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

The study addresses two key challenges in urban water systems: dependence on distant, hazardous iron salt supplies with high costs and environmental/occupational risks, and the low-value utilization of biogas without CO2 removal. The authors hypothesize that an iron-oxidizing electrochemical process can simultaneously upgrade biogas by removing CO2 (and H2S, NH3) and produce an iron carbonate (FeCO3) that can replace conventional iron salts (FeCl2/FeCl3) for wastewater and sludge management. The context includes widespread use of iron salts for coagulation, sulfide and phosphate control, and sludge conditioning, coupled with supply chain vulnerabilities and environmental burdens of current production methods. The purpose is to develop and demonstrate a local, integrated, and environmentally favorable technology that links biogas upgrading with on-site iron salt production and application, yielding system-wide benefits within a circular economy framework.

The paper reviews the extensive use of iron salts (FeCl2, FeCl3, FeSO4, Fe2(SO4)3) in drinking water coagulation, sewer sulfide control, phosphate removal, sludge conditioning, and biogas desulfurization, leading to substantial consumption by the water industry. Current iron salts are typically by-products of metallurgical processes (e.g., steel pickling using HCl/H2SO4 or TiO2 production), sometimes converted to ferric salts via Cl2 or H2O2. Supply chains are long and vulnerable, with recent shortages forcing partial discharge of sewage in some regions. Biogas upgrading to biomethane requires CO2 removal; existing technologies (amine absorption, physical/chemical scrubbing) can be energy-intensive and generate wastes. The proposed electrochemical route differs fundamentally from conventional chemical production and aims to avoid regeneration/disposal issues while creating a local supply aligned with resource recovery trends.

Proof-of-concept experiments were conducted in a sealed 325 mL glass-bottle electrochemical cell with parallel iron plate electrodes (mild steel) in 200 mL NaCl electrolyte (2 g/L), operated at 22±1°C, stirred at 300 rpm. Preparatory phase: current applied without gas feed to generate Fe2+ (anode) and OH− (cathode), elevate pH to a set-point (7.5–9.0), and remove initial inorganic carbon via precipitation as FeCO3. Experimental phase: feed gas (typically ~60% N2 surrogate or CH4 and ~40% CO2, with some tests including ~884 ppmv H2S and ~268 ppmv NH3) continuously sparged via a 0.5 mm needle; current adjusted to maintain pH at set-point. Gas composition sampled periodically; iron species and particle sizes characterized; solids and liquids analyzed for Fe and other parameters. Effects of pH (7.5–9.0) and gas flow (0.6–3.0 L/(L·h)) on CO2 removal and stoichiometric ratios (R_Fe/e, R_CO2/Fe) were evaluated. The FeCO3 slurry produced (E-FeCO3) was tested for applications: - Sewer sulfide control: anaerobic wastewater (DO-free, ~25 mg S/L) dosed with E-FeCO3 at under- (~30 mg Fe/L) and overdosing (~90 mg Fe/L) levels; sulfide and pH monitored. - Activated sludge phosphate removal: mixed liquor (~20 mg P/L target), Fe dosed (~16 or ~70 mg Fe/L), DO controlled at 2–3 mg/L; phosphate and pH tracked. - Anaerobic digestion BMP tests: mixed sludge with sulfate addition (~25 mg S/L), E-FeCO3 dosed (~80 mg Fe/L), incubated at 37°C for ~30 days; biogas quantity/composition, sulfide, sulfate reduction, and dewaterability (SRF) measured. - Flow-on effects: sewage first dosed with E-FeCO3 for sulfide control, then fed to activated sludge for P removal, and subsequently to digestion to assess downstream sulfide control, methane production, settleability (SVI), and dewaterability (SRF). Suspension stability of E-FeCO3 particles under turbulent conditions was also tested, including storage stability (1–4 weeks). Comparative performance tests included C-FeCO3 (commercial), FeCl2, and FeCl3 across sewer, activated sludge, and anaerobic digestion units; pH impacts recorded. Life cycle assessment (LCA): ReCiPe 2016 Midpoint (H) via openLCA for a hypothetical 120 ML/d WWTP comparing Scenario A (E-FeCO3 route; A1 electricity from biogas, A2 grid electricity) vs Scenario B (status quo FeCl2 supply; B1/B2 with 1000/4000 km transport; biogas for CHP). Input-output economic analysis estimated product values and input costs. Statistical analysis employed Student’s t-tests (p<0.05 as significant).

