Environmental implication of soils amended with anaerobically digested sewage sludge in Hong Kong

Rainfall leaching soil column trials were used to characterize the downward movement of potential contaminants through a sandy loam and sandy soil following the application of an anaerobically digested sewage sludge at the rates of 10 and 25% (v/v). Leachate pH did not vary significantly with sludge application except for sandy loam with 25% sludge, while initial electrical conductivity (EC) and dissolved organic carbon (DOC) increased linearly with sludge application and declined shortly to levels found in soils without sludge. A higher initial leaching loss of ammonium (NH₄⁺) was found in sandy soil than loamy sand due to its lower cation exchange capacity. Nitrate (NO₃^-) was the dominant anion in leachates with an average in excess of 10 mg L^-1NO₃^- at all loading rates after 12 weeks. The highest concentration of NO₃^- occurred with the highest sludge application rate. Leachate zinc (Zn) content increased in loamy sand columns at the high sludge loading rate at the end of the experiment owing to the reduced pH following nitrification. No significant difference in leachate copper (Cu) and phosphate (PO₄^3-) contents were noted for both soils receiving various sludge application rates. Evaluation of the soluble nutrients present in the soil profiles at the end of the leaching experiment showed that EC, NH₄⁺ and PO₄^3- increased according to sludge application rate up to a depth of 20 cm. Significant accumulation of NO₃^- was found in sandy loam with sludge application to the depth of 50 cm. Analyses of leachates and soils for the selected contaminants revealed that NO₃^- leaching is likely to occur without plant growth at the current application rate. Therefore, the application rate for sludge should not exceed 10% (v/v), and the provision of vegetation on the amended soil would reduce the leaching loss of NO₃^-. Copyright © 2000 Kluwer Academic Publishers.