Rainwater Harvesting for Landscape Irrigation
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Rainwater Harvesting for Landscape Irrigation

Feb 21, 2020
Video Length:  1:06:57
Presented By:  Mike Warren of Watertronics

The design of rainwater systems can present some distinct challenges. For one, the need to coordinate with multiple vendors and design disciplines often leads to situations not typically found in landscape design. Join us for an in-depth look at this unique design process. We'll review rainwater system components, go over the sizing and design of parts, and outline some best design practices targeted explicitly toward landscape irrigation applications.

Webinar Contents:

Note: The following catalog of content covered in this webinar is time stamped to allow you to follow along or skip to sections of the video that are relevant to your questions. You can also search for content on this page using the FIND command in your browser (CTRL + F in Windows, Command + F in Mac OS.)

  • Intro/TOC

  • Components of Rainwater Systems
  • Pre-Filtration and Sizing
  • What Surfaces are Good for Collection?
  • Water Storage Tank Options and Sizing

0:00 – 11:17: Intro/TOC

Goal of a rainwater system: Develop a water collection and distribution system that harvests water from multiple sources and delivers it efficiently to multiple applications, automatically (4:20)


Water harvesting (5:20):

  • Moving beyond “rainwater” harvesting:
    • Rainwater
    • Stormwater
    • AC condensate


  • Take advantage of all possible sources:
    • Understanding that each source comes with its own challenges
      • Example: Stormwater off a parking lot contains petrochemicals, which must be accounted for in designs.
      • Water from green roofs has extremely high TDS and is not recommended for interior building applications.


  • Defining your intent (7:40)
    • LEED accreditation
    • Rainwater harvesting for which application(s)
    • Stormwater mitigation
    • Government mandate
      • Example: 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).
    • Architect, civil engineer, plumbing engineer, mechanical engineer, landscape architect, irrigation consultant

11:18 – 20:19: Components of Rainwater Systems

Major components (11:18):

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


Visual representation of a rainwater system and its components (12:50)


The three main components (18:00):

  • Pre-filter
  • Storage tank
  • Pump controls and treatment system


Diagram of a below-ground system (19:43)

20:20 – 32:58: Pre-Filtration and Sizing

Pre-filter components (20:20):

  • The best way to filter the water is at the source. The sooner we incorporate filtration, the better the rest of the system operates.
  • Primary particle filtration (TSS – total suspended solids)
  • Oil/water separation (stormwater only – hydro-dynamic separator


Design of pre-filtration components (25:45):

  • All gravity-type rainwater filters work on an efficiency principle.
    • 200 gpm @ 90 percent efficiency = 180 gpm to storage tank.
  • Sizes from 32 gpm 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


Pre-filtration: commercial installation examples (29:40)


Why pre-filtration? (31:18)

The goal is to keep as much debris out of the storage tank as possible in order to ensure the purest water possible.

32:59 – 43:07: What Surfaces are Good for Collection?

Parking lots (32:59)

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 (33:36)


Splash pad collection (37:08)

  • 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


Question: What kind of maintenance is required for the pre-filter and what medium is used for the pre-filter? (38:50)

Answer: 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. For hydrodynamic separators, a vac truck will need to come in and vacuum out the debris, starting at once a month and then possibly once a season.


Question: Is there a particular recommendation of pre-filter for parking lots? (40:19)

Answer: Mike recommends something like a hydrodynamic separator that has a suspended solids removal component and an oil-water separation component.


Question: How do you deal with issues of PH – either too acidic or too alkaline? (41:00)

Answer: 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, since irrigation pipe is generally made of noncorrosive materials that aren’t affected by the water’s PH.

43:08 – end: Water Storage Tank Options and Sizing

Storage (43:08)

  • Storage can consist of any vessel that can hold or retain water.
  • Tanks or ponds
  • Separate containment, or built into building foundation
  • Below or above ground (note anti-flotation when below ground)


Determining tank size, 4-part function (46:58):

  • What is your usage per time period (gallons/day)?
  • Potential of collection
  • Budget
  • Any city code requirements


End usage and end usage pattern can also affect tank size.


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


Integrating items in tanks (53:16)


Mike Warren’s contact info:

  • Phone: +1 414-640-2496
  • Email: mike.warren@watertronics.com


Question: Is it safe to use filtered roof water to top off a pool? (57:10)

Answer: Mike isn’t totally sure of that answer. It will depend on the level of filtration, the pool chlorination, and several other factors.


Question: How do you address freezing in northern climates? (58:25)

Answer: If you have an aboveground storage application, it will have a manual bypass valve on the inlet of the storage tank, which you can close prior to freezes. Some people 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.


Question: What about road salt in the runoff? (1:01:00)

Answer: In northern climates, retention ponds will often have a high concentration of road salt runoff in spring. To reduce salinity, you can blend the water with a normal water source to achieve a better ratio, or use reverse osmosis to remove the salt from the water (expensive and probably not feasible for irrigation).


Accounting for extreme storm events (1:02:33)


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