EcoEng Newsletter No. 11, October 2005

Aluminum composting unit for your garden

Content No. 11/05
Title page / Index
From the editors
Faces: H.v.Bohemen
Review: EE Book
Overview, Etnier
Kirk et al.
Composting (ch. 4)
Fecal composting
Policy Finl, Mattila
Desert infrastruct.
Writers' Fund
Ecosan Durban 05
Good bye T. Rohrer
Various issues:
Joe's Corner
Mailing list

By R. Shanthini, EcoEng-Correspondent and K.S. Walgama, Sri Lanka

Contact Dr. R. Shanthini
Head/ Dept of Chemical Engineering
University of Peradeniya
Peradeniya. Sri Lanka

Chapter 4 of the booklet "How to compost kitchen waste in your garden", by R.Shanthini and K.S.Walgama (ISBN 955-8916-00-5). The booklet is published in a series of articles in this newsletter (ordering information).

Chapter 1: Where did Grandmother's kitchen waste go? (No.8)
Chapter 2: What you must know about turning waste into compost (No.9)
Chapter 3: Barrel as a composting unit for your garden (No.10)

Note: Open forum with Dr. R.Shanthini in

Date: Nov. 22+23, 2005, 9:00 UTC/GMT
Topic: Composting in the tropics - a practitioner's forum

  Following Ms. Batuwitage's footsteps, who introduced us to the barrel [see chapter 3, Newsletter No.10, Dec. 2004], we introduced the aluminium composting unit described in this chapter. The objectives of the design are to facilitate simple construction of a small unit from easily found raw materials, and the convenient handling of the unit. We also had in mind that the unit must be relatively robust to corrosion.

4.1 Getting an aluminium composting unit ready

Figure 4 – Aluminum cylinder in which kitchen waste would turn into compost (not drawn to scale).
Since household composting has not yet become popular in the Sri Lankan urban and suburban households, composting units are not freely available in the market. Therefore, if you get interested in composting your waste in your own garden, you may have to make your own composting unit. This chapter gives adequate instructions about how to make the aluminum composting unit.

Go to a shop and purchase a 100 cm by 75 cm, thin aluminum sheet which would cost you about Rs 150. Bring the aluminum sheet home, spread it on the ground, and make small holes that are placed about 3 cm apart from each other all over the sheet using a nail (3 inch long) and a hammer. Roll this perforated sheet to get a cylinder of about 30 cm diameter and 75 cm height as shown in Figure 4.

Fix the two ends of the aluminum sheet that are touching each other in place by folding them onto each other. Place the aluminum cylinder in a vertical position. The unit in which the kitchen waste turns into compost is now ready.

To hold the aluminum cylinder vertical, we use a wooden frame, which can be made as described below. Purchase or find 4 wooden bars, each having a length of about 90 cm and a cross-section of about 2 inch by 2 inch, and 12 wooden bars, each having a length of about 32 cm and a cross-section of about 2 inch by 1 inch. All the wooden bars would cost you in the range of Rs 200 to 250. Using these wooden bars, prepare the wooden frame shown in Figure 5. Once the frame is made, apply used engine oil that you could obtain from a garage, or wood preservative, to the wooden bars to protect them from any possible future attack by white ants and other insects.

Place the aluminum cylinder erect within the wooden frame as shown in Figure 6, and nail the cylinder to the wooden frame such that the top of the cylinder is aligned with the top of the wooden frame. Wrap the foot of each leg of the wooden frame with, for example, an empty milk powder bag to secure the legs of the wooden frame from possible decay starting from the bottom.

Figure 5 - Wooden frame that would hold the aluminum cylinder vertical (not drawn to scale).
Place the unit on a leveled ground about 3 to 5 meters away from your house, preferably, where direct or diffused sun light falls, and where the rainwater does not stagnate. The unit must be kept on bare ground so that the bottom of the unit has direct access to the soil. Use the hook-and-wire arrangement shown in Figure 6 to firmly hold the wooden frame in place. Or bury 5 cm of each of the four legs of the wooden frame into the ground so as to keep the frame stable.

