Biodegradation of biosolids and specific gravity determination

This paper presents a literature review on the geotechnical behaviour/properties of biosolids and sewage sludge materials, including biodegradation effects, and then focuses on the determination of specific gravity (G _s) for these challenging and difficult geomaterials. For water pycnometry, biodegradation occurring during the course of the test procedure results in apparent higher G _s values, leading to erroneous determinations for the solid, liquid and gas phases per unit volume. As demonstrated in the paper, this issue can be addressed by using kerosene or white spirits as the density liquid. Since the pycnometer method is tedious and time consuming, specific gravity is often correlated with total volatile solids (S _TV), as measured by ignition loss, which is a more straightforward test, having arguably greater reliability. Existing correlations are investigated but found unsuitable for biosolids and sewage sludge materials. Hence, a new G _s–S _TV relationship is established specifically for these materials, avoiding the necessity for direct G _s determinations in practice. Other issues investigated include the effects of intraparticle voids within the test material and the different specimen oven-drying temperatures of 50 and 105°C specified in different codes. It is concluded that oven drying at 105°C should be consistently used for testing of these materials.