The effect of overload on a two-phase high rate anaerobic wastewater treatment system with recycle was determined experimentally and simulated dynamically using a structured model. The experimental system consisted of a well mixed continuous reactor, controlled at pH 6 by sodium hydroxide addition, as the acidification stage and a fluidized bed reactor for the methanogenic stage, with an additional recycle connection from the second to the first reactor. Step changes in the feed concentration as well as in the feed flow rate were investigated and compared to simulation predictions. Operation without recycle was modelled accurately, with the simulation data of most process variables matching the experimental results quantitatively. The application of the same model to recycle operation showed significant discrepancy between the experimental and simulated data for the sodium hydroxide consumption rate and the gas flow rate in the acidification reactor. Although the actual values were different, the qualitative responses of the experimental and simulated step changes were similar. Other simulated variables showed good agreement with the experimental measurements. The model provides a useful tool for design and control studies in systems without recycle and, with some modification, for operation with recycle.

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