Reviving this old thread (apologies if not the done thing... I'm new here!) as looking for advice on reducing risk from large underground rainwater storage systems. One is about 40,000L in a concrete underground tank, which fills over winter, and is to water crops May - Oct, with it almost emptying by end of Oct. It uses a submersible pump to a drip irrigation system delivering water to soil directly, without spraying, once per day, in a field.  The second is about 20,000L, also a concrete underground tank, which feeds an outdoor hydroponic water flow system through drain pipes, for lettuces, herbs and strawberries. This pumps for 7-12 hours per day during daylight, and the system drains back into the main tank with no water remaining in the drainpipe system.

Both are fed from rainwater from roofs. The larger roof is aspestos the smaller tiled. 

Both tanks are concrete with concrete lids/covers and small access hatches. The sun shines onto these concrete areas of lid so I will deploy a temperature monitoring system.... and look at shading/insulating the lids to try to ensure water remains below 20 degrees. 

However, will this suffice? If not does anyone have any suggested mitigation methods to reduce risk further? 

Ideally I do not want to add chlorine as for food. 

Thank you so much for any advice provided!