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Don't Forget the Pipe

by Doug Pushard

The conveyance in a large alternative reuse water is accomplished through piping. Both the inflow and the overflow to and out of the storage system use a variety of piping. Which options are the best and why? Complicating matters, inflow conveyance may use different types of pipes from the outflow conveyance pipes. 

Pipe in trenchWhy are there so many choices? 

Not so long ago, plumbing pipe designs for commercial and residential buildings were limited.  In an effort, to reduce costs new options to solve specific needs were created. 

Rainwater systems can now incorporate these different pipe types to reduce costs but still yield a very productive system.  Depending on the system design, conveyance piping and installation can be one of the more expensive choices of a system.  These new piping options have reduced prices for rainwater systems but necessitate increased the knowledge in designing the right pipe for the right purpose.  

Planning is a requirement to determine the best rainwater piping solution. For example, if rainwater is going to be used in a drinking water system, using piping that meets potable water quality standards (i.e. has an NSF seal on it) for incoming conveyance would be an essential choice. An irrigation system that may water edibles means choosing a pipe that does not leach BPAs is a smart choice. In both of the above use cases the outgoing (i.e. overflow) piping does not have to meet the same standards.  Consequently, it can be a less expensive pipe type.  

                                         Common Plumbing Pipe Types

ABS Corrugated PVC Storm Drain
ABS Pipe Corrugated pipe PVC Pipe SD pipe
Black Black White or Grey White or Green

Piping choices vary in diameter, wall thickness, color, weather proofing, and most important composition. These differences determine when the piping can be used in commercial, residential, potable and waste systems. Today BPA free pipes are recommended for drinking water systems. This is an example of the choice you need to make when planning your design.

In rainwater systems generally, white stormwater piping (i.e. mostly referred to as SD piping) is used for both the incoming and outgoing water flows for the system.  It is thinner walled and less expensive than Schedule 40 PVC DWV (Drain-Waste-Vent) or Schedule 40 potable piping.  Storm drain (SD) piping is available in white and green. 

ABS (Acrylonitrile butadiene styrene) is used in drain-waste-vent pipe systems, sewer systems and is always black while PVC is white. ABS pipe contains BPA while PVC does not and some jurisdictions restrict its use.  It is rarely used in rainwater systems due to cost and the presence of BPA. 

PVC pipe also comes in differing wall thicknesses of the pipe (e.g. Schedule 40, Schedule 80).  There are many schedule numbers used such as:  5, 5S, 10, 20, 30, 40, 60, 80, 100, and several others. The larger the schedule number the thicker the wall size.  Generally, Schedule 40 is used in potable rainwater systems. In commercial systems Schedule 80 is generally required by local plumbing codes. Schedule 80 PVC pipe is gray.

Graywater and blackwater reuse systems are generally used for subsurface irrigation, non-edible plants. Therefore PVC does not need be used and generally these systems incorporate either ABS, SD or corrugated piping.

Trench with pipe installedCorrugated piping is the least expensive and most flexible option.  It comes in both perforated and solid.  Perforated corrugated is commonly used in passive alternative water reuse systems drain fields.  In the picture, both solid and perforated pipe are used.  The perforated pipe is used to let the water drain out of the pipe and it is covered with a white sock to prevent roots from growing into the pipe.  Some type of covering is critical in the perforated pipe otherwise it will become completed clogged in just a few years.

Commonly, either storm drain piping (SD) or flexible corrugated piping is used for the outgoing flow.  Both of these pipes carry the water to the storage tank.  Both types of pipe will work, the corrugated piping is very flexible and easier to install.  However, remember it can easily accidentally be cut with a shovel. 

Due to the variety of piping, the sizes of connectors (e.g. couplers, unions, 90s, Ts, etc.) vary greatly.  Importantly these pipes have different outside diameters and the connectors CAN NOT be interchanged (i.e. a 3” SD coupler will not work on a 3” Schedule 40 pipe*).  Thereby reducing the types of pipes that are used on a design will reduce the number and types of fittings required.  Not to mention the frustration of not having the right fitting.

Important note: All this piping when installed needs to have a slope of 1/8” per foot of pipe per plumbing code to convey the water to it’s destination (e.g. underground rainwater tank or drain field). 

In many systems 3”- 4” pipe * is the piping of choice.  However, the correct piping size is important to the design of the system.  Too small and water will back up and potentially cause a system failure.  Too large and it is a waste of money.  Over-sizing is usually a better option, due to the increasing frequency of 100-year storms.  Conveyance piping size can be correctly determined through the plumbing codes. 

In some cases, the conveyance piping may be part of the rainwater storage system.  These systems are referred to as wet rainwater catchment systems as opposed to dry catchment systems where no water is stored in the pipe system.  To determine the holding capacity of the rainwater system, the below table provides the number of gallons held per foot of pipe.

WATER HOLDING CAPACITY OF PIPE

LENGTH DIAMETER GALLONS PER FOOT
1 FOOT 3 0.30
1 FOOT 4” 0.54
1 FOOT 5” 0.84
1 FOOT 6” 1.02
1 FOOT 7” 1.66
1 FOOT 8” 2.17
1 FOOT 9” 2.74
1 FOOT 10” 3.39
1 FOOT 11” 4.10
1 FOOT 12” 4.88

For example, per the chart if the conveyance system is meant to store/hold water in 100’ of pipe, that would be 65 gallons (i.e. 100’ x .5426 gallons per foot**). More than a typical rain barrel! A 100’ of 6” pipe would hold 146 gallons.

Freezing and location are also considerations in choosing the correct piping. White PVC should not be used in above ground systems exposed to the sun. The sun will over time make this white piping brittle and prone to cracking (i.e. leaks). Schedule 80, greywater PVC, should be used in these situations.

All these pipes may freeze and break in certain conditions. The less rigid the pipe, the less it is prone to freezing. Flexible corrugated pipe will not freeze and break as quickly as Schedule 80 PVC under the same conditions. Burying below the frost line or insulating are options to slow down or prevent freezing. In systems where the conveyance pipe will not hold/store water, freezing is not generally a consideration.

Piping choices have increased and thereby reduced the costs of rainwater systems. However, with these new options it is now possible to use the wrong type pipe. Knowing which pipe is right for what intended or possible use will lead to a better designed system. This article has dealt with the incoming and outgoing portions of the conveyance system. It has not covered the piping for the intended use (e.g. drinking, toilet flushing, indoor plumbing, nor irrigation, etc.) of the recycled water.

____________________________________________________________

4” is not really 4 inches - 4” flexible pipe has an outside diamater of 4.6, 4” Schedule 40 PVC is 4.5", and 4” Stormwater pipe is 4.215.

** Gallons per foot of pipe is calculated by the following formula: Pipe volume = π * radius² * length, where radius = inner diameter/2.

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