Rainwater Harvesting

• Components of Rainwater Systems
• Pre-Filter
• Pump Station
• Controls
• Post Filtration and Treatment
• Backup Water Supply and Level Controls
• Storage
• Cold Weather Considerations
• Related Webinars
• Sources & Surfaces
• Surfaces for Collection

The goal of a rainwater system is to develop a water collection and distribution system that harvests water from multiple sources and delivers it efficiently and automatically to multiple applications.

Thank you to Mike Warren of Watertronics, who provided the information for this page in his 2-part webinar series, Rainwater Harvesting for Landscape Irrigation. See Mike's webinars:

• Rainwater Harvesting for Landscape Irrigation
• Rainwater Harvesting for Landscape Irrigation – Part 2

Components of Rainwater Systems

A rainwater system consists of the following components:

• Catchment
• Pre-filtration
• Storage tank
• Pumping water
• Controls
• Post-filtration

Pre-Filter

The goal of pre-filtration is to keep as much debris out of the storage tank as possible to ensure the purest water possible.The best way to filter the water is at the source. The sooner filtration is incorporated, the better the rest of the system operates.

A pre-filter achieves:

• Primary particle filtration – total suspended solids (TSS)
• Oil/water separation (stormwater only – hydro-dynamic separator)

Design of pre-filtration components:

• All gravity-type rainwater filters work on an efficiency principle.
• 200 gpm @ 90 percent efficiency = 180 gpm to storage tank.
• Sizes from 32 gym to about 4,000 gpm
• Some can be flushed with pressurized water.
• Approximately 350-micron screens (.013 inches)
• Exception: hydro-dynamic separators – 80 percent / 100 micron

All pre-filters have some type of stainless-steel screen. For manual cleaning of a component without an automatic spray head, the screen will need to be pulled out either once a month or once every other month, and then rinsed and possibly wiped off. Hydrodynamic separators will require periodic maintenance in which a vac truck vacuums out the debris, starting at once a month and then possibly once a season.

Pump Station

A pump station is the heart of the water harvesting system and essentially turns the end user into a water manager. It controls all peripheral components (level, flow, pressure, and filter) while providing a way to quantify the user's return on investment and provide data about the system.

Pump stations can incorporate either submersible or above-grade pumps. Submersible pumps are more efficient but potentially require more maintenance.

The needs of the irrigation system will determine the horsepower required for the pump. The pump needs to be designed to provide the needed flow and pressure at the start of the mainline in the worst-case scenario – the pressure needed to deliver irrigation water to the largest zone.

For more information on pump stations, see our pump & pump stations article.

Controls

A control panel includes the following components:

• PLC 24V DC
• 460V disconnect
• Circuit breakers (control power, touch screen, PLC)
• Surge suppressors (analog sensors)
• Lightning arrestor
• 460/24VAC transformer
• 24V DC relays
• Phase monitor and circuit breaker
• 460/120 AC transformer
• Variable frequency drive (VFD) fuses
• VFD

Post Filtration and Treatment

Automatic screen filtration:

• Is controlled via the rainwater control panel
• Reverses flow across the screen
• Includes an internal self-cleaning mechanism
• Flushes on differential pressure, timed interval, or total gallons pumped

Ultraviolet disinfection basics:

• 254nm wavelength UV light is used to render organisms inactive or unable to reproduce
• Water is in contact with light for a period of time. Energy is transmitted to the water (mj/cm2)
• Pick a dose (30mj/cm2) organisms require a certain amount of energy to be deactivated
• Pick a flow rate with a given UVT% (actual UVT of water no known without water sample)

Water quality requirements for UV:

• Dose: Light energy delivered into the water (mj/cm2)
• UV transmittance percentage: light's ability to penetrate the water
• Flow rate: Maximum GPM able to be disinfected at the criteria listed above

General requirements:

• 7 grains or less of hardness
• UVT% must be per the mfg. performance curve
• .05 ppm of manganese (dark black metal)

If the UVT% of the water through the UV unit is different from what you sized the UV for, the dose will not be delivered.

Chlorine recirculation system on a day tank (maintains 2 to 3 ppm residual chlorine level):

• Uses a separate pump in day tank start/stop via timer
• Water sent through CHL Analyzer (PH and CHL sensor)
• Dosing pump insects to maintain a set point of CHL in PPM
• 30-gallon holding tank (uses household bleach/pool shock)

Chlorine will also affect the color of water because it changes the way molecules reflect visible light to the naked eye.

Direct style systems:

In a direct style system, the water in the main storage tank is pumped directly to the given application at the desired pressure. All components on the discharge side of the pump(s) are sized for max flow rate (capacity) and PSI.

