Energy From Solid And Liquid Wastes - Iii

Lecture No: 3 Composition of solid and liquid wastes 3.1.Composition of solid wastes It is not possible to give generally valid values for the composition of overall solid wastes both because data in many cases are not available and because waste composition as mentioned in the previous section varies strongly with level of industrialization, type of society and region of the world. For some of the individual types of waste such as residential wastes some data and available and here it is possible to give some indication of the composition of the materials.

3. 2. Composition of residential solid wastes. The solid wastes generated in residential homes are very often a mixture of several different materials especially if source separation is not implemented. And the waste is therefore often very complex array of materials. Figure 3.1 gives an example of general municipal (mainly residential areas) solid waste components in Denmark. The biodegradable fractions are food waste, garden waste, paper, diapers, cardboard, and newsprint, accounting for almost two third of the combined residential waste stream ( as wet weight). The main reason why especially the food waste account for such a large fraction (34%) of the total amount of wastes generated is its relatively high density and water content. Often biodegradable wastes have higher water contents and bulk densities than the inorganic fractions. This is discussed further in the following chapters. The fractions of the wastes that are considered suitable for treatment and recycling vary with region, tradition, legislation, material types, etc. For instance many of the industrialized countries have already or are currently in the process of developing advanced formal programs for recycling of metal, glass, plastic, paper and cardboard. Food and garden waste are often treated biologically. In Denmark for instance only the organic materials in the food and garden waste fractions are considered for biological treatment whereas only little paper and cardboard enters the biological treatment facilities as these materials are recycled directly. It is estimated that a grand total of 40% (wet weight) of the total mass of residential solid waste generated in Denmark is suitable for biological treatment such as composting or biogas production. This quantity accounts for about 50% of the total amount of residential biodegradable wastes generated.

Fig. 3.1. Composition of residential municipal solid waste in Denmark The composition of solid wastes generated in residential areas and industries is strongly affected by the level of development of the society. The general trend in municipal waste composition across countries with different levels of development is that the percentage of food waste is much higher in developing countries whereas the percentages of paper and yard wastes are highest in countries with a high level of development. Table 3.1 gives the average composition of general municipal residential solid waste for different levels of development and personal income. Some of the reasons for these differences are that in developing countries larger amounts of virgin foodstuffs are used at home in food preparation generating larger amounts of waste whereas in developed countries more preprocessed food is used reducing the amount of food waste generated in the residential areas but at the same time increasing the amount of wrapping materials that needs to be disposed of. In developing countries larger amounts of organic materials from gardens and parks etc. are disposed of on site by for instance incineration or composting, and therefore the contents of these wastes in the general waste stream are lower than in developed countries where they are often handled by the public waste management system. Table 3.1. Percentage composition (wet weight) of municipal residential solid waste as related to regional income( source: Tchobanoglous et al. 1993). Component Food waste Paper/cardboard Plastics Yard waste Other organic Inorganic Sum biodegr

Low- income

Middle- income

High-income

countries 40-85 1-10 1-5 1-5 2-10 1-55 45-25

countries 20-65 8-30 2-6 1-10 2-15 1-45 30-95

countries 6-30 25-60 2-8 10-20 4-15 7-35 45-90

The sum of the fractions of biodegradable materials (on a wet weight basis of food, paper and yard wastes) or the inorganic materials in the general waste stream, however, does not show any significant trends with level of income and development of the society. The last row of Table 3.1 gives the sum of biodegradable wastes as a fractional value for the three levels of income. On average approximately tow thirds of the general waste stream consists of biodegradable materials (compare the data in the last row of Table 3.2 with the Danish data in Fig 3.1). Table 3.2. Factors affecting solid waste generation rates -------------------------------------------Factor -------------------------------------------Long terms trends Seasonal changes Weekly and daily variations Source type Family size Collection practice Infrastructure Population density Economy Statistical properties -----------------------------------------That the fraction of biodegradable wastes is independent on income and development does not necessarily mean that the total quantities of biodegradable wastes generated (for instance measured in terms of tons of biodegradable wastes generated per capita) are the same across different countries of regions of the world since the waste generation rates can be highly different and strongly dependent upon several factors related to culture, tradition, society, etc. Some of the most important of these factors influencing waste generation rates and waste types generated will be discussed in more detail in subsequent sections of this chapter. 3.3. Solid wastes from industry, wastewater and drinking water treatment. Solid wastes from the industry and from water and wastewater treatment plants are, unlike solid wastes produced at residential homes, often very homogeneous. For a given industry

or treatment plant the waste consists of one or at most a few different components. Often the materials from especially industrial production processes are of very high purity and are therefore highly suitable for recycling and therefore usually relatively easy to market with economical gain. An example is food production wastes where significant amounts of the wastes are recycled directly for instance as pet food. Large amounts of solid wastes generated as byproducts in industrial production therefore do not enter the general waste stream but are sold directly to manufacturers that use them for further processing and production. The fraction of industrial waste entering the general waste stream may therefore be as little as 0-10% of the total amount of industrial solid wastes generated. Sludge from water and wastewater treatment sometimes poses more of a problem because they are often contaminated with heavy metals or toxic organic compounds making them more difficult to recycle. Their homogeneity, however, still make them very suitable for biological treatment such as for instance biogas production by anaerobic digestion. 3.4. Quantities of solid wastes The quantity of solid wastes materials (in terms of kg materials per capita per year) entering the general waste stream (that is excluding directly recycled materials) is strongly dependent upon the level of development of the society in question throughout the world. In general developing countries have much lower waste generation rates per capita than have developed countries. Figure 3.2 shows total residential waste generation rates per capita in 1992 and 2000 for a range of urban areas in different countries across the world representing a wide range in the level of development and industrialization. It can be seen from Fig.3.2 that the total waste generation rates per capita in high-income countries such as France, Australia, and South Korea are three to four times higher than solid waste generation rates observed in developing countries such as India and the Philippines. It is also seen that the waste generation rates are in general increasing all over the globe.

