EcoEng Newsletter No. 12, June 2006

Ferro-cement composting unit for your garden

Content No. 12/06
  Title page / Index
  Water from the well
  Phototrophic biofilms
  optimising waste flow
  India: Water supply
  schemes in a slum
  Austria: Sewerless city
  Composting: Ch. 5
  PNG: Ecosan project
  Biopros project
  Ecosan curriculum CD
EcoEng News:
  Joe Swamp
Various issues:
  IEES Writers' Fund
  Mailing list
  EE-Newsletter Flier

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

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

Chapter 5 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)
Chapter 4: Aluminum composting unit for your garden (No. 11)

The ferro-cement composting unit, shown in Photograph 4, is based on the same idea as of the aluminium composting unit, described in Chapter 4. The ferro-cement unit, however, is a more stable structure and has a longer life span than the aluminium unit. The biggest problem with the ferro-cement unit however is its own weight. It is much heavier than the aluminium unit or the barrel, and therefore it is a problem to move the ferro-cement unit from one place to another.

Photograph 4 - Ferro-cement composting unit placed in our (the authors') garden at Peradeniya to compost our kitchen waste mixed with moderate amount of dried leaves. The dark material seen at the bottom of the unit is the composted waste pulled out of the unit, solely for the purpose of displaying the rich compost produced in our garden.

The ferro-cement composting unit is made of a cement mixture with 1 part cement and 4 parts sand, reinforced with a chicken-mesh skeleton. The holes in the structure, seen in Photograph 4, are made with the help of conduit pipe pieces inserted perpendicular to the chicken mesh frame before the application of the cement mixture.

The openings at the bottoms of all four faces of the ferro-cement composting unit shown in Photograph 4, facilitate convenient removal of the composted mass from the ferro-cement composting unit. This Photograph also shows the dark composted mass being pulled out of the unit at its bottom, for the purpose of displaying the rich compost produced from "waste".

The four faces of the ferro-cement unit shown in Photograph 4 were made separately, and then cemented together. It is also possible to tie the four faces together using cords, without cementing them together. You can construct the type of ferro-cement unit that you want with the help of the construction details provided in sections 5.1 and 5.2.

Construction of a ferro-cement unit however demands that you possess certain amount of experience in working with cement. Therefore, you may also get help from a mason to make a ferro-cement unit. It is a good investment since a ferro-cement unit can be used for a very long time.


5.1 Constructing the ferro-cement unit with the four faces of the unit tied together

Figure 8 - The wooden frame used for making the four ferro-cement slabs of the composting unit where the four slabs are tied together. The frame is made using
2.5 cm thick wooden pieces.

The method of constructing the ferro-cement composting unit with the four faces tied together by cords (Photograph 5) is simple, and therefore it is described first. Construct the wooden frame needed to make the ferro-cement slabs, using 1 inch (about 2.5 cm) thick wooden pieces, according to the measurements given in Figure 8.

From a chicken mesh, cut a shape such that it will fit the empty space within the wooden frame of Figure 8. Now, cut six 2.5 cm long pieces from a conduit tube of 15 mm diameter. Insert these cylindrical conduit pieces into the holes of the chicken mesh such that they are placed perpendicular to the chicken mesh along the two long edges of the mesh as shown in Figure 9.

When inserting a conduit piece into a hole in the chicken mesh, care must be taken to make sure that half of the conduit piece is on one side of the chicken mesh and the other half is on the other side, as shown in the isometric view of Figure 9. These conduit pieces shall provide the holes required to tie the ferro-cement slabs together.

Cut several other 2.5 cm long conduit pieces from the conduit tube of 15 mm diameter, and insert them into the holes on the mesh approximately 6 cm apart from each other as shown in Photograph 5. These conduit pieces shall provide the holes required to facilitate air to get in contact with the composting mass within the unit.

Now, place the wooden frame flat on the ground on a few layers of paper. Place the mesh supported by the conduit pieces within the empty space entrapped by the frame, as shown in Photograph 6. Note that the mesh should stay 1.5 cm above the ground supported by the conduit pieces.

Now, pour the cement mixture (4 parts of sand for 1 part of cement) onto the chicken mesh placed within the space bordered by the wooden frame, and level the mixture. Care must be taken not to fill the space within the conduit pieces.

Leave the ferro-cement slab to stay where it is for about three days. Keep watering the slab so that cracks do not develop in the slab. Three days after, remove the frame carefully. What you have now is a ferro-cement slab with holes made by conduit pieces. Make the other three slabs also in the same manner.

One may also use wooden plugs in place of conduit pieces to make holes in the ferro-cement slabs. However, the holes made using wooden plugs may enlarge over time, and thereby exposing the vulnerable chicken mesh for corrosive liquids. We have realised that some may find the construction of the slabs with ventilation holes a somewhat difficult affair. In which case, you may construct the slabs without the ventilation holes. It should however be born in mind that the absence of ventilation holes may lead to foul smelling gases emanating from the composting unit.

