An integrated technology for recovery of energy, nutrients and clean water from cellulosic ethanol stillage

摘要

The downstream section of a lignocellulosic biochemical ethanol process produces a waste stream following the distillation of ethanol from the fermented syrup. This distillery wastewater, referred to as stillage, is a dark colored, acidic, nutrient rich liquid that presents significant problems with its disposal. Unlike conventional distillery wastewater, this stillage may contain recalcitrant products of biomass pretreatment. With US EPA tightening the industrial effluent discharge standards accompanied by decreasing land availability for waste discharge, more intensive treatment approaches must be applied to overcome stillage disposal issues.;In this dissertation, an integrated system was developed to treat stillage produced from a sugarcane bagasse feedstock process. The sugarcane bagasse is first pretreated in a dilute phosphoric acid process followed by enzymatic saccharification and then fermentation. The goals of the integrated treatment system were to recover green energy in the form of biogas, recover nutrients, specifically phosphate via struvite precipitation, and finally polish the effluent using advanced oxidation process to recover clean water for possible reuse in the ethanol plant. The aim of this integrated system was to reduce the carbon and water foot print associated with the ethanol production process.;Thermophilic anaerobic digestion (55 °C) of cellulosic ethanol stillage was investigated using a 5.5 L laboratory scale anaerobic fluidized bed reactor (AFBR). Batches of coarsely separated bagasse stillage (0.425 mm sieve) obtained from the Biofuels pilot plant at the University of Florida with a high and variable soluble chemical oxygen demand (COD) ranging from 11.5 to 63 g sCOD/L was used as feed. The AFBR was operated for 100 days. The methane potential achieved was 12.9 L CH4 (L Stillage)-1 0.288 L CH 4 (g sCODloaded)-1. Soluble organic matter removal efficiency of 93% was achieved at a hydraulic retention time (HRT) of 7.3 days and an organic loading rate of 6.54 g sCOD L-1d -1. The effluent soluble COD was 3.5 g sCOD/L. Organic acids and sugars were monitored periodically during the entire run. No external nutrients (N and P) were added during the operation of the fluidized bed reactor. Typical dilute acid pretreatment products like furfurals and levulinic acids found in the stillage were degraded in the AFBR. Upon operating the AFBR at a loading rate of 8.8 g sCOD L-1d-1, volatile organic acids (especially propionic acid) accumulated and methane production was suppressed. These organic acids were consumed within a couple of days after turning off the feed. Thereafter, the AFBR was operated at normal operating conditions (6.54 g sCOD L-1d-1).;A novel sequential batch reactor (SBR) technology (with fill, react, settle, decant and drain phases) was used to precipitate and recover phosphate from anaerobically digested stillage, in the form of the slow release fertilizer struvite. The use of coarsely separated stillage solids as seed in the SBR to promote settling of struvite was investigated. Results showed that unseeded trials produced a large amount of unsettled fine mineral precipitates, while using 1% (wet w/v) stillage solids as seed material improved settleability of processed sludge. More than 95% of settling occurred within the first 15 minutes of undisturbed settling following the 30 min reaction time. About 99.9% and 56% of orthophosphate-phosphorous and ammonia-nitrogen, respectively, were recovered in the sludge. Seeding also increased the yield of net amount of settled struvite precipitate-containing sludge by 63%. The struvite-containing settled sludge was also tested for its agronomic applicability and nutrient leachability. Results showed markedly improved nutrient uptake by plants and reduced N and P levels in leachate on application of settled sludge as soil amendment for cultivation of sweet sorghum.;A mass and energy balance was developed for a 1 million gallon ethanol plant using sugarcane bagasse as feedstock. The results showed biogasification of stillage produces enough biogas to meet the energy demands in the plant for steam generation and the excess may be used to produce electricity. Phosphates recovered from the process as struvite are enough to supply ∼43% of the fertilizer needs to cultivate sugarcane for bagasse production. And finally polishing the wastewater provides a means for recycling water in the plant. This might help reduce the dependence of bioethanol plants on ground water resources. (Abstract shortened by UMI.).