Branched Drain
Graywater Systems
Go with the flow
A gravity fed system that diverts graywater from sources such as bathroom sinks and showers into the landscape.
Key points
Branched Drain System
Uses standard 1.5”, 2” or larger drain pipe(ABS or PVC-DWV)
Distribution exclusively via gravity
Requires maintaining a 2% downward slope (or ¼ inch per foot)
Utilizes existing house venting system to prevent siphoning within the system
A 3-way diverter valve is installed in the drain line downstream of traps and vents but upstream of any connection to backwater fixtures Captures graywater from the discharge hose of your washing machine
Diversion upstream of sewer drainpipe
An actuator may be installed to automate switching from landscape to septic/sewer
A backwater valve may be installed on the sewer side of the 3-way valve to prevent sewer water from backing up into the graywater system
Typically requires a permit as it requires alteration to existing plumbing
Required Parts
Available from pool supply stores, plumbing-supply, electrical supply and hardware stores
1) 3-way diverter valve (2“ - 3” size valve)(pool supply stores)
2) 7” round valve box (Hardware stores)
3) ABS 1.5” or 2” double ell (twin 90)(plumbing-supply stores)
4) ABS 1.5” or 2” double ell (twin 90) w/inspection/cleanout port
5) 3-way valve actuator (optional)(pool supply stores)
Indoor Assembly
See below for descriptions of each component in detail.
3-Way Diverter Valve
Installed in the drain line downstream of traps and vents but upstream of any connection to backwater fixtures (ie. toilet)
Uses 1.5”, 2” or larger drain pipe
An actuator may be installed on. the valve to allow remote switching from within the house (code required if the valve is not “reasonably accessible”)
A backwater valve (aka “sewer check valve”) is typically installed on the sewer side of the 3-way valve to prevent sewer water from potentially backing up into the graywater system
Backwater Valve
AKA Sewer Check Valve is usually installed on the sewer side of the 3-way valve to prevent sewer water further downstream from backing up into the greywater system. Since the 3-way valve itself shuts off the sewer connection, the backwater valve functions only if the 3-way valve is left partially open, and this component is Not required by all jurisdictions. Must be installed in the correct orientation with flow arrows pointing downstream in order to function correctly.
Valve Actuator
If the 3-way valve is not easily accessible (e.g. tiny crawl space) you will need to install an actuator to operate it remotely from within the house. An actuator is an electrically powered motor that turns the 3-way valve, and is connected to low voltage power and a switch placed in a convenient location. The actuator itself is available from pool supply stores and is designed to fit a particular size and brand of 3-way valve. A few other components available from electrical supply stores or hardware stores are needed to complete the actuator switch:
24 Volt transformer with screw on terminals (plugs into a wall socket and reduces power from 120 Volts to 24 Volts)
A toggle-type switch
A surface mount electrical box and faceplate.
Wire nuts to make electrical connections within box
Low-Voltage wire (12 or 14 gauge)
Shower Drain Pipe Identification
The first step to installing a 3 way diverter valve is to identify the correct pipes to divert. A “p-trap” (a common component of drain plumbing systems that is full of water that prevents sewer gases from entering the building) is usually visible from within the crawlspace and helps identify the shower drain line because sinks and toilets have their p-traps located under the fixture in the house. A 1 1/2”-2” pipe size (inner diameter of pipe (“ID”)) also suggests a shower line as waste water pipes are sized by the amount of water that travels through them and toilets are always a minimum of 3”-4”. A good method of confirming you have the correct pipe is to run HOT water in the drain line until you feel the pipe heat up.
Laundry and Sinks Drain Pipe
Identification
Even though laundry graywater is often sent through an independent “Laundry-to-Landscape” system, it can also be sent through a branched drain or combined into a whole-house system. The drainpipe your laundry discharge hose empties into should be a 2” size and has a non-visible P-trap connected to it located inside the wall. Sink drain pipes can be as small as 1 1/4” or 1 1/2” and sometimes share vents with other fixtures or use an isolated vent such as an auto vent.
Vent Pipe identification
As fluid flows through a drainpipe it pushes air ahead of it and sucks air behind it. Vent pipes provide an outlet and inlet for this air and prevent a vacuum from being formed that could suck the water out of the p-trap (which is meant to stay full all the time). In order for the vent to work though it must be downstream and within a certain distance from the P-trap, known as the “critical distance”. Venting for graywater pipes can be separate from toilet vents or can tie into the toilet vent 12” above the spill point of the highest fixture served by the vent. Vent pipes are easy to confuse as drain lines when one is unfamiliar with plumbing. 3-way diverter valves should always be located downstream of P-traps & Vents, otherwise they would cut off their essential function.
Sizing Drainpipes
Drain line pipes are sized according to the number of “drainage fixture units” they carry. Each type of fixture (ie. shower, sink, toilet) has a different number of “units” assigned to it in the plumbing code.
