Characterization of the physical & chemical nature of sewage sludge with regard to its suitability for disposal in landfills  

Project Title: Characterization of the physical & chemical nature of sewage sludge with regard to its suitability for disposal in landfills

Applicant: Hong Kong University of Science and Technology

Total Approved Grant: $150,000

Duration: 01/02/99 to 31/01/00

Project Status/Remarks: Completed

Scope:

  1. To determine the vane shear strength of dewatered sewage sludge alone and mixtures of sewage sludge and construction waste in varying proportions; and
  2. to investigate experimentally the settling characteristics and the leachability of both dewatered sewage sludge and the mixtures of sludge with construction waste.
Summary of the Findings/Outcomes:

The ultimate objective of this study is to investigate the geotechnical and environmental impact of disposing dewatered CEPT sludge with and without other solid wastes on landfills. The CEPT sludge contains high concentrations of volatile organics and Fe. Both sludge mixtures and sludge at its optimum water content show larger fiction angle under direct shear tests, which is considered to be safer when placing and compacting them on side slopes of landfills. Compaction has proven to be an effective way to reduce the volume of CEPT sludge and its mixtures and improve their engineering properties. The sludge and its mixture behave very similarly to soil compaction characteristics, except they both exhibit lower dry densities.

The hydraulic conductivity of CEPT sludge at its natural water content is about 1.0*10-9 cm/sec, while at its optimum water content is about 1.0*10-7 cm/sec. Moreover, it is interesting to find that the lowest hydraulic conductivity does not occur at its 3% wet of optimum water content. In addition, the hydraulic conductivities of sludge mixtures are four orders of magnitude higher than that of sludge at is natural water content. Thus, these results indicate that a higher amount of leachate might occur from the co-disposal of sludge and solid wastes.

For a given stress increment, sludge at its natural water content undergoes a larger reduction in void ratio during consolidation than that at its optimum water content. Moreover, its compressibility factor (i.e., 0.31) is also the largest one among all other waste samples. As a result, higher compressibility and settlement are expected to occur for sludge at its natural water content.

Leaching of CEPT sludge began after 12 days of infiltration. Among five metals of interest, Zn leaching is found to have the highest concentration while Pb has the least. The leachability of the sludge mixtures at ratios of dewatered sludge is: paper (70% water): sand = 1:6:4 and 1:28:21. Among five metals of interest, Cu and Fe leaching are found to have higher concentration while Pb, Zn, and Ni have the least. It is also found that metal leaching from sludge mixtures occurs at a much faster rate than from CEPT sludge alone. This is most likely due to the fact that the hydraulic conductivities of sludge mixtures are four orders of magnitude higher than that of CEPT sludge at its natural water content. Also, the hydraulic conductivity of sludge mixture at 1: 28: 21 (i.e., k=6*10-5 cm/s) is twice as fast as that at 1: 6: 4 (i.e., k=3*10-5 cm/s).

The settlement of CEPT sludge is about 3% of the total height after 95 days of loading. At this stage, the sludge settlement is not easily discernible between the primary and secondary compression because decomposition of sludge might contribute further to the secondary compression.