Fig. 3.2. Bulk generation of solid waste materials in 1992 (white columns) and 2000 (gray columns) for a range of countries with different levels of industrial development and standard of living

Some of the primary reasons for the large differences in the observed rates of waste generation are that in low-income areas less recyclable materials are let go to waste and

greater amounts materials are recycled directly. In poor areas for instance residential food waste are often used for feeding pigs and chicken kept on the premises. Another reason is that in low-income areas the waste collection system is often not fully developed or nonexistent due to poor infra structure such as lack of roads, poor technology and lack of economic funds. Therefore less waste materials are collected and accounted for, these materials are instead disposed of by other means for instance by recycling, burning, home composting, or illegal dumping. This also means that the potential amount of solid waste materials generated in developing countries can be significantly higher than what is shown in Fig.3.2. The implementation of a better waste collection system will therefore result in increased waste generation rates. In developed countries with a well-structured waste collection system the observed waste generation rates more closely resembles the true amounts of materials generated. 3.5. Wastewater Constituents The physical, chemical and biological constituents found in wastewater and the constituents of concern in wastewater are introduced briefly in the following discussion. 3.5.1. Constituents Found in Wastewater Wastewater is characterized in terms of its physical, chemical, and biological composition. The principal physical properties and the chemical and biological constituents of wastewater, and their sources, are reported in Table 3.3. It should be noted that many of the physical properties and chemical and biological characteristics listed in Table 3.1 are interrelated. For example, temperature, a physical property, affects both the amounts of gases dissolved in the wastewater and the biological activity in the wastewater. 3.5.2.Constituents of Concern in Wastewater Treatment The important constituents of concern in wastewater treatment are listed in Table 3.4. Secondary treatment standards for wastewater are concerned with the removal of biodegradable organics, total suspended solids, and pathogens. Many of the more stringent standards that have been developed recently deal with the removal of nutrients, heavy metals, and priority pollutants. When wastewater is to be reused, standards normally include additional requirements for the removal of refractory organics, heavy metals and in some cases, dissolved inorganic solids. 3.6. Sampling And Analytical Procedures Proper sampling and analytical techniques are of fundamental importance in the characterization of wastewater. Sampling techniques, the methods of analysis, the units of measurement for chemical constituents, and some useful concepts from chemistry are considered below.7 3.6.1. Sampling

Sampling programs are undertaken for a variety if reasons such as to obtain (1) routine operating data on overall plant performance, (2) data that can be used to document the performance of a given treatment operation or process, (3) data that can be used to implement proposed new programs, and (4) data needed for reporting regulatory compliance. To meet the goals of the sampling program, the data collected must be: 1. Representative.

The data must represent the wastewater or environment being

sampled. 2. Reproducible. The data obtained must be reproducible by others following the same sampling and analytical protocols. 3. Defensible. Documentation must be available to validate the sampling procedures. The data must have a known degree of accuracy and precision 4. Useful. The data can be used to meet the objectives of the monitoring plan (Pepper et al., 1996.) Because the data from the analysis of the samples will ultimately serve as a basis for implementing wastewater management facilities and programs, the techniques used in a wastewater sampling program must be such that representative samples are obtained. Table 3.3. Common analyses used to assess the constituents found in wastewatera Testb

Abbreviation/

Use or significance of test results

definition Physical characteristics Total solids Total volatile solids Total fixed solids Total suspended solids Volatile suspended solids Fixed suspended solids Total dissolved solids Volatile dissolved solids Total fixed dissolved solids

TS TVS TFS TSS VSS FSS To assess the reuse potential of a waste TDS (TS – TSS) water and to determine the most suitable VDS type of operations and Processes for its FDS treatment To determine those solids that will settle by

Settleable solids Particle size distribution Turbidity

gravity in a specified time period To assess the performance of treatment

PSD NTU

c

processes Used to assess the quality of treated wastewater

Color

Light

brown, To assess the condition of wastewater (fresh

Transmittance

gray, black %T

or septic) Used to assess the suitability of treated

Odor Temperature

TONd 0 C or 0F

effluent for UV disinfection To determine if odors will be a problem Important in the design and operation of biological processes in treatment facilities

Density Conductivity

ρ EC

Used to assess the suitability effluent for agricultural applications

Inorganic chemical characteristics Free ammonia NH+4 Organic nitrogen Org N TKN(OrgN+ NH+4) Total kjeldahl nitrogen Nitrites NO-2 Nitrates NO-3 Total nitrogen TN Inorganic phosphorus Inorg P Total phosphorus TP Organic phosphorus Org P

Used as a measure of nutrients present and the

degree

of decomposition

in

the

wastewater; the oxidized forms can be taken as a measure of the degree of oxidation

Inorganic chemical characteristics pH

pH= -log[H+]

A measure of the acidity or basicity of an

Alkalinity

aqueous solution HCO3- + CO3-2 + A measure of the buffering capacity of the

Chloride

OH- - H+ Cl

wastewater To assess the suitability of wastewater for

Sulfate

SO-24

agricultural reuse To assess the potential for the formation of odors and may impact the treatability of the

waste sludge As, Cd, Ca, Cr, Co, To assess the suitability of the wastewater

Metals

Cu, Pb, Mg, Hg, for reuse and for toxicity effects in Mo, Ni, Se, Na, Zn Specific

inorganic

elements

and

compounds Various gases

treatment. Trace amounts of metals are important in biological treatment. To assess presence or absence of a specific constituent