Maximum Unit Loading
Add up all the units of the fixtures you want to connect to determine what size pipe is necessary. Whether the section of pipe is vertical or horizontal also determines the amount of units it can carry. The vast majority of single family home graywater systems will be fine with a 2” pipe, and only the largest systems will need a bigger size pipe.
For example, a typical branched drain system may include a shower(2 units), a sink (1 unit), a bathtub(2) and a laundry (3) for a total of 8 units, which still fits in a 2” pipe but is the maximum units allowed in that pipe size.
Minimum Slope
Drain pipe systems need constant proper downwards slope in order to drain correctly without clogging. Quantified slope can be expressed as a percentage, a ratio (rise over run) or an angle(degrees). “2%” slope is the industry standard recommended slope for drainpipes and is the same as saying 2ft of fall over 100 ft of run, or 1/4” per ft. The pipe can NEVER slope up again and because the deeper you release the water in the landscape, the less absorbable it is by plants, (and the less effectively treated it is by the more biologically active upper soil layers) strive to “conserve fall” as much as possible (don’t slope down any more than 2% unless you have to). Preserving fall can also often make it possible for pipes to be able to exit the house through the crawlspace vents, above the foundation wall (to avoid drilling a hole in the concrete).
Flow splitters (double ells)
(aka Twin 90) Passively divide the flow in half to be able to spread out the graywater among multiple outlets. A relatively rare plumbing piece, Double Ells are only reliably available from dedicated plumbing-supply stores. In order to divide the flow evenly, the flow splitters must be level and have a 2' straight section of pipe before the splitter, because if flow enters the splitter at a sharp angle the water won't divide evenly. Double Ells can be multifunctional as clean-outs if they are installed with removable couplings or are outfitted with a clean-out plug, accomplished by drilling a 1 1/4” hole and threading in a PVC 1” threaded Plug. Preserve access for maintenance of the flow splitters by enclosing them within a lidded valve box.
Outlets
Release graywater from the open end of the pipe into a “shield” (valve box) within an infiltration basin (aka mulch basin), created by excavating out the soil and refilling with wood chips, gravel drain rock, or other chunky organic material providing open pore space in which the surge of graywater fills preventing it from being exposed to the surface (which is prohibited).
Estimating Graywater Production
Using the Code formula (1602.8.1) requires knowing the number of bedrooms in the house, which determines the number of theoretical occupants:
1st bedroom = 2 occupants
Additional bedrooms = 1 occupant
Add up the total occupants and then multiply the total number of occupants by the estimated graywater production per-occupant accordingly:
Laundry = 15 GPD (gallons per day) per occupant
Showers, bathtubs, and wash basins = 25 GPD per occupant
See the example at right for a complete calculation process for a four bedroom house.
Sizing Infiltration Areas
The area (square feet) of mulch basins needed is determined by dividing the estimated number of gallons produced per day by the soil type’s maximum absorption capacity (measured in gallons/sq2). Use Code Table 1602.10 at right for common soil types. See examples below for complete calculation for 2 different soil types with an estimated daily graywater production of 125 gpd.
Coarse sand or gravel: 125 gal/day / 5 gal/ft2/day = 25 ft2
Clay w/small amounts of sand or gravel: 125 gal/day / 0.8 gal/ft2/day = 156.25 ft2
Surge Capacity
The open pore space between the particles of wood chips, gravel drain rock, or other chunky organic material in the infiltration basins provides open space in which the surge of graywater fills preventing it from being exposed to the surface (which is prohibited). Approximately 40-60% of the volume in an infiltration basin is occupied by the refill material depending on the exact type used. By calculating the total volume of the excavation and subtracting the volume occupied by the fill material we can get a rough idea of the available surge capacity in Cubic feet and then convert that to gallons (There are 7.48 gallons in a cubic ft3).
(Gallons of estimated graywater discharge per day / 7.48) / porosity (expressed as a decimal) = Cubic ft3 of infiltration basin required
Ensure the available volume of the infiltration basin exceeds the calculated surge volume of the daily greywater production.
Labeling Requirements
To avoid future confusion Graywater distribution piping has to be labeled to identify it as being Non-potable water. The words “CAUTION: NONPOTABLE GRAY WATER, DO NOT DRINK” should be marked on the pipe at intervals not to exceed every 5 ft.
The 3-way diverter valve in the indoor assembly of the Laundry-to-Landscape System is also required to be labeled as to which handle direction sends the graywater to sewer or landscape.
Operation and Maintenance Manual
Required info:
Diagram(s) of the system and location of components
Instructions on operating the system
Details on start-up, shut-down, and deactivation
Applicable testing, inspection and maintenance
Contact info of installer/designer and component manufacturer
Directions that the O&M manual needs to remain with the building for the life of the structure
Other helpful info:
What kinds of soaps to use
A picture of the tubing before it is buried
