10 Characterization of waste

Dr. Logakanthi S

epgp books

 

1.  Objectives:

 

After completing this module, you should be able

  • To understand the concept of waste characterization and its importance
  • To know about the method of waste characterization
  • Characteristic of municipal solid waste in India

 

2. Waste Characterization

 

Waste is described as discarded material that does not have no further use to the primary user. Waste exists in different forms like solid, semi-solid, liquid and gas. Based on its nature and source of origin it is categorized as municipal waste, hazardous waste, biomedical waste, radioactive waste and so on. Different waste streams are composed of different materials and therefore have different health and environmental impacts. Also the quantities to be managed differ from waste stream to stream. Consequently, the methods by which various waste streams are collected, recovered, processed, treated or disposed of may vary broadly.

 

Let us take the case of Municipal Solid Waste (MSW), this stream of waste has various components like organic waste, plastics, metal cans, paper, cardboard, glass, inert materials etc., Some of the constituents in this waste can be recycled, while some can be reduced and used and some others have to be only disposed. Thus, the action taken depends on the nature of the waste and its constituents. This is where the usefulness of waste characterization lies. Waste characterization is a method that is used to determine the types and proportion of materials that are being discarded in a waste stream. Waste characterization is essential in determining the management practices that are to be adopted to manage them. It is especially useful for policy makers and city planners who are involved in identifying and assigning landfill sites and setting up recycling programs.

 

Since the characteristic of waste determines the method of its management it is important and useful to know about the physical, chemical and biological nature of the waste. These characteristics vary depending on the source and type of solid waste and this in turn will affect the leachate and gas production from landfills.

 

2.1 Physical characteristic of MSW

 

Coming to MSW the physical characteristic of the waste includes moisture content, waste particle size, waste density, temperature and pH as these affect the extent and rate of degradation of waste.

 

2.1.1.   Moisture content

 

The moisture content of solid wastes usually is expressed as the weight of moisture per unit weight of wet or dry material. In the wet-weight method of measurement, the moisture in a sample is expressed as a percentage of the wet weight of the material; in the dry-weight method, it is expressed as a percentage of the dry weight of the material. In equation form, the wet-weight moisture content is expressed as follows:

 

Typically in MSW the moisture content varies between 15-40% and this further depends on the composition of the wastes, the season of the year, and the humidity and weather conditions. Moisture content is a critical determinant in the economic feasibility of waste treatment by incineration since energy must be supplied for evaporation of this moisture. Moisture content also plays an important role in other processing methods such as composting and anaerobic digestion. Most micro-organisms including bacteria require a minimum of approximately 12% moisture for growth.

 

2.1.2 Density

 

Density of a waste is its mass per unit volume (kg/m3). It is essential for the design of all elements of the solid waste management system from storage to transportation to final disposal. In high-income countries, the collected waste is typically of low density as it contains more recyclables like cans, glasses etc. Here considerable benefit is derived through the use of compaction vehicles on collection routes, where a 75% reduction of volume is achieved. However, in low-income countries initial compaction are not favourable due to a high initial density of waste. Consequently, compaction vehicles offer little or no advantage and are not cost-effective. Also significant changes in density occur spontaneously as the waste moves from source to disposal, due to scavenging, handling, wetting and drying by weather etc.,

 

2.1.3 Particle size and distribution:

 

Since recovery of waste materials is a key element in solid waste management it is important to have knowledge on the size and distribution of the waste constituents. This knowledge is useful in the utilization of mechanical separators and shredders for waste stream processing. However, the particle size and shape of MSW are challenging to measure due to reasons such as their complex shape, difficulty in the movement of MSW particles along the sieve surface and variation in their area depending on the forces acting on the it (von Blottnitz et. al, 2002).

 

The major means of controlling particle size is through shredding. Shredding increases homogeneity, increases the surface area/volume ratio and reduces the potential for preferential liquid flow paths through the waste. Particle size will also influence waste packing densities, and particle size reduction (by shredding) could increase biogas production through the increased surface area available to degradation by bacteria. However, the flip side is, the smaller particles allow higher packing density which decrease water movement, bacterial movement and the bacterial access to substrate. Therefore, it is important for the particle size to be in line with the treatment method to be adopted.

 

2.1.4 Field capacity

 

The field capacity of solid waste is the amount of moisture that can be retained in a waste sample subject to the downward pull of gravity. The field capacity of waste materials is of critical importance in determining the formation of leachate in landfills. Water in excess of the field capacity will be released as leachate. The field capacity varies with the degree of applied pressure and the state of decomposition of the waste. The field capacity of uncompacted commingled wastes from residential and commercial sources is in the range of 50 to 60 percent.

 

2.1.5    Permeability

 

Permeability is defined as the hydraulic conductivity of compacted waste. It is an important physical property and it governs movement of liquid and gases in landfill. It depends on pore size, surface area and pore size distribution. Permeability is inversely related to density, implying that denser refuse is less permeable. The reported range of permeability of refuse is 10-1 to 10-5 cm/sec.

 

2.2 Chemical characteristic of MSW

 

Chemical composition of solid wastes is important while evaluating alternative processing and recovery options. Especially while looking at waste to energy processes where waste is used as fuel it is important to have knowledge of proximate and ultimate analysis of the substrate.

 

2.2.1     Proximate analysis:

 

A typical proximate analysis includes moisture, ash, volatile matter, and fixed carbon contents.

