Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

Stage 1 focuses primarily on information gathering and assessing its importance, usefulness and reliability.

 

Gather and Assess Available Literature

Information was gathered on the current and future freshwater supplies for the City of Cape Town to establish why it is important to look for alternative methods of collecting freshwater. Information was gathered on various aspects of rainwater harvesting, focusing on its benefits, disadvantages, required components and method of design. Information was sourced to determine at what quality the rainwater should be so that the rainwater can be used in various industrial applications. Case Studies on the implementation of rainwater harvesting were also sourced to show how successful implementation of systems can be advantageous. 

 

Gather and Assess Available Data on Paarden Island

Cadastral Data on Paarden Island

GIS data was required to help establish the geographical boundary of the case study and erven areas. The GIS data also included the location of current municipal freshwater supplies, sewer and stormwater pipes. This data was sourced from the UCT GIS department.

 

Rainfall Data

Rainfall data was not available for Paarden Island and so data was collected from rain gauge stations in Pinelands, Tygerberg and Molteno. Rainfall data for Tygerburg and Molteno was collected for a 30 year period and the rainfall data for Pinelands was for a 20 year period. Figure 4-1 shows the locations of the rain gauges. The City of Cape Town Department of Catchment Stormwater and River Management were able to provide this data. Additional data was collected from the worldclim.org website which contained monthly rainfall data for the entire City of Cape Town. The worldclim.org data is based on extrapolated data from various rain stations throughout the City of Cape Town.

 

The rain gauge rainfall data that was collected was first modified by eliminating all days in which the rainfall was less than 1mm. The first 1mm of rainfall would not contribute to the amount of rainwater that could be collected due to the first flush system.

 

The rain gauge rainfall data was then sorted into separate years and the total annual rainfall, maximum rainfall, mean rainfall per event and the number of days it rained was calculated. The information that was established was then used to help with the analysis of the feasibility of rainwater harvesting systems.

 

The rain gauge rainfall data was then sorted again into separate months and the average of each month for each region was then calculated. The information that was established was used to calculate the amount of water that that could be collected within each month. This information could then be used to determine the size of the rainwater tanks.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4‑1: Location of Rain Gauge Stations

Source: 2011 Census Cape Town Spatial Data

 

Water Consumption for 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. This data was sourced from the City of Cape Town Department of Water and Sanitation, Water Demand Management. The data was sorted and analysed to give monthly estimations of water consumption for different erven. The following modifications were made to the water consumption data:

1. All data between January 2013 and September 2013 were removed due to very high water consumption readings that were given. Most of the data that was given had water consumption readings that were in excess of 50 000 kL and some data showed negative numbers. A new measuring system had been implemented by the City of Cape Town but it was not clear as to how the consumption figures should be read, advice given by the city officials was to ignore these results since they were not reliable;

2. All data that had negative readings were removed from the data set. It was not clear what these reading represented. In some cases it may have been as a result of overcharging for water consumption in the previous months but this was not confirmed;

3. All data that had a 0 value were removed from the data set. This was due to errors that may occur in the calculation of the average water consumption and standard deviation since a 0 value is considered a value by the MS Excel software. Erven with a 0 value would usually indicate that the erf was not being used;

4. Maximum or minimum values of the erf data sets were removed in instances in which the standard deviation was larger than the average. The data point that was the furthest away from the average was removed.

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.

 

Estimated Concentrations of Heavy Metals in Rainwater Runoff

Data on the heavy metal contaminants present in the ground surface runoff in Paarden Island would be required to be determined. The concentration of the heavy metal contaminants would determine if special treatment of the water would be required. This data would was sourced by collecting  samples of ground surface runoff in Paarden Island and then having the samples tested by the UCT Chemical Engineering Department using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), analysis.

 

The water samples for the ground surface runoff was collected in three 2 L plastic bottles that had been previously been washed out using a cleaning agent and municipal water and then left out to dry. When collecting ground surface runoff samples in Paarden Island the plastic bottles were filled up to ¼ full (500mL) and then swirled and rinsed out to remove any possible residue left over from the cleaning process. The plastic bottles were then filled with ground surface runoff rainwater. The plastic bottles were then delivered to the UCT chemical engineering department for testing.

 

There are two standard operating procedures that are followed by the UCT Chemical Engineering Department which are:

• Determination of metals in environmental and ore samples using multi-element calibration standards; and 

• ICP730 series operating procedure.

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Due to maintenance of the auto sampler the procedure was modified to suit a manual analysis.

