The stillage properties depend on the type of feedstock used for ethanol produc­tion as well as the conversion technology employed. The stillage presents high values of biological oxygen demand (BOD5 = 30,000-60,000 mg/L) that is, along with the chemical oxygen demand (COD), a measure of the content of organic matter in an effluent sample. Wilkie et al. (2000) have reviewed the characteris­tics of several types of stillage for various ethanol production processes, includ­ing the stillage yields. The stillage generated in the process employing sugarcane molasses has high polluting loads in terms of both BOD (45,000 mg/L) and COD (113,000 mg/L) as well as in its yield (2.52 liters of stillage per kilogram of molas­ses) and content of potassium, phosphorous, and sulfates. The stillage derived from the process using sugarcane juice exhibits lower levels of BOD, COD, and yield (12,000 mg/L, 25,000 mg/L, and 1.33 L/kg, respectively). The stillage from corn process shows intermediate indicators for BOD and COD (37,000 mg/L and 56,000 mg/L, respectively) but a higher stillage yield (6.20 L/kg). Finally, in the case of lignocellulosic ethanol produced from hardwood, the values of BOD, COD, and yield are 13,200 mg/L, 25,500 mg/L, and 20.4 L/kg, respectively (Wilkie et al., 2000).

If stillage is directly discharged into natural water streams, the microorgan­isms present in them will degrade the organic matter discharged, which uses the oxygen dissolved in the water. This leads to a critical diminution of dissolved oxygen provoking the death of most aquatic organisms. In addition, the utilization of nutrients contained in molasses can lead to the massive propagation of algae on the surface of ponds and lakes causing the blocking of solar light leading to the death of fish and other living organisms. This phenomenon is known as eutrophi­cation. Thus, the stillage discharge can seriously alter the ecological equilibrium of the aquatic ecosystems affected. Residual sugars from fermentation cause still­age COD to reach high values. For every 1% of residual sugar, a stillage COD increment of 16,000 mg/L can be expected (Wilkie et al., 2000). For this reason, full completion of fermentation is desirable in order to reduce stillage volume (greater ethanol concentrations) and minimize the concentration of remaining sugars. For each 1% ethanol left in the stillage (in the case of nonefficient distilla­tion), the stillage COD is incremented by more than 20,000 mg/L.

Due to its elevated organic matter content of stillage, methods for its treatment and economic utilization should be implemented in the industry. Among the most used methods for stillage treatment, irrigation, recycling, evaporation, incinera­tion, and composting should be highlighted. Some of the new treatment methods produce value-added substances. The most important stillage treatment methods are presented in the next sections.