Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

Once stage 1 has been completed stage 2 can start. Stage 2 focuses on assessing, modelling and analysing the data that was collected in stage 1.

 

Monthly Rainfall Data for Paarden Island

Monthly rainfall data for the City of Cape Town was sourced from the Worldclim.org website in the form of GIS GeoTIFF files. Using ArcGIS the Worldclim.org files were imported into ArcGIS with the locations of the rain gauge station locations and the location of Paarden Island. The pixel value of the TIFF files that represent the average monthly rainfall of an area was recorded for each station and Paarden Island for every month.

 

The values of the average monthly rainfall from the Worldclim.org data was compared to the modified monthly rainfall from the rain gauge stations. A modification factor was calculated for each station and corresponding month by dividing the modified rainfall amount by the Worldclim.org rainfall amount.  The average of the 3 rain gauge stations was then calculated for every month which was then used as the modification factor for Paarden Island.

 

The modified average monthly rainfall for Paarden Island was then calculated using the average modification factor. The monthly rainfall amount recorded from the Worldclim.org was multiplied by the average modification factor to give the modified monthly rainfall for Paarden Island.

 

Potential Rainwater from Roof Surfaces that can be harvested in Paarden Island

Using the GIS data that was collected, the roof surfaces of all erven within the case study area were outlined. The area of these outlines were then calculated using the ArcGIS geometry calculator and all roof surfaces that had an area of less than 50m³ were removed from the selection of roof surfaces. The remaining roof surfaces were then assigned to an ERF number which corresponded to a paying municipal water account number.

The roof surfaces were then also assigned a runoff coefficient based on the roof surface that was currently being used by the building and table 3.1.  The monthly rainwater collection was then calculated for each ERF number using equation 3.1. The monthly rainwater volumes were then summed to give annual rainwater volumes for different ERF numbers. The monthly rainfall of each ERF number was then summed to give a total monthly rainwater volume for Paarden Island.  The monthly rainwater volume of Paarden Island was then summed to give an annual roof surface rainwater volume.

 

Potential Rainwater from Ground Surfaces that can be harvested in Paarden Island

Using the GIS data that was collected, the different ground surfaces such as roads, pavements and grassed areas were outlined. The area of these different ground surfaces were then calculated using the ArcGIS geometry calculator. Each ground surface area was then assigned a runoff coefficient based on the ground surface and table 3-2.

 

The monthly rainwater collection was then calculated using equation 3-1. The monthly rainwater volumes were then summed to give annual rainwater volumes for different ground surfaces. The monthly rainfall of each ground water surface was then summed to give a monthly total rainwater volume for Paarden Island.  The monthly rainwater volume of Paarden Island was then summed to give an annual ground surface rainwater volume.

 

Assessment of Possible sources of Contamination from Site Visits

Site visits were carried out to identify possible sources of pollution in Paarden Island due to industrial process that were observed based on visual observations. Contamination refers to any substance that would make the rainwater unfit for consumption. Sources of contamination includes solid particles that are suspended in water such as sand and clay, it also includes biological contaminants from vegetation, algae and mould. Contamination sources can also include heavy metals and other toxic chemicals that can be present due to industrial processes.

 

Sizing of Rainwater Tanks

The sizing of the rainwater tanks were done by comparing the monthly consumptions of municipal water to the potential monthly rainwater capture and calculating the surplus or deficit that would occur. The results of the monthly surplus or deficient resulted in 3 different scenarios:

1. Scenario 1 was a result in which there was a rainwater surplus in every month of the year. In scenario 1 the tank size was estimate to be the equivalent of 2 months’ supply of the average monthly municipal water consumption;

2. Scenario 2 was the result in which there was a rainwater deficit in every month of the year. In scenario 2 the tank size was estimated to be the equivalent of the month with the highest rainwater collection volume; and

3. Scenario 3 was the result in which there was a mixture of rainwater deficits and surpluses for various months of the year. The cumulative rainwater surpluses of the erven resulted in 2 outcomes. In outcome (a) the erven were able to achieve a partial freshwater supply for a certain number of moths; in outcome (b) the erven were able to achieve a complete freshwater supply from the rainwater.

   1. In scenario 3a the tank size was calculated by finding the maximum difference between the cumulative municipal water usage and the cumulative rainfall collection starting with the first month in which a surplus is recorded; and

   2. In scenario 3b the tank size was calculated similar to 3a by finding the maximum difference between the cumulative municipal water usage and the cumulative rainfall collection starting with the first month in which a surplus is recorded. The cumulative surplus and monthly surplus or deficit was added to every month until the starting month was reach again. The maximum cumulative volume for the 12 months was calculated using the MS Excel MAX function. The remaining cumulative rainwater volume from the last month in the series was subtracted from the maximum calculated cumulative rainwater volume to give the surplus tank size. The remaining cumulative rainwater volume from the last month in the series represented rainwater that would not be used again since the following month would be the start of the surplus rainwater months again. The surplus tank size was compared to size of the 2 month water consumption volume of the erf. The maximum volume was chosen as the required tank size.

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Calculating the Feasibility of Rainwater Harvesting Systems for Various ERF’s

The assessment of the economic feasibility of implementing rainwater harvesting system in Paarden Island was done by calculating the profit or loss that would occur if the rainwater harvesting system was implemented for each erf. The cost of the rainwater harvesting equipment was subtracted from the results from the cost savings of rainwater usage that was substituted for municipal water. If a profit was achieved then the rainwater harvesting system was deemed feasible under the conditions in which it achieved those results. In other words if the rainwater harvesting system implementation resulted in a profit for scenario 1 model 3 then the system was deemed feasible for that scenario. This did not mean that the rainwater harvesting system was feasible for all scenarios. The results of the profit and loss calculations are tabulated in table J-3 to J-5. A summary of the results is given in table J-1 and J-2 for 5.0% and 9.5% annual municipal rate charge increases. The rate at which the municipal tariff rate increases was used as a variable to show the feasibility of various erven depending on what the future rate increases might be. The range of tariff rate increase is from 5% to 10%.  The lower limit of 5% represented the mean inflation rate in South Africa over the last 10 years (Tradingeconomics, 2013) and the upper limit of 9.5% represented the current rate of municipal tariff rate increases for 2013/2014 (City of Cape Town, 2013).