90/100mm (inlet only)

In-Ground First Flush

First Flush Diverters
Divert the dirtiest water and transform your Rain Harvesting system from “wet” to “dry” with this in-ground first flush diverter.
Size 90/100mm (inlet only) WDIG01
WDIG01 01 First Flush

Keep the dirtiest water out

By capturing and discarding the dirtiest, most contaminated water from your roof, this first flush diverter reduces the load on your pumps and filters and helps keep hazardous organic and inorganic matter out of your tank so your water remains fit-for-purpose.

Inground First Flush Product Perspective

Prevent anaerobic fermentation

The First Flush In-Ground drains water-filled pipes in your “wet” Rain Harvesting system to prevent anaerobic fermentation and stop fermented water contaminating the rainwater in your tank. This drained water can then be released or used to water your garden.

Inground First Flush Installed

Customise for your needs

Easily customise the first flush diverter’s diversion chamber volume to suit your needs before installing it beneath your garden for maximum space efficiency and aesthetics. You can also customise the destination for drained water and connect the diverter to as many downpipes as necessary.

The length of pipe required to form your first flush diversion chamber will depend upon how much water you need to isolate and capture. The following measurements are given as a guide only.

1 metre of 90mm pipe = 5.7 litres
1 metre of 150mm pipe = 18.8 litres
1 metre of 300mm pipe = 72 litres
3 feet of 4″ pipe = 2 gallons
1 metre of 100mm pipe = 8.8 litres
1 metre of 225mm pipe = 45 litres
3 feet of 3″ pipe = 1.5 gallons
3 feet of 12″ pipe =19 gallons

First Flush diverters help you plan for volume by allowing you to choose your catchment volume with custom catchment size and/ or an advanced release valve.

First Flush Diverters help divert the first flush, through their design. They are installed using a T junction or high flow inlet to which the diversion chamber is fitted. As the chamber fills with the initial dirty water from your roof, a ball rises until it seals the inlet, allowing the rest of your rainwater, which is much cleaner, to flow directly to your tank.

Learn more about the 4 pillars of designing a Rain Harvesting system. 

The Rain Harvesting steps addressed:

Limit sources of contamination     Plan for volume

Filter leaves and debris                    Divert the first flush

Secure the system                             Manage standing water

Consider a safety net                        Monitor and maintain

  • Specifications

    First Flush In Ground Dimensions

  • Installation

    NOTE: Select an installation point for your First Flush In-Ground. Your diverter must be installed on a minimum slope (5% or 1 in 20) to ensure it drains after each rainfall event. The outlet must also be accessible for maintenance and inspection. This may be achieved by running pipe to a location aboveground or installing an access pit (e.g. stormwater pit).

    1. Determine the length of 300mm pipe required for your first flush diversion chamber using the table below and cut accordingly. Ensure all cut edges are clean and smooth.
    2. Attach the upper and lower end caps to your 300mm pipe by applying solvent weld glue to the socket and pipe before inserting the pipe into the end cap and holding until the glue sets. Repeat for the other end cap though ensure your end caps are oriented so that the inlet on the upper end cap is at 12 o’clock and the outlet on the lower end cap is at 6 o’clock. Expect a tight fit.
    3. Select the appropriate flow control washer and fit it into the hose connector with the side marked “TOP” showing. Start by using the Control Washer with the smallest gauge hole (lowest number). Try a larger gauge Washer if experiencing blockages. Save the remaining washers for possible future use.
    4. Insert the plastic filter screen in through the base of the screw cap with O-ring seal and secure by attaching the hose connector and flow control washer.
    5. Apply solvent weld glue to the socket of the threaded coupling and one end of the 90mm length of 90mm pipe. Insert the short section of pipe into the socket and hold until the glue sets. Insert opposite end of this short section inside the lower end cap outlet after applying solvent weld glue.
    6. Place the stainless steel filter in the lower end cap outlet through the threaded coupling, insert the 8mm keeper ring, then attach the screw cap with O-ring seal (and assembled components) to the threaded coupling.
    7. Using a T-junction as a template, measure the pipes at your chosen installation point and cut to create space for the T-junction. If your first flush diverter will be located after the existing 90 degree bend in the pipe that feeds your tank, the T-junction can be used in place of the existing 90 degree bend to direct water vertically to the tank. Otherwise the T-junction can be installed in your existing horizontal pipework. Whichever installation option you choose, ensure all cut edges are clean and smooth.
    8. Place the ball inside your first flush diversion chamber through the upper end cap inlet.
    9. Insert the ball seat into the top of the upper end cap inlet, with the narrow end of the seat facing downstream.
    10. If you are installing your diverter below your existing pipework (i.e. you are not replacing the existing 90 degree bend with the T-junction), connect a 90 degree elbow to your T-junction and install using solvent weld glue.
    11. If you are fitting your diverter to a 90mm T-junction/elbow, cut a length of 90mm pipe to connect your diverter and T-Junction/elbow. Attach the pipe hard down on top of the ball seat, then attach the T-Junction/elbow hard down on top of the pipe.
    12. If you are fitting your diverter to a 100mm T-junction/elbow, attach the 28mm length of 90mm pipe hard down on top of the ball seat and glue in place using solvent weld glue. Fit the T-junction/elbow around the upper end cap inlet or, if required, connect the T-junction/elbow and upper end cap inlet using an additional length of pipe.
    13. Connect the T-junction to the existing pipework using solvent weld glue.
    14. Ensure all subsurface pipe work (including first flush chamber), has been backfilled and supported correctly using appropriate bedding sand or similar after testing for leaks.
  • Maintenance

    It’s important to ensure that your first flush diverter outlet remains clear of any debris. If your outlet becomes blocked, the chamber will not empty and the first flush of water will not be diverted when it rains.

