Rainwater harvesting can be described as collecting water that originates from rainfall. Every time the rain falls, you have a chance to harvest it. If harvested and stored well, the water can be of great use on commercial or residential properties in South Africa.

The rainwater itself is normally relatively pure. This is due to the fact that it is formed after vapour evaporates from the ground and condenses in the atmosphere with no particles or chemicals. Unless it is in places where the air is polluted badly.

Rainwater harvesting therefore majors on collecting the water that falls during rainfall before it touches the ground and becomes polluted by silt and other chemicals from the ground. The water is therefore relatively clean up until this point.

This article is a guide to rainwater harvesting in South Africa.

Benefits of rainwater harvesting in South Africa

  • Harvesting rain water helps in controlling flooding.
  • It also reduces soil erosion.
  • It ensures that people living in areas which have no access to natural water sources like springs or rivers are able to access clean water.
  • If the rainwater is harvested and stored correctly, it provides an adequate source of water for irrigating crops.
  • The rain water collecting systems are normally relatively cheap to install and ends up reducing water bills.
  • It also helps solve drainage problems which are experienced mostly in towns and cities.
  • It also reduces the submergence problem which is mainly experienced in most city areas where rain water is not properly collected and directed into the right places.



Roof Top Rainwater Harvesting

The main method for harvesting rainwater is “roof top rainwater harvesting”. This is a simple cost effective system of harvesting and storing rainwater. Here the rooftops act as the water catchment surface where the rain water is collected. It is then directed to storage tanks or to artificial recharge systems. Roof top rainwater harvesting systems include the following components:

  • Rainwater harvesting catchment system
  • The transportation system
  • The First Flush System
  • The Filter System

What is a Rainwater Catchment System?

The water catchment system is the surface upon which the rain water falls before being redirected. In this case the water catchment surface is the roof top.

Rainfall falls on roof tops of residential and commercial buildings. It then flows down to the gutters which then directs it to the necessary transportation systems attached.

In this case, the larger the surface area of the roof, the larger the amount of rainwater that will be collected within a given time period.

Rainwater Harvesting Transportation Systems

Water is normally transported though down pipes/drains from the roof. The best water pipes to use are those that are UV resistant; ISI HDPE and PVC pipes.

The drains/pipes are normally fitted at the ends of gutters. They should be fitted with wire mesh to filter off the leaves and other substances that may have come down the roof with the rainwater.

First Flush System

This is a very critical component of the rainwater harvesting collection system. It determines the level of cleanliness of the collected water.

The first flush is a device which ensures that the rain water of the first rainfall that falls, after a long period of drought, is diverted away from the harvesting system.

When the first bit of rain lands on your roof, it will wash down all the dirt and debris that has been laying on your roof. This is the kind of water you are looking to avoid when rain water harvesting in South Africa.

Fitting a proper First Flush Water Diverter is very important. It makes sure you get clean quality water. Water Diverters enhance water quality. Reducing the need of regular tank cleaning due to contaminants from the rooftop.

The primary roof top flush

After it rains, water gradually fills up guttering framework before it moves through the downpipe. The primary flush of water from the rooftop can contain a lot of microscopic organisms from decayed creepy crawlies, skinks, winged creatures and as well as droppings and concentrated tannic corrosives.

Rather than streaming it directly to the water tank, these contaminators flows into the water diverter. The water diverter uses a ball and seat system – a straightforward programmed system that does not depend on mechanical parts or manual intercession.

How does it work?

As the water level ascents in the diverter chamber the ball drifts. Once the chamber is full, the ball lays on a seat inside the diverter chamber keeping any further water from entering the diverter. Any other rainwater that streams after this is then guided along the gutter pipe framework to the tank.

For water diverter to work viably, the polluted water that enters into the water diverter chamber must be closed off. This keeps the water from going to the capacity tank and guarantees that the contaminated water does not find its way into the rainwater harvesting storage tanks.

The system has a moderate discharge valve which guarantees that the chamber cleans itself after rain and resets itself consequently.

Rainwater Harvesting Filtration System

Before the collected rain water is stored, it needs to be filtered to remove particles and other contaminating components so that the stored water is clean and safe for use.