- Electrochemical biogas upgrading and FeCO3 production: At pH 8.5, CO2 removal efficiency was ~85.1±0.4%, reducing upgraded gas CO2 to ~6.1% (example), with H2 evolution replacing removed CO2. H2S and NH3 were simultaneously reduced from 884.1±63.5 ppmv and 267.5±26.1 ppmv to 46.2±6.8 ppmv and 54.3±12.3 ppmv, respectively. Stoichiometries at pH 8.5: R_Fe/e = 0.52±0.01 mol/mol, indicating most electrons from Fe oxidation; R_CO2/Fe = 0.84±0.03 mol/mol, indicating most Fe2+ precipitated as FeCO3. - pH dependence: CO2 removal improved from pH 7.5 to 8.5 (78%→84%), but R_CO2/Fe dropped at pH 9.0 (likely Fe(OH)2 formation); optimal around pH 8.5. - Gas flow rate: Increasing from 0.6 to 3.0 L/(L·h) reduced CO2 removal efficiency (88%→70%), highlighting gas residence time limitations. - E-FeCO3 properties: Micron-scale particles with D10=6.9±0.6 µm, D50=20.1±2.3 µm, D90=46.7±3.2 µm; remained suspended under sewer-like turbulence; storage up to 4 weeks did not significantly change size distribution; XRD indicated siderite (dominant Fe2+ fraction 86.2±3.9%), with goethite and hematite present. - Application to wastewater/sludge: • Sewer sulfide control: Under-dosing yielded sulfide removal ratio 0.51±0.04 g S/g Fe; overdosing reduced dissolved sulfide to 0.08±0.02 mg S/L; slight pH increase (~+0.3). • Activated sludge phosphate removal: Under-dosing yielded 0.56±0.02 g P/g Fe; overdosing achieved ~1.62±0.09 mg P/L residual; pH higher than control. • Anaerobic digestion: E-FeCO3 reduced dissolved sulfide and H2S in biogas from 30.5±1.9 mg S/L and 1171.8±269.2 ppmv (control) to 1.8±0.4 mg S/L and 85.7±55.1 ppmv; methane production unaffected. • Flow-on effects: In-sewer dosing led to subsequent P removal in aeration (0.51±0.09 mg P/mg Fe) likely via FeS oxidation and iron regeneration; digesters receiving such sludge showed negligible sulfide despite sulfate reduction; sludge settleability and dewaterability improved by 36.9±2.7% and 39.1±4.5%. - Comparison with other iron salts: Commercial FeCO3 (C-FeCO3) was largely ineffective; E-FeCO3 matched FeCl2 and was similar to slightly below FeCl3 in performance: • Sewer sulfide removal efficiencies: 0.53±0.02 (E-FeCO3), 0.51±0.02 (FeCl2), 0.56±0.03 g S/g Fe (FeCl3); overdosing achieved <0.1 mg S/L. • Digester sulfide/H2S control: all three reduced dissolved sulfide to <2 mg S/L and gaseous H2S to <200 ppmv (~90% reduction); methane production and sulfate reduction not impacted. • Phosphate removal in aeration: 0.47±0.02 (E-FeCO3), 0.51±0.02 (FeCl2), 0.52±0.02 g P/g Fe (FeCl3). • Sludge conditioning: Settleability increased by 36.9±6.2% (E-FeCO3), 37.4±3.4% (FeCl2), 49.7±3.6% (FeCl3); dewaterability improved by 55.0±1.5%, 57.9±5.3%, 63.8±3.2%, respectively. • pH impacts (≈30–90 mg Fe/L doses): E-FeCO3 raised pH slightly (+0.15 to +0.31), whereas FeCl2 and FeCl3 lowered pH (down to −0.49 and −0.66), indicating E-FeCO3 adds alkalinity. - Mass balance: For sewage with bCOD 300 mg/L and ~7% conversion to methane, on-site production could supply ~12 mg Fe/L across the catchment, sufficient for sewer sulfide control, P removal, and sludge conditioning. - Economics: For a 120 ML/d WWTP, upgraded biogas ~1,641 m3/d and E-FeCO3 ~1,470 kg Fe/d yield outputs valued at ~A$2.1 million/year (replacing FeCl2 and gasoline), with inputs (biogas, electricity, NaCl, iron plates) costing ~A$0.64 million/year. - LCA: Scenario A1 (electricity from biogas) showed positive or negligible impacts across all categories and outperformed status quo Scenarios B1 (1000 km FeCl2 transport) and B2 (4000 km) in most indicators. Scenario A2 (grid electricity) still outperformed B1/B2 in 13/18 indicators; performance expected to improve with higher renewable penetration.

