Rainwater harvesting

Harvesting rainwater involves the utilization of some sort of collection system of rainwater for various uses including plumbing, and/or irrigation and various other outdoor and non-potable indoor uses. Whether you want to recover water for use in construction, for plumbing applications, or to use as potable water, NY Engineers will design the ideal rainwater harvesting system.

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Understanding Rainwater Harvesting

Basics Of Rainwater Harvesting

Harvesting rainwater involves the utilization of some sort of collection system of rainwater for various uses including plumbing, and/or irrigation and various other outdoor and non-potable indoor uses.

The Environmental Protection Agency (EPA) in its Municipal Handbook, Managing Wet Weather with Green Infrastructure, that considers rainwater harvesting policies, states that rainwater harvesting also reduces runoff of stormwater and therefore pollution, and reduces soil erosion in urban environments.

As an increasing number of U.S. states have been affected by drought, there has been more focus on harvesting rainwater for consumption. As the EPA document points out, rainwater can:

  • Provide an inexpensive supply of water
  • Augment supplies of drinking water
  • Help with demand management for drinking water systems

Typically, rainwater harvesting systems divert runoff from both residential and commercial roofs and store it. It is then piped or channeled to where it can be used.

However, neither the International Plumbing Code (IPC), adopted with amendments by New York State (NYS), nor the Uniform Plumbing Code (UPC), developed by the International Association of Plumbing and Mechanical Officials (IAPMO), address rainwater harvesting in their stormwater or potable water sections. The codes do though cover water for reuse.

The UPC definitions of water for reuse are:

  • Graywater, which is defined as untreated wastewater that hasn’t had contact with sewage (black water). This includes water from showers, bathtubs, lavatories, and washing machines used for clothing.
  • Reclaimed waterthat has been treated by a public agency to wastewater tertiary standards and is suitable for controlled use. Conveyed in purple pipes, reclaimed water is commonly used for water supply to water closets, urinals, and for trap seal primers that are designed for floor sinks and drains.
  • Harvested rainwater, which is stormwater conveyed from the roof of buildings that is stored in a cistern (or tank) and disinfected and filtered before it is used for toilet flushing. The UPC also states that it can be used for landscape irrigation, but does not list the other possible uses mentioned above.

In spite of the codes, until very recently when the new Rainwater Harvesting System was introduced, there was considerable confusion relating to the reuse of water. Also, state legislation differs, with some U.S. states being more restrictive in terms of how rainwater is used, and some in terms of using rainwater at all. In Colorado, for instance, water rights laws are interpreted as prohibiting rainwater harvesting because cisterns and barrels prevent rainwater runoff from reaching the state’s rivers. This, in turn, decreases downstream users’ allotted water rights, so rainwater capture and reuse are simply prohibited!

At the same time, some states have incentives that encourage rainwater harvesting. For instance, the New York State Senate has introduced a tax bill that plans to offer credits to homeowners and businesses that invest in green infrastructure that manages “wet weather impacts” that provide many community benefits. This includes reducing and treating stormwater at its source rather than utilizing the conventional gray stormwater infrastructure that moves urban stormwater away from the built environment – including our homes. Other initiatives that will likely be rewarded include rain-gardens, green roofs, and permeable pavements.

New Standard For Rainwater Harvesting

Recently, a new standard for rainwater harvesting systems was published by the International Code Council (ICC) and the not-for-profits standards development CSA Group. Titled CSA B805-18/ICC 805-2018 – Rainwater harvesting systems, it is intended for use in both the United States and Canada.

Approved by the American National Standards Institute (ANSI) and by the Standards Council of Canada (SCC), it was developed by a technical committee of experts from both countries comprising plumbing and public health officials, plumbing and water technology engineers, product and system manufacturers, and public safety specialists.

The Standard addresses stormwater and roof-surface rainwater that is being used as source water for both potable and non-potable applications. Specifically:

  • Non-potable applications addressed include toilet and urinal flushing, washing of clothes, irrigation, fire protection, washing of vehicles, hose bibbs (bibcocks), and decorative fountains.
  • Potable applications include drinking water for human consumption, food preparation, oral care, dishwashing, and bathing.

Closely coordinated with health, building, and plumbing codes, the new Standard links rainwater harvesting system designs with relevant building elements including plumbing, roofing, storm management, fire suppression, cooling towers, and irrigation. It also provides tools to establish appropriate water quality for specific end use.

The term “rainwater harvesting” is used generically throughout the Standard in relation to both stormwater and roof runoff. Additionally, the Standard states that rainwater includes water in all its forms including rain and snowmelt.

In basic terms, the new Standard covers systems that provide water for a range of applications:

  • Single-family residences and residential applications
  • Multi-residential applications
  • Non-residential applications

The Standard does not cover water distribution systems developed for commercial agricultural processes, collection of surface water, or rainwater harvesting systems designed for manufacturing or industrial processes.

The Standard is very new, and there are still vast differences in terms of regulations and legislation in different American states.

Rainwater Harvesting Systems

Catchment And Conveyance Systems

Catchment systems for rainwater harvesting

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.

Components Of Rainwater Harvesting Systems

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.

Filtration, Storage, And Distribution Of Water

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

Using Harvested Rainwater

Benefits Of Rainwater Harvesting

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.

Safety Factors For Consumption Of Rainwater Harvested

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.

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