Effects of earthworm mucus and straw charcoal on heavy metals during domestic sludge co-composting

Heavy metals restrict the reuse of municipal sludge. To passivate the activity of heavy metals, reduce sludge toxicity, and create new value, 2 kg of sludge was composted with 40 mL earthworm mucus and 2%, 4%, 6%, and 8% straw charcoal, to investigate changes in the heavy metal mobility in sewage sludge. The results showed that, compared with the control sludge compost (CK), the pH increased by 1.42% (P<0.05) and the total nitrogen and total phosphorus decreased by 7.87% and 14.18%, respectively (P<0.05), after the addition of the mucus to the sludge. After adding both the mucus and straw charcoal to the sludge compost, the sludge gradually became alkaline; furthermore, its electrical conductivity value increased by 5.71%−9.58% (P<0.05), and organic matter content increased by 7.71%−24.60% (P<0.05). Although this enriched the content of soluble ions and available organic matter in the compost, the total nitrogen and potassium contents decreased by 19.10%–30.95% and 7.87%–14.31%, respectively, resulting in the loss of plant nutrients. By adding mucus to the sludge compost, different total heavy metal contents showed different declining trends; these included Cd, Cu, Ni, Zn, and Pb, which decreased by 3.59%, 7.03%, 10.93%, 8.39%, and 5.11% (P<0.05, except Ni), compared to the CK treatment group. The more active forms of Ni, Zn, and Pb were transformed into an unavailable residue form that was difficult to degrade; therefore, the proportion of residual forms increased by 61.81%, 120.19%, and 72.51%, respectively, compared with the CK treatment. When the mucus and different proportions of straw charcoal were added to the sludge, the total heavy metal contents decreased further. The total amount of Cd, Pb, Cu, Ni, and Zn decreased by 37.18%, 67.36%, 6.07%, 59.59%, and 31.82%, respectively, in the mucus plus 8% straw charcoal treatment group (P<0.05). The Ni and Pb associated with the carbonate, Pb associated with iron-manganese, and exchangeable Zn were gradually shifted to the residue form, so that the available contents of Ni, Pb, and Zn were significantly decreased by 28.08%, 42.00%, and 28.31%, respectively, in the mucus plus 8% straw charcoal treatment group, which passivated Ni, Zn, and Pb in the composted sludge. In contrast, organic form of Cu was converted into exchangeable and residual forms. Its available content was increased by 89.82 % (P<0.05) in the mucus plus 8% straw charcoal treatment group, and Cu was activated in the sludge during composting. In the analysis of the effect of mucus and different ratios of straw charcoal on the availability of heavy metals after composting, it was found that the correlation coefficients of straw charcoal addition with the available forms of heavy metals Cu, Zn, Pb, and Ni reached significant levels of 0.906, −0.909, −0.847, and −0.639 (P<0.05), respectively, while the correlation coefficients with Cd were lower. Finally, based on the principal component analysis and stepwise regression equations, mucus in combination with straw charcoal influenced the pH of the sludge compost, affecting the mobility of Ni, Zn, Pb, and Cu. Therefore, mucus plus 8% straw charcoal is an effective approach for treating the heavy metals in the sludge.