8 Rules of Thumb for Rainwater Harvesting for Commercial Buildings in Georgia

For roughing out rainwater jobs:

1. I recommend a tank size of about 1 gallon/square foot of rooftop if you are using rainwater harvesting as a stormwater management tool. This (slightly) exceeds City of Atlanta requirements, and simplifies the math. 1:1:1 is a good ratio, and easy to remember.
   
   1 s.f. of rooftop : 1 gallon of storage : 1 s.f. of turf to irrigate.
   
2. Not interested in stormwater management, and just want to collect what you need? I recommend sizing a tank on a 1 month dry spell (happens almost every year), and collecting from up to twice as much rooftop as irrigation area.
   
   3 gallons storage/1 s.f. of turf, collecting from up to 2 s.f/s.f. of turf.
   
3. Cooling tower makeup requires 7 gallons/year/square foot of air conditioned office space in Georgia. Also, cooling tower useage alone will use all the rainwater typically collected, for a 7 story building. So if you have a 3-4 story building, you only need to collect from half your rooftop, or you have excess that you can use for irrigation or toilet flushing.
   
4. 10 gallons/year/square foot for irrigating turf and other thirsty plants
   
5. 3 gallons/person/day for toilet flushing at an office, or 6 gallons/person/day for continuous occupancy.
   
6. 50" rain falls/year in Atlanta, about 25 gallons/year/square foot rooftop is usable
   
7. Budget Prices: $2/gallon for an above-ground tank is good budget price. This is for a 10,000 - 100,000 gallon tank. That's for tanks, pumps, the extra piping, and the filters needed for irrigation or cooling tower makeup.
   
8. $3/gallon for a below-ground tank, pumps, extra piping, and filters for irrigation or cooling tower makeup.

Concerned about water in Georgia?

Georgia's Inaugural Water Summit

Featuring Governor Nathan Deal

February 26, 2014  -  7:30 - 10:30a

The Loudermilk Center Ballroom
40 Courtland Street, NE
Atlanta, Georgia 30303

Hosted by Atlanta Regional Commission and Metro Atlanta Chamber

To RSVP, please click here or contact Sarah Purser at  spurser@macoc.com or 404.586.8534.

Cheaper, Faster, Better

Is that enough reason to consider point-source stormwater management? Fayette County Georgia is considering a tax (they call it a fee) for impervious surfaces.

http://www.thecitizen.com/articles/01-21-2014/fayette-seeks-route-stormwater-loan

How about instead of a fee, they offer an incentive, or at least a reduction in the fee, for storing the water and using it? As you'll see in the slides of the Stormwater Management presentation, they offer incentives for residences to put in rain barrels. Why not do the same for commercial buildings? Cost per gallon for commercial is lower than residential. And each project will have a bigger impact.

http://www.fayettecountyga.gov/environmental_management/pdf/Stormwater-Management-Overview.pdf

Another important consideration for stormwater management is how the tank is used. If you want to use the water, i.e. rainwater harvesting, then ideally you keep the tank full.  If your purpose is stormwater management, then the tank needs to be empty before the next rainfall. There is a simple way to do this, and an even simpler way.

Simple Way

Equipment Needed - Tank, Piping, Float Switch, Timer, Pump, Irrigation System

Control Sequence - When the float switch detects water in the tank, wait 48 hours. Then, irrigate until the tank is empty.  It's rain to get a major rain event 2 days in a row. This allows the ground to dry out from the rain, so that it can absorb more water.

You get the added benefit of more water for the plants. You'll also need to mow more often, if you are watering grass. Much better idea would be to water vegetables.

Even Simpler Way

Equipment Needed - Tank. Optional: Piping.

Control Sequence - Drill a hole in the tank, or put an orifice plate in the outlet piping. Calculate the hole size to drain the tank in a day or two. Make a note on your calendar to check the hole for clogs every 6-12 months.

Finally Found!