 

Costs of Various Rainwater Harvesting Equipment

The current pricing for rainwater harvesting equipment was sourced from local South African companies which would be used in the calculation of the cost of implementing a rainwater harvesting system. The costs of a rainwater harvesting systems are priced according to 6 separate components which include:

1. Component 1 is based on the cost of implementing leaf screens and first flush diverters. It was estimate that 1 first flush device and leaf screen would be implemented for every 200 m² of roof surface area. It was assumed that these components would all last 10 years provided that they were well maintain and cleaned regularly ;

2. Component 2 is based on the number of rainwater tanks that are require to be implemented to capture the required amount of usable rainwater. The rainwater tanks are all 10 kL storage tanks due to the cost benefit provided by these tanks compared to larger 15 kL and 20 kL tanks. It was assumed that the rainwater tanks would be able to last up to 10 years. The tanks that have been used in this estimation have a warranty of 8 years from the date of purchase but are believed to be able to last much longer than 10 years provided they are well maintained and looked after;

3. Component 3 is based on the cost of implementing a 55 – 400 micron disc filter and water pump. One of each of these components is required to be implemented in a rainwater harvesting system. The expected life span is about 5 years before the filter and water pump would require replacement;

4. Component 4 is based on an Ultra Violet and Cartridge Filtration System post treatment system that can be implemented for the erven that are able to achieve a complete freshwater supply form captured rainwater. The Ultra Violet and Cartridge Filtration Systems require varying maintenance requirements on different components. The future cost of these components will all be adjusted for by the inflation rate each time the components are required to be replaced;

5. Component 5 is based on an additional cost required for pipes, fixtures and fittings that would be required for the installation of a rainwater harvesting system. The additional cost was estimated to be 5% of the total cost of the rainwater harvesting equipment. The lifespan of the pipes are estimated to be 10 years. The life span of the fixtures and fittings are estimated to be the equivalent of the equipment that they are connected to.

6. Component 6 is based on an additional cost required for labour or general cost involved in the installation of a rainwater harvesting system. The additional cost is estimated at 25% of the total cost of the rainwater harvesting equipment up to a maximum of R 100 000. The cost of the installation is a once off value that is only applied once to the total cost.

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The total costs of the 6 separate components are used to estimate the cost of implementing a rainwater harvesting system on an erf over a 10 year period.

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Cost of Municipal Water Charges for Industrial and Commercial Areas

The current cost structure of municipal water and sanitation tariffs was sourced from the (City of Cape Town, 2013). The cost of municipal water that was substituted was calculated for 10 Years. The cost of municipal water was also calculated based on various water restriction tariffs of 10%, 20% and 30% being implemented. To model the outcomes of these tariff structures 5 separate models were chosen as listed below:

1. Model 1: 10% reduction model is applied throughout the 10 year period. This model represents the current normal tariff rate that erven are being charged at when restrictions are not enforced. The 10% reduction level is set based on  the Low Water Demand Curve and has been incorporated into the Water and Sanitation By-Law as a permanent good water demand practice (City of Cape Town, 2013);

2. Model 2: The 20% reduction model is applied throughout the 10 year period. This model represents the tariff rate that will be applied if level 2 restrictions are implemented by the City of Cape Town. This tariff only applies during periods when restrictions have been put in place. This model would normally only be accurate if the City of Cape Town were to experience consecutive seasons of drought;

3. Model 3: The 30% reduction model is applied throughout the 10 year period. This model represents the tariff rate that will be applied if level 3 restrictions are implemented by the City of Cape Town. This tariff model only applies during periods when restrictions have been put in place. This model represents the maximum charge that can currently be implemented by the City of Cape Town;

4. Model 4: A combination of model 1 and 2 are implemented equally throughout the 10 year period. This model is aimed at establishing the cost of having normal level restriction tariffs being applied during the wet winter months and level 2 restriction tariffs being applied during the dry summer months. This model would represent a more appropriate real world comparison since the current water supply and demand don’t have a large safety barrier to compensate periods of drought.

5. Model 5: A combination of model 1, 2 and 3 are implemented at a ratio of 50%, 30% and 20% respectively. This model is aimed at establishing the cost of having water restrictions being put in place due to the new water demands out pacing new water supply sources. This model would represent a situation in which the City of Cape Town was unable to provide sufficient new water supplies, the new water supplies required higher tariffs or higher restrictions have been put in place to help reduce the new demand for water.

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The 5 different models were applied to 3 separate scenarios in which the costs of the rainwater harvesting system could be recuperated. The 3 scenarios are:

1. Scenario 1 considers only the rainwater that will be consumed by the erf. Any excess rainwater will be wasted and not be used by anyone;

2. Scenario 2 considers all the rainwater that will be collected by the erf. Any excess water that is captured will be distributed and sold to other erven that have a deficit supply of rainwater. The costs of the excess rainwater will help to offset the cost of the rainwater harvesting equipment; and

3. Scenario 3 considers the current method of charging for municipal sewage by the City of Cape Town. Currently the amount of sewage an erf produces is based on a portion of the amount of municipal water that is consumed by the same erf. If the amount of municipal water that is consumed is reduced, then so too will the cost of sewage be lowered.  Scenario 3 attributes this reduction in the cost of sewage to the cost of implementing a rainwater harvesting system but does not include rainwater that is wasted.

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The water tariffs are calculated by summing the total rainwater consumption per year for each erf and multiplying this value by the appropriate tariff rate for that year after the correct adjustment for rate increases has been done.