Rainwater harvesting

Originally published Tuesday, January 21, 2014 at 8:45 PM

Seattle Public Utilities seeks steep rate hikes

Seattle Public Utilities says an almost 5 percent increase in rates for water, sewer, garbage and stormwater collection for each of the next six years is necessary to maintain its current level of services.

Now is the time for you to install a rainwater harvesting system to help save you money 

A question to consider:  if thousands of gallons of water fall on your home every year, why do you “import” water from the city to water your garden?

We at Confluence Design+Build asked ourselves this question and found a great answer.

Rainwater harvesting is an economical way to make a big impact on the natural environment while putting money in your pocket.  Until recently, collecting rainwater was illegal: The Washington Department of Ecology owned the rainwater rights over the entire state and a special permit was required to collect it.  Due to the dramatic environmental damage that is caused when it’s Combined Sewer Overflow (CSO) system is unable to handle the runoff during heavy rains, dumping raw sewage into the Puget Sound, the City of Seattle applied for and received a waiver.  This created the opportunity for citizens to collect some of the rain and distribute it during dry times, alleviating some of the overflow issues. In 2009 the Department of Ecology lifted the restrictions permanently and now rainwater harvesting is a viable and logical, not to mention legal, opportunity for people to make a positive impact on the planet and their checkbook.

Large receptacles, called cisterns, are used to collect the water until it is needed.  The water can be used for anything from watering lawns and gardens, to being filtered and used in the home to replace municipal drinking water.  The average roof in the Seattle area receives approximately 20-25,000 gallons of water a year.  Depending on your usage, it is possible to collect enough water to last throughout the drier months of summer to water yards and gardens.  Confluence Design+Build offers many innovative options for water storage, including under decks and in crawlspaces, numerous above-ground and below-ground tanks make of different materials from polyethylene to stainless steel to concrete.  It is possible to store thousands of gallons without negatively impacting the beauty of your home, and Confluence Design+Build specializes in this service.


Can I harvest water from a composite roof?

Yes.  Unless the roof is extremely new, typically only trace amounts of contaminates from the roof make it into the cistern water.  If you are using the water for irrigation only, filtration is rarely required.  If you want to make the water potable a filtration systems using cartridges, activated charcoal and a UV systems will be required, but the filtered water will come out purer than anything you will get from the tap.

Are below-ground tanks a lot more expensive than above-ground versions?

Yes.  To create a cistern that can withstand the pressures of being underground requires a much more robust tank, thus a more expensive one. Plus the installation costs are significantly higher compared to an above-ground unit.  There are advantages to a below-ground tank however and it is worth considering both options.  If the primary concern is hiding the tanks from view, we have several options in addition to burying the unit(s) that you may not have considered.

How do I calculate how much water I can harvest?

Measure the foot print of your roof: length x width, including overhangs, and multiply: this is the square footage of the catchment area for your roof.

Next, multiply the catchment area by 37: the average number of inches of rain we receive in Seattle.

Multiply that result by .623 to determine the maximum number of gallons of water available from your roof.

So the formula is: Annual inches of rainfall x square foot of roof x .623 = gallons of annual rain harvesting possibility.

For example: a 30 feet by 30 feet roof is 900 square feet.  900 x 37 x.623= up to 20,745.9 gallons of free and beautiful water available every year.