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Substituted polycyclic aromatic hydrocarbons released from sewage sludge pyrolysis bring the concern of the clean utilization for the waste. Herein, the roles of calcium oxide on the release of these compounds, especially the heavy ones containing nitrogen and oxygen atoms were investigated. The results showed that calcium oxide significantly decreased light substituted polycyclic aromatic hydrocarbons in the volatiles from 450 to 850 °C and showed obvious inhibitory effects on the formation of heavy substituted polycyclic aromatic hydrocarbons at 850 °C. To understand the impact mechanisms, pyrolysis experiments of sludge model compounds lipid, protein and polysaccharides were conducted. It was found that calcium oxide could catalyze or react with all three components to decrease the content of light substituted polycyclic aromatic hydrocarbons. Heavy oxygen-containing aromatics in sludge derived volatiles mainly resulted from polysaccharides and protein, and they considerably decreased after mixing with calcium oxide. Oxygen-containing aromatics content in the lipid derived volatiles, however, would increase due to polymerization reactions. The decreasing release of nitrogen substituted polycyclic aromatic hydrocarbons from sludge could be attributed to the synergy effects of CaO and other minerals or the effects of CaO on the interaction between protein and other organics.

Thermal treatments of sludge lead to the emission of carcinogenic aromatics, which has already attracted the attention of researchers for years. However, until now the evolution of substituted polycyclic aromatic hydrocarbons (PAHs), especially those with high molecular weight, during thermal treatments of sludge was not paid enough attention even though some of them are more toxic than their parent PAHs. To comprehend the generation of substituted PAHs, in particular that of heavy substituted PAHs, volatiles released from the pyrolysis of three sludge at three temperatures were comprehensively analyzed. The results show oxygen-substituted PAHs (O-PAHs) and nitrogen-substituted PAHs (N-PAHs) are the most abundant light substituted PAHs at 450 °C, while PAHs containing both nitrogen and oxygen atoms (NO-PAHs) and O-PAHs are the dominant substituted aromatics in heavy components. With the temperature increased, condensation degree and content of substituted PAHs in light and heavy components both increase due to condensation effect. Deamination and deoxygenation of NO-PAHs also enhance content of O-PAHs and N-PAHs, respectively. At 850 °C, nearly all heavy components derive from the condensation of light molecules instead of the direct decomposition of sludge. Aromatics, especially N-PAHs, are the dominant compounds in both light and heavy components. Calcium compounds in sludge can greatly affect the formation of substituted PAHs at high temperatures.