Anaerobic Digestion and Phosphorous Recovery from Wastewater Produced in a Lignocellulosic Ethanol Plant – a Techno-Economic Analysis Using Aspen PLUS

Lignocellulosic ethanol plants employing a biochemical platform produce large quantities of wastewater containing undegraded components of plant biomass, catalysts and coproducts from pretreatment, residual nutrients and microbial biomass from fermentation, and condensates from various unit operations. A techno-economic analysis, of a commercial plant employing dilute phosphoric acid pretreatment integrated with anaerobic digestion and phosphorus recovery processes for wastewater treatment, was conducted using a model developed on ASPEN Plus flowsheeting software. The model was calibrated using data from pilot plant and laboratory scale experiments. Biogas produced from anaerobic digestion could displace 40% of natural gas required for process heat. Due to lower capital costs, the breakeven cost of ethanol production (57.85 cents/liter) was comparable to that of a process that recovers undegraded fibers for combustion. The costs are further lowered (55.35 cents/liter) by recovery and sale of phosphorus. If incorporating anaerobic digestion of pretreated biomass to provide all the fuel requirements for plant, the cost increases to 61.67 cents/liter. This is an attractive scenario if a penalty for fossil fuel usage (carbon credits) is levied at current European levels.