O2, CO2, NH3, H2S, To assess presence or absence of a specific CH4

gases

Organic chemical characteristics Five-day carbonaceous CBOD5 Biochemical oxygen Demand Ultimate carbonaceous UBOD

A measure of the amount of oxygen required to stabilize a waste biologically (also A measure of the amount of oxygen

Biochemical

oxygen BODu,BODL)

required to stabilize a waste biologically

Demand Nitrogenous

oxygen NOD

A measure of the amount of oxygen

Demand

required to oxidize biologically the nitrogen

Chemical

in the wastewater to nitrate Often used as a substitute for the BOD test

oxygen COD

demand Total organic carbon TOC Specific organic MBASe, CTASf

Often used as a substitute for the BOD test To determine presence of specific organic

compounds and classes

compounds and to assess whether special

of compounds Biological characteristics Coliform organisms MPN(most

design measures will be needed for removal To assess presence of pathogenic bacteria

probable number) and effectiveness of disinfection process Specific microorganisms Bacteria, protozoa, To assess presence of specific organisms in

Toxicity

helminthes, Viruses

connection with plant operation and for

TUa and TUc

reuse Toxic unit acute, toxic unit chronic

a

Adapted, in part, from Crites and Tchobanoglous (1998).

b

Details on the various test may be found in the Standard Methods (1998)

c

NTU = nephelometric turbidity unit

d

TON = threshold odor number

e

MBAS= methylene blue active substances

i

CTAS = cobalt thiocyanate active substances

Table 3.4 Principal constituents of concern in wastewater treatment Constituent Suspended solids

Biodegradable Organics

Reason for importance Suspended solids can lead to the development of sludge deposits and anaerobic conditions when untreated wastewater is discharged in the aquatic environment Composed principally of proteins, carbohydrates, and fats, biodegradable organics are measured most commonly in

Pathogens Nutrients

Priority pollutants

Refractory organics

Heavy metals

Dissolved inorganics

terms of BOD (biochemical oxygen demand). If discharged untreated to the environment, their biological stabilization can lead to the depletion of natural oxygen resources and to the development of septic conditions Communicable diseases can be transmitted by the pathogenic organisms that may be present in wastewater. Both nitrogen and phosphorus, along with Carbon, are essential nutrients for growth when discharged to the aquatic environment; these nutrients can lead to the growth of undesirable aquatic life. When discharged in excessive amounts on land, they can also lead to the pollution of groundwater Organic and inorganic compounds selected on the basis of their known or suspected carcinogenicity, mutagenicity teratogenicity, or high acute toxicity. Many of these compounds are found in waste water These organics tend to resist conventional methods of wastewater treatment. Typical examples include surfactants, phenols, and agricultural pesticides Heavy metals are usually added to wastewater from commercial and industrial industrial activities and may have to be removed if the wastewater is to be reused Inorganic constituents such as calcium, Sodium, and sulfate are added to the original domestic water supply as a result of water use and may have to be removed if the wastewater is to be reused.

There are no universal procedures for sampling; sampling programs must be tailored individually to fit each situation (see Fig. 3.3). Special procedures are necessary to handle sampling problems that arise when wastes vary considerably in composition.

Fig 3.3. Collection of samples for analysis: (a) collection of an effluent sample from a pilot plant treatment unit and (b) view of an uncapped monitoring well equipped with sampling outlets for four different well depths Before a sampling program is undertaken, a detailed sampling protocol must be developed along with a quality assurance project plan (QAPP) (known previously as quality assurance/quality control, QA/AC). As a minimum, the following items must be specified in the QAPP (Pepper et al., 1996). Additional details on the subject of sampling may be found in Standard Methods (1998). 1. Sampling plan. Number of sampling locations, number (see homework problem 2-5 ) and type of samples, time intervals (e.g., real-time and/or time- delayed samples). 2. Sample types and size. Catch or grab samples, composite samples, or integrated samples, separate samples for different analyses (e.g.., for metals). Sample size (i.e..,volume) required. 3. Sample labeling and chain of custody. Sample labels, sample seals, field log book, chain of custody record, sample analysis request sheets, sample delivery to the laboratory, receipt and logging of sample, and assignment of sample for analysis. 4. Sampling methods.specific techniques and equipments to be used eg manual, automatic, or sorbent sampling}. 5. Sampling storage and preservation.Type of containers (e,g, glass or plastic}preservation methods, maximum allowable holding times. 6. Sample constituents. A list of the parameters to be measured. 7. Analytical methods.A list of the field and laboratory test methods and procedures to be used, and the detection limits for the individual methods. If the physical, chemical and / or biological integrity of the samples is not maintained during interim periods sample collection and

sample

analysis, a carefully performed

sampling program will become worthless. Considerable research on the problem of sample preservation has failed to perfect a universal treatment method, or to formulate a set of fixed rules applicable to samples of all types. Prompt analysis is

undoubtedly the most positive

assurance against error due to sample deterioration. When analytical and testing conditions dictate a lag between collection and analysis, such as when a 24- h composite sample is collected, provisions must be made for preserving samples. Current methods of sample preservation for the analysis of properties subject to deterioration must be used.( Standard methods, 1998) Probable errors due to deterioration of the sample should be noted in reporting analytical data.

