Advanced nitrogen removal from the biological secondary effluent of dyeing wastewater via a biological-ferric-carbon nitrification and denitrification process

It is often difficult for the ammonia nitrogen (NH4+-N) concentration in biological secondary effluent of dyeing wastewater to reach the discharge standard by biological treatment due to the nitrogen-containing auxiliary application. Therefore, a biological-ferric-carbon nitrification and denitrification process is applied in this study. In the ferric carbon system, either under the condition of sludge seeding or not, nitrifying bacteria are accumulated to form a biological-ferric-carbon system, in which the nitrification process is initiated. Under the condition of sludge seeding, a higher NH4+-N removal efficiency is reached in a shorter period. In addition, the biological-ferric-carbon system has advantages both in NH4+-N and COD removal because of mutual promotion effects. In the subsequent denitrification process, an extra carbon source is necessary, and the Fe3+ in the nitrification liquid is beneficial for denitrification, thus a denitrification efficiency of nearly 90.2 +/- 2.1% is achieved in the short period of 2 h, however, only 44.6 +/- 1.5% NO3--N is denitrified during 6 h when Fe3+ is removed by coagulation. Under optimal operation conditions for the biological-ferric-carbon nitrification and denitrification process, the NH4+-N concentration decreased from 32.3 +/- 1.5 mg L-1 to 2.5 +/- 0.2 mg L-1, the effluent total nitrogen was less than 8.8 +/- 1.0 mg L-1, and COD was reduced from 120.0 +/- 6.0 mg L-1 to 49.4 +/- 2.1 mg L-1, which presents a good performance both in advanced nitrogen removal and COD degradation.