(230a) Techno-Economic Feasibility of Biomass Hydrolysate Desalination By Membrane Capacitive Deionization

The bio-based industry is putting significant effort into research and development to replace refined sugars by much cheaper non-food renewable feed stocks for the production of bio-fuels and chemicals. Although this obviously increases sustainability, it also brings about a number of technological challenges due to the higher complexity of these secondary substrates. Biomass hydrolysates, for example, can contain high concentrations of sodium and potassium that inhibit fermentation and hence need to be reduced. The commonly used ion-exchange processes carry high operational costs and generate a waste stream through the use of chemicals for regeneration. Electrochemical pretreatment of biomass hydrolysates with membrane capacitive deionization or MCDI could provide a lower cost alternative with minimal waste generation.

This study considers the electrochemical treatment of biomass hydrolysates by membrane capacitive deionization (MCDI) as an novel chemical free, lower cost alternative in comparison to the commonly used ion-exchange processes. Model experiments with model solutions and a commercial bench-scale MCDI set-up indicated that none of the most abundant hydrolysate components (sugars, organic acids and furans) prohibited MCDI implementation for this application. Compared to water desalination performance was somewhat lowered by the competition for electro-sorption between the protons originating from organic acid dissociation and the cations. This effect was not observed during MCDI treatment of a real biomass hydrolysate sample. Instead, the achieved sodium (Na) and potassium (K) removal and energy usage were comparable to that for a model solution with equal conductivity and sugar concentration.

Based on the experimentally obtained desalination efficiencies, the economic viability of MCDI was investigated for a production capacity of 500 ton sugar/day and a target Na removal from 3 to 0.1 g/kg hydrolysate. Although capital costs were higher for MCDI than for IEX due to the expensive MCDI cells and power supplies, operating costs were lower because less water and chemicals are used and less wastewater is generated. Cost calculations for different initial feed concentrations indicated that IEX was only preferential over MCDI when the feed Na+ concentration was below 0.4 g/kg hydrolysate. Then the higher chemical, water and wastewater treatment costs for IEX no longer outweighed the higher cost of MCDI cells compared to IEX resins.

In conclusion, this study clearly demonstrates the technical and economic potential of MCDI for process streams such as biomass hydrolysates, hereby considerably broadening its potential application field.