Are you asking about collected water--in barrels or other catchment systems--that has been contaminated by wildlife?

I'm not sure why someone would want to disinfect pure rainwater. Rainwater is not a source of viruses, bacteria, or other diseases, that are transmitted to humans through plant tissue. There is no need to disinfect it before applying it to plants unless it is on a container that has been accessed by geese, ducks, flocks of birds, or mammals. 

Yes, you could add bleach to water to kill any potentially harmful bacteria, viruses, algae. However, if the chlorine is not allowed to dissipate, pouring the water on plants could potentially kill the plants. High concentrations of chlorine is fatal for plants (tap water levels is fine).  

If you can clarify your question, I will try to give you a better answer!

Hi,

The rainwater would be collected in rain barrels from gutter systems. The reason I ask about disinfection is because the rainwater will be collected after running off rooftops which can have contaminants from birds, rodents, and other urban wildlife. In my communication with urban farmers, I made sure to state that rain barrels need to have a screen over the inlet area to avoid pest infestations. Thank you for your help. Do you have any thoughts about the solar treatment of rainwater?

I consulted with some of my colleagues. If I hear back from more of them, I will send you information from their responses as well.

I think we are all concerned about the possibility of not only pathogens (virus, bacteria) in the water from wildlife but also chemical contaminants from roofing materials. 

Here is a good article from the University of Arkansas. "Consider using your rain barrel water only for ornamentals, trees and lawns rather than your fruit and vegetable garden if you are selling your produce as part of a regulated business. Metals and other harmful substances from harvested water can end up in irrigated plants and can accumulate in edible portions causing a potential human health risk."

If farmers are using catchment barrels that are not food grade and/or were not designed explicitly to be rain barrels, they can be leaching PFAS into the water they are applying to food crops, contaminating the soil, and then being taken up by the plants and into the produce. 

Apparently, chlorine (bleach) can react with Dissolved Organic Carbon in collected water and create other problems (reduce chlorine's effectiveness, increase bacterial regrowth).

Using solar heat to disinfect water might kill most pathogens, but it will not remove harmful chemicals. 

In addition, solar heat is only a viable option for a small part of the Michigan growing season. Farmers would have to invest in the equipment to ensure the water is adequately heated, including thermometers. 

If the farmers are interested in the solar option, I would recommend that someone do a small pilot program and involve the Wayne County Department of Health, Human and Veterans Services to test for contaminants. 

I think the safest approach is to recommend that farmers only apply rainwater to ornamental crops, like cut flowers or pumpkins, or nursery crops like native plants, trees, shrubs, etc. Applying water to food crops that is potentially contaminated with PFAS or heavy metals, as well as organic materials or pathogens, is risky and not some thing that MSU Extension suggests or recommends.