Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

Data on the freshwater consumption of various erven in Paarden Island was required to determine the amount of water that is used by businesses in the area. This information would be used to determine the amount of rainwater that would be required to offset the current freshwater use. The estimated average monthly water consumption for erven in Paarden Island was calculated based on data that was received from the City of Cape Town Department of Water and Sanitation, Water Demand Management. The data was assessed and modified and then used in the calculation of rainwater tank sizing and water consumption costing.

 

Assumptions

The following assumptions were made concerning the use of the water consumption figures given by the City of Cape Town:

• It was assumed that any erf that is not listed in the water consumption data either doesn’t use water at all, has no access to water or forms part of another erf which does have a paying account;

• It was assumed that any figure that was recorded and seemed reasonable would be used. Water consumption data that was deemed acceptable had to have at least 5 readings that were of the same magnitude, this was check by comparing the average water consumption to the standard deviation of each erf’s data set;

• It was assumed to be acceptable to remove the maximum or minimum data point that was the furthest away from the average water consumption if the data point was not of the same magnitude. These data points were most likely the result of unforeseeable consequences that occurred on the site. Maximum values that were observed may have been due to a burst pipe and a minimum value may correspond to a time in which the erf was not being used.

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Results and assessment of water consumption data

The average, maximum, minimum, range, standard deviation, median and the number of readings were all calculated using the data set that remained after the modifications were made. The results of these calculations can be seen in table F-2. Only 255 out of the 305 registered erven in Paarden Island had registered paying water accounts

The total cumulative average monthly water consumption for Paarden Island was estimated to be 41 000 kL which equates to 492 000 kL per annum. The average monthly water consumption for erven in Paarden Island was estimated to be 163 kL with an average standard deviation of 67 kL. The maximum average monthly water consumption was estimated to be 5 950 kL with a maximum recorded consumption of 6 570 kL. After the modifications were completed only 222 erven were used in the rainwater harvesting implementation feasibility calculations. Table 5-5 gives a summary of the water consumptions for erven in Paarden Island.

 

Table 6‑1: Summary of water consumption in Paarden Island