Underground rainwater harvesting installation

Pre-installation:

Choose a tank

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

Roof p/m²

Tank 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.

Tank placement

The location of an underground tank should be carefully considered and it must be placed on natural 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)

If bearing capacity cannot easily be determined use the provided method of soil classification – which will also assist in determining suitable backfill material.

Soil classification table

Category Visual appearance Squeezed in hand & pressure release Bearing capacity
Rock – Rock, Slate, Shale etc.   – Sufficient
Coarse-grained soil
Gravel
– Coarse to very coarse
– Small stones & particles
– Free-flowing
  – Sufficient
Sand
– Granular appearance (individual grain sizes detectable)
– Free-flowing when dry
– Lighter to brownish colours
– Won’t form a cast when dry & falls apart
– Wet cast crumbles when lightly touched
– Coarse/ Medium sand is Sufficient
– Fine sand is NOT Sufficient
Fine-grained soil
Silt
– 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
– NOT Sufficient
clay
– 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 – Hard homogenous clay may be Sufficient
– Clay is mostly NOT Sufficient
Highly organic soil – 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
– NOT Sufficient

 

 

1. Excavation process

When excavating be aware of:

  • The surroundings Take care not to damage any underground services (telecommunication, electrical cables, water/sewer pipes, gas lines, etc).
  • The natural elements If ground water is present in excavation hole, continuously pump out the water during the installation process.

Location

Description

Excavation 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

– No traffic will pass over the site

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

Type

Thickness

Process

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

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

 

3. Drill hole for inlet

Drill a hole in the designated area (see image above) with 111mm hole saw or a jig saw for pipe to enter at the top of the tank.

 

4. Inlet connection

Fit pipe and elbow to inlet and seal with window putty or pan connector sealer to ensure no leakage once final installation is complete.

 

5. Drill hole for outlet

Drill a hole for the ‘outlet’ with hole saw. The size of the hole will depend on the size of the pipe used (either 20/25mm). Note: the electrical cord also needs to be extended through this hole.

 

6. Place the tank

Carefully place the tank on top of the prepared bedding in the designated excavated area. Note: the direction of flow (inlet) and place according to setup.

 

7. Backfill preperation

NB: the backfill procedure differs based on the size of the conservancy tank used. It is vital to follow the correct procedure to ensure the tank does not get damaged (see steps 8 to 9).

 

8. Backfill for 6000L

For the 6000L tank – Fill the tank with 150mm of water, then backfill 150mm around the perimeter of the tank. Compact the layer and repeat the process until at least half of the hole is filled.

 

9. Backfill for 1250L – 2500L

For the smaller range – fill the entire tank with water, then backfill in layers of 150mm around the perimeter of the tank, compacting each layer before laying the next. Continue until halfway.

 

10. Test compaction

Install an additional elbow and pipe to the overflow, and route to a suitable/required area to ensure no water goes to waste.

 

11. Select pre-filtration

Select desired pre-filtration options.
Pre-filtration will differ depending on preference and what the water will be used for.

 

12. Install leaf eater

Fit the leaf eater to the downpipe (vertically or horizontally).

 

13. First flush diverter

Install the first flush diverter as per instructions provided inside packaging.
Note: the first flush utilizes a dependable ball & seal system, requiring no manual intervention.

 

14. Pipe from first flush

Insert a pipe and extend from the side of the T-junction and connect to the elbow at the top of the tank (precise setup will vary based on installation).

 

15. Pipe to pump connection

a Connect pipe to outlet on pump and extend to the same level as the hole drilled for outlet on tank.
b Submerge the submersible pump (with inserted pipe already in place) into the tank through the lid.

 

16. Plumbing – pump to tap

Connect the pipe from the pump to the pipe that leads to the tap (where you want to use the water).

 

17. Fill lid with concrete

Fill tank lid with concrete (allow concrete to cure) and put the lid in place.

 

18. Backfill 2nd half

Backfill the rest of the hole with suitable backfill material. Refer to steps 8 to 9 to ensure correct procedure is followed (depeding on the size of the tank).

 

19. Pump precautions

Before starting a Submersible Pump ensure that: a There are no signs of damage to the pump or its power cord. b The voltage and frequency specified on the pump coincide with those of the available power supply. c The electric connection is made in a dry place, protected against any risk of flooding. d The electric system is complete with a residual current circuit-braker and an efficient earthing connection. NB: Ensure pump model has a built-in electronic control unit which manages its operation (pump on/off) and prevents damages.

 

20. Pump recommendations

When using a Submersible Pump it is recommended that: a The pump is only used when submersed in water. b The power cord and oating switch is never used to carry/move the pump (always use the pump handle). c The plug is removed from the power socket before taking any action on the pump. To ensure the proper operation of the pump, it is important to comply with the provided recommendations. The recommendations may vary based on specific pump model (please refer to the instruction manual).

 

21. Dressing

Location

Description

Type and thickness

Excavation depth

Wet installation

– Ground water present

Compression ballast
– 450mm
(prevents tank from floating when empty)

Pour 2.8m³ grade 20 MPa concrete on top of the tank (slump around 80) This will give a ballast +- 450mm thick over the entire area of the tank

Non-load bearing

– No ground water will affect the tank – No traffic will pass over the site

Soil – Minimum 200 mm/maximum 750 mm

Shape the soil over the top of the tank to ensure positive drainage

Load bearing

– Traffic will pass over the site

Concrete slab – 150mm
(necessary if vehicles will drive over the tank)

The slab should be 4.2m x 2.9 m and made with 25 MPa concrete, consisting out of two layers

The dressing over the tank will depend on various conditions, use the table above to assist with the appropriate dressing relative to the setup.