The bacteria in rainwater need to be evaluated but our experience is that stored underground where it is very cold there is little to be concerned about.
But everything is dependent on keeping the gutters relatively free of leaves and debris and cleaning out the pre-reservoir sump regularly.
"The top 2 segments are the UV-treated water sample; the bottom half are not. There is one small colony of insignificant growth on the blood-agar of each; otherwise no bacteria at all.
Your water sample results above, both straight from the underground reservoir and the UV-treated look good. The two small colonies constitute a very low bacterial load, and neither were coloforms which would have shown up as purple."
There is considerable resistance by the general public to building underground reservoirs. Part of this is no doubt based on the cost and the hassle-factor, but also the official opinion that it is not potable; in fact potentially dangerous.
There is far greater acceptance of above-ground rainwater tanks, both those of plastic and fibreglass, despite the greater cost and much higher temperature of the liquid. This is in fact more likely to foster a greater risk of bacterial contamination.
After eight years of drinking untreated reservoir rainwater stored underground, with no problems whatsoever, we finally succumbed to family pressure and installed filters and a UV-steriliser. It was the right decision seeing that we have the finance to do it.
There is certainly the potential to have bacterial infiltration of rainwater that has been harvested and stored in tanks of various sorts.
The fact that it doesn't appear to happen to that stored underground we believe is because of the low temperature and that any bugs on the hot-roof of our home are likely to be killed by the sunshine; but still it could happen.
The very misleading assumption is that municipal water that has been treated is perfectly safe to drink; nothing could be further from the truth.
Mysteriously globules of liquid mercury were discharged this week from the taps in Soweto; they were clearly visible but what about all the unseen chemicals and bacteria?
Whilst chlorination was a massive step forwards in preventing serious water-borne diseases, it creates other hazards if there are any organic materials in solution; and it does nothing to prevent contamination in the conduits, or to remove other toxic substances such as plastic microparticles.
In short, nothing is perfectly safe; we consider bacteria from rainwater, especially now that we have inserted filters and a UV-lamp, far less of a concern as compared to that from the utility.
These considerations are weighed in more detail at post-chlorination.
In addition, the availability of our rainwater is far more reliable and comes courtesy of the heavens; it's free for those who have taken the trouble to install an underground reservoir.
Our underground rainwater reservoir contains 27,000 litres; about 7,000 gallons.
If I was to do it again, I would recommend increasing the radius to 2.5 metres, and possibly the depth too, depending on the length of the dry-season.
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The bacteria in rainwater is close to zero but what about in tanks and reservoirs?
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