The objective of this investigation was to determine the effect of aeration in combination with chemical coagulation on the treatment of spent vegetable tan liquor. Chemical coagulation was desirable because it effectively reduced the color and suspended solids of the tanning waste. Aeration was thought to be a possible means of reducing the Chemical Oxygen Demand of the waste by biological stripping of the pyrogallol compound in the spent waste.

Samples collected from the aeration-coagulation system were periodically analyzed for Chemical Oxygen Demand, pH, suspended solids, color removal, volume of sludge produced, and coagulant demand. These values were correlated with those values obtained for the characteristics of the raw spent waste. Color reductions up to 94 per cent were obtained up to an aeration period of 12 hours with a suspended solids reduction of 96 per cent.. Chemical Oxygen Demand reductions of the waste were increased throughout the period of aeration with a maximum reduction of 40 per cent at 36 hours of aeration.

Significance of the five day Biochemical Oxygen Demand value of 25,600 mg/liter for the raw spent waste indicated a 4 to 5 increase over all reported values from the literature.