This lesson gives general considerations on costs of a rainwater system, largely determined by the costs for the storage tank. Examples from different countries show materials lists and prices.
General considerations
Water storage forms the largest single cost component of a RWH system and so is the most likely candidate for cost reduction.
Current state-of-the-art for storage tanks in developing countries is ferrocement construction, either based on a fixed solid mould or on an open framework.
Generally, a solid mould results in a cheaper tank, as the wall thickness of the tank can be more tightly controlled and work is done against an inflexible surface. Open frameworks have the advantage of allowing greater flexibility in tank size but at the cost of plastering work taking place against an elastic backing which lets some mortar through resulting in greater and more variable wall thickness and a higher cost per tank.
Tanks are also made from bricks which can reduce costs by material substitution but the savings are often not great although more of the money remains in the community.
A graph of current costs of various techniques in the three study countries is given below:
(click on the graph to enlarge)
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Notes:
Points represent costs with household labour discounted, error bars represent the extremities when household labour is included in full or ignored altogether.
Lines slopes correspond to a cost:capacity sensitivity of 0.6. A cost sensitivity to volume of 0.6 – 0.7 is the norm, so designs can be usefully compared over a wide range of capacities.
The solid blue lines represent the bounds of normal state-of-the-art. the dotted red line represents the lower limit of a region of “exceptional” low-cost designs such as the Thai jar and the Ugandan tarpaulin tank.
Most of the designs also have a greater reliance on household labour and local materials so compulsory cash costs are kept lower still (as indicated by the bottom extent of the error bars).
Cost of storage tanks
The table below gives the cost of storage tanks as quoted in the literature from Kenya and Sri Lanka as a comparison.
You have to be careful when comparing prices, as they might contain labour, guttering, etc.or might be quoted just as material costs.
System | Volume (cbm) | Cost/cbm | Remarks | Source |
Water jar | 3,4 | 20 | Materials only | Nissen-Petersen (Kenya), 1990 |
Ferrocement tank | 46 | 15 | Materials only | Nissen-Petersen (Kenya), 1990 |
Ferrocement tank | 46 | 19 | Materials with gutters | Nissen-Petersen (Kenya), 1990 |
Ground tank (with concrete roof) | 80 | 11 | Materials | Nissen-Petersen (Kenya), 1990 |
Ground tank (with concrete roof) | 80 | 13 | Materials with gutters | Nissen-Petersen (Kenya), 1990 |
Ferrocement tank | 5 | 25 | Materials and labour | Sri Lanka, 1996 Rainwater Harvesting Forum |
Rock catchment | 13.000 | 1,6 | Materials and labour | IRC-Book (Kenya), 1988? |
Subsurface dam | 3.500 | 2,5 | Materials and labour | IRC-Book (Kenya), 1988? |