Once all four slabs are ready, before they are connected together, coat the edges of each of the slab with a thin cement layer (made by dissolving cement in water to appropriate thickness). It is essential that a cement cover is applied to the edges, since any exposed part of the chicken mesh will be subjected to rapid corrosion if it comes in contact with the various corrosive liquids produced during the intermediate stages of composting.

Connect the four slabs together to form the composting unit using nylon cords inserted through the holes made for that purpose. If you see gaps between the faces of the composting unit, do not worry about that. These gaps are not a problem, and in fact they help improve the air circulation to the composting mass.

As this ferro-cement unit can be assembled, disassembled and reassembled very conveniently, transportation and storage of this unit are not much of a problem.

Figure 9 - The plan and the isometric view of the chicken mesh cut into a shape that would fit the space enclosed by the wooden frame of Figure 8. The approximate positions at which the six 2.5 cm long cylindrical conduit pieces must be inserted are shown. Note that the Figure is not drawn to scale.

Plan view
Isometric view
Photograph 5 - The chicken mesh with
2.5 cm long cylindrical conduit pieces inserted within the holes.

Photograph 6 - The wooden frame for making the slabs placed flat on the ground. The mesh with conduit pieces is placed within the frame. The layers of paper on which the frame and mesh should be placed is not shown in the Figure.


5.2 Constructing the ferro-cement unit with the four faces cemented together


If you wish to construct the unit such that the four slabs are cemented together as shown in Photograph 4, then you will have to follow a slightly different method of construction. In this method, you construct two wooden frames of the dimensions shown in Figure 8, using wooden pieces of 1.25 cm thickness, not of 2.5 cm thickness. These two frames must be placed one on top of the other to get a frame of 2.5 cm thickness.

The chicken mesh in this case is cut in such a way that it is about 4.0 cm wider along the two long edges of the mesh than the size of the mesh discussed in section 5.1. The extra width of the mesh helps us sandwich the mesh between the two wooden frames. The two wooden frames are then screwed together. The frame is laid flat on a few layers of paper and the procedure described in section 5.1 is followed to make the ferro-cement slab with holes made up of conduit pieces.

Note that in this case, the additional six holes made with conduit pieces to insert the cords to tie the slabs together, as described in section 5.1, are not needed.

When the two frames are unscrewed three days after, what we get is a ferro-cement slab with the chicken mesh protruding along the two long edges of the slab. Make the other three slabs also in the manner described above.

Transport the four slabs to the place where you would want the composting unit to be. Then, connect the protruding wires of the slabs in such a manner that the four slabs are brought together to make the composting unit. Apply cement plaster along the connecting edges. Now the ferro-cement composting unit shown in Photograph 4 is ready.


5.3 Preparing the ferro-cement composting unit for kitchen waste disposal


Place the ferro-cement composting unit in an appropriate place in your garden, probably 3 to 5 metres away from the kitchen. Make sure that rainwater does not stagnate at the place of your choice. The unit must be placed in such a manner that the bottom of the unit must be in direct contact with the soil.

You may make a brick layer barrier around the bottom of the composting unit so that any material taken out of the openings at the bottom of the unit will stay confined by the brick layer barrier. A lid to the unit can be made using an asbestos sheet or using a thin ferro-cement slab. Attach a handle to the lid for convenient handling of the lid.

Fill the composting unit with sawdust (or dry soil, only if you cannot find sawdust) until about one third of the height of the ferro-cement composting unit is filled. The area confined by the brick layer barrier could also be filled with sawdust (or dry soil) until the openings on the bottom of the slabs are covered.

Sawdust (or dry soil) plays an important role during composting by absorbing the excess moisture in the kitchen waste. If this excess moisture is not properly absorbed it would lead to foul smell emanating from the composting mass.


5.4 Performance of the ferro-cement unit


Kitchen waste can now be added to the ferro-cement unit from the top. After adding the kitchen waste, remember to put back the lid on the unit to prevent rainwater from entering the unit. A reasonable amount of garden waste added to the composting unit would help improve the C/N ratio towards maintaining good composting conditions within the unit. However, make sure that you do not add too much of garden waste that will result in the unit getting fill with garden waste quickly.

The performance of the ferro-cement composting unit is observed to be very similar to that of the aluminium composting unit. The procedure that should be followed to take the compost out of the ferro-cement unit is very similar to the one used with the aluminium composting unit described in section 4.4.


5.5 Limitations in using the ferro-cement unit


First of all, constructing a ferro-cement unit requires a reasonable amount of experience in working with cement mixtures. Secondly, a ferro-cement unit, once assembled, is very heavy, and therefore it is very difficult to move this unit from one place to another. However, it is a very sturdy unit, and can be used as a composting unit in a house for a very long time. It also has a pleasing appearance.

If you want to reduce the weight of the ferro-cement unit, then you may reduce the height of the unit down to 75 cm from the original 95 cm. Any further reduction in the height of the ferro-cement is not recommended owing to the reduction in its volume.

Cement composting units are also constructed without reinforcement. We do not recommend that since, as we have experienced, such units are not strong and may break during transportation.


© 2006, International Ecological Engineering Society, Wolhusen, Switzerland