It sounds as though this particular plant has a containment problem.

What are the causes of the leaks and what can be done to fix them?

To quote PWSlack "If all else fails, read the instructions" on the container of epoxy. Better yet get technical help from the people who sell you the epoxy. They will be the one's who know how their product should be applied.

It changes as per manufactures requirement. Still it apply in two coats in thickness of 600 micron for each coat.

It goes without saying you should consult and comply with your coatings rep recommendations. Keep in mind, a coatings guy may not have a lot of expertise with this type of surface. Concretes can be tricky to work with based on the environment.

Coatings of all kinds are subject to and sensitive to a concretes chemistry. Outgassing and Moisture Vapor Emissions are real considerations for any coatings longevity on a concrete surface. If your rep recommends a primer or some preparation of a surface prior to coating, ask if this will control alkalies migrating out of the matrix to burn a coating off. Vapor emissions may allow water to collect under your coating and cause everything from bubbles, discoloration or a complete failure.

Also keep in mind that the best coatings are subject to abrasion or nicks that can compromise your coating, leaving your substrate exposed to the acids you speak of in your question. Also be aware that acid etching of a surface exacerbates a host of problems for a coating. Consider stabilizing your concrete to prevent coating failures after you put a tank in service. A lot easier than drawing down a 3MG tank to affect a repair[ assuming you even become aware of a problem ] at a later date!