Usually, in ethanol producing plants, the stillage obtained from the distillation step (whole stillage) undergoes centrifugation in order to recover organic solids, especially yeast debris. The liquid fraction is known as thin stillage. In a typi­cal distillery, more than 20 liters of thin stillage can be generated per each liter of ethanol produced. To minimize the effluent treatment costs, a portion of thin stillage (from 25 to 75%) is recycled in different process steps as fermentation or saccharification (in the case of conversion of starchy materials). In a simi­lar way, the whole stillage can be recycled back to the fermentation step when cane molasses is used. Although this procedure decreases the volume of fresh water employed, it decreases, in turn, the volume of stillage generated; the total amount of organic matter in the stillage (measured as COD) does not change because its concentration increases with the amount of recycled stillage (Wilkie et al., 2000).

The North American ethanol industry that uses corn as feedstock makes use of a fraction of thin stillage to replace a percentage of water required during the mashing process (saccharification step). The thin stillage stream recycle is called backset. The drawback to this practice is that there is an accumulation of undesirable substances, such as lactic acid, produced by contaminant bacteria as well as minerals and nonutilized substrates. In this way, the continuous backset usage results in the buildup of compounds inhibiting the yeast growth and etha­nol production. On a laboratory scale, it has been demonstrated that the use of a backset in recycling 50% of thin stillage during the wheat mashing for five suc­cessive fermentation batches will produce a 60% loss of yeast viability, though the fermenting ability of these microorganisms and the glucoamylase activity is not affected (Chin and Ingledew, 1994). The backset utilization has been evalu­ated in the case of very high gravity (VHG) fermentations of wheat starch. As noted in Chapter 7, Section 7.1.3.2, VHG fermentation requires an available nitro­gen source, but the recycled thin stillage contains no significant amounts of free amino nitrogen. In this sense, the use of fresh yeast autolyzates are obtained by centrifugation of the fermented wort before its distillation. The lysis of yeasts is induced through their resuspension in water and by increasing the temperature up to 48°C for 48 h, followed by pasteurization at 75°C. In this way, the costs associated with the treatment of cell biomass can be reduced in ethanol-producing plants as well (Jones and Ingledew, 1994).

The possibility of implementing a system with zero-discharge of stillage for the process employing starch or starch residues from wet-milling process has been proposed. The stillage is decanted and the resulting thin stillage undergoes ultrafiltration. The solids produced are recycled back to the decanter while the permeate is recirculated to the cooking step of starch. After eight-fold recycling of the permeate, the yield is similar to that of the conventional process, though this system increases the fermentation time from 60 to 90 h (Nguyen, 2003).

For the process based on sugarcane molasses, a bioconcentration system of vinasses has been proposed. This system consists of the recycling of 60% stillage to the fermentation step by replacing part of the water used during the preparation of the culture medium (Navarro et al., 2000). This stillage recycling percentage can increase the ethanol production without provoking inhibitory effects result­ing from the accumulation of by-products released by the yeasts. In the process, 46.2% reduction of fresh water, 66% decrease of nutrients, and 50% reduction of sulfuric acid are attained. The energy balance of the process can be improved if subsequent stillage incineration is implemented since the liquid effluent contains about 24% solids. Moreover, the obtained stillage contains significant amounts of glycerol. In this way, it is possible to generate only five liters of stillage per liter of ethanol produced. These systems require the use of osmotolerant yeasts, such as Schizosaccharomyces pombe, able to grow under high solids concentration in culture medium (Goyes and Bolanos, 2005). The distilleries of CSR (Australia) utilize the biostill process (see Chapter 7, Section 7.1.2.3) implementing such types of recycling and using S. pombe. With this technology, one can obtain still­age with 28 to 30% solids content making it suitable for its direct utilization as a fertilizer (Bullock, 2002). In the distilleries co-located in cane sugar mills, the vinasses can be alternatively used as part of the water for washing the sugarcane or recycled to the molasses dilution step (Sheehan and Greenfield, 1980).

10.1.2.1 Stillage Evaporation

The evaporation represents an intermediate step during the stillage treatment. The evaporated stillage is the starting point for solids recovery, fertilization, and incineration of stillage. Thus, during ethanol production from corn by the dry-milling technology, the thin stillage obtained can be evaporated in order to produce syrup (also called distiller’s solubles). This syrup is combined with the solids from corn to produce a co-product used for animal feed (DDGS). The con­densed water generated in the multiple-effect evaporators contains small amounts of volatile organic compounds and can be employed during cooking and liquefac­tion steps of the corn suspension, but the accumulation of inhibitors impedes the 100% recycling of these condensates. The condensates from evaporators can also undergo aerobic or anaerobic treatment, thus providing all the nutrients required (Wilkie et al., 2000). Palmqvist et al. (1996) proposed, from bench-scale data, to fraction the stillage by evaporation and recirculate only those fractions that have demonstrated having no inhibitory effects on fermentation using Saccharomyces cerevisiae in the case of willow wood hydrolyzates pretreated by steam explo­sion. The nonvolatile fraction has high inhibitory effects presumably caused by the presence of lignin degradation products. Other fractions have low levels of BOD and COD so they can be directly discharged without any treatment. Similar studies were carried out for pine and spruce (softwood) performing the simula­tion of evaporation step using a six-effect evaporation train to optimize the energy consumption (Larsson et al., 1997).