Some of the contaminants which the filtration system is set to remove or reduce from the water are:

  • Volatile Organic Chemicals(VOCs) – These include things such as pesticides, herbicides among several other organic chemicals. Such chemicals are mostly in cities and town centres. If ingested in large quantities in water, they are known to cause liver, reproductive and kidney problems.
  • Metal particles – As the water flows over the rooftops, it may carry with it some metal particles.
  • Organic debris -There are also things like leaves or small twigs that may find their way in to the collected rainwater and also small insects and they require to be filtered.
rainwater harvesting and water filtration

Rainwater Storage Tanks

There are different methods of filtration that can be used with your rainwater harvesting system. In this section we will take a look at the different filtering options available for use.

Sand gravel filter

This is constructed using brick masonry and filled with layers of pebbles, gravel and sand in that order from the bottom.

Charcoal filter

This is a fairly easy to construct. It is a filter that can be permanently constructed or made in a drum. It comprises of layers of pebbles, gravel, sand and charcoal in that order from down.  Then each of the layers separated from the other using a mesh wire to avoid mixing.

Polyvinyl chloride pipe filter

This type of filter is made using a PVC pipe, which is divided into two using a mesh wire and one section filled with gravel while the other one is filled with sand. For purposes of effectiveness in decolourising the water, another layer charcoal in between the two sections of sand and gravel.

The diameter of the PVC pipe used can range from 1-1.2 meters depending on the surface area of the roof. The larger the surface area, the larger the diameter of the PVC pipe used.

At both ends of the filter, the size of the PVC pipe is reduced to fit into the size of the transporting inlet and outlet pipes. This filter can be placed in a horizontal or vertical position in the system.

Sponge filter

This filter is normally made out of a PVC drum that is fitted with sponge. It is actually the simplest of all filters to make.

rainwater harvesting and water collection

Residential Rainwater Harvesting in South Africa

Rainwater Storage Tanks

After the water is filtered, it is then stored in either artificial recharging systems or rainwater storage tanks. The picture above is an example of rainwater storage tanks that have been setup at a residential home.

Commonly used artificial recharging systems

Artificial recharging systems are systems that are constructed by man to aid in the recharging of ground water from the rain water. This is done instead of letting the water flow away as flood water. This allows a specific area have adequate ground water that can be used for other purposes like irrigation.

Recharging of bore holes

Water collected from roof tops is directed using drain pipes into settlement/filtration tanks where solid particles settle and then the water is directed into deep boreholes which had been abandoned and thus recharging the borehole. The filtration/settlement tank is a kind of a filter to ensure that only clean water sips to the bottom to be directed into the borehole.

The size of the filtration tank will depend on the surface area of the water catchment area and the intensity of the rainfall that is normally received in the area under consideration

Recharge pits

This are pits of any shape dug into the ground and filled with filtering material (gravel, sand and charcoal).the walls are lined with bricks or stones. Then the walls are made with holes are rectangular interviews and the top of the pits covered with a perforated cover.

The idea here is filtering the rainwater and letting it to sip back into the ground to recharge underground aquifers and other water deposits.

Recharge shafts/ soaking ways

These are shafts that are dag into the ground where the upper layer is impervious or made of alluvial soil. A hole of a diameter of 30 cm is dug into the ground up to a depth where the impervious layers end at. The hole is then filled with filtering material. For effective infiltration of the rainwater, several such holes are dug at intervals.

Recharging dug wells

This is done by directing the rain water collected from roof tops into filtration beds which are filled with filtering material (layers of sand and gravels).

The water sips through the filtration beds and gets into a perforated tube placed beneath the beds, leading the water into the well. However, the dug wells should be regularly cleaned and salted.

Percolation tanks

This are usually build in gardens or open fields. They are surface water bodies that are artificially created in the ground. The main idea behind these tanks is making a large area of land to be adequately permeable so that water can easily percolate into the ground easily.



Different kinds of Rainwater storage tanks

Another way that rainwater can be stored is by directing the rain water into storage tanks. There are different types of storage tanks that are used by man to store rain water which include:

Polyethylene rainwater storage tanks

These are tanks which are made out of polyethylene. They are the most commonly used tanks especially in homes and institutions.