I'm so excited! FINALLY, after 4 years of searching (admittedly, it wasn't a full time search), I think I have found the key to maximizing ROI on most rainwater jobs for commercial buildings!

If attended one of my rainwater presentations in the past, you've remember page like this:

Where I tried to come up with some reasonable estimate of water used for cooling tower makeup (CTMU). It's my assertion that CTMU is one of the best uses for rainwater in commercial buildings, and provides the best payback. It's relatively easy to pipe, even in a retrofit. The rainwater is cleaner than the water in the cooling tower, so treat the water in the tower, not the incoming rainwater.  

But estimating demand correctly is key to maximizing payback.  Too small and you might buy water that you could have stored. Too big, and you bought more tank than you needed (and you dedicated more space than you ought). So trying to realistically estimate demand, I created a table like this:

Assuming a run time based on maximum and minimum average temperatures in a month. It wasn't the best possible estimate, but it was better than anything else I could find. Using these numbers, we came up with a total building demand that looks like this:

Which showed that for buildings 5 - 10 stories and taller, all available water from the rooftop could be used for cooling tower makeup. That's important, because we could use all available rainwater, but not have the extra expense of extra piping to all the fixtures. Just one riser to the cooling tower, which in many buildings is the way it was designed anyway.

Well, I was never happy with the calculation. I knew that there had to be something better. I mean, an outside air temperature equates to a certain cooling load requirement in the building, which after you go through some efficiency losses, equates to a certain amount of water that needs to evaporate (taking the heat with it) in the cooling tower. It's been a long time since I studied that in school.

Well finally!  I've found a resource to calculate the ACTUAL number of gallons of water used in a cooling tower. Not the maximum flow for the size, but how much is required at a given outside temperature. At 70°F outdoors, the cooling tower doesn't have to work nearly as hard as at 100°. First, the chiller doesn't reject as much heat, because the heating load is smaller. Second, the cooling tower is more efficient.  I always knew that there had to be a way to calculate it, but couldn't find the equation. But here it is! Thanks to SPX cooling towers for creating the page.

We also need to calculate the actual cooling load, but that's much simpler. Stay tuned for more reliable payback calculations!

Benefits of Above-Ground Tanks

1. Cost - Above ground tanks cost approximately 40% less than below ground tanks.  Especially in areas of Georgia with rocky soil or high water table, above ground tanks are worth extra consideration. Cost savings are due to:
   a.  Reduced Shipping Costs
   b.  Reduced Material Handling Costs - carried by hand or skid-steer on the jobsite, not a crane.
   c.  Reduced Sitework - pouring a concrete foundation vs. excavating.
   ​
2. Maintenance - With a properly functioning prefilter, neither tank requires much maintenance. Maybe every few years. In the past, both above and below ground tanks required confined space safety procedures. However, access hatches allowing egress through the liners are the next technological breakthrough for these tanks.
3. Pumping - With positive suction pressure, pumps run smoother and more efficiently. Pumps can be located inside a mechanical room, safe from the environment and eliminating the need to run high voltage electricity outdoors.

Challenges of Above-Ground Tanks

1. Freeze Protection - The biggest challenge with freeze protection is not the tank itself, but any piping with sitting water. The current generation of tanks solved that problem for the most part, with piping penetrations inside the tank.  The mass of water in the tank will freeze on the surface, but it will not damage the tank. Freeze protection in Georgia can be accomplished with smart recirculation, keeping initial costs low.
   
2. Aesthetics - The Great Debate among engineers: are the tanks something to hide or something to show?.  With a tank such as this in full view of passersby and customers, people will certainly know that the business owner is collecting rainwater.  As described inthis Freakanomics article,that might be very desirable indeed.
   
   By far the most common visual option is unpainted galvanized steel. Epoxy coating and urethane mastic is available.  For a different look, the exterior can be clad in wooden planks to simulate staves, or stone or brick.
3. Space - The best tactic for maximizing space is to size the tank for the usage, not expected rainfall. This also reduces the cost of the project. ​