Transfer & day tank style systems:

A complete rainwater system includes 2 storage tanks and 2 pump stations. A smaller transfer pump treats water from a main rainwater storage tank at a lower flow rate to the "day tank," while another pump station to deliver that water at required flow and PSI for the application.

A transfer & day tank system is not advised work for irrigation.

Deciding factors to use direct or day tank systems:

• Pressure required for application (150 PSI max rating / 125 PSI safety factor)
• Filters and UV units have rating restrictions.
• Flow or GPM required to application (for financial savings)
• If the application requires 80 GPM, a day tank system that transfers water at 30 GPM offers little of no cost savings.
• Footprint or available space
• A direct system may be chosen even thous GPM (over 150 GPM) is high due to space constraints.
• Application or usage profile (i.e., irrigation vs. toilet flushing)

Backup Water Supply and Level Controls

Level controls: How to integrate a backup water source:

• Do we need a backup water source?
• Will the backup water source go to the reservoir or direct to the water distribution system?
• Does the backup water source have the proper flow and pressure to satisfy the water distribution system demand?
• The backup water source requirements change based on the following possible scenarios:
• Backup water will go to the reservoir
• Backup water will go directly to the water distribution system

Tank fill:

• Backup water must have the proper flow to the tank. (Pressure is not important.)
• Controls must maintain a low water level in the tank, leaving the most possible room to capture the next rain event. Do not set the fill valve to fill the tank to its full level.

Direct to distribution:

• Local codes must allow for this type of connection.
• Backup water must have the proper flow and pressure at the connection to the rainwater system.
• Controls must have switch back to rainwater based up availability in the tank.

Storage

Storage can consist of any vessel that can hold or retain water, including tanks or ponds. It can be a separate containment structure, or built into a building's foundation. Storage can occur below or above ground, but note anti-flotation when below ground.

To determine tank size, consider the following factors:

• Usage per time period (gallons/day)
• Potential of collection?
• Budget
• Any city code requirements
• End usage and end usage pattern

In an irrigation application, the rainwater system only needs to provide the water needed for plants between the natural rain events.

Cold Weather Considerations

Aboveground storage applications have a manual bypass valve on the inlet of the storage tank, which you can close prior to freezes. Some designers have used heaters, but there isn't an easy equation to determine which heater to use. If you have a below-ground system with a tank that's buried 3 or more feet, the temperature of the ground will prevent the water from freezing.

In northern climates, retention ponds will often have a high concentration of road salt runoff in spring. To reduce salinity, you can either:

• Blend this water with a normal water source to achieve a better ratio, or
• Use reverse osmosis to remove the salt from the water, although this process is expensive and probably not feasible for irrigation.

Sources & Surfaces for Collection

Water can be harvested from:

• Rainwater
• Stormwater
• AC condensate

Ideally, a system can take advantage of all possible sources, under the caveat that each source comes with its own challenges. for example, stormwater collected from a parking lot will contain petrochemicals, which must designs must account for.

Water collected from green roofs has extremely high total dissolved solids (TDS) and is not recommended for interior building applications.

 

 

 

It's important to define your intent when designing a water collection system. For example:

• Are you aiming LEED accreditation?
• Which applications are you planning to achieve with a rainwater harvesting system?
• What type of stormwater mitigation will be required?
• Which government mandates will apply? For example, the City of Tucson, AZ, requires 50% of irrigation be fulfilled b rainwater harvesting on all new commercial buildings.
• Determine your budget.
• Develop the solution.
• Single-source integrated solution is key – don't "split out" the system.
• Several professionals will likely be involved – architect, civil engineer, plumbing engineer, mechanical engineer, landscape architect, irrigation consultant, etc.

 

Surfaces for collection

Possible surfaces include:

• Parking lots: Pre-filtration is especially important for collection systems in parking lots, which lie lower than rooftops and therefore can collect an extremely large amount of debris.
• Green roofs
• Splash pads
• 3K below ground storage tank
• Submersible pump in tank
• Control skid with filtration (outdoors)
• Collects city water used on splash pad
• Fully flooded excavation installation
• Harvested splash pad water for irrigation

Hydrodynamic separators are good pre-filters for water collected from parking lots because they have a suspended solids removal component as well as an oil-water separation component.

A note on PH:

Rainwater is generally around a 6 on the PH scale, depending on location. Different states have different regulations for the type of piping to use in distribution. You'll see it more in plumbing than in irrigation because irrigation pipe is generally made of noncorrosive materials that aren't affected by the water's PH.

Related Webinars

• Rainwater Harvesting for Landscape Irrigation
• Rainwater Harvesting for Landscape Irrigation – Part 2