Use of Ambisystem^® in a Composting Plant.

The term composting means the process of controlled biological maturation, in an aerobic environment, of the organic substance of animal and vegetable residues, resulting in the production of materials with a simpler molecular chain, more stable, hygienised, rich in damp compounds and, in brief, ideal for fertilising agricultural land and for topping up the organic substance in the soil. The process is carried out by different strains of micro-organisms operating in an aerobic environment: bacteria, fungi, Actinomyces, algae and protozoa, naturally present in organic biomasses or artificially introduced with any inoculation material.

The process may be briefly described with the following reaction:

organic material + O₂ > compost + CO₂ + H₂O + NO₃- + SO₄- + heat

The term composting is commonly used, though incorrectly, for the process of aerobic stabilisation of organic material obtained from solid urban waste (SUW) with undifferentiated collection, after mechanical selection: the machinery and plant are essentially the same, but the projectual setting varies and also, above all, the quality of the material obtained. In fact organic material from undifferentiated collection remains destined for the dump; the role of aerobic treatment is traceable only to stabilisation of the organic substance with two objectives:

• to reduce, after disposal at the dump, the anaerobic fermentative processes that create problems of malodorous emissions;

• to reduce the volume of waste to be dumped.

Materials suitable for composting must have such biochemical characteristics as to ensure a regular actuation of the process; in particular they must be easily biodegradable by micro-organisms to such a degree as to ensure the latter sufficient nutriment and a correct process. These materials are:

• vegetable refuse;

• highly biodegradable organic substance-content waste deriving from manufacturing, commercial and services activities;

• organic fractions from domestic use;

• sludge from urban and agri-food purification;

• zootechnical excrement.

The micro-organisms involved in the composting process are many and they alternate synergically in the various phases. A useful example regards the micro-organisms secreting amino acids and vitamins, essential to the life of other microbic strains.

The various micro-organisms operate at defined heat levels and their activity is influenced by the process temperatures to the extent that they can be distinguished in three classes: psychrophilic, mesophilic and thermophilic.

During the composting process the active microbic populations vary in accordance with the variations in biomass temperature: in the first phase of the process, which involves a rapid metabolising of the simplest carbon compounds, initially the psychrophilic and mesophilic micro-organisms operate. Subsequently, due to a raise in temperature following intense metabolic activity, there is great selection among the bacteria populations to the advantage of the thermophilic species whose optimal working conditions are between 50-60°C. If the temperature exceeds the maximum limit of 90°C, microbic activity diminishes progressively until most of the micro-organisms in the organic matrix die off and the composting process ceases.

The micro-organisms are distinguished in aerobic and anaerobic micro-organisms. Composting by definition is an aerobic process featuring the presence of aerobic micro-organisms only. Whereas sometimes, even when process operational conditions appear optimal, certain zones (“pockets”) are formed in the biomass in which the lack of oxygen leads to the triggering of anaerobic fermentations. The gaseous products of these fermentations – including methane and butyric acid, form mixtures with a characteristic unpleasant smell. It is important that these phenomena be kept under control so that product quality is not jeopardised by the prevalence of anaerobic processes.


Phases of the composting process

The phases of transformation undergone by organic substance in the composting process are three in number:

• hygienisation,

• decomposition or destructurisation through bioxidation,

• maturation.

The first phase, known as hygienisation, is limited in time and must ensure a temperature above 55°C for at least 120 hours continuously; the second phase of the bioxidation process ends with the decline of thermophilic fermentations and the achievement of parameters of IR ≤ 1000; the third phase, maturation, is characterised by a prolonged mesophilic process. The main objective of the composting process is to obtain a stabilised product, intending with this adjective the achievement of its biological and agronomic maturity which guarantees the absence of phytotoxic agents.

For the two pivotal phases of the process (bioxidation and maturation) parameters are considered and analysed which permit following the evolution of the process; control of these is a primary objective in carrying it out.


Decomposition of organic substance (bioxidation)

The composting process begins immediately with destructuralisation of the organic fraction that is most easily biodegradable (carbohydrates and hemicelluloses: sugars, organic acids, amino acids) carried out by the aerobic micro-organisms, with the consumption of oxygen and the freeing of carbon dioxide and heat. The temperature of the heaps, due to the energy generated by the oxidation reactions catalysed by the micro-organisms, increases progressively until reaching the thermophilic rate. This phase is known as high rate phase and generally last three weeks (times may vary depending on the characteristics of the substratum and the technique employed). The most rapid increase in temperature, up to 55-60°C, is noted within the subsequent 24-48 hours following the setting up of the heap. The heat must be suitably dissipated because a further increase in temperature could lead to inactivation of most of the micro-organisms.

The thermophilic phase is of great importance also because it leads to devitalisation of the seeds of any infesting plants present in the starting matrix.
In order that the microbic population not be jeopardised it is indispensable to check the temperature and maintain constant the contribution of oxygen to the biomass. . Two techniques can be employed to this end, individually or jointly: the overturning of the heaps and forced aeration thereof.


Parameters and indices of process evolution

The micro-organisms play a fundamental role in the decomposition of organic substance and there is a direct relationship between their activity and the evolution of the composting process. Meaning that progress and speed of the process are strictly dependent on factors that influence optimal conditions for the life of the micro-organisms operating in the different phases of the process.

The main parameters that influence the conditions of life of the micro-organisms, and are therefore normally checked to verify correct progress of the process, are:

• porosity of the substratum;

• humidity of the material;

• oxygen content;

• temperature.

Temperatures

The temperature is not only the best indicator of the correct progress of the composting process but is also the easiest to monitor. As already mentioned, in the composting process there are two distinct phases from the thermal viewpoint: the thermophilic phase (between 55-65°C) which permits hygienisation of the product and the inactivation of seeds of infesting plants that might be present in the heaps, and the mesophilic phase (35-45°C) in which the destructurisation of less easily biodegradable substances takes place.

It should be underscored that in the thermophilic phase the temperatures may reach over 70°C, but this would cause a high mortality rate of the micro-organisms and jeopardise process efficiency.
 

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