1. INTRODUCTION This chapter consists of four main parts: (i) sources of wastes and wastewater in the Asian region; (ii) treatment of both solid waste and wastewater; (iii) recycling as an emerging area of waste minimization; and (iv) remediation of polluted areas. With expanding economic activity and consumption of consumer goods, quantities of wastes are increasing rapidly in the Asian region. Waste is an unwelcome and often non-avoidable side effect of rapid development. At present, the Asian region houses about 60% of the global 6 billion people. Uncontrolled urbanization, rapid industrialization and extensive agricultural practice are generating huge quantities of various wastes. Furthermore, the ecological jurisdiction in many Asian countries is still much more diffuse in comparison with developed countries, thus stimulating different companies to construct branches of waste-generating facilities in the Asian countries without corresponding industrial waste and wastewater treatment capacities. These factors make the waste problem of crucial environmental concern in almost all the Asian countries. We can not here discuss the whole spectrum of waste production and treatment in the Asian region. Only some fundamental principles will be discussed with rather arbitrary selection of waste generation system and waste treatment facilities. You can find more information from additional recommended books and websites of interest at the end of this chapter. 2. TYPE AND SOURCES OF WASTES IN THE ASIAN REGION Primary sources of wastes are municipal, agricultural, commercial and industrial, hospitals and other health-care sectors. Accordingly, major categories of waste are agricultural waste and residues, municipal waste, industrial waste and hazardous waste. Major components of waste are organic and inorganic constituents, nutrients, toxicants and different species of micro-organisms. Wastes are presented in solid, semi-solid (sludge) and liquid forms with one or more of the following characteristics: corrosive, ignitable, reactive, toxic, radioactive, infectious, bioaccumulative, carcinogenic, mutagenic and teratogenic (Figure 1).
2.1. Municipal Solid Waste Types of municipal solid waste In the Asian cities, municipal solid waste (MSW) is generated from residences, commerce, institutions, construction and demolition, cleaning services and treatment plants. Generation rates of MSW vary from city to city and season to season. In most of the Asian cities, these rates range from less than 0.5 to 0.8 kg per capita per day. Some cities have higher generation rates of more than 1.0 kg per capita per day. Generation of MSW depends usually on the level of economic development of the country. The MSW production in high-income countries (Japan, Hong Kong, South Korea, and Singapore) is between 1.5–2.5 kg per capita per day, in middle-income countries (China, Thailand, Malaysia etc.), 0.7–0.9 kg per capita per day and in lowincome countries (like Bangladesh, Sri Lanka, Vietnam, Myanmar etc.), 0.4–0.6 kg per capita per day. The trends in the total MSW production for different groups of countries are shown in Figure 2. Gross annual generation of MSW in some selected Asian countries is depicted in Figure 3.
Composition of municipal solid waste The composition of MSW varies significantly from city to city. The compostable waste fraction represents typically more than 25% in most of the Asian cities, followed by non-compostable waste, paper and cardboard, and plastics, as shown in Figure 4.
The moisture content of MSW is generally between 35 and 55% and the caloric values are also variable, from typical to much higher values of in cities like Bangkok, Jakarta, Manila and Tokyo (ESCAP, 2000). Climate and fuel use are also related to the composition of municipal solid waste. The cities where heating is needed in winter, such as Beijing, Shanghai, Seoul and Tokyo and where coal is the main source of energy, have much greater amounts of ash in their waste during the wintertime. The basic infrastructure brings other variations in cities and towns, such as Calcutta, Dhaka (India), and Hanoi (Vietnam) with unpaved or poorly paved streets that have a large amount of dust and dirt from street sweeping. There are large differences in organic waste fraction among cities according to the number of trees and shrubs in public places. Large and bulky waste items, such as abandoned motor cars, furniture and packing residues are found in the higher-income economy countries (Japan, Brunei Darussalam, South Korea, and Singapore), on the contrary with low-income countries, where such type of wastes are not generally found. The amount of human waste in the MSW is significant in squatter areas of many Asian cities, like Hanoi, Calcutta, Dhaka, etc., where “wrap and throw” sanitation is common or bucket latrines are emptied into waste containers.
