Fate of Micropollutants During Pyrolysis of Biosolids

FATE OF MICROPOLLUTANTS DURING PYROLYSIS OF BIOSOLIDS Approximately 250 tons of organic micropollutants, including pharmaceuticals, antimicrobials, and hormones, are discharged to the environment during land application of wastewater biosolids. Reusing wastewater biosolids is vital to the sustainability of wastewater treatment, but current treatment processes do not remove micropollutants from biosolids in an efficient manner. Pyrolysis―the heating of biomass to temperatures between 400 and 800 °C under oxygen-free conditions―was proposed as a biosolids treatment process that could produce a beneficial soil amendment product, biochar, and remove micropollutants. The objective of this research was to determine the effect of pyrolysis temperature and residence time on the removal of micropollutants in biosolids as well as to characterize the ultimate fate of micropollutants following pyrolysis. Batch pyrolysis experiments were conducted on i) sand samples spiked with micropollutants to quantify the fate and breakdown products in a clean system and on ii) biosolids to determine removal in an actual biosolids matrix. Triclosan, triclocarban, nonylphenol and estradiol were selected for analysis because of their high abundance in biosolids and variable chemical properties. Extraction methods were developed using an Accelerated Solvent Extractor and samples were quantified via liquid chromatography-mass spectrometry. Removal of triclosan and triclocarban (to below quantification limit) was achieved at 300 °C and 200 °C, respectively. Substantial removal (>90%) of nonylphenol was achieved at 300 °C, but 600 °C was required to remove nonylphenol to below the quantification limit. The pyrolysis reaction time to remove 90% of micropollutants was less than 5 minutes at 500 °C. Micropollutant fate studies revealed that pyrolysis both volatilizes and degrades micropollutants. Micropollutants with high vapor pressure were more likely to volatilize before undergoing transformations. Reductive dehalogenation was a suspected degradation pathway for chlorinated organic compounds as dechlorinated triclocarban products and suspected dechlorinated triclosan products were identified. Results from the pyrolysis experiments demonstrate that micropollutants can be removed (to below quantification limit) from the biochar and transferred to the pyrolysis gas and oil where they are destined for combustion. In summary, pyrolysis is a viable biosolids management technology to mitigate the discharge of micropollutants to the environment when land applying biosolids.