Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

There are six important steps involved in a rainwater harvesting system that need to be considered. Depending on what the rainwater will be used for i.e. potable or non-potable water, and from where it was collected i.e. roof surface or land surface runoff, not all the steps are necessary. 

 

Catchment area

This refers to the area or surface from where the rainwater will be collected from. Rainwater is either collected from the roofs of structures or from various types of land surfaces. The quality of the runoff will depend on the surface material, climatic conditions and the surrounding environment (Krishna, 2005). 

 

Roof surfaces

Roof surfaces can be made from various materials such as metal, clay, concrete, composite or asphalt shingle, plastic, wood, tar and gravel (Krishna, 2005). The type of surface that is used can dictate what the rainwater will be suitable for. Some roof materials may leech toxins which are unsuitable for potable use but may still be used to irrigate landscapes (Krishna, 2005). Roof materials that are coated with lead based paints are regarded as unsuitable (UN-HABITAT, 2005). Figures 2-1 to 2-4 show the various types of roofing material that is available. The usability of theses roof material will mostly depend on how clean the surfaces are kept, what constituents are used in the manufacturing of these materials and the constituents of any coating such as paint that is applied. 

 

Land surfaces 

Ground surfaces can vary in composition from concrete paved areas to vegetated fields. The amount of runoff that is observed is dependent on infiltration rate of the surface (Mechell & Lesikar, 2008). Surfaces with low infiltration rates will have high runoff volumes whereas surfaces with high infiltration rates will have low surface runoff but can be used to recharge the groundwater (UN-HABITAT, 2005). The slope of the land can also determine the amount of runoff that will be collected. Figure 2-5 to 2-8 show the various types of surfaces that can be used to collect rainwater runoff. The suitability of these surfaces would depend on the various contaminants that are collected on these surfaces from various sources such as high phosphorus levels from vegetated areas, high levels of hydrocarbons from asphalt surfaces and high levels of settleable solids from gravel surfaces. 

 

Conveyance to storage

This refers to the method of moving the collected rainwater from the area it was captured to where it will be stored. The method used to convey the rainwater will depend on which surface the rainwater was captured off.

 

Roof surfaces

Rainwater that is collected from roof surfaces is conveyed using gutters and downpipes (Krishna, 2005). Gutters are used to catch the rainwater coming from the roof and transfer it in to the downpipes which will then feed into the storage unit. The gutters and downpipes are usually made from PVC, seamless aluminium or galvanised steel (Krishna, 2005). Figure 2-9 to 2-12 shows the various types of roof gutters that are available that can collect rainwater.

 

Land surfaces

Rainwater that is collected from ground surfaces are conveyed using pipes similar to stormwater pipes (UN-HABITAT, 2005) or gravel trenches that have perforated pipes placed inside them (Mechell & Lesikar, 2008). Swales are also able to convey surface runoff. Figure 2-13 to 2-16 shows the various types of ground surface conveyance techniques that can be used. The rainwater can be conveyed above ground using swales or below ground using piped systems. 

                          

Pre-treatment

Pre-treatment facilities are provided in rainwater harvesting systems to help remove large organic matter such as leaves and settleable solids such as dust that is collected by the rainwater when it travels over the catchment area. 

 

Leaf screens

Leaf screens are used to remove various types of large debris that are washed down the downspout with the rainwater (Krishna, 2005). They are usually connected to the downspouts before entering the storage unit and help to prevent the build-up of sedimentation and other debris inside the storage tank. There are various versions of leaf screens available some of which include leaf guards, funnel-type downspout filter, strainer baskets, filter socks and a cylinder of rolled screen. Figures 2-17 to 2-20 show the various types of equipment that can be used to capture or divert large debris before entering the storage medium. 

 

First flush diverters

First flush diverters are used to divert the first flow of rainwater which usually contains the highest amount of contaminants such as dust, leaves, twigs, insects, animal faeces, pesticides and other airborne residues (Krishna, 2005). This will allow the rainwater that is collected to be of a much higher quality. Krishna (2005) suggests that at least 50 litres of the first rainwater that is collected should be diverted and flushed away for every 100 m2 of roof surface area. Figure 2-21 shows the various components of a first flush diverter.

 

Roof washers

Roof washers are placed just before the storage units and are used to remove small debris to prevent clogging of potable systems or drip irrigation systems (Krishna, 2005). These systems are used to remove material as small as 30 microns from the rainwater. Figure 2-22 show the various components of a box roof washer.

 

Sand filters

Depending on the quality of water that is required, sand filters can also be used to treat the rainwater and help to remove fine material (Porter, et al., 2008). Sand filters perform the same function as roof washers but are usually more effective at removing small materials. Gravel and sand based filters are usually widely available (UN-HABITAT, 2005) and could even be home-made (Oregon Department of Consumer & Business Services, 2009) if the costs are too high. Sand filters are used as a low cost option for people who cannot afford specialised filters. Figure 2-23 shows the composition of a homemade sand filter that can be used as a low cost option for people that don’t have a lot of money to spend or if the water is only going to be used for non-potable systems.  

