Effect of precipitating agents on centrifugation and ultrafiltration performance of thin stillage digestate

The EU Renewable Energy Directive 2009/28/EC demands bioethanol plants to reduce their greenhouse gas emissions. This can be achieved by integrating a biogas plant in the bioethanol process and consequently by using the bioethanol by-products (e.g. thin stillage) as feedstock for anaerobic digestion. Due to the large amounts of thin stillage and consequently of digestate an efficient digestate treatment is essential. For this purpose centrifugation and ultrafiltration of thin stillage digestate with and without addition of precipitating agents were carried out. Prior to centrifugation, which was carried out with a disc-stack centrifuge at pilot scale, precipitating agents (iron(III)chloride and carbonated lime) were added. The produced supernatant fractions were then used for ultrafiltration experiments, which were carried out with a αAl2O3-membrane (pore size: 50 nm). Cross flow velocity and trans membrane pressure were adjusted to 6 m s−1 and to 0.8∗105 Pa. The addition of precipitating agents increased the separation efficiency of the suspended solids from 46% to 75%. Furthermore the flux during ultrafiltration increased from 45-50 L m−2 h−1 to 79 L m−2 h−1. In addition, phosphorous could be practically entirely removed from the supernatant with the precipitating agents. For a large-scale bioethanol plant it was proposed to reutilise the digestate fractions as process water. However, depending on its influence on the process and on animal feed quality, different digestate treatment processes should be applied.