Lecture No: 4 Properties- factors affecting - quantity and composition of solid wastes

4.1. Factors affecting the quantity and composition of solid wastes The quantity and composition of the stream of solid waste material at a certain location depends upon several factors and both quantity and composition of the solid wastes often varies considerably over time. Understanding the variations in quantity and composition of the materials is important when designing waste management systems and deciding on the optimal type of treatment and final disposition of the materials. Some of the factors that control the quantity and composition of wastes are listed in Table 3.2 The rate of generation of solid wastes materials often shows a general trend towards increasing quantities with time regardless of the level of development of the area. As discussed earlier this trend coincides with increases in development level and standard of living in the region as also mentioned earlier the relative fraction of food waste decreases and the relative fraction of yard waste increases with development whereas their fractional sum is roughly constant (Table 3.1). As most areas in the world tend toward higher levels of development and standard of living, increased rates of wastes generation should be excepted in the future. Locally, however, there may be activities such as new industries or closing of existing factories to changes in the overall population composition that can mask out these general trends. The composition and quantity of wastes often follows a seasonal cycle. An example is food and garden waste in non-tropical areas that are generated in larger quantities during the summer and early fall seasons. In the summer cooking usually involves the use of larger amounts of fresh vegetables, as these are readily available during this season. Cooking with fresh vegetables usually means production of larger quantities of food waste and, depending on the type of society, infra structure, etc. more or less of these food waste will end up in the general waste stream. Spring summer and fall season are also the seasons where many people are active in gardens and parks and therefore, larger quantities of yard and park waste are produced during these seasons. Figure 4.1 shows example variation in total waste quantities and composition over a one-year period at two sites in the United States.

Fig 4.1. Left: Annual variation in total waste quantities and composition over a one-year period at two sites in the United States, Right: Composition of solid wastes received at a New York Landfill in the 1940’s

The figure shows that total waste quantities can be more than 40% higher in the summer and that the fraction of food waste may vary from less than 5% to more than 45% of the total amount of wastes generated (wet weight). Again these variations depend upon the type of society, level of development and standard of living. The actual seasonal variation in waste materials composition and quantity generated at a specific location depends strongly on the human activities taking place in that location, the climate, the infrastructure, the culture etc. It is therefore very difficult to generalize the trends in the rates of generation of solid wastes from one location to another unless the locations are very similar with respect to these influencing factors. In most cases it is usually necessary to conduct separate investigations at the location in order to adequately determine the seasonal effects on the composition and quantity of waste materials generated. The generation of solid wastes especially those generated in residential areas in residential areas in many cases also follows weekly and daily cycles. These cycles are usually caused by recurring patterns in the behavior of people living in the area such as cooking and cleaning. Often the largest amounts of wastes are generated during preparations of the main meals and during the periods where people are off work. An example is illustrated in Fig. 4.2 that shows the daily and weekly variations in total waste generation in a US high-rise apartment complex. It is seen that the largest amounts of wastes are generated during the evening hours and on weekends (in terms of mass) when most people are at home and have time for cooking and cleaning.

Daily and weekly variations likely have the greatest importance for internal (within the apartment complex or building) waste collection and management, but may or may not have importance on general waste collection and management depending on when and how often waste are collected (usually 0.5-2 times a week depending on climate). Variations in especially quantity and also in composition of waste materials with population density and family size can also in many cases be quite pronounced. Low waste production rates per capita are often seen in densely populated areas whereas waste generation is often significantly higher in rural areas. Waste generation rates per capita are also typically higher in household with few persons compared to households with many persons.

Fig. 4.2 Typical waste discharge rates in apartment complexes with waste chutes. Top: daily variation as a function of the time of the day, Bottom: weekly variations as a function of the day of the week.

Table 4.1 shows weekly waste generation rates per capita for urban, semi-urban, and rural areas in Denmark as a function of the number of persons living in each household. Table 4.1. Total solid waste generation rates per capita in urban, semi-urban and rural areas in Denmark as a function of the number of persons living together in one household. Adopted from Christensen et al. (1998). ---------------------------------------------------------------------------------------------------Household size (persons)

Kg/(person week)

---------------------------------------------------------------------------------------------------Urban

Semi-urban

Rural

1

6.6

7.1

7.8

2

4.5

5.0

5.3

3

3.4

3.8

4.5

4

2.6

3.3

3.6

--------------------------------------------------------------------------------------------------Waste generation rates per capita are significantly lower in household with more persons per household. Households with 4 persons generate approximately 50-60% less waste per person than does households with only one person. Also households in urban areas generate 15-30% less waste per inhabitant than does households in rural areas. The reason for the lower waste generation rates in urban areas is likely that recycling systems are better developed here and therefore less recyclable materials will enter the general waste stream.

This is especially true for general solid waste but may be less so for biodegradable wastes, as these require further treatment and therefore in general have to enter the waste management system in order to be treated and recycled properly. Also if more persons are living in the same household less waste will be generated per person because the amount of materials consumed as a whole in the household usually is not directly proportional to the number of persons living there. Again here the biodegradable wastes may show a less pronounced trend as a person’s food consumption likely is independent of how many others he or she is living together with. There may, however be some effects of being able to buy foods in larger quantities or that households with many persons often include small children that eat less thus reducing the amount of food waste generated. However as no data is available for directly relating the amount of biodegradable wastes generated with the size of the household there is a need for investigating this issue further. The quantities of solid waste that enters the waste management system are as mentioned earlier also strongly dependent upon how user-friendly the waste collection system is, i.e., the ease of bringing the materials to the collection points. The easier and simpler it is to use the collection system the larger the fraction of materials will enter the system and can be treated centrally. This means that infra structure and collection practice both have a major influence upon both composition and especially quantity of the solid wastes that will be collected in areas with poor infra structure and infrequent or nonexistent collection service is especially important if source separation programs are to be introduced, it is important that the system is easy to use otherwise only a small fraction of the materials potentially available will be collected and recycled. So if the objective is to promote central treatment and recycling of the materials it is very important to make the collection system as transparent and simple to use as possible. The impact of collection system structure upon the quantity of materials collected is illustrated for source-separated fractions of recyclable glass and paper in Fig. 4.3 the figure shows the quantities of glass and paper collected per capita in the northern part of Copenhagen as a function of the numbers of collection containers per capita. Of course the collected quantities cannot increase indefinitely as more collection containers are made available. The collected quantities of recyclable materials will asymptotically reach the maximum potential generation rate. At that point the collection rates will become independent on the number of collection containers per capita in case the population density and collection container distribution if uniform within the collection area the collection rate will increase linearly with the number of collection containers for small numbers of

