Friday, March 21, 2014

Rainwater Harvesting at High Schools


Saving the Earth and Saving Money

We have found a few combinations of water supply and demand that are the absolute top when it comes to payback. The common them is:
  1. Ample supply of high quality water. Rooftops are best, especially rooftops made of metal or EPDM.
  2. Significant demand of water that doesn't require much (or any) treatment.
  3. Minimal piping changes.
Here's today's example, a typical high school in the Atlanta area.  On this one, we assumed that:

  1. School is out June and July.
  2. No cooling tower.
  3. We would turn off the irrigation November – March.

The conclusions? 
  1. To store enough rainwater to irrigate 100% with rainwater would require collecting all winter. Over 1,000,000 gallons storage required. Not cost effective.
  2. Using available rainwater for irrigation will reduce total water consumption by approximately 50%, enough to qualify for maximum Potable Water Reduction LEED points.
  3. Toilet flushing is not as cost effective as irrigation. The pumping, treatment, and plumbing are all more expensive.



And the data behind it:

12 Hour Occupants 900  1 Hour Occupants - 900 0
Occupancy Rate 100.00% Building Size (s.f.)                                                                      220,000
Roof Size 120,000 Occupied Hours                                                                                12
Irrigation Area (s.f.) 200,000 GPD Toilets                                                                          2,700
Average RainFall (in) Average Cooling Load Occupancy Days Available Cooling Tower Irrigation  Toilet  Potential Rainwater Storage  Supplemental Water Needed 
Jan 4.2 0.073                 18                   296,856         6,244       48,600                  242,012                    (242,012)
Feb 4.67 0.072                 20                   330,076         6,843       54,000                  269,233                    (269,233)
Mar 4.81 0.121                 22                   339,971       12,650       59,400                  267,921                    (267,921)
Apr 3.36 0.206                 21                   237,485       20,557      496,000       56,700                 (335,772)                     335,772
May 3.67 0.291                 22                   259,396       30,422      496,000       59,400                 (326,427)                     326,427
June 3.95 0.433                   2                   279,186         4,115      496,000         5,400                 (226,329)                     226,329
July 5.27 0.44                   2                   372,484         4,182      496,000         5,400                 (133,098)                     133,098
Aug 3.9 0.44                 15                   275,652       31,363      496,000       40,500                 (292,211)                     292,211
Sept 4.47 0.359                 19                   315,940       32,413      496,000       51,300                 (263,774)                     263,774
Oct 3.41 0.179                 23                   241,019       19,564      496,000       62,100                 (336,645)                     336,645
Nov 4.1 0.151                 20                   289,788       14,351       54,000                  221,437                    (221,437)
Dec 3.9 0.101                 18                   275,652         8,639       48,600                  218,413                    (218,413)
Total:            49.71               202                 3,513,503      191,344   3,472,000      545,400                 (695,241)                     695,241

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