[PDF Notes] What is Mechanical Composting?

As an alternative to windrow composting, it is possible to produce humus within 5-7 days using mechanical systems often the composted material is removed and cured in open windows for an additional period of about 3 weeks. Once the solid waste has been converted to humus, they are ready for the third step of product preparation and marketing.

This method of composting is carried out in different vessels:

(i) horizontal plug flow reactor

(ii) vertical continuous flow reactor

(iii) rotating drum.

Process microbiology

Aerobic composting is a dynamic system in which bacteria, actinomycetes, fungi, and other biological forms are actively involved. The relative predominance of one species over other depends upon the constantly changing available food supply, temperature and substrate conditions. In this process, facultative and obligate aerobic forms of bacteria, actinomycetes and fungi are most active. Mesophilic forms are predominant in the initial stages which soon give way to thermophilic bacteria and fungi except during the final stages of composting when the temperature drops, actinomycetes and fungi are confined to 5-15 cm of outer surface layer. If turning is not carried out frequently, increased growth of actinomycetes and fungi in the outer layer imparts a typical greyish white colour.

Attempts are yet to be made to identify the role of different organisms in the breakdown of different materials. Thermophilic bacteria are mainly responsible for breakdown of protein and other readily biodegradable organic matter. Fungi and actinomycetes play an important role in the decomposition of cellulose and lignin.

Design consideration for composting (aerobic) process

Particle size

For optimum results the particle size in the range of 25-75 mm.


Composting time can be reduced by adding partially decomposed compost (1-5%) or sewage sludge.

Mixing /tuning

To prevent drying, caking and air channeling, materials in the process of being composted should be mixed or turned on a regular schedule.

Moisture content

It should be in the range of 50-60% during composting process.


The optimum temperature for biological stabilization is between 45-55° C.

Carbon nitrogen ratio

The initial carbon-nitrogen ratio between 35-50 is optimum for composting process. At lower ratios and higher pH levels nitrogen is in excess and will be given off as ammonia. At higher ratios nitrogen will be the limiting nutrient.


The pH should be maintained within 8.5 to minimise the loss of nitrogen in the form of ammonia.

Control of pathogens

At the end of composting process temperature should be maintained between 60 and 70°C for 24 to destroy pathogenic organisms.

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