The occurrence and fate of PAHs over multiple years in a wastewater treatment plant of Harbin, Northeast China

Abstract

The occurrence and fate of polycyclic aromatic hydrocarbons (PAHs) were investigated in wastewater, sludge and surrounding air from the wastewater treatment plant (WWTP) in Harbin, Northeast China. The concentration of total PAHs in the influent, effluent and sludge were 4080ng/L, 864ng/L and 8200ng/gdw, respectively. The total concentration of PAHs showed a trend of first rising, and then decreasing over years in the influent, effluent and sludge, which was in agreement with the usage of coal and oil in Harbin. The level of PAHs was 26–560ng/m3 in air from site 1 (the top of the A/O tank), 62–608ng/m3 in air from site 2 (the vicinity of the WWTP) and 61–686ng/m3 in air from site 3 (the urban district of Harbin). In the influent and effluent, the mean concentration of PAHs followed the sequence of summer>winter>autumn>spring, while the sequence was winter>summer>autumn>spring in sludge and air. Rainfall may be the main reason for higher contamination in summer. Coal fired central heating and indoor dust may be reasons for higher PAHs in winter. The mean removal efficiency of total PAHs was approximately 85% (20% of which was adsorbed onto sludge, and 65% volatilized into air or degraded by biodegradation), and 15% of PAHs were discharged through the effluent. There was approximately 6240kg of PAHs imported into the WWTP every year, 1005kg discharged into the Songhua River through the effluent, and 327kg absorbed onto sludge and the rest was degraded or volatilized into air. PCA was applied to identify the sources of PAHs for both heating and non-heating seasons. In general, coal combustion was the main source of PAHs during the heating season and vehicle exhaust was the main source of PAHs during the non-heating season.