    To ensure the flow of water out through your diverter’s outlet, periodically unscrew the outlet to allow debris to fall out. If the diversion chamber is full of water, take care as it empties. Remove the flow control washer, hose connector, keeper ring and filter screens and hose or wash the screens with clean water. Check the flow control washer for any blockages and remove and clean as necessary.

    For best results and minimal maintenance, we recommend installing rain heads such as our Leaf Eater rain heads on all your downpipes to limit the volume and number of leaves and debris that reach your first flush diverter.

  • What's in the box?
    • 2 end caps
    • Hose connector
    • 8 flow control washers
    • Plastic filter screen
    • Screw cap with O-ring seat
    •  90mm length of 90mm pipe
    • Threaded coupling
    • Stainless steel filter screen
    • 90mm keeper filter ring (8mm)
    • Ball
    • Ball seat
    • 28mm length of 90mm pipe
      (Ball seat ring keeper)
  • Tools you might need
    • 300mm pipe (for diversion chamber)
    • Tape measure
    • Marker pen
    • Saw
    • Solvent weld glue
    • T-junction
    • 90 degree elbow (optional)
    • 90mm pipe or 100mm socket or T-Junction
    • Shovel
    • Bedding sand or similar
  • Product FAQ's
    • What comes in my first flush diverter kit?

    The exact components included in your first flush diverter kit will depend upon the product you order and are listed on each individual product page. (For product-specific information, visit the product page for your chosen first flush diverter via the links above.) 

    • Where should I install my first flush diverter?

    Your first flush diverter can be installed anywhere between your roof catchment area and your rainwater tank to isolate and capture the first “flush” of dirty water at the beginning of a rain event. You may choose to install smaller diverters to divert rainwater from each downpipe or you may choose to install larger diverters to divert rainwater from several downpipes. 

    • How much water should I divert?

    In areas with mild pollution (e.g. open fields, no trees, no bird droppings or animal matter, clean environment) you should divert 0.5 litres per square metre of roof (0.0125 gallons per square foot of roof). In areas with higher pollution (e.g. leaves and debris, bird droppings, animal matter/carcasses, pollution) you should divert 2 litres per square metre of roof (0.05 gallons per square foot of roof). These formulas will help you calculate your total diversion volume or, if you’re using more than one first flush diverter, the volume of water you should divert in each device (which will depend upon the size of the roof area that “feeds” each diverter). 

    • How long should my first flush diversion chamber be?

    The length of pipe required to form your first flush diversion chamber will depend upon how much water you need to isolate and capture. The following measurements are given as a guide only. For product-specific information, visit the support page for your first flush diverter.  

    1 metre of 90mm pipe = 5.7 litres 

    1 metre of 100mm pipe = 8.8 litres 

    1 metre of 150mm pipe = 18.8 litres 

    1 metre of 225mm pipe = 45 litres 

    1 metre of 300mm pipe = 72 litres 

    3 feet of 3″ pipe = 1.5 gallons  

    3 feet of 4″ pipe = 2 gallons 

    3 feet of 12″ pipe =19 gallons 

    • Can I run two or more pipes into one first flush diversion chamber?

    Yes. Just remember that you need to size your first flush diversion chamber based on the size of the roof area that’s upstream from your diverter. If two or more pipes (aka lines) feed into a single diverter, you should size your diversion chamber accordingly by considering the total roof area that feeds both pipes. 

    • Which flow control washer (valve) should I use?

    Your first flush diverter comes with a range of flow control washers (valves). Each washer has a different sized hole. The size of the hole determines the rate at which your first flush diversion chamber will empty. A smaller hole will create a slower flow rate and release less water during a rain event, while a bigger hole will create a higher flow rate and release more water during a rain event. (Blockages excepted, any size washer will eventually fully empty your diversion chamber after each rain event.) Washers with a smaller hole will become blocked sooner than washers with a larger hole. Choosing your flow control washer is therefore a trade-off – lower wasted water during rainfall means more regular maintenance, while moderate wasted water during rainfall means less maintenance is required. However, when choosing your flow control washer, remember that the volume of water flowing from the largest washer, which has a 2mm diameter hole, will still be insignificant when compared to the rainfall flow rate. 

    • Which way should the ball seat be oriented?

    Your diverter’s ball seat is like a funnel: \  /. The narrow end is installed facing down so it can be sealed by the black ball. 

    • How do I plumb the trickle feed outlet on my first flush diverter?

    You can use 12mm irrigation tubing to plumb the trickle feed outlet to a location away from the diverter. 

    • How do I clean my first flush diverter?

    Slowly unscrew the black hose fitting from the bottom of your first flush diverter. If the diversion chamber is full of water, take care as the water empties. Remove the plastic filter (it may come out when you remove the black hose fitting) and wash under clean water. Check the flow control washer for any blockages and wash with clean water, then replace all these parts. 

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Your selection

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