As the demand for biofuel products continues to rise along with the need for tertiary wastewater treatment processes to facilitate the removal of phosphorus, increasing importance is placed on the design of cultivation systems for the production of algal biomass. In addition to traditional open ponds and photobioreactors, attached algal growth systems can provide high rates of biomass production while simultaneously removing nutrients from wastewater effluent. The potential economic viability of these attached growth systems is dependent on the optimization of bed flow characteristics, inflow water quality parameters, and hydrodynamic regimes in order to maximize biomass productivity. This study monitored the productivity and nutrient removal rates of a pilot-scale attached growth system (AGS1) used to remove phosphorus from wastewater effluent at a municipal wastewater treatment plant in Fayetteville, Arkansas. These results were compared with the results from a similar system in the same watershed that was monitored in a previous study (AGS2). The performance of AGS2 was documented in a companion article in Ecological Engineering. In spite of the similarities between the systems’ locations and influent characteristics, the productivities of the two systems were very different. The rates of biomass production in AGS1 and AGS2 were 4.4 ± 4.8 and 26.7 ± 16.0 g dry weight m−2 d−1, respectively. Potential reasons for the dramatic differences in performance between the two systems are explored in this article.