In the waste water system, the drain pumps are responsible for moving incoming water from the house into the constructed wetland or gray water tank. To do this, they must pump water through 5-10m of horizontal pipe and 1.5m of vertical pipe. Because the water stuck in the vertical pipes would flow back into the drain tanks once the pumps shut off (which would then activate the float switches, turning the pumps back on and starting a never ending cycle), check valves were installed in series with them. Unfortunately, during testing it was found that pumps didn’t have the power to overcome the static pressure on the other side of the check valve due to the column of water.

One of the solutions to this problem was to invest in larger and stronger pumps, but this would mean having to redesign the drain tanks, which wasn’t ideal.

An alternate solution (and the one that was chosen) involved replacing the check valves with solenoid valves. The idea was that when the pumps turned off, the solenoid valves would close, preventing water from flowing backwards into the drain tanks. When the pumps needed to turn on, the solenoid valves would open first and intentionally let the water flow back into the drain tank. Once the column of water is gone, the pumps switch on and are able to force the water back up the vertical pipes and into the constructed wetland or gray water tank. There were concerns that allowing water to flow back into the drain tank while more water was coming in from the house would cause the water to back-up the house drains, but volume and flow-rate calculations verified that this would not be the case.

The deadline to ship the American built components to Morocco prevented this solution from being tested, so the Solar Decathlon competition will be its big debut.

John McDowell
Water Systems