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The anaerobic single-stage reactor was evaluated to treat vinasse and to evaluate its stability. This bench reactor was filled with dolomitic limestone with a horizontal plug flow to simulate a drainage channel. The experiment lasted 129 days while the reactor was submitted to different applied organic concentrations (chronologically applied: 3.0; 5.0; 12.0; 9.0 and 7.5 g L-1 as COD, chemical oxygen demand). COD removals were 50% and 9% with 3.0 and 7.5 g L-1, respectively. With 12.0 g L-1, reactor efficiency increased to 33%, with an abrupt drop to 3% on the 84th day. Therefore, in order to avoid reactor collapse, a remedial measure was necessary. The system remained in batch without feeding for 19 days (from the 85th to the 104th day) with 9.0 g L-1. Afterwards, it was observed that the performance of the system tended to stabilize, reaching 47% with 7.5 g L-1 in the 118th day. At the end of the experiment, the potassium content of the wastewater decreased from 800 mg L-1 to 594 mg L-1 (on an average 25%) and calcium and magnesium increased within the reactor liquor. The dissolution of the limestone inside the liquor reactor probably caused this result. After the treatment with limestone, the average pH value of the effluent increased from 4.9 to over 6.0 in all organic concentrations. It could be concluded that the reactor filled with dolomitic limestone in these operational conditions assured a low efficiency in COD removal, potassium reduction, increasing values of pH, alkalinity, calcium and magnesium. The instability was observed when there was increase in organic load to 12 g L-1 with subsequent recovery.