Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

Rainwater is considered to be relatively pure substance but can absorb small quantities of airborne contaminants. Most of the contaminants present in rainwater harvesting is due to its “collection, storage and distribution” (TCEQ Regulatory Guidance, Water Supply Division, 2007, p. 8). Pure water is considered a strong cleaning agent and will clean a wide variety of minerals, metals and organic matter (TCEQ Regulatory Guidance, Water Supply Division, 2007, p. 8).

 

Pollution Sources

Napierlska & Goldyn (2013, p. 483) found that highest concentration of contaminants were measured in the first 15-60 minutes of a rainfall event.

 

Atmosphere

There are many different gases that are introduced into the atmosphere that come from industrial furnaces, car exhausts and even bush fires and most of these sources contain chemicals that react with water in the atmosphere which are then returned to the earth’s surface during rain events. 

 

Exhaust gases can cause the rain to become acidic. The acidity of rainwater can usually have effects on dams and trees but the rainwaters’ degree of acidity is usually tolerable to humans (Thomas, 1998, p. 98).  The acidity would be dependent on the relative location of pollution sources, the quantity of contaminants they produce and the location of where the rain falls. I.e. rural areas that are far from industrial polluters would have less acidic water content than compared to industrial erven that are located adjacent to a refinery.  

 

Roof surfaces

The quality of rainwater harvesting from different roof surfaces is dependent on the roof material, climatic conditions and the surrounding environment (Krishna, 2005). If roofs are well maintained and cleaned of any particulates that gather on the roofs before the rain events then the water quality should be of a higher standard.

 

Research done in Spain by Farreny, et al (2011) showed that the location of the runoff surface was very important. Their research found that areas where there was no direct source of pollution had a higher quality of rainwater than areas that did. Farreny, et al (2011) also found that the runoff quality of different roof slopes was relevant and that steeper sloped roofs had a higher quality of rainwater as compared to flat roofs. 

 

According to Thomas (1998, p. 98) rusted corrugated iron roofing doesn’t give an unacceptably high iron content in rainwater runoff. 

 

Research done by Bannerman, et al (1993, p. 252) looked at the galvanised roofs in industrial areas and found that they were a major source of zinc contaminants but contributed a relatively small amount of the other forms of contaminants that were tested. 

 

The quality of runoff from roofs may not be of the highest standard but in most cases depending on the contaminants in the surrounding environment rainwater runoff is acceptable to consume. Rainwater runoff from roofs can be used as drinking water without any treatment (UN-HABITAT, 2005) if it is the only available water source and the roof surfaces are kept clean. Drinking untreated rainwater is usually the only option for poor people that live in areas that aren’t supplied with another potable source of drinking water or the financial ability to afford to pay for the treatment of the rainwater.

 

Land surfaces

The runoff quality of land surfaces are dependent on the profile, land use, the amount of impervious surfaces, the amount of vegetation and the frequency and length of the rainfall event (Napieralska & Goldyn, 2013, p. 481).

According to Bannerman, et al (1993, p. 251) who carried out experiments on the quality of rainwater from different runoff surfaces (streets, driveways, lawns, parking lots and roofs) in different areas (residential, commercial and industrial) found lawns in residential areas had between 2 to 18 times as much concentration of total phosphorus than from any other source. Bannerman, et al (1993, p. 258) also found that streets followed by parking lots were critical areas which had the highest concentration of contaminants in all areas except phosphorus and zinc. Large concentration of zinc found in roof runoff indicated that galvanised roofing materials were a source of zinc (Bannerman, et al., 1993).

 

Quality Requirements

The final end use of rainwater will determine to what level of treatment the rainwater will be required to undergo and the acceptable level to which the rainwater must be treated to so as to make it usable. Potable water uses will require a much higher level of quality than non-potable water uses. Table 3-3 gives values of water quality standards as set out but the Department of Water Affairs & Forestry for different uses. The values that are recorded in the table represent to the highest quality that the water should be treated to. There are certain allowances given by the Department of Water Affairs & Forestry for qualities that are of lower standards and only impose minor health risks.

 

Industrial use of water is broken down into 4 different categories depending on what types of processes are required to be completed. Categories 1 and 2 have a higher water quality requirement than domestic water for certain contaminants. Category 3 is assumed to be the equivalent of domestic water and category 4 is used for processes that do not require additional treatment. Each industrial category is further broken down into 6 process types which are cooling water, steam generation, process water, product water, utility water and wash water. Table 3-3 gives values of water quality standards as set out but the Department of Water Affairs & Forestry for different industrial categories.