What is best design for a branched drain system?

System Layout and Sizing

A branched drain system typically consists of a central manifold that distributes the greywater from the source (e.g., shower, sink, washing machine) to multiple branches. Each branch should be sized according to its flow rate, taking into account the number of fixtures connected, the flow rate of each fixture, and the length of the pipe run. A general rule of thumb is to size each branch to handle 1/4 to 1/2 of the total system flow rate. For example, if the total system flow rate is 10 gallons per minute (GPM), each branch should be sized to handle 2.5 to 5 GPM.

Branch Design and Slope

The slope of each branch should be between 1/4 and 1/2 percent to ensure proper drainage and prevent backpressure. The slope should also be consistent throughout the system to prevent uneven drainage. In a radial layout, the branch pipes should be sized to match the flow rate and slope requirements, taking into account any changes in elevation or direction along the pipe run. For example, if a branch needs to traverse a 10-foot elevation change, the pipe should be sized to handle the increased pressure drop, which can be estimated using the Hazen-Williams equation.

Manifold Design and Installation

The central manifold should be designed to handle the total system flow rate and be sized to accommodate the largest branch. The manifold should also be installed with a minimum of 1/4 inch of clearance between the manifold and any surrounding surfaces to allow for easy maintenance and cleaning. When designing the manifold, consider using a manifold with individual shut-off valves for each branch to facilitate maintenance and troubleshooting. The manifold should also be installed at a convenient location, such as near the source of the greywater, to simplify maintenance and repairs.