Dear Mayor Milton,  one of ...

Dear Mayor Milton,  one of the questions that needs to be asked of the company is from which surface the rainwater is collected.  This may be from the factory roof, in which case they should install a "first flush disposal system".  This allows the first 10 minutes or so of rainfall to be disposed of - as this would contain most of the dirt (including bird droppings) to bypass their collection tank and go to drain.  After this the rainwater will enter the collection tank and be fairly clean - although a sieve should also be installed at the entrance to the collector tank to catch leaves and any other large debris.  The rainwater may still contain some fine particles and even some organic solids.  These should be filtered out with a filter as suggested by other responders, and then as a final step disinfected with UV or ozone.  The factory will need to decide whether they need to adjust the pH for the optimum product and shelf life.