Ozone treatment of process water from a dry-mill ethanol plant

Fuel ethanol production in corn dry milling plants is a rapidly expanding industrial sector. Whole stillage, the residue from the distillation of the fermented corn, is centrifuged and the concentrate, thin stillage, is found to have a chemical oxygen demand (COD) of approximately 75,000 mg/L. This thin stillage is partly recycled, but much of it needs to be evaporated to concentrate the solubles for addition to the animal feed coproduct from corn dry milling. This research is an exploration into lowering COD from thin stillage using ozonation as a simple single-step unit process to facilitate a larger reusable fraction. The ozonation would usually be a pretreatment before additional flocculation or biological treatment. Also, COD removal by ozonation with and without a catalyst has been studied. Three different application rates of ozone O3,1 = 7 mg/min, O3,2 = 21 mg/min, and O3,3 = 33 mg/min were used for 8 h into samples of 2 L each of three dilutions 20×, 30× and 40×. COD removal of 85% was observed with an ozone dosage of 4000 mg/L into a 40×-diluted sample. This would correspond to about 0.5 mg COD removed per mg ozone dosed. However, at lower dosages and smaller dilutions, more than 1 mg COD removal was achieved per mg ozone dosed. Two different catalysts, Fe(II) and Fe(III), were used and the samples were ozonated for 4 h. Five different dosages of each of the two catalysts were used and better COD removal was observed compared to ozonation alone with all 5 dosages. COD removal rate was increased from 45% to 74% with Fe(III) and was increased to 77% with Fe(II). Up to 10 mg/L COD was removed per mg O3 dosed well below the maximum dosage. Both the catalysts resulted in almost the same improved COD removal rates when compared to ozonation alone.