (HCl, H2SO4, Cl2, H2O2) and long-distance transport, creating environmental and economic burdens. Biogas upgrading is widely implemented in WWTPs for bioenergy recovery, but the low value of raw biogas necessitates upgrading to higher-value products (transport fuel or injection into natural gas grids), requiring CO2 removal. Existing CO2 removal methods are often energy-inefficient and produce toxic waste. This paper proposes an integrated approach linking these traditionally separate processes.

The study employed an electrochemical cell with iron plates as electrodes and a NaCl electrolyte to remove CO2 from biogas. The process involved iron oxidation at the anode (Fe → Fe2+ + 2e−) and hydroxide ion (OH−) production at the cathode (2H2O + 2e− → 2OH− + H2). CO2 reacted with the generated OH−, forming carbonate (CO32−), which precipitated with Fe2+ to form FeCO3 (siderite). The experimental design included a preparatory phase (CO2 removal from the initial electrolyte) followed by an experimental phase (CO2 removal from the feed gas). The influence of pH (7.5, 8.0, 8.5, 9.0) and gas flow rate on CO2 removal efficiency and FeCO3 production was investigated. The characteristics of the electrochemically produced FeCO3 (E-FeCO3) slurry, including particle size and composition, were analyzed. The efficacy of E-FeCO3 in removing sulfide and phosphate from anaerobic sewage, aerated activated sludge, and anaerobic digesters was assessed through batch experiments. Comparative studies were conducted using commercially available FeCO3 (C-FeCO3), FeCl2, and FeCl3 to evaluate the performance of E-FeCO3. Flow-on effects of E-FeCO3 dosing to sewer networks on downstream wastewater and sludge treatment processes were also investigated. Finally, life cycle assessments (LCAs) compared the environmental impacts of the proposed E-FeCO3 production and biogas upgrading process with the conventional FeCl2 supply chain.

The electrochemical cell achieved high CO2 removal efficiency (up to 88%), simultaneously producing E-FeCO3. Optimal pH was found to be 8.5. E-FeCO3 effectively removed sulfide from anaerobic sewage (0.53 ± 0.02 g S/g Fe), anaerobic digesters (reducing H2S in biogas by nearly 90%), and phosphate from aerated activated sludge (0.47 ± 0.02 g P/g Fe). E-FeCO3 significantly improved sludge settleability (36.9 ± 6.2%) and dewaterability (55.0 ± 1.5%). The performance of E-FeCO3 was comparable to or slightly lower than FeCl3 but significantly better than C-FeCO3. E-FeCO3 dosing increased wastewater pH, unlike FeCl2 and FeCl3, offering additional alkalinity beneficial for nitrification and digester stability. LCA showed that the proposed integrated system (Scenario A1, using biogas for electricity) had positive or negligible environmental impacts across various categories, significantly outperforming the status quo (Scenario B). The input-output analysis suggested that the economic benefits of the integrated system (producing E-FeCO3 and upgraded biogas) would exceed the input costs.

The findings demonstrate the feasibility and advantages of integrating biogas upgrading and iron salt production for IUWM. The electrochemically produced FeCO3 proves a viable and sustainable alternative to traditional iron salts, offering comparable or superior performance in wastewater and sludge treatment while contributing to CO2 fixation and bioenergy recovery. The positive environmental impacts, as indicated by LCA, highlight the potential for significant reductions in greenhouse gas emissions and other environmental burdens compared to conventional practices. The increased alkalinity provided by E-FeCO3 offers additional benefits for various wastewater treatment processes. The economic analysis indicates potential cost savings, although further research is needed to validate it for full-scale implementation. The study showcases a promising approach to a more circular economy in urban water management.

This research successfully demonstrated an integrated urban water management strategy that couples iron salt production with biogas upgrading. The electrochemically produced FeCO3 effectively replaces conventional iron salts, offering comparable performance in wastewater treatment while enhancing biogas quality and reducing environmental impacts. The economic and life-cycle assessments suggest significant advantages over current practices. Future work should focus on full-scale implementation, further optimization of the electrochemical cell design, and broader evaluation of the process under diverse operating conditions.

The study was conducted using laboratory-scale experiments. Scaling up the process to full-scale WWTPs requires further engineering and optimization. The economic and LCA analyses were based on a hypothetical scenario and may not fully represent the variability inherent in real-world applications. While the flow-on effects were investigated in batch experiments, long-term operational performance in full-scale systems remains to be assessed. The study focused primarily on the performance of E-FeCO3 in wastewater treatment; its long-term effects on the overall ecosystem require additional studies.