Small towns that operate wastewater treatment lagoons struggle to meet ammonia limits in cold weather. Here we report the performance of a lagoon, retrofitted with submerged attached-growth reactors (SAGRsTM), to provide insight on ammonia effluent compliance and optimal SAGR sizing as functions of water temperature. The lagoon-SAGR water resource recovery facility (WRRF) removed 95% of incoming ammonia with 94% attributable to the SAGRs. The high treatment capacity of the two primary SAGRs, evidenced by nearly continuous dissolved oxygen saturation and exceedingly high ammonia removals, suggested the two secondary SAGRs were essentially unnecessary and that all four SAGRs should be reduced in size. Furthermore, without the secondary SAGRs, the primary SAGR effluent would have exceeded the permitted ammonia discharge limit only four times in the 2.5 year study. At its current size, the lagoon-SAGR WRRF never exceeded permitted ammonia limits, but size reductions should be used for future retrofits. To further understand cold-weather ammonia removal in the lagoon-SAGR WRRF, we investigated the effect of increased ammonia loading on biomass and the effect of biofilms on microbial abundance. When ammonia loading to the SAGRs was increased in the fall, the lagoon-SAGR WRRF never exceeded its ammonia permit limit, the kinetic coefficients were maintained (0.5-0.8 d-1) and the NH3 removal rates improved (0.25 kg d-1 in baseline loading to 0.45 kg d-1) despite a large temperature decrease (25 °C to < 16 °C). In the biofilm, ammonia-oxidizing archaea abundance was 10 times greater than the ammonia-oxidizing bacteria abundance suggesting the potential importance of ammonia oxidizing archaea in biofilm mediated systems. Additionally, the ammonia and nitrite transforming microbes in the SAGRs had a diverse range of dissolved oxygen affinities and were more abundant in the biofilm in comparison to the wastewater. Anaerobic ammonium-oxidizing bacteria were abundant in the biofilm even though the film constantly interacted with high dissolved oxygen. We found that two components of a successful lagoon-SAGR WRRF were increased biomass in the SAGRs before cold-weather due to elevated ammonia loading and diverse oxygen affinities in the microbes related to ammonia removal.