 

(a) Moisture: The moisture content of the sample is determined by drying 1 gram of sample at 1050 C for one hour. Weight loss is expressed in % of initial weight of sample as

 

Moisture (%) = (weight loss/weight of sample) ×100

 

(b) Volatile matter: Volatile matter is the weight loss obtained on heating 1 gm sample of substance at 950oC for 7 minutes in the absence of air

 

Weight loss due to VM = Total weight loss – moisture

 

Volatile Matter (%) = (weight loss due to VM/ weight of sample) ×100.

 

(c) Ash: Ash is the weight of the residue obtained after complete combustion of one gram of the substance at 700-750 o C.

 

Ash (%) = (weight of residue/weight of sample) × 100

 

(d)   Fixed carbon: Fixed carbon is the material, other than ash, that does not vaporize when heated in the absence of air. It is usually determined by subtracting the sum of the first three values (i.e) moisture, ash, and volatile matter in weight percent from 100 percent.

 

Fixed carbon (W% wet basis) =100− (%M+%Ash + %VM)

 

Table 1 summarizes the proximate analysis of typical municipal solid waste generated in India. As mentioned earlier the moisture content of municipal solid waste is in the range of 15-40%. Likewise, the volatile matter and fixed carbon is 40-60% and 5-12%, respectively.

 

Table 1 Proximate analysis of municipal solid waste

 

Source: http://nptel.ac.in/courses/120108005/4

 

2.2.2    Fusing point of ash:

 

The fusing point ash is defined as that temperature at which the ash resulting from the burning of waste will form a solid (clinker) by fusion and agglomeration. Typical fusing temperature for the formation of clinker from solid waste range from 1100 to 1200oC.

 

2.2.3    Ultimate Analysis of Solid Waste Components:

 

The ultimate analysis of a waste component typically involves the determination of the percent C (carbon), H (hydrogen), O (oxygen), N (nitrogen), S (sulphur), and ash. Due to the concern over the emission of chlorinated compounds during combustion, the determination of halogens is often included in an ultimate analysis. The results of the ultimate analysis are used to characterise the chemical composition of the organic matter in MSW. They are also used to define the proper mix of waste materials to achieve suitable C/N ratios for biological conversion processes. Analysis for solid waste for carbon, hydrogen, nitrogen and sulphur can be done using CHNS analyser.

 

2.2.4    Heat content:

 

Calorific value is the amount of heat generated from combustion of a unit weight of a substance, expressed as kcal/kg. The calorific value is determined experimentally using Bomb calorimeter in which the heat generated at a constant temperature of 250 C from combustion of a dry sample is measured. Since the test temperature is below the boiling point of water, the combustion water remains in the liquid state. However, during combustion gases remain above 1000 C so that the water resulting from combustion is in the vapour state.

 

Table 2 Heating value of municipal solid waste

Source: http://nptel.ac.in/courses/120108005/4

 

2.2.5     Essential Nutrients and Other Elements:

 

Where the organic fraction of MSW is to be used as feedstock for the production of biological conversion products such as compost, methane, and ethanol, information on the essential nutrients and elements in the waste materials is of importance with respect to the microbial nutrient balance and in assessing what final uses can be made of the materials remaining after biological conversion.

 

2.3   Biological characteristic of MSW

 

The most important biological characteristic of the organic fraction of MSW is that almost all of the organic components can be converted biologically to gases and relatively inert organic and inorganic solids. The production of odours and the generation of flies are also related to the putrescible nature of the organic materials found in MSW (e.g., food wastes). The organic fraction of MSW (excluding plastic, rubber and leather) includes water-soluble constituents (such as sugars, starches, amino acids, and various organic acids), hemicellulose, cellulose, Fats, oils, and waxes, lignin, lignocellulose and proteins. While evaluating incineration as a means of disposal or energy recovery, it is important to keep in view the following facts:

 

Organic material yields energy only when dry;

 

The moisture contained as free water in the waste reduces the dry organic material per kilogram of waste and requires a significant amount of energy for evaporation; and

 

The ash content of the waste reduces the proportion of dry organic material per kilogram of waste. It also retains some heat when removed from the furnace.

 

4 Summary

 

To summarize, in this chapter we discussed about the waste characteristics

  • Physical characteristics
  • Chemical characteristics and
  • Biological characteristics

 

you can view video on Characterization of waste

Reference:

 

  1. UNU (2014), https://i.unu.edu/media/unu.edu/news/52624/UNU-1stGlobal-E-Waste-Monitor-2014-small.pdf
  2. United Nations Environment Programme (2002), Division of Technology, Industry and Economics, State of Waste Management in South East Asia, Types of Wastes – Sources and Composition
  3. Annepu, Ranjith Kharvel (2012), Sustainable solid waste management in India, Columbia University, New York
  4. Maria Gaviota Velasco Perez Alonso, Nickolas Themelis (2011), Generation and Disposition of Municipal Solid Waste in Mexico and Potential for Improving Waste Management in Toluca Municipality. Waste-to-Energy Research and Technology Council (WTERT)
  5. Kumar, Sunil (2010), Effective Waste Management in India, INTECH, CROATIA
  6. Von Blottnitz, H., Pehlken, A., Pretz, Th., “The description of solid wastes by particle mass instead of particle size distributions,” Resources, Consevation and Recycling 34(2002) pp.193-207.
  7. United Nations Environmental Program (2013). “Guidelines for National Waste Management Strategies Moving from Challenges to Opportunities”
  8. http://www.indiawaterportal.org/sites/indiawaterportal.org/files/Manual%20on%20munici pal%20solid%20waste%20management_%20MoUD_GOI_2000.pdf