Evaluation of the environmental and plant growth effectiveness of a new substrate consisting of municipal sludge and fly ash.

In order to seek a safe, sustainable, and low-cost method for reuse of municipal sewage sludge, four species of native plants, i.e., Forsythia suspensa, Sophora japonica, Cotinus coggygria, and Ailanthus altissima were planted in flowerpots containing 4 growth substrates consisting of raw sludge and fly ash at volume/volume ratios of 20:80, 40:60, 60:40, and 80:20, respectively. The results showed that the physiochemical characteristics of the sewage sludge and fly ash were complementary. The sludge supplied the nutrients and the fly ash maintained air permeability in the mixed substrate. The mixed substrates containing 40-60% sewage sludge that belonged to sand clay loam were suitable for the seedling growth of the four species. After the end of the growing season, the electrical conductivity, pH, and contents of organic matter, nitrogen, potassium, and heavy metals in the four growth substrates decreased significantly. Moreover, most of the heavy metals were removed from the substrates by seedling root system. A. altissima grew best, and heavy metal enrichments of F. suspense and C. coggygria were stronger than other two species. The results indicate that the new substrates containing 40-60% sludge exhibiting good physiochemical properties, are environmentally friendly, and suitable for landscape planting.