Water Cistern at Casa Cantalagua, 2018

A Rain water harvesting cistern at under 2 Pesos/ l (Mexico) or 8 Rupees/ l (India)

This is one of the points on the learning curve of the several attempts of making cost effective and sustainable rain water cisterns. This is a scaleable design of a underground water tank of 12,000 L that cost less than 24000 pesos (excluding plumbing). Add 8000 pesos for drains and plumbing.

It can be scaled up or down proportionally and will work from 3000l to 18000l at a price under 2 pesos/l.

Note: We have decided to waterpoof the cistern with only slaked lime. We shall store slaked lime in this cistern for the next 4 months, while using it for the construction of the house. We shall not do the inner ferrocement water proofing nor the ferrocement floor on the top.

In this post, I will focus on the capacity calculation of a rain water cistern

My case: Here in Mexico, like in India there is no public supply of safe drinking water. Drinking water is mainly consumed from “garrafones” 25 l refillable plastic water containers. The refill costs 30 pesos as of today (March, 2018).

A family of 4 (assuming 2 l a day), consume at least 10 garrafones a month, amounting to 300 pesos a month, i.e. 3600 pesos a year. All this for just clean drinking water.

In addition, my daily water consumption has been about 100l per day, or 3000l per month, or 3 m3 per month. That costed me about 95 pesos / month. A family of 4 shall be consuming 4 times, or 380 pesos/month or 4560 pesos / year.

Total water expense per year for a family of 4 is = 3600 + 4560 = 8160 Pesos/ year.

Here is my calculation for Contepec Michoacan, with a 144 sqm roof (compensated for runoff).

My Case: If we built a cistern that costed less than 2 pesos / l of storage, we would save money within the first 4 years of its functioning.

1. In Queretaro with 532mm annual rainfall, If every household with roof area more than 40sqm is connected to a 3000l tank, the annual saving by avoiding canned drinking water is 3600 pesos / year. Cost of Cistern is less than 6000 pesos. Payback is less than 2 years.
2. In Contepec, Michoacan with 830mm annual rainfall, if every household with roof area more than 40sqm is connected to a 10000 l tank, the annual saving is avoiding drinking water + 6 months free rainwater for daily use. Cost of cistern is less than 20000 pesos. Payback is less than 4 years.