Weighted comprehensive risk assessment and pollution analysis of long-term printing and dyeing sludge landfills in an industrialized Chinese city

Sludge landfilling is widely used in China, accounting for approximately 65% of total sludge disposal, due to its simplicity and cost-effectiveness. However, with increasing land scarcity and stricter environmental regulations, the Chinese government has emphasized reducing sludge landfilling. Despite these efforts, sludge historically disposed of in landfills continues to pose risks, including heavy metal leaching and contamination of groundwater and soil. Comprehensive risk assessments for multi-contaminant industrial sludge in long-term landfills remain limited. This study conducted a risk survey and pollution assessment at a long-term sludge landfill in a typical industrial city in China. The sludge at the site was found to be mildly alkaline (pH 7.30-8.29) and had a low calorific value (1.37E+03-1.61E+03 kcal/kg), with significant dewatering challenges (1.23 × 10¹⁵ s²/g). The overall pollution load was relatively low, with a regional Pollution Load Index (PLI) of 0.65. However, due to extreme pollutants, the Nemerow Pollution Index (NPI) reached 9.49, indicating severe pollution. The single-factor index assessment indicates high pollution levels for Zn, Sb, BaP, and total petroleum hydrocarbons (TPHs). Ecological risk is primarily associated with Hg, Sb, BaP, InP, and TPHs. However, the weighted comprehensive risk assessment method proposed in the study identifies Zn, Pb, Sb, and TPHs as priority pollutants, with the main pollution hotspots being SZ4, SZ5, and SZ9. Some pollutants, such as Ni, antibiotics, polychlorinated biphenyls, and certain polycyclic aromatic hydrocarbons, were not detected in the sludge. Through principal component analysis and positive matrix factorization analysis, multiple sources of pollution at the landfill were identified, including the dyeing industry, electronics manufacturing, traffic emissions, fuel combustion, and urban runoff. This analysis highlights heavy metal pollution (Cr, Hg, Pb) and organic pollutants (PAHs, TPHs, DBA) resulting from industrial and urban activities. The results offer key insights to guide future sludge treatment and disposal at this landfill.