Process Optimization and Environmental Risk Assessment of Vermicomposting Co-composted from Sludge, Kitchen Waste and Stored Waste

The resource of sludge, kitchen waste and stock waste play an important role on the organic solid waste management in the city based on the waste classification. In this study, the process optimization and environmental risk assessment of vermicomposting co-composted from sludge, kitchen waste and stock waste (VCSKS) were studied by the orthogonal test, the plant growth experiment and potential ecological risk model. The results indicate that the VCSKS could significantly improve physical and chemical properties of composting and the growth of earthworms. However, the addition with more than 50% kitchen waste could restrain the growth and reproduction of earthworms. The best parameters of the VCSKS is 7∶3 mass ratio of sludge and kitchen waste, 5% addition of stock waste and 33 pieces per kilogram earthworms inoculation rate. Both the quality of vermicomposting and its mixed soil met with both the requirements of the organic fertilizer and the planting soil for greening. When the vermicomposting addition proportion was 20%, the growth of ryegrass was the best. When the vermicomposting addition proportion reached to 50%, the organic matter, total nitrogen, total phosphorus and available potassium were the largest, which increased to 107.00 g/kg, 6.69 mg/g, 5.96 mg/g and 0.42 mg/g, respectively. As the vermicomposting addition proportion increased from 0 to 50%, the potential ecological risk index of single heavy metal increased in soil, and the combined potential ecological risk factor increased from 17.42 to 33.82, with the environmental risk all to be at low ecological hazard level. Cu had the greatest potential ecological hazard (Er=17.69). With thirty times addition of vermicomposting at 20% proportion, of the fertilizer the ecological risk index of Cu could be increased from 10.69 to 35.04, and after then the accumulation effect would become non-obvious (Er=35.09). Therefore, vermicomposting co-composted from sludge, kitchen waste and stock waste will be a safe and effective route for the municipal organic solid waste management in cites.