Investigation into the distribution of polycyclic aromatic hydrocarbons (PAHs) in wastewater sewage sludge and its resulting pyrolysis bio-oils

- We investigated 16 US EPA PAHs in sewage sludge and the resulting pyrolysis bio-oil.
- We examined the factors influencing the distribution of different rings of PAHs.
- Most of the 16 target PAHs were enriched in the sludge and its resulting pyrolysis oil.
- LMW PAHs in bio-oil are correlated with MMW PAHs in raw sewage sludge at best.
- Temperature and residence time in sludge pyrolysis evidently impact PAH emission.

This study firstly investigated the distributions of 16 US EPA priority controlled polycyclic aromatic hydrocarbons (PAHs) in seven kinds of different wastewater sewage sludges and bio-oils from the sludge pyrolysis. A lab-scale tube furnace was used to simulate sludge pyrolysis and retrieve condensed oils. PAH determination was conducted with the extraction, concentration, and purification of PAHs in sludge samples and the resulting bio-oils, and then GC-MS analysis. Then, the factors influencing the distributions of different rings of PAHs in pyrolysis bio-oil, such as the chemical characteristics of raw sewage sludge and pyrolysis condition, were analyzed. It was noted that the total amount of PAHs in raw sludge is evidently varied with the sludge resource, with values ranging between 9.19 and 23.68 mg/kg. The middle molar weight (MMW) PAH distribution is dominant. PAH concentrations in sludge pyrolysis bio-oil were ranged from 13.72 to 48.9 mg/kg. The most abundant PAHs were the low molar weight (LMW) PAHs. It could be found that the concentration of LMW PAHs in bio-oil is correlated with MMW PAHs in raw sewage sludge at best, which the correlation coefficient is 0.607. For MMW and HMW (high molar weight) PAHs, they are significantly correlated with HMW PAHs in raw sewage sludge, which the correlation coefficients are 0.672 and 0.580, respectively. The concentration of LMW PAHs in bio-oil is also relatively significant and correlated with the volatile matter content of raw sludge. In addition, it was proved that final temperature and residence time have important influences on PAH generations during the pyrolysis of sewage sludge.