(697g) Integrated Wastewater Treatment Design for Biochemical Cellulosic Ethanol: Process Economics and Sustainability

The NREL 2012 design case for biochemical ethanol is a detailed technoeconomic model of one potential conversion process conceptually based upon our core research in dilute sulfuric acid pretreatment, enzymatic saccharification, and co-fermentation. Ancillary processes for product recovery, wastewater treatment (WWT), and residue combustion for combined heat and power are also included in the model. Recent research in process integration prompted a switch from lime to ammonia as the alkaline conditioning agent applied after pretreatment. Use of ammonia avoids significant sugar losses associated with the liming process and permits conditioning of the hydrolyzate as a whole slurry, eliminating costly solid-liquid separation steps.

During a subsequent update of the 2012 design case, it was determined that this front-end process change had forced a major redesign of the process back end. Where previously the ethanol stillage was concentrated into a syrup and burned, in the current design soluble neutralization products in the stillage required treatment of this stream in WWT, possibly with additional ammonia and sulfur cleanup steps upstream of aerobic/anaerobic digestion. Together with Harris Group and Brown and Caldwell, NREL set about designing a WWT system to handle this material using data from an actual stillage sample generated by the process described above. In the design study it was determined that sulfate ions would be removed as H₂S from the anaerobic digesters, requiring flue gas desulfurization with lime after combustion. Ammonium ions would be removed as a nitrate brine following nitrification in the aerobic lagoons and concentration via reverse osmosis and evaporation. A significant amount of caustic was required meet the biological alkalinity demand in nitrification. This new WWT design was incorporated into the technoeconomic model with all capital and operating costs.

Using the integrated process model, this paper will discuss the economic trade-offs associated with the switch from lime to ammonia conditioning, i.e., trading reduced sugar loss and solid-liquid separation capital for increased chemical costs and WWT capital. Partial LCA data will be presented that compares plant-wide GHG and other emissions between the conditioning technologies. This paper will outline NREL’s future research path to address the issues highlighted by the WWT design study.