Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

Rainwater represents a potentially substantial source of usable water in the urban environment – but it largely goes to waste at present. In 2010 the City of Cape Town had a freshwater supply of 556 million m3 and demand of 511 million m3, with a growth rate of 2% per annum it is estimated that by 2014 the demand for freshwater would be higher than the available supply. The City of Cape Town is currently considering new ways of sourcing freshwater to meet the growing demand.

 

One of the ways the City of Cape Town is considering includes the use of desalination plants to convert saline sea water into drinkable freshwater. Even with many advances in recent years desalination plants are still a relatively expensive process to implement and they require large amounts of energy to effectively convert saline water to freshwater (WWCW in the 21st Century, 2000, p. 17).  

 

The second way in which the City of Cape Town is considering sourcing new freshwater supply is through the use of groundwater supplies which are pumped out of the ground. The system utilises treated sewer and stormwater which is pumped into large detention ponds and allowed to infiltrate into the ground (Tredoux & Cain, 2010). The problem with using this system is that the quantity of water that is infiltrating into the ground must be equivalent to the amount of groundwater that is being extracted otherwise a deficit will occur and the surrounding water table will drop. After 30 years, the sources of groundwater supply only make up around 2% of the total supply but this is expected to increase to 7% in the future (City of Cape Town Water and Sanitation Department, 2010).

 

Both these methods of freshwater sourcing require the water to be transported long distances from where the freshwater is captured to where the freshwater is stored and then again to where the freshwater is used. The transportation of water results in additional higher costs involved with the conveyance of freshwater. The long distance conveyance of freshwater tends to make these systems unsustainable over long periods.

 

The need to find sustainable alternatives of freshwater supplies in City of Cape Town is becoming more and more of a necessity. One of these alternative options includes the use of rainwater harvesting systems. Rainwater harvesting systems are a proven method of collecting, storing and using rainwater at the source of collection. The reduction in the length of how far water needs to be conveyed makes rainwater harvesting systems a lot more sustainable than when compared to other existing alternatives.

 

This study will consider an industrial site in the City of Cape Town such as Paarden Island and try to establish whether or not it will be feasible to implement rainwater harvesting systems to either completely supply Paarden Island or at least supplement some of its water usage during the rainy months.