The effluent from bioethanol production still contains a large amount of or­ganics that are not composed of carbohydrates. Anaerobic digestion (AD) has been used for a long time to treat organic waste streams with a high concen­tration of organic matter. The benefits of anaerobic treatment are stabilization of the waste stream, the high reduction of organic matter, and the production of methane, which can be used as energy source [25]. This gives an overall positive energy balance of the waste treatment process compared to aerobic waste treatment. The income from the methane produced after bioethanol production constitutes a value corresponding to a lowering of the ethanol production price by 34%.

The effluent from the fermentation step of bioethanol production con­tains low-molecular weight lignin degradation products primarily generated during the physical-chemical pretreatment. These aromatic compounds are generally difficult to degrade under anaerobic conditions. Furthermore, a re­peated reuse of the process water has the potential to cause a build up of these fermentation inhibitors. It is therefore important to achieve an anaero­bic purification technology that is able to remove these compounds from the process water. Experiments in our laboratory have shown that all problematic organic components can be removed in the anaerobic step. The low hydraulic retention times and the removal of organics are of great importance, looking at the overall process feasibility [26].

The Maxifuel concept has been implemented at pilot scale at the Techni­cal University of Denmark, DTU (Fig. 5) and the concept is planned to go into demonstration phase in 2008.

The plant is dimensioned to convert 150 kg dry biomass/day, and con­sists of 17 tanks (fermentation, reactors, and holding tanks). The ethanol fermentation takes place in two 2700 L fermenters. The plant includes all pro­cesses from straw to ethanol, and was brought into operation in the autumn of 2006.

4.6