The OECD guidelines 308 and 309 define simulation tests aimed at assessing biotransformation of chemicals in water-sediment systems. They should serve the estimation of persistence indicators for hazard assessment and half-lives for exposure modeling. Although dissipation half-lives of the parent compound are directly extractable from OECD 308 data, they are system-specific and mix up phase transfer with biotransformation. In contrast, aerobic biotransformation half-lives should be easier to extract from OECD 309 experiments with suspended sediments. Therefore, there is scope for OECD 309 tests with suspended sediment to serve as a proxy for degradation in the aerobic phase of the more complicated OECD 308 test, but that correspondence has remained untested so far. Our aim was to find a way to extract biotransformation rate constants that are universally valid across variants of water-sediment systems and, hence, provide a more general description of the compound’s behavior in the environment. We developed a unified model that was able to simulate four experimental types (two variants of OECD 308 and two variants of OECD 309) for three compounds by using a biomass-corrected, generalized aerobic biotransformation parameter (k′_bio). We used Bayesian calibration and uncertainty assessment to calibrate the models for individual experimental types separately and for combinations of experimental types. The results suggested that k′_bio was a generally valid parameter for quantifying biotransformation across systems. However, its uncertainty remained significant when calibrated on individual systems alone. Using at least two different experimental types for the calibration of k′_bio increased its robustness by clearly separating degradation from the phase-transfer processes taking place in the individual systems. Overall, k′_bio has the potential to serve as a system-independent descriptor of aerobic biotransformation at the water–sediment interface that is equally and consistently applicable for both persistence and exposure assessment purposes.