Changes in Heavy Metals, PCBs, and Selected Organic Compounds in Sanandaj Digested Sewage Sludge after Composting with Sawdust

This study compared the concentrations of heavy metals, polychlorinated biphenyls (PCBs), and selected organic compounds in digested sewage sludge from the Sanandaj wastewater treatment plant before and after composting with sawdust. Digested sludge and sawdust were mixed at a volumetric ratio of 2:3 (v/v) and composted for approximately two months, with the mixture turned two to three times daily. Heavy metals were measured by atomic absorption spectrometry, PCBs by gas chromatography with an electron-capture detector (GC-ECD), and selected organic compounds by gas chromatography. After composting, the reported concentrations of arsenic, copper, lead, and zinc decreased from 4.13 to 1.56, 211 to 40.5, 34.4 to 7.0, and 875 to 148 mg/kg, respectively. Cadmium and mercury concentrations declined from 1.0 and 0.6 mg/kg to below reporting thresholds (<0.5 and <0.1 mg/kg, respectively). Conversely, chromium increased from 56.9 to 93.4 mg/kg (~64.1%), while nickel remained relatively stable (from 21.8 to 22.6 mg/kg), which is primarily attributed to organic matter decomposition and dry mass loss. Total PCBs were below 0.2 µg/kg both before and after composting, and the twelve selected organic compounds remained below analytical reporting limits. These findings describe changes in measured concentrations and illustrate compliance with land-application thresholds, though process monitoring and mass-balance replication were not reported.