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Facility for Composting Organic Component of Municipal Solid Waste

 

1. Introduction

Deposition of organic waste materials often leads to significant problems due to their instability. It is known that anaerobic degradation develops about 100 kg/ton of methane, but according to the Kyoto convention it is allowed to deposit organic materials only in sanitary dumps. At the same time, during the decomposition of these materials the volume of discharged waste waters is significantly increased as well as unpleasant odor emission around the dump site.

The need to process the waste of organic origin (food waste, bio waste) in the aim to reduce the existing dumps and to revitalize part of their content, has led to significant development of the composting technology, i.e., mechanical-biological processing. Table 1. displays the overview of the most common components of municipal solid waste, and their physical characteristics. We find 96 chemical elements in nature, but only 18 elements participate in plant structure, of which the most important are hydrogen, oxygen, nitrogen, carbon, phosphorus and sulphur. All elements except carbon, plants obtain from the soil, so that in time depletion of these elements occurs, i.e., they need to be restored. Compost incorporates many of these elements, thus implying its usefulness. Naturally, which compost and in what concentration should be used for particular soil and particular plant culture depends on the features of land such as porosity, humidity, pH value and other chemical and biological parameters. Compost use decreases land erosion and pollution caused by overtreatment with artificial fertilizers.

Municipal waste component Concentration kJ/kg Carbon Evaporable content Nonevaporable content Humidity
Paper and cardboard 41% 13490 35% 37% 44% 19%
Bio waste 15.3% 9300 23% 21% 33% 46%
Food waste 6.8% 7560 18% 17% 23% 60%
Plastics 9.8% 26980 56% 20% 65% 15%
Wood 3.8% 16050 41% 60% 44% 16%
Textiles 2.2% 15350 37% 36% 39% 25%
Rubber, leather 2.4% 19538 43% 24% 66% 10%
Glass 7.6% 0 0% 0% 100% 0%
Metal 9.0% 0 0% 0% 100% 0%
Mix 2.1% 0 25% 33% 33% 33%
Average value 100% 11235 27.5% 24.7% 50.9% 24.4%

Table 1.

2. Definition of Composting

Composting is a controlled biological decomposition and stabilization of organic substrate, under the condition that it is primarily aerobic and that it allows for the development of thermophilic temperature as a result of biologically generated heat. The process generates the final product which is sanitary and stabilized, has high concentration of humus substances and can be usefully applied to the land.

In other words, composting is a natural process that transforms organic waste into clean and useful product.

3. Composting Process

Composting process is the application of a natural process of putrefaction. Significant difference is in that enough volume of organic material should be gathered and that in this way generated heat is preserved. This has a favorable effect on the process speed and performs selective sterilization of potential pathogens and weed seeds known as sanitation, pasteurization or disinfection.

Composting is performed under the influence of microbiological processes. Commonly the process is divided into four phases: heating, thermophile decomposition, mesophile decomposition and maturation. This classification corresponds to temperature ranges under which certain microorganisms develop or are especially active.

Compost is made of easily degradable organic material and of sufficient mineral nourishing substances, especially nitrogen substances. Typical composting process is displayed in Figure 1.

Figure 1. Compost

Figure 1. Temperature changes (full line), population of mesophile fungi (dashed line) and thermophile fungi (dotted line) in compost. Left ordinate indicates the logarithm of the volume of formed fungi colony per gram of compost and the right ordinate - temperature at compost center

4. Optimization of the Composting Process

Efficient composting needs appropriate raw material base which is "food" for microbes that initiate the process. Sufficient amount of easily degradable carbon is necessary. This is sometimes called the "evaporable solid content" (Table 1.) and is important for fast heating.

Materials with high nitrogen content C:N
Cut grass 19:1
Mud digested
16:1
Food
15:1
Cow manure 20:1
Horse manure 25:1
Materials with high carbon content C:N
Leaves 40-80:1
Tree bark
100-130:1
Paper
170:1
Wood and saw dust 300-700:1

Table 2.

Also it is necessary to achieve balance of other materials, for example nitrogen. This precondition is often designated as the C:N ratio. Table 2. lists some of the materials that are processed during composting.

Operating range is pretty wide: range 25:1 - 40:1 is often designated as ideal.

