First Flush Delta In Ground | Rain Harvesting by Blue Mountain Co

First Flush Delta In-Ground

First Flush Delta Diverters
Divert the dirtiest water and transform your Rain Harvesting system from “wet” to “dry” with this large volume inground first flush diverter. Utilising 100mm pipes for the chamber makes installation simple and the advanced release valve reduces outlet maintenance. 1m of Delta Chamber = approximately 73 litres*
Size 100mm WDIG10
First Flush Delta Extra Chamber

Delta High Volume Chamber

Easily divert large volumes of first flush with the Delta High Volume Chamber. Using multiple 100mm pipes to create the diversion chamber, the Delta’s revolutionary design makes installation simple. The Delta chamber is customizable up to 2 metres. Every 1m of Delta chamber = 73 litres of diversion.

For larger volumes, consider the Delta manifold installation configurations.

Delta Inground Kit

Integrated ball and seat

This unique design ensure that the first flush ball remains close to the seat, even in high flow events, ensuring the diverter seals as soon as the chamber is full. No more lost balls when cleaning your outlet either!


Arv Web Kit

Divert larger volumes

By allowing you to determine the frequency of first flush, the Advanced Release Valve ensures that the Delta’s first flush diversion is optimized. The transparent, rapid release exit funnel reduces build up and blockages while allowing for easy


How to install:

The Rain Harvesting steps addressed:


  • 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. 

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

  • Installation
    1. It is a requirement to install a rain head upstream of any down pipe feeding the Delta In-Ground First Flush. Large debris must not enter the First Flush chambers to prevent blockages and damage to the Advanced Release Valve.
    2. Select an installation point for your Delta In-Ground First Flush. Your diverter must be installed on a slope (minimum 5% or 1 in 20) to ensure it drains after each rainfall event. Your first flush T-junction should be at the lowest point in your Rain Harvesting sub-surface pipework or at a location to allow for easy installation and draining of the Advanced Release Valve. The T-junction can also be installed to replace the 90° bend at the base of the vertical pipe feeding your rainwater tank. If the Rain Harvesting sub-surface pipework is not part of a “wet” system, the T-junction will need to be installed with the branch below the main line and a 90° bend to direct first flush of water to the Delta. The outlet must also be accessible for maintenance and inspection. This may be achieved by running pipe to a location above ground or installing an access pit (e.g. stormwater pit) (refer to Figure 1 for suggested installation locations)

      Figure 1
      Suggested Installation Locations

    3. Remove Delta First Flush components from packaging and lay out parts ready for assembly.
    4. Determine chamber length using the calculation chart provided, based on your Rain Harvesting roof collection area and considered pollution level (see Delta Diversion Chamber Calculator Tab below.
    5. Using a tape measure, mark, cut and deburr 6 equal lengths of 100mm (4”) pipe to be used as the Chamber Pipes

      NOTE: It is critical that all the Chamber Pipes are exactly equal length. It is also recommended to apply a small chamfer to the outside ends of the six Chamber Pipes to improve ease of insertion into the Chamber Sockets.

    6. Using priming fluid, clean all internal sockets of both Delta End Caps and each external ends of the six Chamber Pipes.
    7. Working with one Delta End Cap, apply solvent weld glue internally to a Chamber Socket and then externally to one of the Chamber Pipes. Bring the two together ensuring the pipe is inserted fully into the socket and hold until firm. Repeat this step for all remaining pipes until all six Chamber Pipes are glued into one Delta End Cap.

      NOTE: All sockets of the Delta End Cap are stepped internally. The inner socket is for use with 100mm UPVC pipe and the outer socket for 4” Sch. 40 pipe. Only apply solvent weld glue to the socket relating to the pipe in use.

    8. Slide the Chamber Support Spacer over the open end of the Chamber Pipes and position approx. 200mm (8”) from the unglued end.
    9. Before completing the next step consider the installation position of your Delta and how the inlet and outlets should be oriented (Figure 2 – Delta In-Ground Inlet/Outlet Position). The invert of the Delta inlet should be in the high position & the invert of the Delta outlet should be in the low position.

      Figure 2
      Delta In-Ground Inlet/Outlet Position

    10. Working quickly, apply solvent weld glue to each of the six internal Chamber Sockets of the remaining Delta End Cap and then externally to the six Chamber Pipes. Quickly bring the Delta End Cap together with the six pipes by first aligning three pipes and sockets on one side and then rolling onto the remaining three pipes. Using some force, push the Delta End Cap down onto the Chamber Pipes ensuring the pipes enter the socket fully and hold in position until secure (Figure 3 – Delta Diagram).

      NOTE: refer to solvent weld glue manufacturers specifications and curing times.

      Figure 3
      Delta Diagram

    11. Move the Chamber Support Spacer down the Chamber Pipes to approximately the half way position ensuring the pipes will be supported evenly.
    12. Insert Cage/Seat & Ball into the inlet (upstream) Delta End Cap, ensuring it is oriented correctly (Figure 4).

      Figure 4
      Inserting the Cage/Seat and Ball

    13. Excavate the installation location for the T-junction, assembled Delta Chamber and adjoining pipe work. Trenches should be wide enough to include a minimum of 75mm bedding and overlay support and a minimum of 100mm side support. In non-trafficable areas allow for a minimum of 300mm cover (measured from the top of the Delta/pipe to the ground surface). See Figure 5 for installation. For further information we recommend using AS3500.2:2018 as a guide.

      Figure 5
      Delta In-Ground Installation

    14. Using a T-junction as a template, measure the pipes at your chosen installation point and cut to create space for the T-junction. Using priming fluid and solvent weld glue, install the T-junction and fittings as required and extend pipe work ready to install your Delta.
    15. Place your Delta in position and using priming fluid and solvent weld glue connect your Delta to the extended pipe work ensuring you push the pipe hard up against the Cage/Seat & Ball. Backfill your Delta using bedding sand capable of passing through a 2mm sieve, as per State and Local Government guidelines and information provided in the previous step.
    16. The Transparent Rapid Release Exit Funnel needs to be installed in an accessible location (above ground where the ground slopes away or in a stormwater pit for access. Using a min. 170mm of 100mm pipe (3.5″ of 4″ pipe) and solvent weld glue, attach the 100mm-90mm (4”-3”) Socket Reducer (100mm/4” End Coupling) to the outlet (downstream) Delta End Cap. Screw the Transparent Rapid Release Exit Funnel onto the threaded end of the Socket Reducer.
    17. Install the Primary Filter Screen, Transparent Rapid Release Exit Funnel, and Advanced Release Valve by following the instructions in this guide.
  • Delta Diversion Chamber Calculator

  • 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 Advanced Release Valve, periodically remove from the Transparent Rapid
    Release Exit Funnel to check for any build-up of matter. Remove primary filter plus ball, and clean if required.

    Periodically check that the Advanced Release Valve batteries have charge. This is indicated by the flashing light.

    To protect your Advanced Release Valve from freezing or “winterising”, remove the timer prior to the first frost or freeze
    and store it indoors until spring. Remember to remove the batteries from the battery compartment.

    For best results and minimal maintenance, rain heads with 0.955mm aperture mesh such as Leaf Eater Rain Heads must be installed upstream of the Delta First Flush to limit the entry of debris that can reach your diverter.

  • Photo Gallery

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