In situ electrocatalytic nitrate-to-nitrite conversion-driven anammox in MBRs for extremely efficient ammonium-containing wastewater treatment.

Nitrogen removal is crucial for global wastewater recycling and reuse. Anaerobic ammonia oxidation (anammox) is a revolutionary wastewater treatment technology, offering sustainable and cost-effective nitrogen removal solutions. However, the generation of 11 % nitrate (NO3-) as a byproduct remains a critical bottleneck, as it limits nitrogen-removal efficiency and requires external electron donors for further denitrification. To overcome this limitation, we propose an innovative integration of a green electrocatalytic reduction system, featuring a copper mesh-embedded membrane cathode, into the anammox biological process. This integration enables simultaneous selective NO3--to-NO2-(nitrite) conversion and NO2- recycling for anammox reactions. Over 190 days of operation in a submerged membrane bioreactor (MBR) treating synthetic wastewater (total nitrogen: 2200-2400 mg/L), the electrocatalytically improved anammox system achieved a maximum nitrogen-removal efficiency of 94 %, significantly surpassing the control MBR (86 %). This improvement was driven by considerably higher NO2- selectivity (88.6 %) over ammonium (5.9 %) during NO3- reduction at a low current density of 0.1 mA/cm2 in the anammox operational environment, which facilitated effective NO2- accumulation and reintroduction into anammox. Theoretically, the integration of anammox and NO3--to-NO2- conversion enables N2 production to reach 99.6 % of total nitrogen products, advancing anammox application in mainstream wastewater treatment, where achieving a stable NO2- shunt from partial nitrification remains challenging. Furthermore, the coupled system exhibited a significantly prolonged average fouling cycle of 31.4 days-twice that of the control (15.7 days)-attributed to synchronous electro-Fenton oxidation on the electrocatalytic membrane, while simultaneously reducing energy consumption by 21 %. The significantly mitigated membrane fouling enhanced the retention of valuable slow-growing anammox bacteria, ensuring sustained high-efficiency nitrogen removal. This work demonstrates an extremely efficient and environmentally friendly wastewater treatment paradigm through the synergy between anammox and electrocatalysis, supporting sustainable wastewater management and carbon neutrality goals.