Pre-treatment, measurement execution procedure and waste characteristics in the rheology of sewage sludges and the digested organic fraction of municipal solid wastes

Abstract

The rheology of a wide range of sludges coming from wastewater treatment plants and from the digestion and codigestion process of sewage sludge and organic fraction of municipal solid waste (OFMSW) is measured. Sieve pre-treatment is necessary for selecting the solid fraction (<0.84 mm) in order to carry out measurement avoiding bob locking in a rotational viscosimeter. All sludges, except those from double phase digestion of source selected OFMSW, have a minimum of 80% TS and 70% TVS in this fraction. By using a plastic model, the flow curve offers a yield stress with a high standard deviation particularly at low solid content, while the rigidity coefficient s.d. is less than 10% at 90% probability. The plastic model can be proposed for all sludges since it offers a standard error of data fitting of less than 5%, in addition this model is not significantly different from the pseudo plastic one. Rheological properties cover a four order of magnitude range and follow a power type relation with TVS for the whole sector of sewage sludge and an exponential one for the OFMSW sector. Between 5–55°C, a positive temperature sensitivity is shown, while the activation energy is not significantly different from that of water. A high thixotropic nature is found in sewage and a low one in OFMSW sludge. A power relation with the TVS content is verified only for the sewage sludge sector.