Municipal wastewater Municipal wastewater is a combination of liquid and water-carried waste from residences, commercial buildings and institutions. This is actually the wastewater arising from human sanitary water use and a combination of water coming from kitchen, toilets, bathrooms, laundries, etc., where it may have been used for washing, cooking, food preparation and cleaning, showers, toilet flushing, etc. The important parameters for proper management of municipal wastewater are the quantity and the chemical composition of wastewater. The quantity can be estimated using a number of factors, such as population, water supply data, housing type and likely presence of washing machines and dish-washing machines, presence of hotels, restaurants, hospitals, offices, schools, colleges and universities. Using this information, approximate wastewater quantities can be calculated to design a proper wastewater treatment plant. Various countries of the Asian region produce different quantities of municipal wastewater and sewage (Table 1). We can see that developed countries, like South Korea, produce much more wastewater in spite of much less population (44 million in 2000) than multi-million China. The composition of municipal wastewater is quite variable, being highly dependent on the local conditions. In addition to any materials present in the home water supply, it will contain different contaminants, like feces, urine, paper, soap, oil and grease, dirt and soil, food waste, and home used chemicals. Typical composition of an untreated municipal wastewater includes dissolved solids, , organic nitrogen, ammonia, organic phosphorus, inorganic phosphorus, , chloride, oil and grease, These waters contain also a significant number of various microorganisms, including some poisoning species (ESCAP, 2000).
2.2. Industrial Waste We will consider briefly sources of both industrial solid waste and industrial wastewater in the Asian region.
Industrial solid waste Various industries in the Asian region can be grouped into following broad categories: 1. iron and steel industries; 2. chloralkali industries; 3. sulfuric acid and nitric acids plants; 4. hydrochloric acid and fluorochemical plants; 5. petroleum oil refineries; 6. fertilizer industries; 7. mining industries; 8. metal fabricating industries; 9. surface coating industries; 10. pesticide industries; As a result of relevant industrial activities, huge quantities of solid waste are generated in the Asian region with a very wide spectrum of waste materials of different environmental toxicity. Due to the absence of a regularly up-dated and systematic statistical database on solid waste industrial generation from different groups of industry, the exact rates of generation are very uncertain for individual industries. However, we can conclude that the typical waste products include rubbish, paper, packaging materials, waste from food processing, oils, solvents, resins, paints and sludge, glass, ceramics, stones, metals, plastics, rubber, leather, wood, cloth, straw, abrasives, etc. Ratio between municipal and industrial solid waste generation varies significantly from country to country. For example, in China this ratio is three to one, whereas in Bangladesh, Sri Lanka and Pakistan, much less. In developed countries, like Japan, this ratio is equal to 1 : 8. Based on an average ratio for the Asian region, the industrial solid waste generation is approximately 1.5 billion ton per annum (ESCAP, 2000). These data are depicted in Figure 5. The amount is expected to increase significantly with planned economic growth in the region, doubling the current generation rates. This incremental growth will pose very serious environmental problems because existing industrial solid waste collection, processing and disposal systems are grossly inadequate and the required improvement of solid waste treatment is unlikely in the near future. 11. pharmaceutical industries; 12. natural rubber industries; 13. palm oil mills; 14. dyes and intermediates industries; 15. textile industries; 16. leather tanneries; 17. fermentation industries; 18. sugar industries; 19. food processing industries, and 20. jute and coir industries.
Industrial wastewater Industrial wastewater is produced by all industries, which use water in different technological processes. The generation of this wastewater varies depending upon nature of industries, their production processes and water usage. The present estimate shows
that huge quantities of wastewater are generated both in developed and developing countries of the Asian region. However, the generation rates are very different. China, for example, generates over 2 kg of wastewater for an industrial production equivalent of one US dollar, followed by Japan, 1 kg, South Korea, 0.5 kg, Hong Kong, 0.3 kg and Singapore, 0.2 kg (ESCAP, 2000). The composition of industrial wastewater is variable as flow rates with type and size of industry. The typical chemicals of environmental concern in wastewater are shown below (the list is incomplete): acrylonitrile, arsenic, and aldrin/dieldrin; benzene, benzidine, benz(a)pyrene, beryllium, bis(2-chloroethyl) ether, bromodichloromethane, bromoethane, and bromoform; cadmium, carbon tetrachloride, chlordane, chloroform, chloromethane, chromium, copper, cyanides; DDT, dibromochloromethane, dibutyl phtalate, 1,2-dichloroethane, 1,1-dichlrodiethyl phtalate, di-n-butylphtalate, 2,4-dinitrophenol, 1,2-diphenylhydrazine;
heptachlor, heptachloroepoxide, hexachlorobutadiene, hexachlorocyclopentadiene, hexachloroethane, hydrogen cyanide; iron, isophorone; lead, lindane; malathion, mercury, methylene chloride, molybdenum, mirex; nickel, nitrobenzene, N-nitrosodimethylamine, N-nitrosodi-n-propylamine; pentachlorophenol, phenol, PCBs, PAHs, photomirex; total hydrocarbons; VOCs; 2,3,7,8-TCDD, 1,1,2,2-tetrachloroethane, trichloroethylene, toluene, toxaphene, 1,2,3-trichlorobenzene, 1,1,1- and 1,1,2-trichloroethylene, 2,4,6-trichloroenol, etc. We can add here, that this typical chemical composition is complicated by high concentrations of both dissolved and suspended solids, nutrients, acids, oil and greases.
