Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

Assumptions

The following assumptions were made about the sizing of rainwater tanks:

• It was assumed that all erven would be able to collect all the rainwater that falls onto their roof surfaces. This assumption is valid for erven that have roofs and drainage equipment that are well maintained. Most the erven in Paarden Island are in a state of disrepair and require maintenance to be carried out on broken roof surfaces, gutters and down pipes;

• It was assumed that all erven would be able to accommodate the storage volume of rainwater tanks that would be required to collect the rainwater. Due to available space certain erven that only require 1 or 2 rainwater tanks would mostly likely be able to accommodate the rainwater tanks. Erven that have more than 5 rainwater tanks could encounter issues of placing rainwater tanks due to a lack of available space;

• It was assumed that the rainwater tanks will be able to accommodate the average monthly rainfall for each month. Based on rainfall data that has been previously recorded certain years have recorded rainfall that have been about 50% higher or lower than the estimated average; and

• It was assumed that all erven could afford to pay for the installation of the required rainwater tanks. The cost of purchasing all the rainwater tanks could be very high initial capital expenditure which some business in Paarden Island would not be able to afford.

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Analysis of required tank sizes

The results of the monthly surplus or deficit of rainwater collection for each month can be seen in table G-1 to G-4 which is used in the sizing of the rainwater tank. The tank sizing was done in conjunction with table B-1 which provided the monthly rainwater collection for each month for all the erven and table F-2 which was able to provide the average monthly water consumption for each erf.

 

Scenario 1: Erf achieved a monthly rainwater surplus throughout the year

In Scenario 1 10 erven were able to able to achieve a rainwater surplus throughout the year. It was noted that 7 of these erven had an average monthly water consumption of less than 10 kL. It is not clear whether the values of the monthly water consumption figures are correct or not. ERF numbers 17 691 and 17 734 only had 5 usable water consumption figures while all the other erven had 10 or more water consumption readings. A possibility for the low water consumption figures may be due to the erven already using an alternative water supply or the erf does not use a lot of water, further research would be required. The erven that had higher water consumptions also had large roof surfaces. ERF 149 437 for example had a water consumption of 96 kL a month but also had a total roof surface of 26 500 m2.

 

It was also estimated that about 18 000 kL of rainwater would be wasted from these erven per year. This wasted rainwater supply represents about 9.1% of the total average rainwater that could be captured in Paarden Island. It would thus be recommended that the erven with large surpluses should try to share this rainwater with surrounding erven. The erven with surplus rainwater could either charge surrounding erven for the use of their rainwater or enter into partnerships to help reduce the cost implementing rainwater harvesting systems. 

 

Scenario 2: Erf was not able to achieve a monthly rainwater surplus throughout the year

In Scenario 2 96 erven were not able to achieve a rainwater surplus for any of the months throughout the year. It was noted that 3 erven; ERF 17 710, 158 155 and 107 891 had a monthly water consumption of 1 500 kL, 2 400 kL and 3 700 kL respectively which were all extremely high compared to the average water consumption for the area of only 160 kL. The same erven also had roof areas of 13 400 m2, 13 000m2 and 2 580 m2 respectively which indicated that these erven were larger than most erven in the area which have an average of 1 300 m2 roof area and thus may actually use the recorded amount of water consumption.

 

59 of the erven in Scenario 2 had roof surfaces below the average roof area of 1 300m2 ranging from 304 m2 to 1 279 m2  of which 49 had a roof surface below 1 000m2. This meant that erven would not be able collect enough rainwater to meet the requirements on the monthly water consumption. 42 of the erven also had monthly water consumption figures between 169 kL to 3 700 kL which was higher than the average water consumption for the area of 160 kL. This meant that certain erven would not be able to meet all their water consumption requirements from rainwater due to either a small roof area or the water consumption of the erf was too high to be accommodated. The erven in Scenario 2 have a total rainwater supply deficit of 245 900 kL per annum.

 

 The erven that have a higher than average water consumption should consider how their water is being consumed. Reduction in consumption can be achieved by identifying areas which use excessive water or waste large amounts of water. Water efficient systems could then be put in place to reduce the amount of water that is consumed or water recycling could also be implemented. The erven could also look at trying to source water from the erven listed in Scenario 1 which will have an excess amount of rainwater supply.

