Removal of polycyclic aromatic hydrocarbons and heterocyclic nitrogen compounds in a municipal treatment plant

Abstract

Emissions of polycyclic aromatic hydrocarbons (PAHs) and heterocyclic nitrogenous compounds (HNCs) were measured at a publicly owned treatment works (POTW) receiving a dry weather flow (DWF) of 409 ML/d. Samples of raw sewage, primary and secondary effluents were monitored over three sampling periods and were screened for 16 PAHs and 24 HNCs. The PAHs tended to partition to the solid phase while the HNCs were predominantly found in the liquid phase. Higher levels of PAHs were observed in the raw wastewaters than typically seen at POTWs, suggesting the influence of industrial discharges. Despite removal efficiencies of 98.3% for PAHs and 96.3% for HNCs, average daily effluent mass discharges of 0.2 kg PAHs/d and 1.0 kg HNCs/d were measured. A fate of contaminants model, Toxchem, was used to forecast the impact of different process strategies for minimizing the discharge of PAHs. Analysis suggested that increases of up to 50% in primary or secondary clarifier capacity would not likely improve PAH capture. A 100% increase in the solids retention time of the activated sludge system similarly would not likely improve PAH removal.