Large-scale composting of solid organic residues

LARGE-SSCALE COMPOSTING OF SOLID ORGANIC RESIDUES

1. INTRODUCCION

Composting is a process of aerobic decomposition of organic matter by means of microorganisms and fungi, to obtain compost that is a natural fertilizer. During the composting process, it produces carbon dioxide, water, minerals, compost and also heat, which can destroy pathogens. This transformation is carried out in any place under controlled conditions, since it does not require any mechanism, engine or maintenance, only a composter is necessary.

This process contributes to the reduction of the amount of garbage that they bring to landfills or to the recovery plants. At the same time, it is possible to reduce the consumption of chemical fertilizers, as it promotes natural fertilization. Of each 100 kg of organic waste 30 kg of compost are obtained.

At home, of the total garbage, 40% is organic matter, which can be recycled and returned to the soil in the form of humus for plants and crops. It should also be noted that domestic composting emits 5 times less greenhouse gases, compared to industrial composting, for the same organic matter.

With regard to large-scale composting, in the early 1990s, most public officials and businessmen saw the remnants of food as a problem. But, at present, this problem becomes environmental benefits.

2. COMPOSTING

A large fraction of the waste stream consists of organic residuals that can be turned from a waste into a useful soil amendment through composting. CWMI* addresses a broad range of residuals including manure, yard and food wastes, and mortalities and a wide array of audiences including households, schools, farms, municipalities and private entities.

*CWMI: Composite water management index

2.1 COMPOSTING MICROORGANISMS

Microorganisms such as bacteria, fungi, and actinomycetes account for most of the decomposition that takes place in a pile. They are considered chemical decomposers, because they change the chemistry of organic wastes. The larger decomposers, or macroorganisms, in a compost pile include mites, centipedes, sow bugs... They bite and transform materials into smaller pieces. Of all these organisms, aerobic bacteria are the most important decomposers. They are very abundant; there may be millions in a gram of soil or decaying organic matter.  They are the most nutritionally diverse of all organisms and can eat nearly anything (Friend, M et al, 2018).

Bacteria utilize carbon as a source of energy and nitrogen to build protein in their bodies. They obtain energy by oxidizing organic material, especially the carbon fraction. This oxidation process heats up the compost pile from ambient air temperature.

This bacteria are poor of complexity, and they can not survive when environment is unfavourable (changes in oxygen, moisture, temperature, and acidity). Aerobic bacteria need oxygen levels greater than five percent. They produce the most rapid and effective composting. They also excrete plant nutrients such as nitrogen, phosphorus, and magnesium. But when oxygen levels fall below five percent, the aerobes die and decomposition is done by anaerobic bacteria. These bacteria reduce the process velocity and sometime produce toxic.

There are different types of aerobic bacteria that work in composting piles. Their populations will vary according to the pile temperature. Psychrophilic bacteria work in the lowest temperature range (less 20ºC). They give off a small amount of heat which help to build the pile temperature to the point where mesophilic bacteria, start to take over.

Mesophilic bacteria rapidly decompose organic matter, producing acids, carbon dioxide and heat. Their working temperature range is generally between 12 ºC to 45 ºC. When the pile temperature rises above 45 ºC, the mesophilic bacteria begin to die off or move to the outer part of the heap. They are replaced by thermophilic bacteria.

Thermophilic bacteria continue the decomposition process, raising the pile temperature to 70ºC, where it usually stabilizes. High temperatures typically last no more than three to five days. As the thermophilic bacteria decline and the temperature of the pile gradually cools off, the mesophilic bacteria again become dominant. The mesophilic bacteria consume remaining organic material with the help of other organisms.

Figure 1: Composting phases according to temperature

2.2 Comparison between large scales composting with small scale composting

When large-scale composting has been studied, the results have been seen and the conclusions cannot always be applicable to small-scale composting. Next, an experiment made by some students of the Higher Technical School of Agronomists of the Public University of Navarra will be explained.

Large-scale composting could be compared to that of small scale to see the differences; In an experiment where it is intended to compare this, two containers are used. The same type of agri-food waste was composted in tumbled piles of more than 1300 tons and in the same time in 320 L plastic containers, usually used for domestic composting. The evolution of temperature, humidity and weight reduction was monitored throughout the first 18 weeks of the composting process. In both cases, 5 monthly turns were made and then every 2 months. After 295 days of processing, the weight and volume reductions were calculated with respect to the initial material contributed. Storino et al.(2014)

Over time the temperature varied greatly depending on the treatment. The maximum temperature was higher than 60ºC, which was very similar in the two compostages.

The first is the thermophilic one that with the hard composting only the first 15 days, in the large-scale composting hard until the first 125 days, therefore, the average temperature was higher and this is due to a lower dissipation of heat and a greater thermal inertia.

Small-scale composting required 7 additional watering due to higher losses and greater aeration (Figure 2). And in the large-scale composting, only one irrigation was necessary in the 295 days of the process. Storino et al.(2014)

Figure 2: Evolution of humidity during composting processes.

Small-scale composting showed a lower pH and also a lower specific weight, and in the large-scale composting it had a lower degree of maturity, being able to see remains of straw without decomposing (Figure 3). Storino et al.(2014)

Figure 3: Small-scale composting (left) and large-scale composting (right) at the 4th month of the process.

With the results seen in the 4th month, it is observed how the small scale composting has a greater intensity in the process, due to this factor I present a greater weight reduction during the 295 days, and a lower C / N ratio. The final values ​​of the parameters after 295 days (Figure 4) of processing were similar in the two compostages. Storino et al.(2014)

Figure 4: Reduction of mass during the composting processes at the domestic and industrial levels

The experiment is summarized in that the two composting reached very similar temperatures, the only thing that in the small scale composting the duration of the thermophilic phase was shorter, first 15 days. And with regard to maturation, they observed that in large-scale composting maturation was lower during the whole process until the seventh month that the values ​​became similar. And also as previously observed, small-scale composting has a greater capacity to reduce the weight of composted materials.

With all these results and observations it is concluded that on a small scale the degradation and decomposition process was more intense in the early stages of composting, regardless of the average temperatures observed. Storino et al.(2014)

3. CONCLUSIONS

In conclusion, we would see that, although the two processes reached similar temperatures, the duration of the thermophilic phase in large-scale composting lasted much longer and also that there was less ripening and a lower humidity of the Organic matter and had similar values ​​in the composting of smaller scale (domestic) in the seventh month.

And with all this we could summarize that the process of degradation and decomposition in small scale composting was more intense in the first stages regardless of temperatures.

4.BIBLIOGRAFIA

• Friend, M, H Johnson, and A Smith. 2018. ‘The Science of Composting - Composting for the Homeowner - University of Illinois Extension’. https://m.extension.illinois.edu/homecompost/science.cfm (November 3, 2018).

• Barrena R., Font X., Gabarrell X., Sánchez A., 2014. Home composting versus industrial composting: Influence of composting system on compost quality with focus on compost stability. Waste Management (November 2,2018).

• Li, Yangyang, Ashish Manandhar, Guoxue Li, and Ajay Shah. 2018. ‘Life Cycle Assessment of Integrated Solid State Anaerobic Digestion and Composting for On-Farm Organic Residues Treatment’. Waste Management 76: 294–305. https://www.sciencedirect.com/science/article/pii/S0956053X18301673?via%3Dihub (November 2, 2018).

• Composting - Cornell Waste Management Institute’. http://cwmi.css.cornell.edu/composting.htm (November 2, 2018).

Cora Garcia, Mireia Diez, Anna Oriol, Aida Pelegrin.