The biomass was digested using a mixed culture of microorganisms (derived from silage, rumen fluid and compost) at a high solid loading rates. This system attempts to mimic the biomimetic conversion system of a cow. Pretreated willow wood was digested in a batch process at a solid loading rate of 75 DM g/L. Air-dried immature switchgrass was digested in a semi-continuous mode with 5 DM g/L/day and 10 DM g/L/day with daily replacement of 10% reactor volume, for equivalent loading rates of 50 and 100 g/L respectively over a 10 day mean residence time. Our results indicated that lignocellulosic biomass could be converted into carboxylic acids at high yields. Small and medium chain carboxylic acids including acetic, lactic, propionic, butyric, valeric and hexanoic acids were detected in the fermentation liquid at different concentrations. Nanofiltration was used as a separation and downstream strategy. Our results also indicate that a continuous carboxylic acid production and separation strategy can be developed using these process design parameters.