Enhanced nitrogen removal in the upgrading of municipal wastewater treatment plants by using zero-valent iron-modified biofilm carriers and clinoptilolite-modified biofilm carriers

A municipal wastewater treatment plant (WWTP) was initially operated as an anaerobic/aerobic (A/O) process and designed with the second grade of the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002, Table S1). To upgrade for meeting the first grade A of the GB18918-2002 (COD, BOD5, NH4+-N, and TN are 50 mg/L, 10 mg/L, 5/8 mg/L, and 15 mg/L, respectively), the original process was reformed to a process of anaerobic/anoxic/aerobic-integrated free-moving biofilm and activated sludge (A2O-IFFAS) by filling zero-valent iron (ZVI)-modified biofilm carriers into anoxic bioreactors, as well as clinoptilolite-modified biofilm carriers into aerobic bioreactors. ZVI-modified biofilm carriers were used to enhance denitrification, whereas clinoptilolite-modified biofilm carriers were used to enhance nitrification. The effluent COD, BOD5, NH4+-N, and TN of the new process were 26 +/- 14 mg/L, 7 +/- 1 mg/L, 1.3 +/- 0.2/4.7 +/- 0.7 mg/L (summer/ winter), and 12.1 +/- 2.4 mg/L, respectively, and their removal efficiencies were 95 %, 96.1 %, 90.1 %/97.3 %, and 78.8 %, respectively, which meet the first grade A of the GB18918-2002. Compared with flocculent sludge, the biofilm of ZVI-modified biofilm carriers increased the relative abundance of Hyphomicrobium (facultative autotrophic denitrifying bacteria) from 1.5 % to 8.7 %, the biofilm of clinoptilolite-modified biofilm carriers increased the relative abundance of nitrifying bacteria (composed of Nitrosomonas and Nitrospira) from 4.0 % to 39.2 %. The case demonstrated that the A2O-IFFAS equipped with ZVI-modified biofilm carriers and clinoptilolite-modified biofilm carriers could significantly increase the nitrogen removal efficiency and had a good prospect for upgrading municipal WWTPs.

Automated summary

A municipal WWTP upgraded its process to A2O-IFFAS by filling ZVI-modified biofilm carriers into anoxic bioreactors and clinoptilolite-modified biofilm carriers into aerobic bioreactors. The upgraded process achieved high nitrogen removal efficiency and met the discharge standard requirements.