Washington, DC's First Living Machine™

The empty tanks and trough at the beginning of July 2000.
The empty tanks and trough at the beginning of July 2000. Photo (c) P. Bogdonoff

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Water Volumes, Flow, System Cycle Times

Water Volumes

Nitrifier Tank
The nitrifier tank is 48 inches tall by 12 inches diameter with water only coming up to a height of 44 inches. The total possible water volume is therefore 44 x pi x 6**2 = 4976 cubic inches.

The lava rock in the tank fills 28 inches, reducing the volume of water in that section by about half, or 14 inches. So the effective height of the water is 44 - 14 = 30 inches. Hence, the actual water in the tank is 30 x pi x 6**2 = 3393 cubic inches.

4976 / 1728 = 2.880 cubic feet; 3393 / 1728 = 1.963 cubic feet

2.880 x 7.481 = 21.545 gallons possible; 1.963 x 7.481 = 14.685 gallons actual.

Invertebrate Tank
The invertebrate tank is 60 inches tall by 18 inches diameter with water only coming up to a height of 56 inches. The total possible water volume is therefore 56 x pi x 9**2 = 14250 cubic inches.

14250 / 1728 = 8.247 cubic feet; 8.247 x 7.481 = 61.693 gallons.

Fish Tank
The fish tank is 48 inches tall by 30 inches diameter with water only coming up to a height of 44 inches. The total possible water volume is therefore 44 x pi x 15**2 = 31102 cubic inches.

31102 / 1728 = 17.999 cubic feet; 17.999 x 7.481 = 134.648 gallons.

Marsh Trough
The "Marsh Trough" has a potential capacity of 100 gallons. Water, on average, fills the trough about halfway. The trough is wider at the top than at the bottom. Our guesstimate is that only 40 gallons of the trough is filled, and about half of that is with gravel. Hence, we guess the volume of water in the trough is about 20 gallons.

Total Volume of Water in the System (in gallons)
Nitrifier Tank14.7
Invertebrate Tank  61.7
Fish Tank134.6
Marsh Trough20.0
TOTAL231.0

Total weight of the water is 231 g x 8 lbs/g = 1,848 lbs.


Pumps, Flows, and System Cycle Times

The First Pump
The first pump we used was rated at 210-215 gallons per hour at 5 feet head height (average 212.5 g/h). Therefore, with that pump all the water in the system cycled through the pump once every 231 g / 212.5 g/h * 60 m/h = 65 minutes.

Update #1: A Smaller Pump
We are now using a pump that has about 1/3rd the power in order to extend the system cycle time. It lifts about 85 gallons per hour up the 5 feet of height from the marsh trough to the top of the nitrifier tank. A side effect of the less powerful pump is that the water level in each of the tanks has dropped about 2 inches because the reduced rate of flow means less water backs up trying to exit through the outflow pipes. (Initially this water drop accumulated in the marsh trough and covered the gravel by several inches. We re-adjusted the total volume to keep the level of water in the marsh trough just below the surface of the gravel.) The recalculated volumes (in gallons) are:

Nitrifier Tank14
Invertebrate Tank  60
Fish Tank130
Marsh Trough20
TOTAL VOLUME224

Now, with the smaller pump all the water in the system cycles through the pump once every 224 g / 85 g/h * 60 m/h = 158 minutes (approx. 2.5 hours). Total weight of the water is now 224 g * 8 lbs/g = 1,792 lbs.

Update #2: Solar Powered Cycle
On August 30th, 2000 the photovoltaic system was completed and we disconnected the system from the "grid"! The water and air pump are now running on battery-stored solar power. We set the timer to turn the pumps on for 40 minutes 6 times during a 24 hour cycle (every 4 hours for a total of 240 minutes). With the timer the system is not circulating water most of the time and we had to remove a few more gallons of water from the trough in order to keep the "marsh" from being flooded while the pumps are stopped. Our current guess is that there are about 215 gallons of water in the whole system (1,720 lbs.). So, the water circulation through the system is 240 / 60 * 85 g/h = 340 g/day. Turnover is therefore 340 g/day / 215 g = 1.6 times/day. Or, on average, the water circulates through the system once every 15 hours.

Equations Used

1) The volume of a cylinder is height x pi x (radius)**2.
2) A cubic foot is 12 x 12 x 12 or 1728 cubic inches.
3) Multiply cubic feet by 7.481 to get gallons.
4) A gallon of water weighs 8 lbs.

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Copyright (c) 2000 Philip Bogdonoff.
http://pbogdonoff.home.igc.org/LivingMachine/volumes.html
12 August 2000