An oil/grease separator separates a lighter liquid from a mixture of the heavier liquid and a lighter liquid. A housing has an inner volume, an inlet to receive influent flow, an outlet to discharge the heavier liquid, which defines a static liquid level, and an outlet port above the static liquid level to discharge substantially all of the lighter liquid and portions of the heavier liquid. The lighter liquid floats on the heavier liquid, so that large volume flows into the inlet raises the liquid level in the housing, flowing the floating lighter liquid through the outlet port.