**3. Physical-chemical parameters monitored during the composting process**

The principal physical–chemical parameters of the process monitoring are summarised in the parameters that condition the good development and progress of microbiological activities, and their monitoring is essential to test the effective conduct and behaviour of the composting process [9, 10]. This is achieved by optimising nutrient supply and regulating pH, temperature, water content and aeration conditions.

### **3.1 Grain size of wastes**

*Humic Substances*

Composting is a biochemically continuous phenomenon of organic matter mineralisation or oxidation in the presence of oxygen. The mineralisation or oxidation is achieved by microorganisms that use oxygen from the air and organic carbon for all their all-metabolite biosynthesis. To make the degradation or the oxidation easy, two operations may be implemented. These two operations are always considered as optional. First, the waste can be sorted to separate the fermentable fraction from the non-recyclable one. Second, the waste may be mechanically shredded to improve the structure of the waste mass; thus, on the one hand, the waste shredding facilitated the biodegradation, reduced the treatment time and make handling more pleasant; and the second hand to make homogenisation of the waste mass easy by allowing a uniform distribution of the different waste components. Once the residual substrate has been prepared, fermentation that resides at the heart of the process, is started. This fermentation leads to a rapid decomposition of easily biodegradable organic matter that generates some fewer complex molecules. Subsequently, the substrate biodegradation leads to slower maturation. These steps are commonly known as the processes of humification and stabilisation of the compost [5]. Once the compost has reached maturity, it will undergo screening and sieving. This operation is allowed to give two products: a commercial product known as compost and a refusal product to refine and/or to landfill. All these mentioned operations are well conducted in all common composting processes. Despite, some precise differences could be existed and lie in the location of the screening phase and the choice of the fermentation system. The originality aim of this book chapter is to describe, in the composting plant implemented under semi-arid pedoclimatic conditions, a description of new and simple process of composting and co-composting of municipal solid wastes, olive mill wastewater, waste farm and garden cutting, straw and sewage sludge. All these residual materials resulting from the main human activities commonly known at the present state in modern societies, with especially the olive mill wastewater resulting from oleiocultural activities, will be considered in this a new process of composting description, mainly charac-

terized as simple to implement and to monitor [6, 7].

**2. General composting process operation**

to the biosynthesis of humic compounds by fungi.

fermentation), respectively.

Indeed, the process is directed in two principal successive steps: step of prefermentation (uncontrolled fermentation) and step of maturation (controlled

Composting is a complex bio-physical–chemical operation that comprises biodegradation of organic waste under controlled conditions of temperature, humidity and aeration. Two important following one another biophysical phenomenon could include the common composting process. The first process brings the organic residues to the state of fresh compost. An intense aerobic degradation concern essentially the decomposition of fresh organic matter at a high temperature of 50–70°C under the action of thermophilic bacteria; while the second process is done by a less sustained degradation mainly achieved by mesophilic bacteria. These bacteria transform the fresh compost into a mature compost, rich in humus. This maturation phenomenon, which takes place at lower temperatures of 35–45°C, leads

The composting process of organic residues takes place in three distinct and important phases. First, the temperature rises rapidly to around 40°C or 45°C following the respiration of aerobic mesophilic microorganisms; in parallel, the most degradable compounds such as sugars and starch are consumed. Second, the temperature rises progressively to around 60°C or 70°C and the mesophilic

**48**

The waste particle size is an important parameter to consider in the composting process since (i) it determines the size and volume of the pores created by the arrangement of the particles in the waste matrix, (ii) it is involved in increasing the specific surface area of the raw organic matter, (iii) it facilitates the diffusion of oxygen inside the compost waste matrix, thus allowing homogenisation of the waste, and at last (iv) it is the site of main microbiological activities that take place on the surface of the organic particles.

#### **3.2 Interstices oxygen rate**

This parameter shows the real proportion of oxygen in the interstices of the waste mass. It is critical to the oxidation of the organic matter, and directly related to the size, humidity and aeration of the waste during composting. Oxygen requirements decrease along composting whether they are proportionate to the organic matter gradually disappearing over the mineralisation process. However, maintaining and preserving good aeration avoids the start of an anaerobic process that could induce the generation of malodorous compounds. Moisture in the waste mass always interacts negatively with the system aeration. The supply of oxygen allows the drop in humidity. If this humidity is high, a probable temperature rise will take place leading to a significant improvement in the substrate mass homogeneity. The minimum threshold of oxygen needed to maintain aerobic conditions is of the order of 5% as reported by Jammes [9].

