4/30/11

Water Wheel Conclusion

In February, I began testing a prototype of a water wheel biological filter (see original post describing wheel here). After about a month and a half of testing, I found that it provided decent biological filtration but that it had mechanical flaws. I’ve since decided to retire the project, at least for the summer season. Here’s the scoop:

My neighbor and I built the wheel from ¼” thick, untreated plywood. While pressure treated or stained wood would have resisted rot and water absorption, I did not want to risk chemicals leaching into the water. We also experimented with some Plexiglas, which proved too difficult to cut as it fractured easily.

The wheel was connected to two screen-covered drums by PVC pipe, the pipe was supported by a wooden dowel, and the dowel was suspended by strings at each end. As you can see in the video on the original post the wheel turned well initially; however, after a few days, the wheel noticeable slowed down, eventually coming to a stop. I found that the dowel bowed due to the weight, creating extra friction, so I replaced the dowel with a metal pipe. Nevertheless, after a while the wheel slowed to a stop, too. The wood had absorbed water which weighed it down and it’s possible that bacteria growth also added to the weight. Interestingly the wood did not warp noticeably, but after the trial period, it was covered in little black dots of mold – probably not good for water or our air quality. 

Black mold growing on water wheel paddles (left) and screen drum (right)
Meanwhile, I was designing my flood and drain and deep flow technique (DFT) systems* for the outdoor season this summer. The DFT would use the water wheel as its biofilter*. During my research, I learned...
....that I would also need to build a swirl filter** to settle out solids. I also found that a downside of the DFT is that the shallow water in its pipes can heat up easily, stressing the plants. I considered building the swirl filter, improving upon the water wheel, and taking my chances with warm water, but two things stopped me. First, the wheel introduced more moving parts to my system, and I wanted something very reliable with few possibilities for error. Second, the DFT is a continuous flow system, meaning my pump would run 24/7. That’s a lot more energy pumping water than a flood and drain system that pumps for 15 minute cycles 5 to 7 times a day.

Of course, the DFT does offer some advantages over ebb and flood, a significant one being that you avoid dealing with the weight and volume of the gravel. Now, if I decide to pursue DFT again the future, I’ll have a great model from which to work. And I really did enjoy the pleasant sound a water wheel makes as its paddles fill, turn, and release their load. For now though, I’ll save some energy, buy a Sounds of the Rain CD, and stick to a flood and drain system. 

*Many aquaponic systems plants grow in gravel and these are typically ‘flood and drain’ systems in which water is periodically pumped into the system then drains out. In those cases, the gravel provides the surface area for nitrifying bacteria to live on. These bacteria provide biological filtration: they convert fish waste, ammonia, to plant food, nitrate. In a deep flow technique system, the plants are grown in small cups partially submerged in water. Usually, 2”-3” PVC pipes are used to hold the cups and water. The surface area of these cups is much too small to support enough bacteria to convert the all ammonia in the tank to nitrate. Therefore, an additional system is needed to grow the bacteria. This is called a biofilter. When it is attached to a wheel, it is a biowheel.

**In aquaponics, solid fish waste needs to be removed from the system. It can be captured with fine mesh or other filters, but it is likely that many fine particles will escape so another method is needed. In a flood and drain system, the gravel will catch the majority of these solids so they do not return to the tank. Overtime, the gravel will need to be cleaned or have composting worms introduced to break the solids down. Deep flow technique lacks the gravel to act as a secondary filter, so systems often use a swirl filter. These can be employed in flood and drain systems too as extra filtration. The swirl filter (shown below) has an inflow pipe from the fish tank that is lower than the outflow pipe to the grow beds. The solids to fall to the bottom and only clean water at the top of the filter can enter the grow beds. A drain on the bottom can be opened to periodically empty the solids. Learn more at http://www.ecofilms.com.au/2010/08/20/diy-swirl-filter-for-aquaponics/

Do -It-Yourself Swirl Filter

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