The performance of a laboratory-scale anaerobic filter treating soybean processing wastewater with and without effluent recycle was investigated. Three organic loading rates (OLRs), 8.16, 11.3 and 13.5 gCOD/1 d, and six recycle ratios (1.0-6.0) were selected for recycle tests. There was an optimum recycle ratio, r(max), which varied depending on the OLR. Operation with recycle around r(max) enhanced chemical oxygen demand (COD) removal. At OLRs of 8.16, 11.3 and 13.5 gCOD/1 d, the r(max) values were 3, 2, 2, respectively. At a high OLR there was a threshold recycle ratio. Recycle below this value led to the increased COD removal, whereas recycle with a ratio beyond this value resulted in a higher COD than that without recycle. The threshold recycle ratio for OLR of 13.5 gCOD/1 d was 5. The methane percentage decreased with increased recycle ratio, e.g. at OLRo of 8.16 gCOD/1 d, from 75% without recycle to 51% at r of 6. The carbon dioxide percentage had an opposite trend to that of methane. Total biogas production rate and the effluent suspended solids level both increased with recycle ratio, but the methane production rate, total volatile fatty acids concentration and their distribution in the effluent were not significantly affected by recycle. (C) 2002 Elsevier Science Ltd. All rights reserved.