#### **3.3 Prevailing humidity in the waste mass**

Humidity is both a raw material-related parameter and a monitoring parameter. It hosts the development of the microbial flora within the compost. The optimal water content during composting is around 60%. However, high water content promotes anaerobic fermentation. If the water content exceeds 70%, the water fills the voids and space, making oxygen exchange very difficult. On the other hand, if this prevailing humidity drops below 20%, the decomposition of organic matter will be inhibited.

#### *Humic Substances*

The quantity of water lost by vaporisation during the release of heat exceeds those formed during the reaction of oxidation; therefore, to compensate this lessening, it is tolerated and needed watering materials during composting. Although it is difficult to determine the volume to be added, water can be added as long as no runoff appears under the pile of waste or the waste mass.

#### **3.4 C/N ratio of the waste mass**

The C/N ratio of the waste mass qualifies the biodegradability of organic waste by ensuring the trophic balance necessary for the flora optimal development. Suitability of fermentable waste for composting is determined by the C/N ratio of the mixture of their various constituents (fermentable, paper and cardboard). Putrescible materials whose C/N are of the order of 15, are substrates easily biodegradable, while paperboards, with C/N ranging from 60 to 107, are substrates hardly biodegradable.

During aerobic fermentation, microorganisms consume carbon 15 to 30 times more than nitrogen. The initial C/N ratio is around 30 to 35, while that of the final product is less than 15. Sometimes the C/N ratio of waste can be so low that it is unsuitable for composting. This can be remedied by adding a specific substrate with a high C/N ratio, which brings the initial C/N value back towards the optimum [9]. If the initial ratio is less than 30, nitrogen losses are accompanied by the odour nuisance. If the ratio is low, ammonia nitrogen losses may reduce pH. **Tables 1** and **2** showed examples of the C/N values of some compostable materials.

#### **3.5 Temperature developed inside the waste mass**

The increase in temperature is caused by microbiological activity. During the degradation of organic matter, there is energy initially contained in the chemical bonds of the constituent molecules, which are released, part of which is recovered by the metabolism of microorganisms, and the other part is dissipated into the atmosphere. Therefore, the minimum temperature is necessary for degradation and the evolution of the temperature during composting is allowed distinguishing four successive distinct phases [1].


#### **Table 1.**

*Average C/N ratios of some fermentable components.*


**51**

composting sites.

*Co-Composting of Various Residual Organic Waste and Olive Mill Wastewater for Organic Soil…*

Mesophilic microorganisms (especially bacteria and fungi) invade the raw material, so their activity causes a rise in temperature (from 10 to 15°C to 30–40°C),

During this phase, the temperature reaches 60 to 70°C, values to which only thermo-tolerant microorganisms like actinomycetes and thermophilic bacteria could remain in operation in this very hostile environment, and the degradation activity of resistant fungi will be stopped. Along this hostile phase, the nitrogen mineralized as NH4+ will be lost as NH3. This hostile environment takes place specifically inside the waste mass centre, hence leading to the need to turn over the

This phase is followed by the operation of turning and watering the mass of waste being composted. Thus, it is mainly characterised by the reappearance of ambient temperature and mesophilic microorganisms that decompose materials remained intact during the previous phase and nitrogen of some complex

The microbiological activity regresses considerably during this phase, and the waste receives new colonisers that are the macrofauna, particularly earthworms. The organic matter becomes stabilised and humified compared to their initial state. It should be pointed out that a temperature above 70°C should be avoided, as it leads to extreme drying, a very significant loss of material and even a halt in the process

The hydrogen potential known as pH is a measure of the chemical activity of protons or hydrogen ions in solution inside each medium. The pH largely influences the development of the microflora responsible for the waste decomposition. Its value is determined and imposed by the raw material used, but varies according to the progress of the composting process. The pH monitoring is mandatory since it provides information on the different phases of the process. The optimal pH prevailing in the waste mass during the composting is on average between 6 and 8.

The composition of municipal waste may show undesirable non-biodegradable

elements that could affect the process and the ultimate product quality. These elements may be notable as packaging and special wastes, rich in metallic elements. Therefore, these undesirable materials should be separated by sorting. The operation of sorting can be made at the domestic level known as source sorting prevailing and well distributed in very advanced modern societies and/or in

**3.7 Undesirable and non-biodegradable waste products**

a significant release of CO2 and subsequently a decrease in the C/N ratio and

waste mass for ensuring homogeneous and disinfected products.

*DOI: http://dx.doi.org/10.5772/intechopen.97050*

*3.5.1 The mesophilic phase*

*3.5.2 The thermophilic phase*

*3.5.3 The cooling phase*

*3.5.4 The maturation phase*

by the microflora destruction.

**3.6 pH or acidity degree**

components.

acidification.

