bad water smell removal for a copper boiler

[azertyuiop]

I have a problem with our office rain water recuperation and underground storage. That water gets used for various office consumers (office toilets , cleaning , warm water generation) , to massively reduce city water consumption in our office building. I have emptied the three 15 000 liter tanks , let them be cleaned , and waited a week to get it sufficiently refilled with rain water. The really bad smell hasn't got away , the water looks clean , but after refilling a couple of buckets , the stench is still apparent , altough the filter in the water entry stays clean. We have lots of birds in the vincinity , their disposals in the gutter , flowing into the tanks may be the source of contamination.

Is there a product or method to get rid of that smell , other than a carbon filter system in the feeding line? That water also gets injected in a boiler with copper heating circuit , so decontamination with chlore or similar commercially available products is probably not an option (corrosion).

Any bright engineer available with a simple solution?