The findings validate the central hypothesis that an iron-oxidizing electrochemical cell can couple biogas upgrading with the local production of a reactive FeCO3 slurry suitable for multiple wastewater management tasks. By removing CO2 (and H2S, NH3) at controlled alkaline pH and precipitating FeCO3, the process upgrades biogas for higher-value uses (e.g., vehicle fuel or grid injection) while generating an iron reagent that effectively replaces FeCl2/FeCl3 for sulfide and phosphate removal and improves sludge settleability and dewaterability. The comparable performance of E-FeCO3 to conventional chloride-based salts, with the added benefit of alkalinity provision, addresses operational needs such as sulfide odor/corrosion control, P removal, nitrification support, and digester pH stability. System integration yields cascading benefits: in-sewer sulfide control, downstream P removal via regenerated iron, and reduced sulfide in digesters without compromising methane production. Economic and LCA results indicate the integrated approach can reduce costs and environmental burdens relative to current practices, especially when electricity is sourced from biogas. The work advances circular economy goals by creating a secure, local iron salt supply chain and valorizing biogas. Remaining engineering challenges include optimizing CO2 mass transfer (reactor design, gas retention, bubble size), scaling electrode systems, ensuring manageable maintenance (iron plate replacement), and validating performance under full-scale, variable field conditions.

This study demonstrates a practical, integrated urban water management strategy that links electrochemical biogas upgrading with on-site production and application of an FeCO3 slurry (E-FeCO3). The process achieves high CO2 removal (~85% at pH 8.5), co-removes H2S and NH3, and produces a reactive iron salt that effectively controls sulfide, removes phosphate, and improves sludge conditioning, with performance comparable to FeCl2 and approaching FeCl3, while adding alkalinity. Mass balance, economic, and LCA analyses suggest the approach can meet catchment iron salt demands, lower operating costs, and reduce environmental impacts versus conventional supply chains and biogas CHP use. Future work should focus on reactor engineering to enhance CO2 mass transfer, optimization of gas distribution and electrode configurations, assessment of long-term operation and maintenance (electrode replacement intervals), and full-scale demonstrations across diverse sewer and WWTP conditions. Additional exploration of operational niches (e.g., alkalinity-limited plants) and integration with nutrient recovery pathways (e.g., vivianite) could further enhance system-wide benefits.

- Proof-of-concept scale with a small glass-bottle reactor; performance depends on scale-up of gas–liquid mass transfer and reactor hydrodynamics. - CO2 mass transfer is rate-limiting; efficiencies vary with gas flow rate and reactor configuration. - Stoichiometric shifts at higher pH (e.g., 9.0) suggest competing Fe(OH)2 precipitation; requires careful pH control (~8.5). - Comparative advantages over FeCl3 in coagulation/flocculation may be smaller due to the oxidative and stronger flocculating capacity of Fe3+; E-FeCO3 cannot replace ferric dosing in certain applications (e.g., primary settling or tertiary P polishing). - Economic analysis is preliminary (input–output) and site-specific; LCA outcomes depend on electricity mix (A2 impacted by coal-heavy grid). - Maintenance requirements (iron plate replacement) and long-term slurry handling/storage need validation at scale.