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Mantis ComposT-Twin

Review Summary

Priced at $499
Reviewed by Leon Frechette
08/01/05 (First Reviewed)
Updated 03/06/07

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As an active gardener, I know that compost guarantees happy and healthy gardens. Compost is black gold to a garden because it's a rich source of the nitrogen, phosphorus, and potassium that plants thrive on.

It helps soil retain water and can be used as a mulch to help block weed sprouts.

Even though I know all this, I must confess that I have never composted, partly because of space constraints and also because it's easy to run down to a local farm and pick up steamy manure loaded with red worms.

Now that I'm a little older and calmer, I want to try my hand at creating some of this black gold from my green grass clippings. When Mantis contacted me in 2005 about their ComposT-Twin, I was very excited for a couple of reasons.

First they've designed a product that looks good no matter where it's placed. During the winter months when it's not in use, a cover (accessory item) protects the unit. Second, its off-the-ground design makes it more user-friendly.

Now that I have gone through the first year, I discovered that the cover, while well constructed, only has four grommets to tie it to the support frame. As it stands now, the wind could lift the cover. It would be helpful to have two more grommets: one dead center between each of the two existing grommets. I also recommend that two bungee cords with three hooks each be included with the cover at the time of purchase to properly secure the cover to the frame.

I have noticed small amounts of paint flaking off the support frame stand when I wash it with a garden house. For winter storage, it would also be helpful to raise the unit off the ground using four hollow concrete blocks 8"x8"x8" with a groove in the center of one surface. The support frame will fit nicely in this groove to help keep the unit in place and off the ground.

When washing each compartment for winter storage, I found it almost impossible to empty all the water out of each bay. The two vertical rib stiffeners in the door opening keep both compost material and water trapped in the bays. I recommend that the design be modified so that at least one of the stiffeners can be removed, redesigned so stiffeners are not required, or a "V" notch is added in the center of the two stiffeners. The "V" notch would help the user empty water—but not compost material—from the drum.

When you order the ComposT-Twin, it arrives in several boxes, leading you to expect the worst, but the owner’s manual clearly lays out the contents of each box and walks you through the different phases. Assembly of this unit is a two-person project and would be difficult without the detailed drawings and photos in the owner's manual.

Mantis ComposT-Twin composter I find the two-chamber design interesting: one "cooks" while you fill up the second with more yard waste. For fastest composting, a chamber should be at least two-thirds full. Together, the chambers hold up to 25 cubic feet of raw material. I've been testing the system to see how long it really takes to create this black gold I've heard so much about. I've come to the conclusion that while I achieved black gold, it took longer that it should have because I was unable to take advantage of a southern exposure. It takes full sun to cook the waste, so correct placement of the unit will enhance its performance.

The ComposT-Twin has an easy-to-turn handle and gear system for fatigue-free mixing. The old-fashioned way was to turn the compost once a day with a pitchfork. Now I just turn the handle to rotate the barrel once a day, which mixes and aerates the material. This helps to build up a heat core for fast decomposition.

The handle and poly gear system work very smoothly, but I don't like the handle's plastic grip. It needs a large flange designed into the unit at one end (the end up against the push end cap) to prevent your hand from coming off the end and getting pinched up against the push cap nut on the end of the metal handle and the plastic grip. Also I noticed that the spring pins that hold gears in place are starting to rust—apparently they are not made of stainless steel.

If you wish, you can add a compost activator to help speed up the decomposition process. Mantis offers a 20-inch thermometer to measure up to 220°F how hot your batch is cooking and a moisture meter to help you determine if you need to add water.

Mantis ComposT-Twin composterKeeping the galvanized drums off the ground keeps pests out and prevents odors. Two aerator vents per door allow essential oxygen into the mix, and the poly end caps are vented for even air flow and proper drainage. In about three weeks (depending on heat, moisture, and content) the compost is ready for your garden. Put a wheelbarrow under the unit, remove one of the doors, and turn the handle until the opening is over the wheelbarrow. It's that simple!

The door openings are roughly 14" wide by 16" high, which is just about big enough to work in; however, if it were about another inch wider, I could easily dump in my grass clippings right from the lawnmower's bag.

Now I insert the bag diagonally in the opening to dump the grass, an awkward and uncomfortable maneuver. I discovered by accident that I can flip the door all the way open up against the drum and it will stay there as long as the opening is parallel with the ground. This is great so you don't have to remove the doors to check the compost, however I don't recommend it because it rubs off paint on both sides of the top of the door opening. A better approach would be to get in the habit of removing the doors.

If you want finer compost, e.g., for indoor plants, then check out their accessory sifter screen. It replaces one of their solid doors and allows finer particulars of compost to fall through the screen into your wheelbarrow.

The unit sits 32 inches off the ground and has an overall size of 65" wide by 66" long by 41" deep, including the sturdy tubular steel frame. The overall drum size is 33 1/2" in diameter by 51" long.

Mantis goes the extra mile and includes a Guide to Composting as a free gift. It points out that compost is how nature recycles its trash and helped me rethink the whole idea of composting. It explains the composting process in detail and gives the recipe for success.

It's time to leave behind those unattractive collapsing chicken-wire bins and upgrade to a compost system. The two-drum design of the Mantis ComposT-Twin assures that you'll have plenty of finished compost on hand while another batch cooks. This is truly a unique and well-designed system that you and your garden will surely appreciate!

Despite some of the deficiencies I discovered, overall the unit is still well designed both in construction and function. If you are concerned about any of the issues I mentioned, you can easily correct some of them yourself if the manufacturer doesn't beat you to it. The product is still worth the investment!

Copyright © 08/01/05 & 03/06/07 LAF/C.R.S., Inc. All rights reserved.

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