"Treated" to produce what?  What is the quality of the raw waste as far as some basic parameters, such as total solids, TSS, sulfide or total sulfur, etc?  What quality does the effluent need to meet?

If the waste is in fact "spent caustic," what is the percent of NaOH?

If the caustic is loaded with captured sulfur compounds, such as sulfide and mercaptan, you need to be cautious about acidifying the caustic as that would generate very high levels of gaseous hydrogen sulfide and mercaptain, which are highly toxic.

I would look into the possibility of use of a Multiple Effect Evaporator to obtain reusable Caustic solution of greater than 25% concentration. This concentrate could be reused or sold for other useful purposes.