Large-scale microalgae cultivation for biodiesel production is expected to be performed utilizing open air growth infrastructure that will inherently introduce ash into the system. High ash biomass represents a significant challenge for the production of biofuel as it increases processing capital and operational costs. This study assesses the economic viability of pretreatment to remove ash from biomass grown with an algal turf scrubber (ATS). An engineering process model of biofuel production was developed based on an ATS growth system followed by an ash removal process and conversion of the biomass to fuels through hydrothermal liquefaction. The model was validated with literature data for the growth and conversion processes and with experimental data for 14 de-ashing processes using water washing or alkaline extraction. The engineering process model was integrated with techno-economic modeling to investigate the impact of ash on biomass and fuel selling prices. Capital costs associated with biofuel conversion doubled as ash content increased from 0% to 70%, increasing fuel selling price by 21%. Integrating ash removal resulted in reduced conversion capital costs (14-42%) based on the reduction of total mass processed. However, only water washes scenarios at 25 degrees C and 50 degrees C were found to reduce overall fuel selling price, with alkaline extraction scenarios showing a significant increase. Operational expenses associated with alkaline extraction de-ashing including wastewater treatment, chemical costs, and heating the microalgae slurry were found to significantly increase the overall fuel selling price of the microalgae biofuel by 17-92% depending on the operational scenario.