#### **Table 2.**

*Average C/N ratios of main fermentable components.*

*Co-Composting of Various Residual Organic Waste and Olive Mill Wastewater for Organic Soil… DOI: http://dx.doi.org/10.5772/intechopen.97050*

#### *3.5.1 The mesophilic phase*

*Humic Substances*

The quantity of water lost by vaporisation during the release of heat exceeds those formed during the reaction of oxidation; therefore, to compensate this lessening, it is tolerated and needed watering materials during composting. Although it is difficult to determine the volume to be added, water can be added as long as no

The C/N ratio of the waste mass qualifies the biodegradability of organic waste by ensuring the trophic balance necessary for the flora optimal development. Suitability of fermentable waste for composting is determined by the C/N ratio of the mixture of their various constituents (fermentable, paper and cardboard). Putrescible materials whose C/N are of the order of 15, are substrates easily biodegradable, while paperboards, with C/N ranging from 60 to 107, are substrates hardly biodegradable. During aerobic fermentation, microorganisms consume carbon 15 to 30 times more than nitrogen. The initial C/N ratio is around 30 to 35, while that of the final product is less than 15. Sometimes the C/N ratio of waste can be so low that it is unsuitable for composting. This can be remedied by adding a specific substrate with a high C/N ratio, which brings the initial C/N value back towards the optimum [9]. If the initial ratio is less than 30, nitrogen losses are accompanied by the odour nuisance. If the ratio is low, ammonia nitrogen losses may reduce pH. **Tables 1** and **2**

runoff appears under the pile of waste or the waste mass.

showed examples of the C/N values of some compostable materials.

The increase in temperature is caused by microbiological activity. During the degradation of organic matter, there is energy initially contained in the chemical bonds of the constituent molecules, which are released, part of which is recovered by the metabolism of microorganisms, and the other part is dissipated into the atmosphere. Therefore, the minimum temperature is necessary for degradation and the evolution of the temperature during composting is allowed distinguishing four

**Categories Season Average C/N** Fermentables Spring 18.3 Paper-Cardboard Spring 63.4 Fermentables Summer 13.1 Paper-Cardboard Summer 59.2 Fermentables Autumn 15.6 Paper-Cardboard Autumn 107.5 Fermentables Winter 15.2 Paper-Cardboard Winter 79.1

**Municipal wastes Cattle manure Sewage sludge Olive pomace**

C/N < 20 20 11 49.3

**3.5 Temperature developed inside the waste mass**

successive distinct phases [1].

*Average C/N ratios of some fermentable components.*

*Average C/N ratios of main fermentable components.*

**3.4 C/N ratio of the waste mass**

**50**

**Table 1.**

**Table 2.**

Mesophilic microorganisms (especially bacteria and fungi) invade the raw material, so their activity causes a rise in temperature (from 10 to 15°C to 30–40°C), a significant release of CO2 and subsequently a decrease in the C/N ratio and acidification.

#### *3.5.2 The thermophilic phase*

During this phase, the temperature reaches 60 to 70°C, values to which only thermo-tolerant microorganisms like actinomycetes and thermophilic bacteria could remain in operation in this very hostile environment, and the degradation activity of resistant fungi will be stopped. Along this hostile phase, the nitrogen mineralized as NH4+ will be lost as NH3. This hostile environment takes place specifically inside the waste mass centre, hence leading to the need to turn over the waste mass for ensuring homogeneous and disinfected products.

#### *3.5.3 The cooling phase*

This phase is followed by the operation of turning and watering the mass of waste being composted. Thus, it is mainly characterised by the reappearance of ambient temperature and mesophilic microorganisms that decompose materials remained intact during the previous phase and nitrogen of some complex components.

#### *3.5.4 The maturation phase*

The microbiological activity regresses considerably during this phase, and the waste receives new colonisers that are the macrofauna, particularly earthworms. The organic matter becomes stabilised and humified compared to their initial state. It should be pointed out that a temperature above 70°C should be avoided, as it leads to extreme drying, a very significant loss of material and even a halt in the process by the microflora destruction.

#### **3.6 pH or acidity degree**

The hydrogen potential known as pH is a measure of the chemical activity of protons or hydrogen ions in solution inside each medium. The pH largely influences the development of the microflora responsible for the waste decomposition. Its value is determined and imposed by the raw material used, but varies according to the progress of the composting process. The pH monitoring is mandatory since it provides information on the different phases of the process. The optimal pH prevailing in the waste mass during the composting is on average between 6 and 8.

#### **3.7 Undesirable and non-biodegradable waste products**

The composition of municipal waste may show undesirable non-biodegradable elements that could affect the process and the ultimate product quality. These elements may be notable as packaging and special wastes, rich in metallic elements. Therefore, these undesirable materials should be separated by sorting. The operation of sorting can be made at the domestic level known as source sorting prevailing and well distributed in very advanced modern societies and/or in composting sites.
