Biological and Chemical Conversion Technologies

Biological and Chemical Conversion Technologies

Chapter 14

Presentation Outline

Introduction

Properties and Classification of Hazardous Wastes

Sources, Types and Quantity of Hazardous Wastes Found in MSW

Significance of Hazardous Waste in MSW

Physical, Chemical and Biological Transformations of Hazardous Waste Constituents Found in MSW

Management of Hazardous Wastes in MSW

Introduction

The purpose of this chapter is to introduce and review the biological and chemical processes that can be used to transform the organic fraction of MSW into gaseous, liquid and solid conversion products

The major focus of this chapter is on the biological process because they have been used most commonly for the transformation of organic waste materials

Biological Processes considered in this chapter include o aerobic composting o low – solid anaerobic digestion o high – solid anaerobic digestion

Biological Principal

Nutritional Requirements for Microbial Growth Carbon and Energy Sources

Biological Principal

Nutritional Requirements for Microbial Growth

Nutrient and Growth Factor Requirements o Nutrients may at time be the limiting material for

microbial synthesis and growtho The principal inorganic nutrients needed by

microorganisms are N, S, P, K, Mg, Ca, Fe, Na, Cl. o Minor nutrients of importance include Zn, Mn, Mo, Se,

Co, Cu, Ni and W o The major growth factors (organic nutrients) fall into

the following three classes amino acid purines Vitamins

Types of Microbial Metabolism

Organisms that generate energy by enzyme – mediated electron transport from an electron transport from an electron donor to an external electron acceptor are said to have a respiratory metabolism

Fermentative metabolism does not involve the participation of an external electron acceptor

Obligate aerobic depends on Oxygen to meet their energetic needs

Anoxic depends on oxidized inorganic compound such as nitrate and sulfate

Facultative anaerobs has the ability to grow in either the presence or the absence of Oxygen

Types of Microorganisms

Microorganisms are commonly classified on the basis of cell structure and function

The procaryotic groups are of primary importance in the biological conversion of the organic fraction of solid waste and are generally referred to as bacteria

Eucaryotes important in biological conversion of organic wastes include bacteria, fungi, yeasts, actinomycetes

Environmental Requirements

Environmental conditions of temperature and pH have an important effect on the survival and growth of microorganisms.

Table 14-4 presents some typical temperature ranges for various bacteria

The optimum value of pH for bacterial growth lies between 6.5 and 7.5

Moisture content is another essential environmental requirement for the growth of microorganisms

The moisture content of the organic wastes to be converted must be known, especially if a dry process is to be used.

Environmental Requirements

Aerobic Biological Transformation

The general aerobic transformation of solid waste can be described by means of the following equation

The amount of oxygen required for aerobic stabilization of the biodegradable organic fraction of MSW can be estimated by the following equation

If complete conversion is accomplished, the corresponding expression is


The amount of oxygen required for the oxidation of ammonia to nitrate can be computed by the following equations

computation of the amount of oxygen required for the stabilization of prepared solid waste is illustrated in example 14-1


Anaerobic Biological Transformations

Process Microbiology The biological conversion of the organic fraction of MSW under

anaerobic conditions is thought to occur in three stepso Hydrolysis of higher molecular mass compounds into

compounds suitable for use as a source of energy and cell tissue

o The bacterial conversion of the compounds resulting from the first step into identifiable lower molecular mass intermediate compounds

o The bacterial conversion of the intermediate compounds into simpler end products, principally, methane and carbon dioxide


Insert Figure 14-1


Biochemical Pathways It is important to know that methane bacteria can only use a

limited number of substrate for the formation of methane.


Environmental Factors To maintain an anaerobic treatment system that will stabilize

an organic waste efficiently, the rector content should be avoid of dissolved oxygen and free from inhibitory concentrations of free ammonia and heavy metals

The pH should range from 6.5 – 7.5 Sufficient alkalinity should be present to ensure that the pH

will not drop below 6.2, because the methane bacterial can not function below this point

A sufficient amount of nutrients must also be available to ensure proper growth of the biological community

Temperature is also another important environmental parameter


Gas production The general anaerobic transformation of solid waste can be

described by means of the following equation

For practical purposes, the overall conversion of the organic fraction of solid waste to methane, carbon dioxide and ammonia can be represented with the following equaiton


Biological Process Selection

Aerobic and anaerobic processes both have a place in solid waste management. The relative advantages of aerobic and anaerobic processes are summarized in Table 14-5


Aerobic Composting Aerobic composting is the most commonly used biological

process for the conversion of the organic fraction of MSW to a stable humus-like material known as compost

Application of aerobic composting include o yard waste o separated MSW o commingled MSW o co-composting with wastewater sludge


Process Description All aerobic composting process are similar in that they all

incorporate there basic steps: 1. preprocessing of the MSW, 2. aerobic decomposition of the organic fraction of the MSW, 3. product preparation and marketing

The three principal methods used for the composting of the organic fraction of MSW are o windrow o aerated static pile o in-vessel


Design and Operational Consideration




Processing Compost for Market The economics of compost systems are greatly enhanced if the

compost can be sold To be marketable, compost must be of a consistent size, free

from contaminants such as glass, plastics, and metals and free of objectionable odors.

shredding and screening are commonly used to produce a more uniform product

Selection of Aerobic Composting Process because the performance of properly operating window,

aerated static pile and in-vessel composting process is the same, the selection among alternative process is based on capital and operating costs, land availability, operational complexity and potential for nuisance problems.


Low – Solid Anaerobic Digestion

Low – solids anaerobic digestion is a biological process in which organic waste are fermented at solids concentration equal or less than 4 to 8 percent.

One of the disadvantages of the low-solids anaerobic digestion process is that considerable water must be added to waste to bring the solid contents to the required range of 4-8 percent.

The addition of water results in a very dilute digested sludge which must be dewatered prior to disposal


Process Description


Process Description


Process Design Consideration

Important design considerations for the low-solids anaerobic digestion of the organic fraction of MSW

High – Solids Anaerobic Digestion

Two important advantages of the high solids anaerobic digestion process are lower requirements and higher gas production per unit volume of the reactor size.

The major disadvantage of this process is that limited full scale operating experience is available

Process Description The principal difference is at the end of the process, where less

efforts is required to dewater and dispose of the digested sludge


Process Design ConsiderationImportant design considerations for the high-solids anaerobic

digestion of the organic fraction of MSW


Chemical Transformation Process

Chemical transformation process include a number of hydrolysis process which are used to recover compounds such as glucose and furfural and other chemical compounds

The chemical processes are not used routinely for the transformation of the organic fraction of MSW, because these compounds can also be manufactured from other cellulose – containing waste

Chemical Transformation Process

Acid Hydrolysis

Methanol Production from Methane

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Biological and Chemical Conversion Technologies