Rainwater harvesting underground

The installation of an underground tank that is filled automatically when it rains. This will require the installation of suitable guttering and piping to transport rainwater to the tank, as well as a submersible pump to supply water as required.

The Benefits

It is simply, easy and offers advantages like self-sufficiency and the convenience of not being dependent on regulated water resources.

It presents the opportunity to replace high quality drinking water with rainwater, for low grade uses such as toilet flushing, irrigation, household cleaning and car washing. As a result you will save water, reduce your water bill and have access to an adequate water supply when none is available.

 
 

Underground Rainwater Harvesting Component Structure

For a basic system you will require the following:

Underground/conservancy tank (depending on size required).

  1. Pre-filtration
    • Rainhead i.e. Leaf Eater
    • First Flush Diverter
    • Pipes – 3 x 110 mm SV pipes
  1. Lengths and quantity will vary depending on installation.
    • From bottom of leafeater to slow release control valve.
    • From side of T-junction to top elbow.
    • From top elbow to bottom elbow.
    • From bottom elbow to last elbow entering into tank.
  1. Pipes – HHDPE– 110mm
    • 20/25 mm depending on pump model.
    • From pump to elbow (inside tank)
    • From elbow to tap
  1. Elbows
    • 100 MM
    • Sizes according to pipes
  2. Submersible pump
 

How To Maintain Your Rainwater Harvesting System

General Maintenance

Once an underground tank is installed access is limited (particularly for the tank and components that are underground). Therefore it is of utmost importance to ensure all installation guidelines and manufacturer’s specifications are followed to ensure proper functionality. A properly installed underground tank requires very little maintenance.

Here are few things that will help prolong your system’s life, reduce the potential for damages and ensure you get maximum benefit from it.

Excavation/Backfill

  1. Underground tanks should be placed on natural soil with a bearing capacity greater than 120kPa.
  2. Avoid installing the tank:
  • In water saturated clay or an area that is frequently flooded
  • Where depth to bedrock is less than 2.5m
  • In the path of vehicles/heavy equipment
  • Where underground services are expected (i.e. electric cables, water/sewer pipes and gas lines).
  1. Excavation, bedding, backfill and dressing will vary depending on the location, type of soil and the size of the tank. Follow manufacturer’s installation guidelines relevant to your setup to ensure proper functionality and to prevent damages.

Tank Maintenance

Tank Lid - Check that the lid of the tank remains in place. The tank lid should be filled with concrete (when installed) to ensure it stays in place during all weather conditions.

Fittings & Connections - Generally check all fittings and pipework connections (above ground) at least every 3 months for leaks and signs of damage.

How To Maintain Your Underground Rainwater Harvesting System

  1. Rooftops - Rooftops should be cleaned regularly to avoid contamination of the runoff. During dry weather, dust, leaves, animal excrement and dead insects accumulate on the roof. Make sure that the roof is cleaned before the rainy season starts, followed by a monthly check to keep it clean.
  2. Overhanging Branches - Check that branches overhanging the roof are regularly trimmed back.
  3. Gutters - Ensure that the gutters are free from leaves - consider installing gutter mesh.
  4. Pre-Filtration - Regularly check the pre-filtration device/s to ensure that it is free from leaves and that rainwater can pass through.
  5. First Flush Diverter - If a first flush diverter is installed, check and clean it out on a monthly basis.Ensure the diverter is dripping out sufficiently to reset the system.
  6. Pump Efficiency - Check to see if the submersible pump is functioning properly and that there are no signs of damages to the pump or its power cord. To ensure proper pump operation it is important to comply with the provided recommendations and specifications (see instruction mannual).
  7. Keeping your tank Clean - Inspect the inside of the tank every 3 months to make sure that leaf litter has not built up in the bottom of the tank. A properly designed system should not allow leaves and other material to get into the tank.
  8. Water Quality – Please remember that water collected from a roof will not be clean. If the intent is to use the rainwater for drinking purposes, consult an installer with experience in installing water filtration systems. For irrigation purposes however, the water would be perfect.
 

How To Install your Underground Rainwater Harvesting System

Pre-installation:

Choose a tank

How to calculate what size tank would be sufficient for Rainwater Harvesting:

Roof p/m²Tank Size
50 – 100 1250 L – 2000 L
200 – 400 2500 L – 6 000 L

The 6000L is the largest underground tank. For something smaller, use any of the conservancy tanks ranging from 1250L to 2500L.

The biggest downfall of Rainwater Harvesting is lack of storage capacity.