Polyethylene tanks can be grouped into two major groups; the above-ground polyethylene tanks and the in-ground polyethylene tanks.

Above-ground polyethylene rainwater storage tanks

These are polyethylene tanks which are placed above the ground. They range from twelve gallons to fifteen thousand gallons in volume and they are made in an array of shapes as well as styles. Also known for their durability, lightweight and long-lasting characteristics.

They can either be place at the ground level or at a raised level like on top of buildings.

In-ground polyethylene rainwater storage tanks

These are polyethylene tanks which are placed underground; buried inside the ground. They have ribbed designs so as to increase their strength.

They are a bit costly to set up due to the excavation charges.

Galvanised metal rainwater storage tanks

These are normally made out of steel which is galvanised with zinc and the inside coated with a layer of a food-grade liner like PVC, epoxy paint or polyethylene.

They are usually a bit expensive and at the same time less durable since the galvanizing layer wears out with time and the tanks starts to rust.

Stone/concrete rainwater storage tanks

These are tanks which are made out of either pure concrete or a mixture of concrete and stones. They are usually constructed where large storage tanks are required.

They are normally custom designed and built and can be built to any size. These tanks are more durable since they are not affected by weather. Also, they ensure that the water remains cool even in hot climates.

Another thing with this tanks is that they can either be constructed in-ground or above the ground. However, they are quite expensive to construct.

Wooded rainwater storage tanks

These are tanks made of wood/timber. They are not very common. They are mainly made of redwood, pine, cypress or cedar and rapped with steel tension cables for strength. The inside is then lined with a United States Department of Agriculture (USDA) approved liner.

The wood tanks are very appealing and they are mainly used for their aesthetic value.They can range from 700 to over a million gallons in capacity.

Fibreglass rainwater storage tanks

These tanks are manufactured in standard capacities of 50 to 50,000 gallons. They can be either vertical cylinders or low-horizontal cylinders in configurations. These tanks are used for portable although they are primarily used in in-ground applications.

Bladder/pillow rainwater storage tanks

These are collapsible rain water tanks made of extra strong geotech fabric attached to a heavy gauge galvanized steel frame. The bladders are placed either under the house or deck.

The rain water from the roof top is diverted through downpipes to a common in-flow pipe which then pours the water into the bladder.

To avoid overfilling and bursting of the bladders, when the bladders are full, the excess water is diverted to a storm water collecting system.

If there is enough space, several bladder tanks can be installed attached end-to-end or side-by side to allow maximum rainwater storage.

Normally bladder tanks are designed to stretch to a maximum height of 600 millimetres. The bladders can easily be transported and they are a great choice for crawl spaces or outdoor locations like in construction sites.

Onion /pumpkin rainwater storage tanks

These tanks are designed with open tops and self rising tank walls. The main advantages of these tanks are that they can virtually be moved and set up in minutes. These are great tanks especially for military use or when hiking and people require to harvest and store rainwater.

Folding frame rainwater storage tanks

These tanks are made with a strong outer frame as well as interior liner. The interior liners are made from a wide range of fabrics.

When it is not raining and the tank is empty, these collapsible tanks are folded flat and stored.

When rain is anticipated, they are easy and quick to unfold and install.

Advantages of rainwater harvesting in Johannesburg, Pretoria, Durban and Cape town

  • Harvesting rain water helps in controlling flooding.
  • It also reduces soil erosion.
  • It ensures that people living in areas which have no access to natural water sources like springs or rivers are able to access clean water.
  • If the rainwater is harvested and stored correctly, it provides an adequate source of water for irrigating crops.
  • The rain water collecting systems are normally relatively cheap to install and ends up reducing water bills.
  • It also helps solve drainage problems which are experienced mostly in towns and cities.
  • It also reduces the submergence problem which is mainly experienced in most city areas where rain water is not properly collected and directed into the right places.

Storm water collection systems

Storm water is different to rainwater. They both originate from rainfall. However, the storm water is that water which has already reached the ground and it is normally contaminated with silt and other components found in the soil.

There are various methods of collecting storm water and storing it. Furthermore, it can be used at a later time instead of letting it to flow away on the ground and at times even cause damage in terms of erosion.

