Process Intensification at Raglan WWTP Using Integrated MABR and MBR for High Rate Nutrient Removal

[b]Objectives[/b] Raglan, New Zealand, is upgrading its municipal wastewater treatment plant (WWTP) to meet stringent nutrient discharge limits, accommodate population growth (from ~4,850 to ~7,000 PE by 2030 and onward), and enable land discharge preferred by the local community. The objective was to design and implement a compact, energy-efficient biological nutrient removal (BNR) train by integrating membrane-aerated biofilm reactors (MABR) upstream of a membrane bioreactor (MBR) within a four-stage activated sludge process (ASP), and to document design criteria, operational configuration, and early commissioning outcomes. [b]Background & Rationale[/b] Legacy oxidation ponds predominate across New Zealand but typically fall short of advanced nitrogen removal required by modern consents. To future-proof Raglan WWTP, the owner opted for a four-stage ASP with OxyMem™ OxyFAS MABR modules in the pre-anoxic zone (for simultaneous nitrification/denitrification, SND) and MBR polishing and solids retention downstream. The approach targets stringent nitrogen limits today while reserving headroom for load growth beyond 2055 and supports cultural preferences for land discharge in lieu of marine discharge. [b]Design & Methods[/b] The upgraded process operates in two parallel lanes, each comprising: (1) pre-anoxic MABR, (2) three-pass aerobic tank, (3) post-anoxic, and (4) MBR. Ten OxyMem modules are installed (5 per lane), delivering a total active membrane area of 14,520 m² (1,452 m² per module), with provisions for additional modules to 2055. Pre-treatment consists of 3-mm screening and grit removal. Process air is supplied to the dense PDMS hollow-fibre membranes at approximately 14 Nm³ h⁻¹ per module and ~320 mbar(g) inlet pressure; daily coarse-bubble scour at ~165 m³ h⁻¹ per module controls biofilm thickness. Module off-gas powers integral airlifts to generate ~20–30 m³ h⁻¹ vertical crossflow per module, enhancing biofilm–substrate contact while avoiding moving parts in the tank. The counter-diffusional biofilm establishes aerobic (nitrifying) inner layers and anoxic outer layers, enabling true SND within a compact volume. Plant hydraulics for the 2030 horizon: PDWF ≈ 3,500 m³ d⁻¹ (design), with additional wet-weather capacities; a separate design daily flow figure of 3,586 m³ d⁻¹ appears in project documentation and is consistent with ≈3.5 ML d⁻¹ sizing. Per-lane inventories are approximately pre-anoxic 216 m³, aerobic 512 m³, post-anoxic 132 m³, and MBR 136 m³ (≈ 996 m³ per lane). [b]Influent & Effluent Requirements[/b] Design influent (representative) includes COD ~750 mg L⁻¹, BOD₅ ~360 mg L⁻¹, TSS~380 mg L⁻¹, NH₄-N ~44 mg L⁻¹ at ~14 °C. The MBR is sized to achieve effluent TN ≤ 4 mg L⁻¹ and NH₄-N ≤ 1 mg L⁻¹ at the 2030 load, with MABR providing margin and resiliency toward 2055. At the module level, the design anticipates NH₄-N removal capacity ≈ 31 kg N d⁻¹ (average areal rate ~1.8 g N m⁻² d⁻¹). [b]Process Performance Considerations [/b] Dense PDMS membranes provide bubble-less oxygenation driven by concentration gradient, permitting low-pressure operation and high oxygen transfer efficiency (OTE) in shallow tanks (~25–30%), and a Standard Aeration Efficiency (SAE) on the order of 4–12 kg O₂ kWh⁻¹ (vs. ~1.4 kg O₂ kWh⁻¹ typical for conventional fine-bubble). This supports process intensification (decoupled SRT/HRT) and reduced blower power while maintaining robust nitrification. [b]Commissioning & Early Outcomes[/b] Ten MABR modules were installed in July 2025; all modules were operational by day 3, with biofilm acclimation in progress at the time of reporting. The integrated MABR–MBR BNR train is expected to meet current discharge consents and facilitate land discharge once biofilms mature. The anoxic zones include reserved positions for additional modules (future expansion), thus future-proofing capacity to 2055 while maintaining nitrogen polishing and resilience during seasonal peaks. Figure 1 (A) Anoxic tank and piping installed in preparation of MABR module installation, the background lane was included for future expansion (B) Anoxic tank with MABRs fully installed (walkways included over each module to allow for easier monitoring of MABRs) Figure 2 MBR installation of Memcor MBRs downstream of the MABR [b]Conclusion[/b] Raglan demonstrates a scalable retrofit pathway to advanced BNR for small-to-mid-size communities migrating from lagoon-based treatment. Key benefits include: (i) minimal footprint and civil works via drop-in modules; (ii) energy efficiency from low-pressure, bubble-less oxygenation; (iii) process stability via intrinsic SND in counter-diffusional biofilms; and (iv) modular expandability aligned with growth and decarbonization objectives. Such integrated MABR–MBR trains provide a pragmatic template for municipalities seeking stringent TN compliance and reduction of greenhouse gas intensity while maintaining operational simplicity.