Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

Assumptions

The following assumptions were made concerning the calculation of municipal water supply:

• It was assumed that the cost of municipal water would be proportional to the average annual rainwater that is used by an erf;

• It was assumed that the cost of water would increase by an average of 9.5% per annum which is equivalent to what was proposed in the 2013/2014 City of Cape Town budget. The cost of water may in future increase at a rate that is higher or lower than the 9.5% that was stipulated. If current water supply is not able to meet the current water demand then one of the first measures that will be implemented to reduce water demand would be to increase the cost of water;

• It was assumed that an average increase of 5% per annum could also be used to establish a lower bound estimation of the rainwater usage cost that could be achieved through out the life span of the rainwater harvesting system. The lowest limit of 5% represented the mean inflation rate in South Africa over the last 10 years; and 

• It was assumed that each year had a fixed tariff for the water supply which started from January to December. In reality the year starts from April to March the following year. This assumption was used mainly to make calculations easier.

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Calculation of the cost of municipal water substitution

Table I-3 to I-6 shows the current and future estimated costs of municipal water and sewage removal for different rates of inflation that were used in the calculations of municipal water substitution. The cumulative total that is calculated after 10 years is used to establish the feasibility of a rainwater harvesting system that has a life span of 10 years. The results of the 10 year rainwater consumption for each of the 5 models due to 9.5% inflation are given in table I-7to I9.

 

Assessment of municipal rainwater substitution

Based on table I-1 it can be observed that the potential cost savings form the use of rainwater as a supply source is quite substantial. Scenario 3 can offer the highest benefit to erven but it is still not clear what the actual legal implications may be if scenario 3 can be implemented as a cost saving option. At present rainwater harvesting is still a very small scale freshwater supply in most municipalities and as such is not able to currently influence the supply of freshwater. Municipalities have not yet made modifications to the sewage billing system to account for the substitution of municipal water usage by rainwater. Sewage is still produced by the erf but the erf currently does not have to pay for it. The current method of sewage billing will have to change in future when rainwater harvesting becomes more large scale. Table I-1 shows that the municipality would lose at least R 20 million in revenue if the billing structure is not changed. Comparing Table I-1 to I-2 it can be seen that a difference of 3% in annual increases result in an average 15% difference in the amount of cost savings that can be achieved by implementing rainwater harvesting equipment.

 

Table 6‑5: Summary of cost savings from municipal water usage