Methods of collecting storm water

There are several methods of harvesting storm water. Some of these methods are used to trap storm water to enable it infiltrate into the ground while some are used for storing the collected water for domestic and commercial use. Those methods which are used for collecting storm water for domestic and commercial use have several stages which are:

Harvesting

Storm water is harvested/collected from a drain. This is the rainwater which has reached the ground and it flows into the drains. The drains are usually placed in the sub surface and they help in collecting the rain water that sips into the sub soil layer.

Storage

The collected water is then temporarily stored in either underground or above-ground systems.

Treatment

In the temporal storage systems, the collected storm water is treated so that it can be safe for use. Filters and hydrodynamics separators are used.

Filters help in removing things like phosphorus. There are several types of filters to consider when designing a storm water filtration system.

Hydrodynamic separators on the other hand are used for removing solids. It is a very effective way of removing large solids, debris and trash.

Distribution

After the water is treated and it is safe for use, it is then distributed to the various places where it is required to be used. The methods which act as traps for enabling the storm water to sip into the ground include:

Building infiltration trenches

Infiltration trenches are superficial excavations filled with homogeneously crushed stone to create underground reservoirs for storm water or runoff. They are similar to soak pits. The walls and the top of the infiltration trenches are lined with geo-textile to prevent sediment from penetrating into the trenches. In other cases, the trenches are designed to include vegetative cover and other features, in an effort to make it a bio-filtration area.

In most cases these trenches are constructed next to outdoor parking lots or streets. The trapped storm water then slowly infiltrates through the bottom of the infiltration trench into the subsoil layer and then gets into the water table.

Constructing grass filter stripes

Grass filter stripes also commonly known as filter stripes or grassed filters are usually thickly vegetated, uniformly graded areas where surface flow from adjacent impervious areas flows into.

They help in slowing the speeds of runoff, providing a self-effacing way for the runoff to infiltrate and trapping sediment which are in most cases increases fertility of the land.

Constructing grassed swales/vegetated swales

These are open grassed channels in which storm water finds its way into. They mainly act as a way for slowing down and partially infiltrating the surface runoff along their course. The channels are also designed to have check dams.

The vegetation and check dams trap sediments and allow sedimentation while giving time for the water to slowly infiltrate into the ground.

These are mainly used in roadside drainage systems.

Construction of pervious pavements

These are permeable pavement surfaces with an underground stone reservoir. The stone reservoir stores the collected surface runoff for a short time before the directing it to other drainage systems or storage systems.

They are mainly used in town and cities where most of the ground is covered by pavements. They help in reducing flooding of towns and cities.

Above ground water tanks in South Africa

There are different rainwater harvesting storage tank options. You can either get an above ground water tank installed or an underground water tank installed. Both of these two options come with advantages and disadvantages. In this section we will do a comparison of the water tank options.

One of the primary advantages of the above ground water tank is the fact that is the more cost effective solution to get installed. This is because of the cost involved with digging the system into the ground.

The above ground water tank might be cheaper to get installed, but you have to remember that they will be effected by the outdoor elements. If you live in a particularly cold climate you could have the water in the pipes freeze on you.

Another thing to take into consideration is the cleaning of the water tank. The above ground water tank is much easier to clean out because you have sufficient access to the tanks. Obviously and underground water tank would be much harder to flush out and clean.

The above ground water tanks in South Africa take up quite a lot of space around your home. Some homeowners might consider these tanks an eyesore.

Underground water tanks in South Africa

Underground water tanks are tanks that have been buried underground. It costs quite a bit more to get an underground water tank installed on your residential or commercial property in South Africa. This is because of the work involved with getting these huge water tanks buried underground.

One of the advantages of the underground water tank system is the fact that you can enjoy more space around your house and in the garden. They take up absolutely no space around the house.

The best time to get an underground water tank installed in South Africa is during the construction phase. It’s the most cost effective way to go.

A water pump is also required with this system to pump the water from your tank storage underground to the surface.

Finally, if you decide to get an underground water system installed in South Africa, make sure it has sufficient reinforcement. Remember you want the water tank to last for many years underground. Make sure your installers have handled the underground pressure problem. You don’t want to end up with cracks in your new rainwater harvesting storage tank system.