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We developed a lab-scale aerobic–methane oxidation bioreactor (MOB)–anoxic system, combining a MOB and the aerobic–anoxic denitrification process, and evaluated its potential for advanced nitrogen treatment in wastewater treatment plants (WWTPs). The MOB used biogas generated from a WWTP and secondary-treated wastewater to support mixed methanotroph cultures, which mediated the simultaneous direct denitrification by methanotrophs and methanol production necessary for denitrifying bacteria in the anoxic chamber for denitrification. Compared to the aerobic–anoxic process, the aerobic–MOB–anoxic system with an influent concentration of 4.8 L·day−1 showed a marked increase in the reduction efficiency for total nitrogen (41.9% vs. 85.9%) and PO4−3-P (41.1% vs. 69.5%). However, the integrated actions of high nitrogen and phosphorus consumption are required for methanotroph growth, as well as the production and supply of methanol as a carbon source for denitrification and methane monooxygenase-mediated oxidation of NH3 into N2O by methanotrophs. After three months of continuous operation using actual wastewater, the total nitrogen removal rate was 76.3%, equivalent to the rate observed in a tertiary-advanced WWTP, while the total phosphorus removal rate reached 83.7%.

Biochars prepared from macro-algae have a lower C/N ratio compared to lignocellulosic biochar, which is advantageous for direct nutrition. In particular, Sargassum, a marine macro-algae, has a high Mg content; hence, it can be expected to adsorb P and N simultaneously. In this study, Sargassum horneri biochar (SB), pyrolyzed at 400, 500, and 600 °C, was doped with innate Mg through water leaching, and nutrient recovery from the wastewater-mimicking solution was confirmed. The biochar pyrolyzed at 600 °C showed maximum Mg adsorption during water leaching, and the efficiency of K and Na removal was also high, at 92.7% and 91.9%, respectively. The addition of MgCl2 during pyrolysis and high ion exchange did not show distinct advantages for surface modification and nutrient adsorption. X-ray photoelectron spectroscopy analysis confirmed the participation of biochar in the surface adsorption of Mg and PO4 recovery. The PO4 sorption capacity of biochar reached >120 mg·g−1, while the sorption capacity for NH4 was low, at 22.8–28.2 mg·g−1, suggesting that Mg-surface-doped SB presented excellent phosphorus recovery. Hence, upgrading an adsorbent as a wastewater-treatment material and soil ameliorant that recovers nutrients using innate Mg from Sargassum is possible through appropriate surface modification.