Tank placement

The location of an underground tank should be carefully considered and it must be placed onnatural soil with a bearing capacity greater than 120 kPa.

Avoid installing the tank

  • In water saturated clay or an area that is frequently flooded
  • Where depth to bedrock is less than 2.5m
  • In the path of vehicles/heavy equipment
  • Where underground services are expected (i.e. electric cables, water and sewer pipes, gas lines etc)
 

Soil classification table

Category Bearing capacityVisual appearanceSqueezed in hand & pressure release
Rock Sufficient Rock, Slate, Shale etc. -
Coarse-grainedSoil        
Gravel Sufficient • Coarse to very coarse Free-flowing
• Small stones & particles
• Free-flowing
 
Sand Coarse/ Medium sand is Sufficient  • Granular appearance (individual grain sizes detectable)   • Won’t form a cast when dry & falls apart
  Fine sand is NOT Sufficient • Free-flowing when dry
• Lighter to brownish colours
• Wet cast crumbles when lightly touched
Fine-grained soil        
Silt NOT Sufficient • Very little fine sand Cloddy when dry & soft flourlike feel 
• Readily pulverizes to powder 
• Darker colours (green, blue & black)
• Cast can be handled without breaking
• Readily puddles when wet  

Clay

Hard homogenous clay may be Sufficient  • Clay is mostly NOT Sufficient • Fine textured
• Breaks into very hard lumps when dry
• Difficult to pulverize into soft flourlike powder
• Cohesive when moist & shrinks when drying
• Darker colours (green, blue, black & orange)
• Cast can be freely handled without breaking
Highly organic soil  NOT Sufficient • High organic content (often decomposed)
• Plant remains/woody structure easily recognised
• Mineral soil finely divided with fibrous remains
• Occur in lowlands, swamps or swales
• Dark or black in colour

 

 

Installation Guide:

1. Excavation process

When excavating be aware of:

  1. a) The surroundings

    Take care not to damage any underground services (telecommunication, electrical cables, water/sewer pipes, gas lines, etc).

  2. b) The Natural Elements

    If ground water is present in excavation hole, continuously pump out the water during the installation process.

LocationDescriptionExcavation depth
Soft in-situ soil Fine sand, silt or clay Min 420mm / max 720mm deeper than the tank’s height
Non-load bearing No ground water will affect the tank Min 120mm / max 720mm deeper than the tank’s height
Load bearing • Ground water will affect the tank
• Traffic will pass over the site
Min 420mm / max 720mm deeper than the tank’s height

Note: In the example the measurements are for a 6000L underground tank. Allow between 300mm to 600mm on all sides of the tank. Use the table provided to assist with the appropriate excavation depth relative to the setup.

2. Prepare the tank bed

TypeThicknessProcess
Rocks/rocky soils • Minimum 150mm crusher dust or a coarse sand/gravel mixture 
• Bedding to be thicker where hollows or uneven areas are levelled out
Remove all large/loose rocks and objects that could interfere
Dry installation gravel/coarse sand (no ground water) • 150mm crusher dust or a coarse sand/gravel mixture 
• Stabilise bedding layer with 3% cement (1 ½ bags of 50kg cement)
• Spread cement evenly and compact, using mechanical compactor
• Do not wet cement, as the natural moisture form the ground will do this over time
• If the soil is too dry to form a lump when pressed in your hand, add a little water but ensure lump still crumbles when lightly touched
Dry installation soft in-situ soil – fine sand, silt or clay (no ground water) • 2 x layers 150mm crusher dust or a coarse sand/gravel mixture each
• Stabilise bedding layer with 3% cement (1 ½ bags of 50kg cement)
• Spread cement evenly and compact, using mechanical compactor
• Do not wet cement, as the natural moisture form the ground will do this over time 
• If the soil is too dry to form a lump when pressed in your hand, add a little water but ensure lump still crumbles when lightly touched
Wet installation (ground water will affect the tank) • Drainage bedding layer of 150mm crushed rock (19mm), use crusher dust or a coarse sand to bind, followed by
• 150mm crusher dust or a coarse sand/gravel mixture
• Stabilise each bedding layer with 3% (1 ½ bags of 50kg cement)
• Spread cement evenly and compact, using mechanical compactor
• Do not wet cement, as the natural moisture form the ground will do this over time
• If the soil is too dry to form a lump when pressed in your hand, add a little water but ensure lump still crumbles when lightly touched

Note: The bedding is the foundation (inside the excavated area) on which the tank will be installed. The type and thickness of bedding will vary, choose the correct process relevant to your setup.

 

 

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