Composting is a multi-stage process

Composting is a multi-stage biological process in which microbes break down organic material - generally biowaste or sewage sludge. The process requires moist, aerobic conditions with adequate thermal insulation. The final products of the process are carbon dioxide, water, material containing stable humus and inorganic salts, and heat. Composting involves a significant rise in temperature.

Composting progresses in stages. To begin with, bacteria and fungi attack easily decomposable organic materials - sugars, starch and fats - utilizing readily soluble carbon as their “building material” and energy source. The process of decomposition gives rise to new organic compounds, some microbes die off, and the temperature starts to rise.

Microbes that are adapted to the new conditions utilize the less-readily decomposed waste and dead microbial material. This stage again produces new biochemical compounds, dead microbial cells, and new microbes which, in turn, are suited to the even higher temperature levels and changed chemical conditions.

In the next stage proteins are broken down, which releases significant amounts of ammonia. The pH of the material increases and its temperature rises significantly. In the final stage macromolecular fibrous material - cellulose and hemicellulose - are broken down. The material which remains consists of very slowly decomposing lignin and other stable humus. When decomposition has progressed this far, the temperature starts to fall.  

Process know-how

The design and construction of efficient composting plants requires expertise in all aspects of the composting process. Vapo has acquired this expertise through its intensive research work, which it has been conducting since the late 1980s. This know-how is utilized in composting plants which are designed so that the composting conditions can be optimized throughout the process. This is achieved by means of an automatic aeration system, which provides the ideal oxygen content, temperature and moisture levels for each composting stage.

Stable, hygienic, mature and odourless

In the Wastech system the composting process takes place in two stages. The material undergoes active composting in the tunnels for a period of 2-3 weeks. It is then put in windrows for post maturation, which generally takes 3-6 months. To ensure that the desired results are achieved, the material should not be transferred from the active stage to post maturation until it is sufficiently stable, hygienic, mature and free from odours.

The material is stable when it is no longer undergoing change. During the initial stage, in particular, compost is in a state of constant biological and chemical change. As composting progresses, biological activity decreases and the processes of chemical and physical (i.e. temperature) change diminish. Sufficient stability has been achieved when the final product can be stored for long periods without odour or hygiene problems. Post maturation has a significant role to play in this respect.

The product can be described as hygienic when the number of microbes in the compost has decreased to the extent that handling operations do not involve any health risks. In Wastech composting plants this is achieved by maintaining the composting material at a temperature of more than 55oC for at least 48 hours.

The material is mature when the organic matter has decomposed to the extent that the final product can be utilized in a growing medium without any adverse effect on the germination, growth or root development of the plants. During the composting process, any harmful material evaporates or is decomposed, the carbon/nitrogen ratio decreases and the proportion of humus material increases. The most obvious sign that this has occurred is that the compost starts to smell like soil.

Odour is a very subjective matter, which can be described in terms of odour units. Air may normally contain 2 000 odour units per cubic metre, and a concentration of 1 000 odour units, for example, disperses in the surrounding air to become odourless in a radius of a few metres. The accurate process control at Wastech composting plants ensures that no odour problems are caused by the material transferred into windrows for post maturation. Malodorous gases produced during the composting process are removed by means of a biofilter. These biofilters - developed by Vapo - operate on the basis of microbial action and can eliminate up to 95% of malodorous gases.