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