Improvement of the rainwater storage system
This article is a follow-up to the one on rainwater harvesting.
Since I am currently renovating underground cisterns, several ideas have come to me to optimize their design. I won't be able to put it into practice completely because I'm working on old tanks. But if I had the opportunity, here's what I would do.
Points to watch out for
Foremost, you have to anticipate a certain number of things to avoid modifying the installation too often afterwards:
what to do if it rains too much
what to do if a tank needs to be repaired
what to do to avoid frequent tank repairs
how to make sure that the tanks can be pumped easily
how to ensure that tanks are both accessible and remain fresh
While examining the damage to one of the cisterns on the farm where I live, I realized that its location was very judicious to keep water fresh (underground) and to make sure that water from the gutters could be easily collected, but that it really lacked intelligence when it came to repairing the cistern and especially to avoid having to do it too often.
The two main problems with underground cisterns (I mean literally underground, not just a little below the ground surface) are the pressure exerted by the soil on the walls (which can lead to collapses) and the roots of trees, which not only can be deep but are attracted by the water percolating through the porous cistern walls.
The tree roots will go so far as to pierce the walls and create leaks on all sides, which are extremely difficult to repair, as this would require digging to remove the roots (which would eventually come back anyway), presenting a very high risk of collapse.
Some people have the idea of removing the trees near the cisterns, but not only can some of the roots be more than 50 meters long in certain contexts, but the reduction of trees causes other problems in most regions (lack of shade and coolness, hydrological problems, decrease in food resources, biodiversity, weakening of electromagnetic protection, etc.). It is therefore not a synergetic solution even if it can be advised in some cases to not have too many trees near one’s house (risk of falling, too much shade, too much humidity, problems with insurers).
Another common problem is to be able to intervene in the tanks at any time in case of problems, even if it is raining and without interrupting the harvesting nor the pumps.
Finally, access to the tanks, whether for repairs or to lower the pumping hose, is rarely satisfactory. As a result, people are reluctant to go down to the tank, for example, preferring to postpone the maintenance task.
Optimized solution proposal
Here is the most functional solution I have found so far, and which completes the scheme I have drawn in the article on rainwater harvesting.
First, you have to create a room that will be cool all year long. It will therefore necessarily be partially buried (not entirely, we will see why farther) and will serve as a cellar for storing food. In this basement, at least three cisterns will be installed, raised above the ground by feet high enough to allow a person to slide in if repairs are needed (it shouldn’t in theory).
The cisterns should be made of rock and mortar or concrete if not possible. Bricks could do the job too but don’t hesitate to check if the mineralization of the water is correct (ideally between 20 and 50 mg/L).
Each tank can be isolated from the other two for maintenance purposes and can be emptied completely without interrupting the rainwater harvesting system.
An overflow will be installed in case of heavy rainfall. This overflow must be able to be emptied outside by gravity, which is why the basement must not be completely buried. The other reason is that it is easier to ventilate a room that is at least partially open to the outside.
The pumping pipe must be able to be maneuvered from one tank to another without hindrance.
A simple ladder should be sufficient to check the tanks and to climb down into them if necessary.
For a family of 4, each tank should have a capacity of at least 10 cubic meters in a temperate climate (with seasonal rainfall but still spread out over time).
This corresponds for a square tank to 2*2*2.5 meters or for a round tank 2.5 meters in diameter and 2.5 meters high.
It is possible to add a little margin in height for practical purposes and remain on a total of 30 cubic meters of water available (30,000 liters).
Don’t forget also the capacity of the upper domestic tank where the pump ends: 1000 liters should be fine for a family of 4.
Here is a more precise diagram of the tanks, then put in the global context of rainwater harvesting:
It is not necessary to connect all three tanks to the gutters and overflow, since maintenance is normally carried out one tank at a time, there is still at least one tank connected.
To empty one tank during a maintenance operation, the use of the pump goes without saying. If it is not possible for some reason, a simple system of communicating vessels with a flexible hose should suffice to empty the tank into another until the equilibrium between them is reached. Then it will be necessary to use something else such as a bucket to finish emptying it.