Activated Sludge Mineralization and Solutions in the Process of Zero-Valent Iron Autotrophic Denitrification

Abstract

We studied the inoculation of activated sludge from domestic sewage treatment plants. The reaction of reducing nitrate by zero-valent iron was started in the up-flow anaerobic reactor. After 52 d of operation, a nitrogen removal rate of 29.3 g·(m3·d)-1 was achieved. Ferric iron and iron oxide coated on the sludge formed in the operation process caused the mineralization and slowly decreased the sludge activity. The methods of the "supplement and replacement of the sludge in the reactor" and "changing the reflux mode of the reactor" were applied. Using the method of the supplement and replacement of the sludge in the reactor, by discharging a part of the sludge in the reactor and adding the extra dosage from the anaerobic denitrifying sludge for the treatment of wastewater, after 22 d of operation, the conversion rate of nitrate-nitrogen was 33.0 g·(m3·d)-1 and the concentration of nitrite-nitrogen was 16.50 mg·L-1. The effluent average concentration of ammonia-nitrogen decreased from 12.38 mg·L-1 to 3.58 mg·L-1 and the nitrogen removal rate was recovered from 9.9 g·(m3·d)-1to 15.0 g·(m3·d)-1. The biological reaction weakened the chemical reduction of nitrate by zero-valent iron. Using the method of changing the reflux mode of the reactor, the reflux tank was arranged outside the reaction column using hydraulic circulation. The increase in the erosion of the upper layer of the sedimentation tank would bring out ferric iron and iron oxide with the outflow water and they would be deposited in the reflux tank. The corresponding ferric iron of transformation of nitrate settled in the external reflux tank was 58% at the ascending velocity of 3.49 m·h-1. The nitrate-nitrogen conversion rate was 34.3 g·(m3·d)-1, the effluent concentration of nitrite-nitrogen was 0.22 mg·L-1, and ammonia-nitrogen was 0.75 mg·L-1. Ammonia and nitrite did not extensively accumulate. The nitrogen removal rate was 33.4 g·(m3·d)-1, which solved the problem of the mineralized sludge in the long-term reactor operation. In summary, the method of reforming the reflux mode of the reactor performed better than the method of the supplement and replacement of sludge in the reactor.