Economically viable domestic roofwater harvesting

The virtues of domestic roofwater harvesting (DRWH) are well known. However against these virtues, whose value varies greatly with location, must be set a weakness of DRWH, namely that it is usually unsuitable as the sole source of domestic water. This is partly because the total resource available to a household (the product of rainfall and the house's roof area) is limited but mainly because storing water in very large cisterns is expensive. At present the capital cost of a DRWH system that will supply the bulk of a household's water in the favourable humid tropics usually lies in the range $20 to $30 per person. In the Monsoon tropics (Summer rains only) these costs may double. A water agency might compare such costs with some investment norm such as $16 to $20 per capita and conclude that DRWH is not cheap and is to be used only where other sources are particularly difficult. Typical forms of such difficulty are unfavourable topography or highly mineralised groundwater. A householder will weigh the cost against the time and money that possessing DRWH would save or might compare the system cost with that of say a bicycle ($50). Household and community surveys recently undertaken in 9 locations in Ethiopia, Sri Lanka and Uganda have indicated that excessive cost is considered the largest impediment to take-up of DRWH among poor households. However willingness to pay appears sufficiently high that if costs were significantly reduced, unsubsidised DRWH would be accepted. The principal component of cost for all but the smallest DRWH system is the water storage device, which we may call the cistern. So reduction of this component's cost will yield the greatest benefit. Such savings can be applied to make more cisterns, bigger cisterns or simply to speed up the payback. For example a 33% reduction in cistern cost could either increase internal rate of return by 50%, or permit a 100% increase in cistern size. However when modelled using climate data from Western Kenya, representative of the Equatorial zone, the latter volume increase projected into an increase in harvested water value of only ca 13%. This paper will show that through design innovation tank costs can be reduced by 30% to 50% below current norms. In particular we explore three strategies for making such savings: (i) reducing unjustified cistern size and thereby sys tem performance (ii) streamlining the production process (iii) reducing superfluous construction quality