Process liquid recirculation initially stimulated one-phase anaerobic digestion of alfalfa silage in two semi-continuously fed and stirred tank reactors. Thus, with increased pH, alkalinity and stability it was possible to increase the organic loading rate to 3 g VS L -1 d -1, as compared to 2.25 g VS L -1 d -1 in a control reactor without recirculation. However, the recirculation of liquid eventually caused an accumulation of organic and inorganic substances, leading to an inhibition of hydrolysis and methanogenesis. This inhibition of microbial activity was prevented in one of the processes by replacing 50% of the recirculated process liquid with water during the second half of the operation period. A multiple linear regression model of principal components using seven input variables explained the variance in output variables nearly as well as the original model using all 23 measured input variables. The results show that it is necessary to adjust the degree of liquid recirculation to reach an optimal process. © 2005 Elsevier Ltd. All rights reserved.