The aluminum cylinder is 75 cm in its height and the wooden frame is 90 cm in its height. Since the top of the cylinder is aligned with the top of the wooden frame, the bottom of the cylinder does not touch the ground, and there is about 10 to 15 cm space between the bottom of the cylinder and the ground, depending on whether or not you buried 5 cm of the legs of the wooden frame into the ground.

The space between the bottom of the aluminum cylinder and the ground is maintained to facilitate removal of the composted mass from the bottom while we continue to feed the unit with waste from the top.


4.2 Preparing the aluminum unit for kitchen waste disposal

Figure 6 - Aluminum cylinder is placed within the wooden frame such that the tops are aligned. The foot of each leg of the wooden frame is wrapped in an empty milk powder bag, and the wooden frame is held firmly in place with a hook-and-wire arrangement shown.
Since there is an open space at the bottom of the aluminum cylinder, the following preparation is needed before starting to add the kitchen waste to the unit. Get some water absorbing material, sawdust or dry soil, and feed it into the aluminum cylinder. Sawdust (or dry soil) is fed to the aluminum cylinder until the space between the ground and the bottom of the aluminum cylinder is filled.

Keep adding the sawdust (or dry soil) until 1/3 of the height of the aluminum cylinder is filled. A sawdust (or dry soil) base for the aluminum unit is necessary to absorb the excessive amount of water present in the kitchen waste, and also for preventing rats and other small animals from entering the composting mass from the bottom to pull the composting kitchen waste out of the aluminum cylinder from below.

If you wish, you can make a brick layer around the bottom of the aluminum unit as shown in Figure 7 or in Photograph 2. The brick layer serves as a barrier to the sawdust (or dry soil) added to the aluminum unit, and also it improves the appearance of the aluminum unit set-up.

Purchase or look around in your garden for a 40 cm by 40 cm sheet of waterproof material to be used as a lid to the composting unit. Nail a small piece of wood to the lid so that it is easy to hold the lid. The lid is placed on top of the wooden frame as shown in Figure 7, to cover the mouth of the aluminum cylinder to prevent rainwater from getting into the composting mass.

Since kitchen waste contains very high amount of moisture content, it is essential to make sure that no additional water gets into the composting mass containing kitchen waste. If it happens, it will clog the air path, and that will lead to foul smelling gases emanating from the composting unit.

The lid also helps in maintaining the composting mass warm by preventing possible escape of heat generated during the composting action via the mouth of the composting unit. And of course, the warmer the composting mass, the better is the composting action. Now the composting unit is ready for your kitchen waste to be disposed into.


4.3 Composting kitchen and garden waste in the aluminum composting unit

Figure 7 - Aluminum composting unit with the brick layer barrier, and a lid with handle placed on the top.
You may add the compostable portion of your kitchen waste to the aluminum unit once a day or twice a day, or as it fits your convenience. You may also add a portion of your garden waste to the composting mass, perhaps on a daily basis, since it would greatly assist the composting process by improving the carbon/nitrogen ratio of the composting mass.

However, if you add a lot of garden waste to the aluminum composting unit, then you will fill the aluminum unit quickly with the garden waste alone. We therefore recommend from our own experience that the amount of garden waste added to the aluminum unit per day be not more than 5 liters in volume (which is equivalent to the volume of a plastic bucket in which laundry detergent or paint is sold).


4.4 Taking the compost out of the aluminum unit

Photograph 2 - An aluminum composting unit in use in a true household in Peradeniya. Observe the sawdust at the bottom of the composting unit, and the brick layer barrier surrounding the sawdust.
As soon as about 3/4 of the height of the composting unit is filled with the composting mass (which takes about six months from the day you start operating the unit), the waste contained at the bottom of the composting unit would have transformed into a dark, soil-like material. Using a small garden spade or a hard stick or an iron bar, pull this dark, soil-like material at the bottom of the composting unit out through the space between the bottom of the aluminum cylinder and the ground.

When you do that, you can observe that the top surface of the composting mass within the composting unit goes down. Pull the composted mass from the bottom of the unit till the top surface of the composting mass goes down to a level such that 1/3 of the height of the aluminum cylinder is still filled with the composting mass. Now, add about 15 - 20 cm of sawdust (or dry soil) onto the top surface of the composting mass, and then go on adding fresh kitchen waste and garden waste.