containers, and be almost constant with a value approximately equal to the potential material generation rate for a large number of containers. The actual slope of the collection rate curve as it approaches the potential generation rate depends among other factors upon the behavior of the population living in the area. At present the knowledge in the relation between collection system configuration and collected quantity is rather limited.

Fig 4.3. Quantities of glass and paper collected per capita in the northern part of Copenhagen as a function of the numbers of collection containers

4.2. Legislative control of solid waste management and reuse. In addition to the factors discussed above the generation and reuse of solid waste is often regulated by legislation. This is done to promote recycling or to ensure that the wastes are disposed of in an environmentally sound manner. In many European countries for instance there are extensive legislation requiring that recyclable materials are recycled for instance by requiring source separation of these materials. In Denmark for instance legislation requires that recyclable paper, glass, and metal is separated from the general waste stream and recycled. Also onsite disposal is in many countries heavily regulated. In densely populated areas of Europe for instance the incineration of garden and park waste is prohibited due to nuisance generated by the smoke and toxic substances such as dioxin released by the incineration process. Such legislation is often backed up by economic incentives such as fees and fines and can have a major positive influence upon the management of the wastes. As a result of such legislation and economic incentives many central and northern European countries have developed or are in the process of developing plans and systems for treating and recycling the wastes. Too strict legislation, however, can also result in unwanted effects. For instance if excessive fees for waste disposal and treatment are imposed increased illegal dumping of the waste in vacant lots, parking lots, or other public areas may occur with

environmental degradation as a result. Control of waste management by legislation is therefore a matter of finding the right balance between the desired goals and what is practically possible to achieve.

4.3. Characterization of solid wastes. Information about the characteristics that is the quantity; composition, density, etc. of the solid wastes is important when planning the treatment and management of the residuals from international to community level. It is therefore necessary to collect representative data characterizing the materials that enters the waste management system. Often new problems occur within existing waste management systems or the composition of the waste stream changes due to change in the activities in the region. Collection of new and location specific data are therefore in many cases required. The following sections gives a brief overview of some of the general parameters used for characterizing solid waste materials with respect to quantity, composition and physical and chemical properties. Solid waste materials can be generally characterized at four different levels of detail. These levels are defined with respect to type, components(composition), chemical composition, and physical properties. The material type usually relates to the source of the material, examples of types are industrial waste and household waste materials. Components are the specific materials contained in the general waste stream such as paper, food waste, yard waste, etc. These somewhat general components may be broken further down into more specific components. The chemical composition is the content of different chemical elements such as carbon(C), hydrogen(H), oxygen(O), nitrogen(N), heavy metals, etc. Physical properties are characteristics of the waste materials that are important for the handling of the materials. These characteristics include water content, bulk density, energy content, hydraulic conductivity, compressibility, etc.

4.3.1. Data collection Collection of the data necessary for the characterization of the materials in the waste stream is very often a tedious task due to the highly variable and complex nature of the materials. Several different strategies for data collection may need to be employed to gather all the necessary data. Some of the more important strategies that have been used widely in the past are (1) data collection at the source of the waste material, (2) data collection at the central collection, processing and treatment plant input side, (3) data collection at the central

treatment and processing plant output side, and (4) the use of existing databases, e.g., existing literature, the internet, reports, etc. Data collection at the source where the waste materials are generated is usually one of the most time and effort-consuming strategies because a large number of material samples are necessary in order to get a good representative data set. Usually there are a large number of sources within the region of interest and it is therefore necessary to select a representative of sub-set of these sources. It is also necessary to conduct the sampling over a longer period of time to capture temporal variations in quantity and composition of the materials that are generated. The advantages of sampling at the source are that all characteristic parameters can be determined on all components of the material in as much detail as desired because source separation of the materials can easily be conducted. Some of the disadvantages are that in addition to the large over a long enough period of time to mask out seasonal variations in composition of the biodegradable wastes. This means that this type of data collection strategy can become and usually is very costly and such investigations are therefore not often used in larger areas. Data collection at the input side o the central treatment plant(s) is significantly much less time consuming and costly and it is usually possible to provide adequate data concerning the quantity of the total stream of wastes entering the facility. Data collection is, usually conducted by random selection of incoming car or truckloads of the waste or at least parts of these loads followed by subsequent analysis of the specific contents. Again to mask out the influence of seasonal variations in material composition, the sampling must be conducted several times over a long enough period of time to improve data quality and reliability. The advantage of this method is that a relatively high level of characterization of the materials can be achieved with a minimum of cost and effort. The disadvantage is that the different materials usually are mixed together making it difficult to fully assess the purity and physical properties of single individual components of the materials, which is possible when sampling is conducted at the source. Often certain materials or chemical compounds of interest are only found in very low quantities or concentrations in the incoming material stream such as for instance certain heavy metals or toxic or hazardous organic compounds. In such cases it is often easier To conduct sampling and data collection on the output side of the treatment facility. Here the material stream containing the compound of interest is usually smaller and more homogeneous than on the input side such as for instance after incineration. This means that smaller sample volumes taken at the output can be used to represent larger volumes of input

