For the ones who likes to know the detail!

•  Composting Theory
The production of compost occurs by the interaction of microbes with organic material, oxygen and water. Because composting is an aerobic process (needing oxygen), it can only occur when enough air is available. If layers of organics are thicker than about 60 cm, the decomposition process becomes increasingly anaerobic (without oxygen).

The criteria to keep the composting process aerobic are an oxygen content of more than 6% and a water content of 40 to 60%. Below a content of 6% oxygen and over 60% water the process becomes anaerobic. Below 40 % water content microbial activity stops.

During the composting process about 65 % of the organic material is digested by microbes. Glucose (C 6 H 12 O 6 ) is commonly used as a model for the aerobic microbial decomposition:

C 6 H 12 O 6 + 6 O 2 --> 6 CO 2 + 6 H 2 0 (product = carbon dioxide and water)

The remaining 35 % of the organic material is not digested and results in compost. With the help of oxygen, microbes produce first highly soluble and colored fulvic and humic acids that react later to less soluble humic compounds, the basis of humus.

The anaerobic decomposition follows the following process:

C 6 H 12 O 6 -> 3 CO 2 + 3 CH 4 (product = carbon dioxide and methane)

The prevention of methane, which is 21 to 23-times stronger as greenhouse gas than carbon dioxide, makes aerobic composting eligible as a Clean Development Mechanism (CDM).

•  Research into Composting Technology
As mentioned above, air and thus oxygen can aerate composting organics only in layers up to about 60 cm. This passive aeration is generated by heat convection and oxygen dissociation. Therefore, such passive aeration is not suitable for large compost piles. This is why the Gianyar Waste Project has chosen an active forced aeration process, where air is forced into the composting material with help of a system of blowers and perforated pipes.

Traditionally composting is done in windrows, which is a discontinuous batch process. Because compost piles must be turned about weekly to be watered and mixed, windrows require a lot of space. To protect windrows from heavy rain in Indonesia , they must either be covered with a roof or semi-permeable covers, both are expensive. The Gianyar Waste Project is investing time and funds to develop a more efficient semi-continuous process.

To improve the composting technology, the Gianyar Waste Project conducts research since 2004 and focuses on the following topics:

•  Optimal pile size and shape to take advantage of economies of scale
•  Optimal system for forced aeration to reduce operating cost
•  Active moisture control to reduce the frequency of turning the piles
•  Reduction of processing time and thus increasing capacity
This ongoing research resulted in the table composting system discussed below.

•  Table Composting Technology
The project is faced with the challenge to process daily 42.5 tons of organic matter into high quality compost. Aerated composting can be done in windrows or in a table pile as depicted below. Windrows need a lot of space and have the disadvantage of a two-dimensional airflow, when aerated from a central air pipe. In such windrows, air speed and supply decrease 20-fold with every meter distance from the pipe. Composting tables need less space and have a one-dimensional laminar air flow, resulting in more homogeneous pile conditions. In the Gianyar, the compost table is 3 to 4 m high, 30 m wide and 60 m long.

Compost tables also have the advantage that they allow a semi-continuous composting process, while windrow composting is a batch process. By the regular turning of the compost table, the organic matter is moved from one end where new shredded organic matter is added and to the other end where the finished compost can be removed. The project team evaluated both alternatives and decided clearly for table composting based on space required, investment and operating cost. The graph below, illustrates the semi-continuous table process. The attachment at the end shows the layout of the composting plant. There the aeration system with the blower, duct and the 2 m apart perforated pipes can be seen.

For the periodic turning of the compost, three alternatives are available and tested:

1. Manual Turning: Very slow but job creating. The compost table is turned manually with handheld tools. We use this method until another one proves to be better.
2. Using a Front Loader: Very quick but uses expensive equipment. The organics tend to be compacted, which is not desirable as it increases the aeration back pressure.
3. Using a Pneumatic Conveyor: Relatively quick and cheaper than front loaders. Results in a fluffy compost table, which is desirable and might become the method of choice.

While the project favors the job creating manual turning, time and economic consideration may dictate turning with a front loader or a cheaper pneumatic conveyor.

To support an optimal microbial activity, the moisture content must be kept between 40 and 60 % and a temperature of about 65 °C should be maintained as long as possible. However, during the whole process an oxygen content of at least 12 % will be assured, because the process turns anaerobic below 6 % oxygen. The microbial digestion produces heat. Blowing air into the compost table also serves to cool it to the optimal 65 °C as long as excess heat is produced. However, this also removes moisture. Therefore, the compost table must be turned, mixed, watered and loosened up about once per week. The project has all the necessary instruments to measure temperature, water and oxygen content electronically.

The air requirement is very high (average about 6.0 m 3 air per hour and m 3 organic matter) during the first composting phase of about 45 days while the organic matter is digested. When the compost is curing during the second phase of about 45 days, the air requirement drops quickly (to about 0.6 m 3 air per hour and m 3 organic matter).

During the whole processing cycle, the organic matter is shrinking. Shredding reduces the volume of the delivered organics to about 52 %. This volume decreases again to about 23 % during composting. Therefore, the finished compost has only about 12 % of the volume of the delivered organic waste. The weight also drops to about 35 % Thus, the daily 42.5 tons or 183 m 3 of organic waste are processed into 15 tons or 22 m 3 quality compost.