The dark soil-like material that you pulled out towards the bricks can stay there until earthworms and other similar life forms come to live in it. At this point what you have is mature compost, which is dark in color, uniform in its texture, and gives off a clean, earthy smell. This compost is now ready for your plants.

When 3/4 of the height of the composting unit gets filled again, repeat the procedure described in the preceding paragraphs once again.


4.5 Performance of the aluminum unit

  We were able to germinate tomato seeds in the medium of composted mass collected from the bottom of the composting unit three months after the unit was started in a real household. Seeds could not have germinated if the compost was not matured enough. Therefore, we could say that the waste added to the composting unit turned into mature compost in three months time.

To check the maturity of your compost, you don't have to try to germinate tomato seeds in your compost. You just check for the presence of earthworms in your compost.

The composting mass gave out hardly any smell. However, there was a significant population of fly larvae present in the composting mass. Even though it was a problem to be concerned with, a neat solution to this problem was provided by Mother Nature, herself.

She sent the birds. Birds of several kinds flocked the composting unit to feed on the larvae that came out of the composting unit. Certain species of birds even put up a permanet shelter by the composting unit.

If you are concerned with the presence of larvae, despite the birds that visit your composting unit to feed on the larvae, you may bury the legs of the aluminum composting unit until the bottom of the aluminum cylinder firmly touches the ground. It is found to discourage the larvae from leaving the composting unit.

The problem would now be it is impossible to remove the composted mass from the unit while the unit is in operation. A suggestion to solve this problem is provided in section 6.6 [to be published in Newsletter No. 13 in 2006, the editor].


4.6 Experimental results obtained with the aluminum unit

  In an experiment carried out with an aluminum composting unit in our (the authors') home garden, the unit was fed with a total of about 190 kg of kitchen and yard waste over 160 days. At the end of the 160 days, the composting mass was filling only about a 40-litre volume of the composting unit, which had a total volume of 60 liters. The temperature of the composting mass within the unit remained, in general, in the range of 40 to 50°C, which were about 15 to 25°C above the ambient temperature. Feeding the composting unit with a larger load at a time that includes adequate garden waste, and turning the composting mass using a long stick well before feeding a new load were found to raise the temperatures of the composting mass to values as high as 50 to 60°C.

4.7 Limitations in using the aluminum unit

Photograph 3 - An aluminum composting unit in operation for about an year and a half at one of our colleague's garden. The unit has been protected from possible attacks by wild pigs by the mesh surrounding it.
The aluminum composting unit described in this chapter is, in our opinion, the most convenient unit to be used for composting kitchen waste from a household with moderate amount of garden waste. It is also the simplest unit to construct if you have to construct your own composting unit. There is however one problem. When the rats, porcupines, and pigs have their minds set upon getting hold of the food contained within the aluminum unit, it simply succumbs to the wills of those wild animals. But then, we don't mind feeding the wild animals now and then! What about you?

One of our colleagues at the Faculty of Engineering, University of Peradeniya, Mr. Sarath Seneviratne, has an interesting idea to protect the aluminum unit from possible attack by wild pigs and other animals. Following an attack by wild pigs, which toppled his composting unit, he has encircled his unit by a mesh as shown in Photograph 3. Note that the mesh used is not chicken mesh, but one that is harder and stronger than chicken mesh is.

Mr. Seneviratne also said that once when he removed the composted mass from the bottom of the aluminum composting unit, he had removed too much of it. That had resulted in the fresh kitchen waste reaching levels that were too close to the bottom of the composting unit to attract wild pigs to look for their food in his composting unit. Because, he reasoned, that it was only once during the entire one and a half year period of running the aluminum composting unit in his garden that the pigs had tried to get their food from his unit.

It is therefore possible to keep the wild pigs and other animals away from your composting unit if you do not allow the fresh kitchen waste to reach levels that are very close to the bottom of the composting unit. That is, care must be taken to make sure that 1/3 of the height of the aluminum cylinder is always filled either by sawdust (or dry soil) or by the composted mass itself.


© 2005, International Ecological Engineering Society, Wolhusen, Switzerland