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This paper is built on a study performed to investigate the possibilities to take care of fiber sludge from a former pulp mill in the city Tampere, Finland. A pilot plant biorefinery process has been developed by Finnoflag and tested during 2018-2019 for the conversion of the cellulosic material into useful chemical products by the help of a microbial process. The results from this are used as input to a study on downstream processing performed through lab and pilot plant tests of different technologies, as well as system simulations. An economic analysis is also performed with respect to installation costs for the different alternatives of downstream processing – ion exchange, distillation and membrane filtration. It was found that all three processes would successfully concentrate the main product (lactic acid), however distillation would likely be energy intensive, and ion-exchange costly for materials. Therefore, the recommendation from this study is to focus on reverse osmosis after ultrafiltration, with the addition of evaporation if necessary. Besides the resulting chemicals fraction the biorefinery produced hydrogen, and the residues could be converted effectively into methane. These gases provided energy for the industrial unit and could be converted into energy, too. On the other hand, the feasible production of the chemicals gave an economic justification for the energy producing units.