The disposal of increasing quantities of dewatered sewage sludge in landfills may cause geoenvironmental problems such as leachability, total and differential settlement, slope stability of landfill sites and leachate contaminations. The objectives of this study are to investigate the geotechnical, physio-chemical and leaching properties of the dewatered sewage sludge generated from chemically enhanced primary treatment (CEPT) plant at Stonecutters Island in Hong Kong, and to assess the feasibility of sludge disposal in landfills....[ Read more ]

The disposal of increasing quantities of dewatered sewage sludge in landfills may cause geoenvironmental problems such as leachability, total and differential settlement, slope stability of landfill sites and leachate contaminations. The objectives of this study are to investigate the geotechnical, physio-chemical and leaching properties of the dewatered sewage sludge generated from chemically enhanced primary treatment (CEPT) plant at Stonecutters Island in Hong Kong, and to assess the feasibility of sludge disposal in landfills.

With regard to the geotechnical aspect of dewatered CEPT sludge, it is characterized by high water contents, high organic content, and high compressibility. Compaction test results show that CEPT sludge exhibits compaction characteristics similar to clayey soils. However, the lowest hydraulic conductivity of CEPT sludge does not occur at 3% wet of its optimum water content. Pore water pressure dissipation of sludge during consolidation does not follow Terzaghi's theory probably due to its fibrous microstructure. Under an overburden pressure higher than 24 kPa, the relationship between the logarithm-of-hydraulic conductivity (log k) and void ratio (e) is linear. Results from the direct shear strength tests reveal that slopes of 20° can probably be constructed in a landfill site without causing any sliding problems. Compared with the disposal of pure CEPT sludge, the co-disposal of dewatered sewage sludge with municipal solid wastes results in a lower compressibility, a smaller settlement, but a higher hydraulic conductivity.

On the other hand, from the environmental point of view, experimental results from four bench-scale column tests show that leachate from landfill disposal of CEPT sludge is characterized by high nitrogen, Fe and Cd content, which exceed the drinking water standard. Sludge compacted at a lower water content generates leachate with a higher concentration and a higher leaching rate than sludge compacted at its natural water content. If CEPT sludge is disposed in landfills, Cr, Fe, Zn and Cu might be released at a higher rate than other metals in its leachate. Compared with the disposal of pure CEPT sludge in landfills, the co-disposal of sludge with solid wastes has higher average leaching rates of Kjeldahl-N and metals, but a lower biodegradation rate.