 

Storage

There are various storage techniques available for storing rainwater for later use. Methods include the use of tanks and groundwater.

 

Tanks

There are many different types of tanks available that have varying shapes, sizes and are made from a variety of different materials. Storage tanks can be made from fiberglass, polypropylene, underground polypropylene, wood, metal, concrete, Ferro cement, and clay. Storage tanks can be placed above ground or below ground. The storage tank is the most expensive part of a rainwater harvesting system (Krishna, 2005). The size of a storage tank is dependent on the rainwater supply, the projected number of dry days without rain, the catchment area, aesthetics, personal preference and the available budget. Figures 2-24 to 2-27 show the various types of storage tanks that are available. Theses storage tanks con be made from various types of materials and can be construct into various shapes with cylinders usually being the most efficient. The storage tanks can also be placed in various locations both above and below ground level. 

 

Groundwater storage

Captured rainwater can be stored in the ground by using infiltration techniques to help recharge the groundwater to be used at a later date. 

The selection of a location for infiltration systems depends on (Mechell & Lesikar, 2008): 

• The geographic location: which will determine the climate and rainfall patterns;

• Topography: this determines the direction of flow of surface runoff which must be diverted away from homes and to an area when the water can be infiltrated;

• Vegetative cover: helps to remove air from soil as well as help to prevent erosion from taking place. Care should be taken not to have trees or woody plants near the infiltration basin since their roots may block the system;

• Separation distance: helps to not cause structural instability of nearby building and water contamination that might occur;

• Water movement: it is important to know from where the underground water is coming and to where it is going; and

• Soil characteristics: this will determine the rate at which the soil is able to infiltrate the water into the ground.

 

Delivery system

If above ground storage tanks can be elevated, they will be able to provide a gravity fed distribution system. Underground tanks and groundwater storage will require the use of pumps to allow the water to be used. Hand pump systems could also be used as an alternative low cost option if the end user is unable to pay for an automated/electrical pump system. Pump systems are also used to provide an additional pressure head to the water delivery of elevated storage tanks if the pressure that they provide is too low. Krishna (2005) states that most underground storage unites usually require pressure tanks and pumps to be connected between the storage tank and the treatment system. Figure 2-28 shows two different types of pressure vessels connected to an automated pump system. 

 

Final treatment

The treatment of non-potable rainwater harvesting systems does not require this step as the pre-treatment of the rainwater before storage is usually enough (Krishna, 2005). If the rainwater is to be used for potable water, then further treatment will be required. This treatment usually consists of disinfecting the water by removing disease causing pathogens that are present in rainwater as well as micro particles still left in the water to improve the turbidity of the water.

 

Cartridge filters and ultraviolet (UV) light

Cartridge Filters are usually placed in series with a 5 micron filter followed by a 3 micron activated charcoal cartridge filter which is followed by the UV light. The filters are used to (Krishna, 2005)improve the turbidity while the UV is used to kill most microbiological organisms that pass through them (UN-HABITAT, 2005). This form of disinfection is mainly used in the USA (UN-HABITAT, 2005). Figure 2-29 shows what a typical UV and cartridge filter system looks like.

 

 Ozone

The Ozone acts as a strong oxidizing agent that is used to reduce colour, eliminate foul odours and is used to reduce the total organic carbon (TOC) from the system (Krishna, 2005). Ozone is mainly used in European counties (UN-HABITAT, 2005). 

 

Membrane filtration (reverse Osmosis and Nanofiltration)

Membrane filtration causes water under high pressure to filter dissolved solids such as salts. A certain amount of water is also lost as waste which contains the dissolved contaminants which is referred to as brine (Krishna, 2005). Figure 2-30 shows a commercial reverse osmosis unit. 

 

Chlorination

Chlorine is used to disinfect the water by killing the bacteria that is present in the water (Krishna, 2005). Even though most households will filter their water the use of disinfectants is seldom used (UN-HABITAT, 2005). 

 

Boiling 

10-20 minutes of boiling is enough to kill most biological contaminants (UN-HABITAT, 2005). This method of disinfection is also energy intensive due to the energy required to boil water. This method is mostly used by people that cannot afford to buy expensive post treatment equipment. 

 

Buffering

The use of Baking Soda can help to improve the pH levels of collected rainwater that is slightly acidic due chemicals in the air such as sulphur and carbon that cause acid rain to form (UN-HABITAT, 2005).  Figure 2-31 shows a sample of the chemical that is used by small home aquariums to balance the pH of the water. Potable rainwater systems would require the use of similar chemicals to balance the pH of the water before use.