Interested in Capturing Rainwater? Here’s a few things to consider

On October 17th I had the pleasure of co-presenting at the Sustainable Living Fair on “Saving Water in your Home or Workplace.” My co-presenter, David Mars, Energy Specialist with Columbia Water & Light, and I shared ways that companies and homeowners can save water.

My portion of the presentation focused how SOA’s 10,000 gallon tank system works, along with a few additional thoughts we would suggest building owners consider, if they are interested in capturing rainwater. Here are a few highlights from the presentation:

A few basics on SOA’s system:

  • Rainwater runoff from our building’s 12,000 sf roof is captured in a 10,000 gallon tank on the west side of the building
  • Installing the rainwater tank allowed for a smaller bio-retention basin, which enabled more buildable area on the oddly-shaped site
  • Runoff flows through internal roof drains and then goes through a cyclonic filter that separates debris from the water
  • Rainwater is stored in the tank and then used for landscaping irrigation and toilet flushing

From observing the system’s use for almost a full year, we estimate that we save:

  • 100 gallons / day from toilet flushing
  • 320 gallons / day for landscape irrigation, during peak usage

The tank is sized to accommodate 100% of the combined flushing and irrigation needs most of the year, and about 80% during the hottest months of the year, assuming typical rainfall – which leads to our first consideration:

#1 What is the largest tank the project can afford?

While 10,000 gallons may seem like a lot of water, it only takes a 1.33″ rain event to fill our tank. Weather, being what it is, tends to bring big rainstorms and then periods of drought. As a result our tank has been empty, especially in the last few months. Knowing this, we would advise building owners to size their tank as large as their space and budget would allow. The larger the tank, the more resilient the system will be during dry spells. The ability to “budget” the water in the tank during those dry periods is addressed by consideration #2.

#2 What type of tracking system is appropriate?

The amount of control a building owner would like over his or her rainwater collection system is a main consideration in deciding what type of tracking system, if any, is appropriate. Our rainwater collection system did not include a tracking system when it was first installed. However, after about 6 months of use, we installed an Aquatel Fluid Level Monitor, so that we could know the current water level of the tank (without opening up the manhole and looking in!). The monitor uses ultrasonic signals to detect the water level and then wirelessly transmits that information to a small display indoors. Knowing the water tank level could be critical for a building owner that is relying solely on rainwater for irrigation, and must budget its use accordingly.

A flow meter is the second type of monitoring system that building owners might want to consider. A flow meter keeps track of the gallons of water that enter (and/or exit) the tank. The flow meter does not report the current water level in the tank, but instead keeps track of how many gallons flow through the system. For building owners that need to accurately know how many gallons of rainwater are used at a time, say, for use in a manufacturing process or cooling tower, then a flow meter would be appropriate. A flow meter also allows a building owner to calculate, rather than estimate, the payback period on a rainwater collection system.

#3 What will the landscaping be like around the tank?

The manhole at the top of the tank is one location that we would advise careful landscape planning. Because landscaping was slow to get established in the area around our tank there was a period of time where loose topsoil was being washed under the manhole lid during heavy rains. As a result, the rainwater in the toilets occasionally appeared slightly dirty. Once the landscaping was established and eroding soil no longer an issue, the color in the water disappeared. While this was really more of a nuisance than a problem, on future projects we will recommend either locating the manhole 6″ or so above grade, and/or being careful to establish landscaping as soon as possible. We have also noticed that at certain times of the year, pollen, dirt or leaves collecting on the roof will discolor the water as they are washed off. For a building owner that is especially concerned about discolored water, a “first flush diverter” might be appropriate. This device prevents the first flush of water (containing the majority of the pollen, dirt and leaf particles) from entering the tank, so the water stays cleaner.

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