The increase of studies relaying on gas to liquid mass transfer in digested sludge (shear thinning fluid) necessitates a better understanding of the impact of apparent viscosity (mu(a)) and rheology in process performance. Mass transfer retardation due to mu(a) variations was investigated in a pilot scale absorption bubble column for Newtonian and shear thinning fluids with varied superficial gas velocities (U-G). A non-linear reduction of mass transfer efficiency with increasing mu(a) was observed, being the impact higher at low mu(a) ranges and high U-G. An increase of 114 cPo in mu from 1.01 to 115 cPo in glycerol solutions saturated with U-G = 1.73 cm s(-1) led to a reduction of 96% in k(L)a (alpha = 0.04), while a comparable raise from 115 to 229 cPo implied a reduction of 52% (alpha = 0.02). Slug-annular flow regime was identified for shear thinning fluids of high mu(a) (1.0% and 1.5% carboxymethyl cellulose sodium salt solutions), where bubble buoyancy was conditioned by the mu of the fluid at rest and the active volume for mass transfer was reduced because of the presence of stagnant areas. Conditions imitating the rheological variability of anaerobically digested sewage sludge were included within those tested, being a reduction in gas transfer efficiency of 6 percentage points (from 7.6 +/- 0.3% to 1.6 +/- 0.1%) recorded when increasing mu(a)' from 130 to 340 cPo. It is thus recommended that rheology and mu(a) variability are accounted for within the design of gas to liquid mass transfer systems involving digested sewage sludge, in order to avoid reductions in process performance and active volume. (C) 2015 The Authors. Published by Elsevier B.V.