I really did think that if I wrote about rainwater harvesting we would have some significant rain. No such luck, but a few light showers have been collected and are now held in our underground water cistern. This means we have some water but not enough to stop wishing for some more rain. But, possible thunderstorms are forecast for today so who knows, maybe this post will tempt fate and the weather gods enough for us to have a good downpour and harvest the rainwater we need.
Whilst I accept not many folks will be digging up their basement to install an underground rainwater cistern, that is definitely the best way to harvest rainwater. Underground cisterns can hold as much water as you like so provide an efficient way of harvesting most if not all the available rains. Most importantly, being situated underground keeps the water at a constant temperature. Whilst the water stored in our above ground tanks makes for energy saving hot showers it is the cooler water in the underground cistern which makes for the best refreshing cold shower at this time of year.
Of course siting the rainwater cistern underground does induce a few dilemmas regarding overflows and pumps. Perhaps the most energy efficient solution is to still use a roof space cistern to hold some water to feed the shower, washing machine and anything else with a heater attached. Using this double system of water storage means the pump which extracts water from the underground cistern does not have to be too expensive or powerful. Ours is a very cheap and cheerful 12volt battery powered pump. It can shift the water three metres vertically with ease, but has trouble doing that at the pressure required to make the hot water boiler kick in.
As well as working out how to get the water out of your cistern it is important to have an access hatch and an overflow. It’s true the access hatch is not needed that often, but it’s always good to be able to see how much water you have left before inviting guests to enjoy a long relaxing soak. Plus there are times when maintenance may be needed. A few years ago I spent several days painting the inside of our underground rainwater cistern with waterproofing paint. Ours had seen no maintenance in the hundred years or so since its construction so it seemed sensible to give it a lick of latex paint.
More important is the overflow. This must be at least as large as the inlet pipe. But more important is where it goes. You want to divert any overflow well away from your building as the concentrated rain from your entire roof could destroy footings rapidly.
Rainwater cisterns can be built from concrete block work, rather like a swimming pool with a reinforced concrete top, or you can buy fiberglass liners if you’re not quite so confident about the strength and waterproofing capabilities of your rendering.
If sited underground there is far less chance of a concrete cistern cracking. This can be a real problem in the summer with above ground versions which dry out and are then exposed to the heat of the sun.
Always make sure the downpipes carrying rainwater into the cistern are relatively long, going a good way beneath the mid depth. Our cistern was originally fed with terracotta tiles which fed the rain water to the roof of the cistern and then let it plummet in. This is an exceptionally noisy method of capturing rainwater. If you run the downpipes to below the usual water level you may hear some bubbling but not an entire cascade of water whenever it rains. Most disconcerting when the cistern is underneath the living room.
Periodically even an underground rainwater cistern will need a good clean out. Filters are a good idea on both the rain inlet, overflow and most importantly the pump, but no matter what you do debris will eventually collect in your cistern. It’s not my favorite job but much easier if done relatively regularly.