materials. The sampling method is most applicable at for instance incineration plants where the gases(smoke) or the solid residuals (slag) produced by combustion can more easily be sampled and analyzed because of their homogeneity. This type of data collection method is less than that of incineration plants. The disadvantage of sampling on the output side of the treatment facility is that the range of compounds that can be analyzed for often is restricted to the basic elements or relatively simple chemical compounds whereas material fractions of the incoming wastes such as paper, plastics, etc. cannot be measured as they are often destroyed by the treatment process. The easiest and also the most inexpensive approach is to use existing databases, for instance books, articles, reports, the internet, etc. it is important when using this approach that the data are screened in a very critical manner with respect to the quality of the data. This is necessary as the data can be biased because they were collected in a different region or country and at a different time and, thus, may not be representative for the region under consideration. The data can also be erroneous due to poor sampling and data interpretation and it is therefore important to regard data that are borrowed from other sources as approximate values that should be used with caution. The best approach is always to conduct measurements in the region of interest.

4.3.2. Types of solid wastes. The bulk stream of solid waste can as discussed earlier be divided into a number of types depending on the source of generation. Some of the types usually considered are residential wastes, materials from small business such as restaurants public kitchens, markets and stores, industrial wastes, and materials from parks and garden etc. Depending on the number and diversity of waste material sources and the selected definition of the different material types the material stream can consist of few or many types. Figure 4.4 shows the composition of the total potential amounts of biodegradable wastes generated in Denmark as distributed between four different very general material types. For comparison the biodegradable wastes generated by the agriculture (primarily animal wastes) are also included. Each of the five types shown in Fig.4.4 may easily be broken further down into more sub-types. For instance the industrial biodegradable waste can be distributed between the dairy industry, the sugar industry, the margarine and oil industry, the potato industry, slaughterhouses, etc. the number of types in which to divide the main stream of wastes depends on several factors. Some of the main reasons for dividing into material types are that the characteristics of the materials, i.e., the composition and physical properties very often depend upon their source of origin. Because the optimal method of treatment of the

wastes and the intended subsequent use of the treated materials in turn depends on the physical and chemical properties of the materials themselves the optimal selection of treatment method and use of the treated wastes are therefore source dependent.

Fig 4.4. Types of biodegradable waste produced in Denmark. Percentages indicate relative quantity based on the wet weight of the materials

4.3.3. Material components in the waste stream. The general stream of solid wastes generated within an urban region normally contains many different types of materials as indicated in the previous section. Each of these types of wastes consists of one or more specific component materials also called material fractions. These components are materials with different distinctive physical and chemical properties and possible subsequent uses. Examples of such fractions in biodegradable residential waste are paper, vegetable food waste, food waste of animal origin, etc. the fractions or components are usually considered based upon their physical and chemical material properties and especially their recyclability, and the optimal method of further treatment. Knowledge of the material components of a stream of solid wastes is therefore important for instance if source separation programs with the intension of increasing reuse and recycling are to be implemented. For instance if there is a proposal for implementing a program in which paper is to be recycled it will be necessary to determine the quantities of the paper component in the main stream of materials to evaluate the feasibility of implementing the recycling program. In Fig.4.5 the biodegradable wastes produced in residential areas in Denmark presented in Fig 1.8(3%) are broken further down into a series of different material components depending on their physical properties and recyclability. It is stressed that the

material components shown in Fig 4.5 do not at all constitute all possible components that may be considered given the material types in Fig. 1.8 but it is merely an example of an approach that is used in practice. The number of fractions to be considered in an actual case will of course depend upon the actual material types present and the available methods or recycling or treatment in the region of interest.

Fig. 4.5. Fractions of residential biodegradable wastes produced in Denmark 4.3.4. Chemical composition of solid wastes. The highest level of detail with respect to the composition of solid wastes is the chemical composition. To characterize the materials at this level of detail involves the determination of the composition in terms of the quantities of the different elements that make up the materials. The elements usually considered in such characterization are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), chlorine (Cl) and in certain cases also a range of heavy metals especially in the case of hazardous materials or materials under suspicion of being contaminated with heavy metals. These elements are also the most important with respect to thermal or biological treatment processes (these will be discussed in later chapters of this compendium). Table 1.5 shows some typical chemical compositions for a range of wastes often contained in the waste stream or encountered at central waste treatment facilities. Table 4.2. Chemical composition of different materials. Percentages indicate relative quantity as related to the dry weight. Source US EPA (1997). -------------------------------------------------------------------------------Component

% C

H

O

N

S

Cl

--------------------------------------------------------------------------------Food waste

44.8

6.5

32.3

2.8

0.3

1.0

Garden waste

42.4

5.3

31.8

1.6

0.4

0.2

Newsprint

48.8

6.3

42.4

0.1

0.5

0.1

Magazines

39.2

5.5

39.2

0.1

0.2

0.1

Wood

49.0

6.0

41.2

0.2

0.1

0.1

Paper

42.1

5.8

38.8

0.4

0.3

0.8

Rubber

47.9

6.0

12.9

1.4

1.3

5.6

Textiles

49.6

6.7

36.1

4.1

0.4

0.4

Plastics

66.4

9.2

9.5

1.1

2.5

0.4

Cardboard

46.0

6.4

44.3

0.1

0.3

0.1

Mixed waste

35.7

4.8

26.8

0.6

1.0

0.6

------------------------------------------------------------------------------4.3.5. Physical properties of solid wastes. In addition to their types, components and chemical composition, the solid wastes are also characterized by their physical properties. Knowledge of these properties is important as they determine the possible method of the handling (transport and storage) and treatment of the wastes. In the following sections some of the most important physical properties of solid waste materials are discussed. Some of the most important properties are listed in Table 4.3. Because the physical properties determine how the materials are handled and treated it is therefore often necessary to measure the physical properties before making decisions with respect to the selection of specific treatment methods.

