Anaerobic bioprocessing of organic waste streams is common, converting organic matter to biogas, a combustible mixture methane, and carbon dioxide. At wastewater treatment plants, anaerobic digestion (AD) is commonly used to treat the concentrated settled solids from settling tanks - i.e. a side stream of sewage. AD-based Upflow Anaerobic Sludge Blanket (UASB) reactors, which can treat mainstream sewage or a range of other organic wastestreams, are common in some locations like Brazil and India. Regardless of feedstock, the liquid effluent from such anaerobic digestion dependent reactions contains high dissolved methane ranging from 20 to 25 mg/L plus any micro bubbles are trapped in the effluent water. This effluent also contains very high levels of ammonia (tens of mg/L in sewage up to 1000 mg/L in AD effluent) with low C:N ratio. These AD-effluents waste stream presents a unique challenge to treat, especially if considering greenhouse gas emissions, and nitrogen removal to N2 gas. To simultaneously address methane and ammonium removal, this study proposes a novel system based on the synergetic interaction between methanotrophs and anammox bacteria. It has been established that methanotrophs with particulate methane monooxygenase (pMMO) can both catalyze methane oxidation as well as ammonium oxidation, exhibiting a competitive inhibition between these substrates. The high oxygen affinity of pMMO in methanotrophs also makes co-metabolic nitrification possible not only without a nitrifier present but also under hypoxic conditions (DO < 0.3 mg/L). Such conditions facilitate the growth of anammox bacteria. Therefore, methanotrophs were employed in this system as functional substitutes for conventional nitrifiers (ammonium-oxidizing bacteria, AOB) to facilitate partial nitrification and achieve an ammonium-to-nitrite ratio of approximately 1:1.32, thereby enhancing anammox-driven nitrogen removal. This system is termed Methanotrophy/Anammox (Me/A). To test the feasibility of Me/A, continuous flow experiments were conducted in membrane bioreactors where methane and oxygen were fed via pressurized membranes. Two different methane feedstocks were tested: purified methane versus 70% CH4/30% CO2 as it would be produced as the UASB biogas. A negative control reactor had membranes without pressurized gas delivery. All membrane bioreactors were fed with UASB Effluent (40-100 mg/L COD and 100-150 mg/L N) for several months and biofilms in experimental reactors grew thick on the O2 membrane but thin on the methane membrane, suggesting O2-limited conditions, which were confirmed using GC analysis. By varying the oxygen pressure in the membrane, shifts in N removal patterns were revealed both in nitrification products nitrite and nitrate. Under the optimum experimental conditions, the N removal reached 88% with 32 mg N/d being removed, demonstrating its efficiencies on N removal. qPCR analyses were used to quantify key community members in the bioreactors during continuous-flow operation: methanotrophs, anammox, nitrifiers and denitrifiers. Methanotrophs were identified as the dominant population, accounting for over 90% of the summed abundance among nitrifiers, anammox bacteria, and methanotrophs. No denitrifiers (nosZ gene copies) were detected in any of the samples, suggesting dentirifiers are not outcompeting anammox for nitrite. Additionally, ammonium-oxidizing bacteria (AOB) were strongly outcompeted by methanotrophs under hypoxic conditions in two experimental groups, consistent with the theoretical framework of the Me/A system. To quantitatively describe the Me/A system, a simplified biological model incorporating only methanotrophs and anammox bacteria was developed. By simulating scenarios with varying methane and oxygen fluxes, the model identified the optimal methane-to-oxygen supply ratio for maximizing nitrogen removal as well as the corresponding nitrogen distribution among NH4+, NO2-, and NO3- under different operating conditions.