Composting is an aerobic process, and sufficient volume of oxygen must be applied in order to enable process realization. Air must be enabled to reach all parts of the deposits. At the overall level this implies application of a rotating mechanism or injecting air. Water concentration is also a critical factor. Water concentration should be between 40-60%. Higher humidity levels decreases porosity which disables the maintenance of the necessary oxygen level. At a lower humidity level biological activity is insufficient for process continuation.

For efficient microbiological activity, these conditions should be present at the micro level rather than on the overall score. Necessary parameters of waste material and process are:

Particle dimension 25-75 mm, carbon to nitrogen ratio C/N=25-50, humidity 50-60%,
temperature 50-55C, pH control pH=7-7.5, sufficient aeration


5. Quality Standards

Standards differ by their content, but generally they require that products are safe for humans to handle and for the environment which is being fertilized by it. Standards can also have components that imply quality appropriate for application.

Usually components of standards include:
  • contamination with heavy metals and chemicals. Heavy metals are omnipresent in the environment, but can be a problem in compost, especially those separated from biosolids and mud for processing. Chemical contaminants such as pesticides are also limited by some standards.
  • Hyman pathogens: Typically salmonella and coliform are also limited. Since these requirements can be determined by direct testing, commonly specific temperature of the composting process is demanded. Typically it is 3 days above 55C. 
  • Weed seeds : composting process should destroy majority of the seed population, this aspect is also controlled by some standards.
  • Reduction of the attraction vector: degradable waste is  usually attractive for a whole range of disagreeable animals including birds and rodents. Complete composting eliminates attraction of the material. This requirement is usually based on time and temperature.
  • decreased plant growth: As discussed above, compost that has not matured can be toxic for plants. Herbicide transfer is also a problem occasionally. Obviously this is a significant factor where the product is used as a component of an agent for plant growth.
  • Physical contaminants: metal, glass, plastics and gravel are possible contaminants of compost and their concentration reflects the quality of the raw material base.
Some of the most important standards are: USEPA 503 and AS/NZ 4454.

6. Automated Facility for the Mechanical-Biological Treatment of Municipal Solid Waste

Facility is based on the Bed minster's procedure and is fully automated.

Municipal solid waste is sorted following waste receipt whereby materials that can be recycled such as paper, plastics, ferromagnetic materials, aluminum particles, and non-biologically degradable materials are extracted. Material obtained in this way is directed to the reactor where it undergoes mechanical-biological treatment whose final objective is compost generation.

From the start of processing compost is isolated from the environment so that permanent monitoring and high quality control is possible. At the same time, since the process is fully automated, it is possible to perform continuous treatment and maximum manufacturing productivity. It is necessary to stress that the compost generating process is performed continuously in the reactor that rotates at 1 rotation per minute. After 1800 rotations the compost is screened, pulverized, freed from metal particles, gravel and other heavy components and is directed for fermentation and maturation.

Facility performance
Rated yearly capacity for processing of municipal waste 25000 t
Maximum daily capacity ( mud and municipal waste) 100 t
Necessary yearly mud volume 2000 t
Yearly compost output 10000 t

Bio reactor parameters
Length 42 m
Diameter 4.2 m
Wall thickness 16 mm
Bio reactor mass 222000 kg
Number of rotations of the reactor pipe 1 min-1
Plant rotation power 2x75 kW/ 1500 min-1

Technological units of the composting facility are displayed on the following diagram:

Composting facility units

 1.Facility Tipping Floor
 2.Materials-Recovery Building
 3.Materials-Recovery Staging Area
 4.Presorting Materials Tipping Floor
 5.Employees' Lockers, Cafeteria
 6.Facility Management Office
 7.Digested Tipping Floor
 8.Bio reactor
 9.Primary Screening Building
10.DE-Stoning Building
11.Bio filter and Emissions-Control Equipment
12.First-Phase Composting
13.Second-Phase Composting & Curing
14.Final Screening & Compost Load-Out
15.Biosolids Storage Bunker and Pumps

Material Balance

Per 1000 kg material in the bio reactor with 50.55% humidity (mud and water) after 1800 rotations of the reactor pipe the following decomposition occurs:
150 kg gas component
30 kg ferromagnetic material
450 kg refuse material
370 kg compost


Material Flow

Per 1000 kg material in the bio reactor with 50.55% humidity (mud and water) after 1800 rotations of the reactor pipe the following decomposition occurs:

Material flow

Figure 2. Material flow in the automated facility for mechanical-biological treatment of municipal solid waste

Dr Stojković Slobodan


 
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