Table 1.6 Important physical properties of solid waste -------------------------------------------------------Parameter

Unit

-------------------------------------------------------Water content

cm3/cm3

Air content

cm3/cm3

Porosity

cm3/cm3

Bulk density

g/cm3

Solid content

% weight

Ash content

% weight

Compaction ratio

%

Particle size distribution

Cm

Hydraulic conductivity

m/d

Field capacity

cm3/cm3

Energy content

MJ/ton

------------------------------------------------------The physical properties are normally determined on a sample of wastes (mixture or separated material) of known volume (Vtotal). The total volume of waste (Vtotal) in the sample is the sum of the volume of solids (Vsolids ) and volume of pores or voids (Vvoids) as illustrated in Fig.4.6.

Fig 4.6. Schematic of the components of organic matter with respect to water, air and solids

When dealing with solid waste materials under normal circumstances the voids will normally be filled with either water or air and, thus, the volume of voids is the sum of the volume of (Vair) and the volume of water (Vwater ) contained in the sample. This three-phase concept that is similar to that found in other porous media such as for instance soils is shown in Fig.4.6. The volumetric water content (0, cm3 water/cm3 wastes) is the relative volumetric volume of water, i.e., volume of water per volume of total waste that is contained in the sample of wastes. The volumetric water content is calculated as

θ= V water = Mwater = Mtotal - Msolids Vtotal

Vtotal

-----------

(4.1)

Vtotal

where Vwater = total volume of water (cm3), Vtotal = total volume of wet waste in the sample (cm3), Mwater = total mass of water in waste(g), Mtotal = total wet weight (g), and Msolids = total dry weight of residual. The dry weight (Msolids ) is normally measured by drying the sample at 105°C until no more weight loss is found (usually 24 -28 hours). It is noted here that drying a 105°C does not remove all of the water as a small fraction of the water is very strongly bound to the solids and cannot be removed unless the temperature is raised to 600°C at which temperature organic wastes will have been combusted. Normally this fraction of strongly bound water is very small and may for practical purposes be neglected. The water content can also be expressed on a gravimetric (often expressed as mass of water per mass of total) rather than a volumetric basis. The gravimetric water content (w,g, water/g wet waste )is found as w = M water = Mtotal -Msolids Mtotal

-----------

(4.2)

Mtotal

where Mtotal = total mass of the wet sample. The volumetric air content or air-filled porosity (ε, cm3 air/cm wastes) is the relative volumetric amount of air contained in the sample of wastes. The volumetric air content can be found as ε =Vair = 1 – Vwater - Vsolids Vtotal

Vtotal

-----------

(4.3)

The porosity (Ф, cm3void space/cm3 wastes) is the relative volumetric amount of total void space (space not occupied by solids) in the sample of wastes. The porosity is determined as follows Ф = Vsolids = Vwater + Vair = ε+ θ Vtotal

-----------

(4.4)

Vtotal

The porosity can be estimated by measuring the weight loss by drying of a sample of waste with all void spaces completely filled with water(this situation is not as easy to obtain as it sounds). Water content, air content and porosity are especially important when aerobic composting is considered as a treatment method because these parameters often control the rate of the biological processes occurring during composting (more about this later). The parameters are also important during general handling, transport, deposition and compaction in landfills. The water content is important when determining the energy content of organic materials as water consume a significant amount of energy for evaporation during combustion of the organic materials. An alternative method for determining total or air-filled porosity is the pychnometer method. This method uses two air-filled containers(containers 1 and 2) with known volumes (V1 and V2) at different known air pressures (P1 and P2). Container 2 contains the waste sample with volume Vtotal . The two containers are then brought in contact allowing air to flow freely between them equilibrating the pressure difference and yielding and overall final pressure (Pfinal). The air-filled porosity of the sample can then be determined from the initial and final pressures and the known volumes of the two containers using the ideal gas equation. ε= Vtotal –( Pfinal – P1 V1 +V2 (Pfinal) – P2

-----------

(4.5)

______________________ Vtotal The total porosity can then be determined if the water content is known. The bulk density (ρb , g of waste/cm3 waste) of the wastes is normally calculated both on a wet and a dry weight basis. The wet bulk density(ρb wet, g of wet waste/cm3 waste) is given as ρb wet = Mtotal = Mwater + Msolids Vtotal

Vtotal

-----------

(4.6)

Typical wet bulk densities for municipal mixed waste is approximately 200 kg/m3 for loose waste dumped in to waste bins, 500 kg/m3 in compactor trucks and about 700 kg/m3 when compacted into landfills.

