Aerobic and two-stage anaerobic–aerobic sludge digestion with pure oxygen and air aeration

The degradability of excess activated sludge from a wastewater treatment plant was studied. The objective was establishing the degree of degradation using either air or pure oxygen at different temperatures. Sludge treated with pure oxygen was degraded at temperatures from 22 °C to 50 °C while samples treated with air were degraded between 32 °C and 65 °C. Using air, sludge is efficiently degraded at 37 °C and at 50–55 °C. With oxygen, sludge was most effectively degraded at 38 °C or at 25–30 °C. Two-stage anaerobic–aerobic processes were studied. The first anaerobic stage was always operated for 5 days HRT, and the second stage involved aeration with pure oxygen and an HRT between 5 and 10 days. Under these conditions, there is 53.5% VSS removal and 55.4% COD degradation at 15 days HRT – 5 days anaerobic, 10 days aerobic. Sludge digested with pure oxygen at 25 °C in a batch reactor converted 48% of sludge total Kjeldahl nitrogen to nitrate. Addition of an aerobic stage with pure oxygen aeration to the anaerobic digestion enhances ammonium nitrogen removal. In a two-stage anaerobic–aerobic sludge digestion process within 8 days HRT of the aerobic stage, the removal of ammonium nitrogen was 85%.