I tested the brewer with an Eco Plus 7 air pump, an old hot tub air jet blower, and a DR 083 Rotron regenerative blower .
The Eco Plus 7 is capable of 200 liter/minute. It handled the 48" depth very well.
The hot tub blower puts out a lot of air, and also handles the depth very well. But it's noisy, I may switch between the two, and use the hot tub blower during the day and the Eco Plus during the night.
The regenerative blower delivers a massive 18 cubic feet per minute, but it's incapable of pumping air deeper than 24" so it will not work for this application.
Four air lift pumps were too many, and even the hot tub blower worked best with just 2 air lifts. That's OK because 2 air lifts rather than 4 simplifies the design, and cuts the cost by quite a bit as well.
I might build a very simple brewer out of the left over parts. I'm thinking of one that would simply fit down inside of a barrel. One air lift and a center pickup. Keeping it from being swept away might be the biggest obstacle.
This is a concept drawing of my brewer where a vortex is created.
After testing for a while I finally decided to use only one air lift and the Eco Plus 7. Other people have made good compost tea with less, and I felt the noise from the hot tub blower was excessive. The 200 lpm Eco Plus is a substantial amount of air, and the vortex is still plenty strong.
I have also seen brewers made with a small drill press mounted to a piece of plywood. The drill press is then placed on top of the barrels, and a long stir rod is inserted down into the liquid to create a vortex. This design lacks the extreme air provided by an air lift, but this design is being used at the Earthworm Soil Factory with good results.
A common design is to simply drop an air stone into the batch and let it bubble over night. This too seems to provide adequate aeration and movement, but I felt that the air lifts were the best of all the designs I had seen, and worthy of the extra effort.
My first batch worked well. The microbes cause a lot of foam on top at first. I would advise keeping the liquid level at least 12" below the top of the barrel. I'm told, adding more molasses will extend the useful life of the tea.
I'm pleased to say even at 1/3 full the air lift continues to circulate the remaining tea. This is good because it keeps the tea steered up while I'm draining the tank. I used an old barrel with previously drilled holes for my prototype, and ended up with a 1" drain. I will use a 2" drain next time.
When I fill my bucket I place a paint strainer bag inside of a 5 gallon bucket unless I'm just dumping the tea on top of the soil. This lets me use the tea in my watering can; otherwise the spout gets clogged. The second time I made a batch of tea, I placed the dry compost in strainer bags. This also worked but not as well. Below is a commercial brewer with a similar, but better idea.
I have been using the tea during initial seed planting. It also appears to help avoid transplant shock.
I like this machine because it uses a bag which keeps the tea clean of debris. Also the spray guns are great.