A general concern that anaerobic treatment of dilute wastewaters is limited by the inability of methanogenic and related syntrophic organisms to reduce substrate concentrations adequately was evaluated using a 35 degrees C granular activated carbon-containing laboratory-scale fluidized bed reactor fed an acetate-propionate equal chemical oxygen demand (COD) mixture synthetic wastewater. Contrary to general expectations, effluent acetate and propionate concentrations remained near or below their detection limits of 0.4 mg COD/L with influent COD of 200 mg/L, 17 min hydraulic retention time, and organic loading as high as 17 kg COD/m(3) d, or with influent COD values ranging from 45 to 2010 mg COD/L and organic loadings of 4.2-4.5 kg COD/m(3) d. The effluent acetate concentrations in these well-fed systems were at or much below reported threshold limits for starving non-fed cultures, suggesting that a better understanding of threshold values and factors affecting treatment efficiency with anaerobic treatment of dilute wastewaters is needed. (C) 2012 Elsevier Ltd. All rights reserved.