Rainwater Harvesting System Design

Catchment systems of rainwater harvesting systems make the first contact with rainwater and direct it into the conveyance system. Usually, catchments are roof surfaces and they provide a certain yield based on the amount of water that normally falls. But some materials produce a higher yield and therefore increase the ability of the rainwater harvesting system to harvest water.

Roofs are, of course, made from a variety of materials and this affects the properties (and quality) of the catchment area. Generally, metal roofs are best for rainwater harvesting systems that are designed for potable water because they shed contaminants easily and there is less buildup of debris, all of which keeps the water cleaner. Additionally, steeper sloping roofs allow water to run off more quickly, which gets rid of contaminants more quickly.

Metal roofs also have low permeability and a high runoff coefficient which results in a high yield of storm- or rainwater. Just one caveat: catchment systems designed for potable water or irrigation should never be installed where metal flashing contains lead.

Having identified the catchment area, a conveyance system must be designed to channel the water to a storage tank. This is usually achieved by using a series of gutters and downspouts.

There are two types of conveyance systems mentioned in the NYS Rain Water Harvesting Guide: wet and dry.

• Wet conveyance systems convey water from the catchment area to below ground, from where it is channeled to an above-ground tank. One disadvantage is that water in the piping can get stagnant between periods of rainfall.
• Dry conveyance systems run water directly from the catchment to the tank so that there is only ever water in the conveyance system when it is raining.

Ordinary rain barrels suitable for rainwater harvesting are relatively inexpensive and offer an easy retrofit to reduce stormwater runoff and provide homes with irrigation water.

Generally, dark-colored materials are preferred for rain barrels and cisterns because they minimize penetration of light which helps to prevent the growth of algae. If they are used to catch potable water, rain barrels should be rated for potable water use, and, once installed, they should be screened from directed sunlight. Additionally, a reduced pressure backflow assembly or an air gap should be installed to prevent any cross-contamination of the municipal water system.

Other typical rainwater harvesting system components include:

• Gutters that commonly incorporate leaf screens.
• Piping that is separate from potable water pipes. These pipes are usually purple in color for easy identification and local regulations often require them to be clearly labeled.
• Roof washers that divert at least 10 gallons of rainwater away from the cistern. These would typically contain 18 inches of coarse sand, filter fabric, and about six inches of pea gravel, all of which ensure good filtration of the water. Note that roof washers are not generally required for water that is collected from pervious surfaces and green roofs.
• Pumps are used to pressurize systems if required. They may be located within the storage tank or in-line.
• Pressurized tanks are sometimes used to avoid having a pump that constantly turns on and off.

As design engineers, we also need to ascertain how much rainwater any harvesting system will be able to collect. The first step will be to check the normal annual precipitation figures available from the National Weather Service. These will show how much, on average, falls in different parts of any one state. But that is not enough information on its own. We also need to determine the catchment area of the roof as well as the runoff coefficient that depends primarily on the surface type of the roof.

Then we are able to establish what the rainwater harvesting potential is for the system for a year.

Before any decisions are made in terms of the type of rainwater harvesting system to be used, it is essential to determine exactly what is required and why so that the system size and installation needs can be identified.

As discussed earlier, rainwater harvesting system design involves the specification of roof downspouts that convey rainwater into some sort of catchment system, usually cisterns or some sort of rain barrels. Irrespective of whether a dry or wet conveyance system is used, we need to ascertain whether the harvested rainwater will be used immediately or whether it will be stored for use later.

Actual use is also important. For example, if the rainwater harvesting system is going to be used to spray crops we will need to design a system with high pressure. If it’s going to be used to water crops using a drip system, we will design a low-pressure system.

We also need to ascertain how much water will be required.

To prevent debris from getting into the conveyance system, we usually install a leaf screen over the top of the downspout. Then the water should pass through a first flush diverter that stops contaminants from entering the cistern or rainwater tank. An inline sediment filter is designed to remove sediment once the water flows out of the tank before it gets to the outlet.

Guidelines from the EPA’s Municipal Handbook on Rainwater Harvest Policies provide suggested treatment options for rainwater that will be reused. For example, water required for:

• Potable indoor use should be:
  • Pre-filtered with a first-flush diverter
  • Filtered through a 3-micron sediment cartridge followed by filtration through a 3-micron activated carbon filter
  • Treated with UV or via a chlorine residual of 0.2 ppm to disinfect the water
• Non-potable indoor use should be:
  • Pre-filtered with a first-flush diverter
  • Filtered through a 5-micron sediment cartridge
  • Treated with UV or disinfected using household bleach
• Outdoor uses require the water to be pre-filtered with a first-flush diverter

The Syracuse University’s Environmental Finance Center published the New Your State Rainwater Harvesting Guide in 2015 which, amongst other things, discusses the benefits of rainwater harvesting.

In addition to simply harvesting “free” water, these benefits include:

• Decreased erosion from runoff caused by stormwater, which, in turn, can increase agricultural productivity.
• Reduced charges relating to utility bills.
• Reduction of potentially harmful runoff that might contain fertilizers, pesticides, or sediments. This is particularly important when runoff occurs on a farm or near to some sort of body of water.
• Availability of rainwater for cleaning machinery, supplying water to irrigation systems, for washing out animals pens, or for providing animals with drinking water.

A primary concern with rainwater harvesting systems is, not surprisingly, water quality and its impact on human health. The new CSA B805/ICC 805 Rainwater Harvesting System Standard provides tools that are designed to establish appropriate water quality based on the quality and cleanliness of the source water as well as the desired categories of end use.

While rainwater is generally regarded as being “clean” water, if it runs off the roof, it commonly picks up pollutants either from roofing materials or from nutrients from the atmosphere or even bacteria from bird droppings. This is why it is so important to consider the end use of the water and to ensure it is filtered and/or treated if necessary.

Key information is provided in the new CSA/ICC Standard for developing water safety plans as well as techniques that can be used for sizing systems. Related system components including storage tanks, water tanks, and disinfectant systems may be evaluated in accordance with other existing standards established by various bodies including the America Society of Mechanical Engineers, NSF International, ASTM International, and the CSA Group that played a significant role in the establishment of the new Standard.

Because reclaimed water, which includes harvested rainwater, needs to be filtered or treated in some other way to ensure it is safe for human consumption, local authorities commonly require that rainwater harvesting pipework and outlets that provide water closets, urinals, hose bibbs, irrigation outlets and so on with reclaimed water are very clearly labeled.

However, the end use of rainwater that has been harvested is also important. For instance, if it is to be used for residential irrigation, it doesn’t usually need to be treated in any way at all. If it is to be used for commercial irrigation or indoors for non-potable use, some kind of treatment will usually be required, depending on the demands of the local authority concerned. Most require some sort of disinfection (for example UV or chlorine) that will get rid of bacteria.

Another safeguard relates to plumbing systems rather than human health and safety. This is why when we design rainwater harvesting systems we ensure that there is some level of screening and filtration that will prevent debris and particles from getting into the plumbing system.

Ultimately, while municipalities will inspect rainwater harvesting systems during installation, and sometimes inspect backflow prevention systems annually, the general operation and maintenance of these is the responsibility of individual property owners.