In the present study the increase of the organic loading rate from 2.11 to 4.25 kg VS m(-3) d(-1) in a two stage, agricultural biogas plant was investigated. The process enhancement resulted in the doubling of the plant capacity from 500 M to 1000 kW retaining the same digester volume. Efficiency criteria showed good performance throughout the study. At the end of the monitoring, biogas yield was on the same level as before the enhancement, while volume related biogas productivity almost doubled from 1.50 to 2.91 Nm(3) m(-3) d(-1). However, as a consequence of the higher transfer of poorly degraded organic material into the effluent, the residual methane potential of the effluent multiplied by the factor 10. The results of this study show, that most agricultural biogas plants in Austria have a great potential for a significant capacity increase. However, to avoid atmospheric emissions, the effluent storage of high loaded processes has to be integrated into the gas-tight system of the digesters. (c) 2007 Elsevier Ltd. All rights reserved.