 

Scenario 3a: Erf achieved a partial freshwater supply from monthly rainwater surpluses during certain periods of the year

In Scenario 3a 57 erven were able to achieve a partial freshwater supply due to some months achieving a rainwater surplus. The majority of these surpluses were achieved during the wet season in Cape Town which peaks in July. 27 erven were able to achieve 4 consecutive months of rainfall surpluses while the other erven were only able to achieve 3 months or less of consecutive rainfall surpluses. These erven had varied monthly water consumptions of between 20 kL to 394 kL and roof surface area of between 550 m2 and 10 000m2. Figure G-1 shows that there is a good relationship between the average monthly water consumption of erven and the available roof surface.

 

The erven in Scenario 3a unfortunately still have a total water deficit of 19 700 kL per annum which will be have to be compensated for by the municipal water supply. It should be noted that the storage tanks range from 43 kL to 1 150kL. The larger rainwater tanks would require large amounts of usable floor space to be used. Certain erven in Paarden Island would not easily be able to accommodate this additional required floor space. The use of underground tanks can be used in erven that have available yard areas. New modulated rainwater tanks have been developed that are able to withstand the loading caused by large trucks. Erven that don’t have yard space could opt for the use of segmented storage areas where by the storage of rainwater tanks are not fixed to a single area but are instead distributed throughout the building. It is a lot easier to find space for a single rainwater tank than to make room for a large collection of rainwater tanks.

 

Scenario 3b: Erf achieved complete freshwater supply from monthly rainwater surpluses during certain periods of the year

In Scenario 3b 58 erven were able to achieve a complete freshwater supply from only having certain months on the year contributing surplus rainwater. The majority of these surpluses were achieved during the wet season in Cape Town which peaks in July. These erven had varied monthly water consumptions of between 8 kL to 540 kL and roof surface area of between 640 m2 and 20 500m2. Figure 6-2 shows that there is a good relationship between the average monthly water consumption of erven and the available roof surface.

 

The erven in Scenario 3b have a total rainwater surplus of 15 900 kL per annum which will not be used and will have to be wasted. The additional supply of rainwater could be used to help compensate erven that have rainwater deficits like those in scenario 2 or 3a. It should be noted that the storage tanks range from 15 kL to 1 570 kL. The larger rainwater tanks would require large amounts of usable floor space to be used. The same issues and solutions raised in Scenario 3a could also be used for scenario 3b regarding rainwater tanks.

 

Comparisons of different scenarios

Figure 6-2 shows a regression analysis done on the 4 different outcomes that were achieved. It can be seen that there is a good relationship between the available roof surface area and the monthly rainwater consumption. The higher the ratio of roof surface area to monthly water consumption the more likely an erf was able to achieve a complete freshwater supply. It was estimated that at about 1 kL of monthly water consumption would require 30 m2 of roof surface area to be able to achieve a complete freshwater supply from rainwater in Paarden Island. This estimation was achieved by finding the lower limit of scenario 3b and the upper limit of Scenario 3a. Paarden Island only has 66 erven that are able to achieve this value. Figure 6-1 shows the location of the various freshwater supply scenarios and their relation to one another.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6‑1: Location of various freshwater supply scenarios

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6‑2: Regression analysis comparison of all erven in Paarden Island

 

There is an uneven distribution of rainwater supply and usage. Scenario 1 and 3b have a combine rainwater surplus of 33 900 kL of rainwater which represents 17.6% of the total collectable rainwater per annum. This rainwater will be wasted if it is not distributed to the other erven that have rainwater supply deficits. An even greater amount of rainwater could be lost if the funding and available space requirements for rainwater storage are not met. The storage of water can take up valuable factory floor space. Water has volume of 1 m3 per 1 kL which means that a 100 kL underground storage tank that has a 1 m depth will still require a floor area of 10 m x 10 m. Nine polypropylene tanks that have a volume of 10 kL, a height of 4.2 m and a diameter of 2.6 m would still require a floor area of 64.6 m2 as shown in figure 6-3 for 90 kL of Storage. A total of 92 erven will require rainwater storage of 100 kL or more. The largest rainwater storage tank would be 1 570 kL.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6‑3: Possible layout of rainwater tanks for 90 kL storage