The production of artificial soil mix from coal fly ash and sewage sludge

The scarcity of land in Hong Kong makes landfilling an unattractive means for the disposal of sewage sludge and coal fly ash. Therefore, reutilisation of these solid wastes might ease the disposal problems. A glasshouse pot leaching study was performed to evaluate the feasibility of using alkaline fly ash as a stabilisation agent for sewage sludge and the final product being used as a potting medium. Sludge was amended with ash at 0, 5, 10, 35 and 50% (w/w). Each mixture was then mixed with a loamy soil (1:1 v/v) and leached with 600 mL of deionized water prior to plant growth experiment usingAgropyron elongatum (Tall wheat grass). Soil pH and electrical conductivity following leaching increased consistently with an increase in ash amendment, from 6.3 to 8.2 and 1.6 to 2.3 dS m⁻¹ respectively. Pots amended with 35% ash showed a significant reduction in Zn, Cu and Cd availability but an increase in B contents. The total dry weight yields of ash amended pots were significantly increased as compared to the control without ash amendment. Addition of ash also significantly reduced the uptake of Zn, Cu and Mn but increased shoot B contents. However, the high tissue B contents had no adverse effect on plant growth. The experimental results affirm that alkaline ash is effective in reducing the metal availability of sludge, and acts as a good stabilisation agent for sewage sludge.