One interesting alternative to the usual end-of-pipe processes for treating waste water produced during cheese-making involves its fermentation to sodium lactate. Then, electrodialysis can be used to recover free lactic acid from the fermentation broth. The design and optimization of such a process is aided by a suitable mathematical model. This work presents a model of the performance of an electrodialysis stack to be used in such a process. Its reliability has been assessed by comparison with experimental data for free lactic acid recovery from sodium lactate solutions. A suitable procedure for estimating some process-dependent parameters is also discussed.