Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

Rainwater Harvesting Potential Capture for Paarden Island

Assessment of the annual rain gauge rainfall data

Rainfall around Cape Town can be highly variable. The rainfall data showed that the highest rainfall event average of 43 mm accounted for 8% of the total annual modified average rainfall of 522 mm. The median rainfall event of 5.7 mm was offset to the mean rainfall of 8.5 mm that was recorded. The data also showed that on average only 60 days of the year have rainfall events that are able to contribute to the collection of rainwater.It could then be concluded that most of the rainfall that occur are from a few large rainfall events. Majority of rainfall events that occur have very little rainfall which could result in problems with the collection of rainwater since for over 50% of the rainfall events would have an efficiency of about 80% or less due to the first flush device alone. The higher rainfall events that occur could result is loss of rainwater due to storage tanks being already partially full during these rainfall events and then being filled to capacity before the rainfall event is over resulting in loss of rainwater. The overall modification factor of 0.66 showed that about 1/3 of the rain that falls in Cape Town would not be able to contribute to fresh water supplies from roof surface runoff.

 

Calculation of potential rainwater that can be harvested from roof surfaces

The total roof area in Paarden Island was estimated at 528 000 m2 and would be able to provide up to an estimated 198 000 kL of rainwater supply per annum. the yearly collected amount of rainwater can vary by as much as 28% from the average. The estimated collectable rainwater per annum varies from 142 500 kL to 253 000 kL.The roof surface rainwater supply could on average account for almost 40% of the required freshwater supply. 

 

Calculation of potential rainwater that can be harvested from ground surfaces

A total of 232 000 kL of rainwater can be collected from the ground surface runoff. 94% of the ground surface rainwater comes from the tarred and paved surfaces. Ground surface rainwater is able to supply almost 44% of the water demand for Paarden Island. A the yearly collected amount of rainwater can vary by as much as 28% from the average. The estimated collectable rainwater per annum varies from 167 000 kL to 297 000 kL. Ground surface runoff rainwater does have a number problems associated with it.

 

Ground Water Surface Runoff Quality

Possible Sources of Contamination

The following are list of some contamination sources that were observed: 

• Petroleum Products;

• Total Phosphorus and Orthophosphate sources;

• Biological contaminates; and

• Suspended Soildids.

 

Estimated Concentrations of Heavy Metals

The chemical elements of Arsenic, Cadmium, Chromium, Copper and Lead all had results that were inconclusive due to the concentration of these elements being too low for the ICP-OES to detect. The concentration of Magnesium, Sodium and Zinc were all below the target water quality requirements for all uses. The concentration of Iron from 2 of the 3 samples indicated that the iron was above the recommended water quality requirement for domestic consumption and industrial categories 1, 2 and 3.