The dry bulk density (ρb dry, g of dry waste/cm3 wastes ) is determined as ρb dry = Msolids = Mtotal - Mwater= ρb wet – θ Vtotal

Vtotal

-----------

(4.7)

Bulk density especially wet bulk density is important when designing storage, collection source/central separation and transport equipment as the equipment often is designed to hold certain volumes of waste and therefore also must be able to withstand the weight of the materials. The dry solids density (ρs) (that is the density of the dry matter particles themselves) of most organic materials equals approximately 0.8g/cm3. The solids content or dry matter content (g of dry waste/g wet wastes) is calculated as Dry matter content = Mdry = ρb dry = 1- Mwater = 1 – w----------Mtotal ρb wet

(4.8)

Mtotal

Solids content of biodegradable wastes is often important when designing anaerobic biological treatment (digestion) facilities as the course of the biological process and the design of the equipment often depend upon the solids content of the organic material. Certain biological digestion processes require the solids content to be within very specific limits. The ash content (g o ash/g wastes) is the quantity of inorganic solid matter remaining after combustion of the waste material at 550°C. The ash consists mainly of mineral matter contains in the wastes. Ash contents for most types organic wastes are usually very low compared to the wet weight of the materials because most of the material mass is lost during combustion. The ash content is normally determined based on both wet and dry weight basis. Table 4.4 lists ash content values for a range of wastes. The ash content is important to consider when planning thermal treatment (incineration) facilities, as the ash is the main solid component in terms of both weight and volume that is produced at incineration facilities. If incineration is considered as a means of reducing the volume of the wastes, for instance with the aim of subsequent deposition of the materials at a landfill the ash content is the most important parameter to consider. The ash content can also be used to get a first rough estimate of the carbon content of the wastes (Diaz et al. 1996) as % Carbon = 100 - % Ash

-----------

(4.9)

Laboratory investigations indicate that Eq.(4.9) gives values of carbon content that are within 2-10% of the exact values and it can therefore provide a useful tools in cases where more exact determination methods are not available. Table 4.4. Physical properties of residential biodegradable wastes. Source: Elmlund et al. (1980) Component

Wet bulky density (kg/m3)

Solids content Ash content Energy content %

% of solids

MJ/kg solids

Veg.food

250

25

12

18

Anim. Food

250

50

25

18

Newsprint

150

85

1-25

18

Magzines

150

88

25

16

Diapers etc.

180

50

1

18

Napkins etc.

150

45

1

18

Clean paper

30-70

90

10

17

Dirty paper

75-190

60-90

5

18

Garden

100

55

3

8

The compressibility ratio (P,%) of the wastes for a given applied pressure is determined by applying the desired pressure to a column of uncompressed waste of know height (h1) as illustrated in Fig 4.7. The height (h2) after compaction is measured and the compressibility or compaction ratio can be calculated as

P = h2 = V2

-----------

(4.10)

h1 = V1 Compressibility ratio may be measured as a function of pressure over a range of different pressures as illustrated in Fig 4.8 for two different types of waste (paper and mixed refuse). The compressibility of refuse materials is important in many instances for instance when determining the transport capacity of compactor trucks or when designing landfills where the deposited materials are typically compacted when deposited in the landfill.

Fig 4.7 and Fig 4.8 Fig 4.7 Illustration of the determination of the compressibility ratio (P) fro the solid wastes Fig 4.8. Compressibility ratio (P) as function of pressure for paper (white symbols) and mixed residential refuse (black symbols) The particle size distribution is determined by sieving the wastes through sieves with different decreasing known mesh sizes and subsequently measuring the mass of materials retained in each sieve. The effective particle diameter (Sp, cm) is normally determined as the smallest mesh size through which the material can pass. In special cases for instance when dealing with very large or bulky items such as for instance furniture or tree branches that cannot easily be analyzed by sieving the effective particle diameter can alternatively be calculated using one of the expressions in Eq. (4.11) below. Sp= L,

Sp = L +W 2

Sp = (L W )0.5 ---------

(4.11)

where L = length and W= width of the particles. Typical ranges of Sp values for different components in organic waste are shown in Table 4.5. The particle size distribution is important with respect to for instance mechanical separation of the materials based on sieving that separates the materials based on their size. Size is also important with respect to biological treatment methods where smaller particle sizes usually are required. In these cases it is often necessary to reduce the particle size by means of shredding or other mechanical action in order to increase the rate of biological transformation and degradation of the materials. Knowing the particle size therefore determines if a material is directly suitable for biological treatment or if some size reduction is necessary. Table 4.5. Typical ranges and average values of Sp for different organic material. Source : Tchovanglous et al. (1993)

---------------------------------------------------------------------Component

Range(cm)

Average(cm)

---------------------------------------------------------------------Food

2-18

8

Paper

15-45

30

Cardboard

20-60

45

Yard

2-30

8

Wood

2-25

12

--------------------------------------------------------------------The saturated hydraulic conductivity of the waste (Kw, cm/d) is the capacity of the wastes to conduct water under a given pressure gradient or hydraulic head at fully water saturated conditions, i.e., with the air-filled porosity ε=0. Fully saturated conditions can often be difficult to achieve due to entrapment of air and measured values of saturated hydraulic conductivity will therefore generally be somewhat lower than the true value. Hydraulic conductivity is usually measured by placing the ends of a water saturated sample of waste with cross sectional area A (cm2) length Δx (cm) in contact with two water reservoirs with a height difference of Δh (cm) and then measuring the volumetric water flow Q (cm 3/d) through the sample. The saturated hydraulic conductivity can then be calculated using Darcy’s law as Kw = Q Δh A Δx

(4.12)

Other methods of determination of saturated hydraulic conductivity based on infiltration rate are available, but they are in general more applicable to soils and similar more fine-textured porous media than to the coarser textured refuse materials. The saturated hydraulic conductivity is especially important in connection with outdoor storage of waste or in connection.