Energy From Solid and Liquid Wastes - III

8/14/2019 Energy From Solid and Liquid Wastes - III

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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.


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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 Low- income

countries

Middle- income

countries

High-income

countries

Food waste 40-85 20-65 6-30

Paper/cardboard 1-10 8-30 25-60

Plastics 1-5 2-6 2-8

Yard waste 1-5 1-10 10-20

Other organic 2-10 2-15 4-15

Inorganic 1-55 1-45 7-35

Sum biodegr 45-25 30-95 45-90


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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


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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


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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


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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/

definition

Use or significance of test results

Physical characteristics

Total solids TS

Total volatile solids TVS

Total fixed solids TFS

Total suspended solids TSS

Volatile suspended solids VSS

Fixed suspended solids FSS

Total dissolved solids TDS (TS TSS)Volatile dissolved solids VDS

Total fixed dissolved solids FDS

To assess the reuse potential of a waste

water and to determine the most suitable

type of operations and Processes for its

treatment

Settleable solids To determine those solids that will settle by

gravity in a specified time period

Particle size distribution PSD To assess the performance of treatment

processes

Turbidity NTUc Used to assess the quality of treated

wastewater


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Color Light brown,

gray, black

To assess the condition of wastewater (fresh

or septic)

Transmittance % T Used to assess the suitability of treated

effluent for UV disinfection

Odor TONd To determine if odors will be a problemTemperature 0C or0F Important in the design and operation of

biological processes in treatment facilities

Density

Conductivity EC Used to assess the suitability effluent for

agricultural applications

Inorganic chemical characteristics

Free ammonia NH+4Organic nitrogen Org N

Total kjeldahl nitrogen TKN(OrgN+ NH+4)

Nitrites NO-2Nitrates NO-3Total 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 characteristicspH pH= -log[H+] A measure of the acidity or basicity of an

aqueous solution

Alkalinity HCO3- + CO3

-2 +

OH- - H+

A measure of the buffering capacity of the

wastewater

Chloride Cl To assess the suitability of wastewater for

agricultural reuse

Sulfate SO-24 To assess the potential for the formation of

odors and may impact the treatability of the

waste sludgeMetals As, Cd, Ca, Cr, Co,

Cu, Pb, Mg, Hg,

Mo, Ni, Se, Na, Zn

To assess the suitability of the wastewater

for reuse and for toxicity effects in

treatment. Trace amounts of metals are

important in biological treatment.

Specific inorganic

elements and

compounds

To assess presence or absence of a specific

constituent

Various gases O2, CO2, NH3, H2S,

CH4

To assess presence or absence of a specific

gases


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Organic chemical characteristics

Five-day carbonaceous CBOD5Biochemical oxygen

Demand

A measure of the amount of oxygen

required to stabilize a waste biologically

Ultimate carbonaceous

Biochemical oxygen

Demand

UBOD (also

BODu,BODL)

A measure of the amount of oxygen

required to stabilize a waste biologically

Nitrogenous oxygen

Demand

NOD A measure of the amount of oxygen

required to oxidize biologically the nitrogen

in the wastewater to nitrate

Chemical oxygen

demand

COD Often used as a substitute for the BOD test

Total organic carbon TOC Often used as a substitute for the BOD test

Specific organic

compounds and classes

of compounds

MBASe

, CTASf

To determine presence of specific organic

compounds and to assess whether special

design measures will be needed for removal

Biological characteristics

Coliform organisms MPN(most

probable number)

To assess presence of pathogenic bacteria

and effectiveness of disinfection process

Specific microorganisms Bacteria, protozoa,

helminthes, Viruses

To assess presence of specific organisms in

connection with plant operation and for

reuseToxicity TUa and TUc 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)

cNTU = nephelometric turbidity unit

d TON = threshold odor number

e

MBAS= methylene blue active substancesi CTAS = cobalt thiocyanate active substances

Table 3.4 Principal constituents of concern in wastewater treatment

Constituent Reason for importance

Suspended solids Suspended solids can lead to the development of sludge

deposits and anaerobic conditions when untreated

wastewater is discharged in the aquatic environment

Biodegradable Organics Composed principally of proteins, carbohydrates, and fats,biodegradable organics are measured most commonly in


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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

Pathogens Communicable diseases can be transmitted by the

pathogenic organisms that may be present in wastewater.

Nutrients 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

Priority pollutants 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

Refractory organics These organics tend to resist conventional methods of

wastewater treatment. Typical examples include

surfactants, phenols, and agricultural pesticides

Heavy metals 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

Dissolved inorganics 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.


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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.


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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 towardsincreasing 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 quantitiesduring 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 1940s


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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.


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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.


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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 persons 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


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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


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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


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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


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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 veryoften depend upon their source of origin. Because the optimal method of treatment of the


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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


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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


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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.


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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


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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

= V water = Mwater= Mtotal - Msolids ----------- (4.1)

Vtotal Vtotal 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

105C until no more weight loss is found (usually 24 -28 hours). It is noted here that drying a

105C 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 600C 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 ----------- (4.2)

Mtotal Mtotal

where Mtotal = total mass of the wet sample.

The volumetric air content or air-filled porosity (, 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

=Vair= 1 Vwater- Vsolids ----------- (4.3)

Vtotal Vtotal


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The porosity (, 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

= Vsolids = Vwater+ Vair= + ----------- (4.4)

Vtotal 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). Theparameters 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.

= 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 (b , g of waste/cm3 waste) of the wastes is normally calculated both on a

wet and a dry weight basis. The wet bulk density(b wet, g of wet waste/cm3 waste) is given as

b wet = Mtotal = Mwater+ Msolids ----------- (4.6)

Vtotal Vtotal


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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 (b dry, g of dry waste/cm3 wastes ) is determined as

b dry = Msolids = Mtotal - Mwater= b wet

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 (s) (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 = b dry = 1- Mwater= 1 w----------- (4.8)

Mtotal b wet 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 550C. 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)


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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 Solids content Ash content Energy content

(kg/m3) % % 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 (h 1) 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.


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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) andmixed 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 Sp = (L W )0.5

2 --------- (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)


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----------------------------------------------------------------------

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 =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 x (cm) in contact with two water reservoirs with a

height difference of h (cm) and then measuring the volumetric water flow Q (cm3/d) through

the sample. The saturated hydraulic conductivity can then be calculated using Darcys law as

Kw = Q h

A x (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.

Documents

Energy From Solid and Liquid Wastes - III