Raw liq. sewage is injected into a reservoir which provides a single horizontal closed llop path. The liq. circulates at a minimum speed of 0.8 ft/sec. to prevent settling out. The liq. is aerate to promote aerobic decompsn. and formation of nitrate cpds. The liq. is circulated for a distance until the liq. is substantially devoid of oxygen when it reaches again the point of raw sewage injection. The liq. containing nitrates and the raw sewage are circulated together for a second distance in which denitrification proceeds until it reaches the next point of aeration. Liq. is continuously removed for clarification at about the same rate as the rate of injection of raw sewage. A portion of the settled sludge is returned to the reservoir. The average retention time in the reservoir is 24 hrs. The second distance is at least 10% of the sum of the first and second distances. For treating sewage so that its organic contents are broken down and the nitrates so formed are further broken down into their basic elements.
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