Abstract This study elucidates the co-composting of biosolids and spent activated clay (SAC) using physio-chemical, bioassay, and spectroscopic methods. A pilot-scale pile of blended limed biosolids, SAC, and rice husk was composted for 15 weeks. The changes in temperature, pH, Fourier-transform infrared (FT-IR) spectra, C/N, and germination index (GI) of Chinese cabbage ( Brassica chinensis) seeds with time support the goal of producing a mature compost with a decline in the SAC acidity of associated with biosolids. Cadmium, Cr, Cu, Ni, and Pb in the initial biosolids were converted from labile fractions into relatively immobile phases upon maturation. Temperature, moisture, pH, C/N, and GI were used to separate the composting process into three phases – initial, thermophilic, and cooling, based on a score plot of principal component analysis (PCA). The values of the parameters of interest reveal that the compost fulfills the requirements of compost maturity in the literature.