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Currently, thermophilic membrane biological reactors (TMBRs) are used to treat industrial wastewaters and biological sewage sludge with the aim of nutrients recovery. The performance of the biological process is strongly influenced by rheological behaviour of the thermophilic biological sludge (TBS) inside the reactor. Considering the high concentration of heavy metals in matrices fed to the reactors, this work aims to evaluate the influence of heavy metal types and concentration on rheological properties of TBS. Sludge has been extracted from a full-scale TMBR and conditioned with Fe3+, Cu2+, and Al3+. Rheological measures have been conducted and constants k and n of Herschel–Bulkley model were used to define the rheological properties of TBS. Rheological properties of high consistency TBS (0.06 ≤ k0 < 0.2 and 0.6 ≤ n0 < 0.8) were not significantly affected by the conditioning with Fe3+, Cu2+, and Al3+. In case of TBS with initial low consistency (0.02 ≤ k0 < 0.06) and behaviour more similar to Newtonian fluids (0.8 ≤ n0 < 1), Fe3+ and Al3+ determined a significant increase in consistency. On the contrary, the addition of Cu2+ reduced k of conditioned TBS with a lower impact on the distance for Newtonian behaviour (n). This work demonstrates the strong influence of Fe3+, Cu2+, and Al3+ on the rheological properties of TBS depending on the initial consistency of the sludge, and the types and dosage of heavy metals. [ABSTRACT FROM AUTHOR]