2.3. Agricultural Waste Agricultural waste and residues The expanded production of a range of agricultural crops has naturally resulted in the generation of annually increased quantities of waste (including manure and animal dung) and residues in the form of livestock waste, agricultural crop residues and agro-industrial by-products. The approximate estimate shows that among the Asian countries, China produces the largest quantities of both agricultural waste and crop residues, followed by India (Table 2).
Plastic waste Another form of agricultural waste produced in some countries of the Asian region is plastic. On-farm plastic use appears to be rapidly increasing in the form of low-density polyethylene (LDPE) agricultural plastic film. The use of this film is extensively practiced in South Korea, China and Japan as a means to maintain soil temperature favorable to plant growth, retain moisture and reduce weeds. LDPE agricultural plastic film is an increasingly popular alternative to more traditional methods of storage and cover. It is used in agricultural operations for silo bunker cover, silage bags, bale
wrap, greenhouse cover, haulage cover, row cover, and mulch film. Crops for which LDPE is used include cotton, maize, tobacco, juvenile rice and fruit tree seedlings, melon, vegetables and groundnuts. In China alone, the use of plastic in agricultural practices has increased almost 70 times since 1990. Similar numbers have been shown for South Korea (ESCAP, 2000). Aquaculture waste In the Asian region, marine and freshwater aquaculture farms are producing almost 90% of the World’s farm-bred seafood. China accounts for about half of the region output, followed by India, Thailand, Vietnam and Japan. Huge quantities of wastewater are produced from freshwater aquaculture. At present, the exact values are not available, but it is expected that these figures will be doubled by 2010.
2.4. Hazardous Waste With rapid development of industry, agriculture, hospital and health-care facilities, the Asian region is consuming significant quantities of toxic chemicals and producing a large amount of hazardous waste. Each year, another 1000 new chemicals are added to the market for industrial and other uses. Most hazardous waste is the by-product of a broad spectrum of industrial, agricultural and manufacturing processes, nuclear power plants, and hospitals. Primarily,
high-volume generators of industrial hazardous waste are the chemical, petrochemical, petroleum, metals, wood treatment, pulp and paper, leather, textiles and energy production plants, especially coal- and lignite-burning plants. Municipal waste also contains hazardous acids and solvents, dry cleaners and pesticide residues, photochemical waste, plastics, medical waste and paint sludge. Among the prominent groups of hazardous waste that are generated in the Asian region are waste solvents, chlorine bearing waste, and pesticide formulation waste and residues. Chlorine is the primary ingredient of PVC formulation. In the Asian countries, chemical industries are using about 20% of the global production of chlorine and caustic soda, 10 and 12 million tons respectively for the formulation of around 11,000 organochlorine species. The estimated quantities of hazardous waste in some selected Asian countries are shown in Table 3. We can see a significant increase of hazardous waste production in planning for China, India, Indonesia, Philippines and Thailand by year 2010. Huge amounts of petroleum waste are produced annually in some countries of the region, like Brunei Darussalam, China, India, Indonesia, Malaysia, Japan and South Korea. Two latter countries and Taiwan produce significant quantities of radioactive waste.
Hazardous waste could be solid, semi-solid (sludge), liquid and gaseous and possesses variable characteristic properties such as toxicity, reactivity, flammability, explosivity, corrosivity and radioactivity.
2.5. Transboundary Movement of Hazardous Waste to the Asian Region The Asian region has become the favorite dumping ground for hazardous waste exporters. From 1994 to 1997, industrialized countries have sent totally 3.5 million tons of hazardous waste or harmful trash to the Asian countries. For example, waste export to China has been documented from United State, Japan, South Korea and Taiwan (ESCAP, 2000). Another example is related to India. Cheap labor, poor environmental standards, a sieve-like import regime and a growing market for cheap raw materials are the main driving forces. Ignoring its law courts, India is thus helping developed countries to beat the International Basel Convention on the ban on the damping of toxic industrial waste in developing countries. Thousands of tons of toxic waste are being illegally shipped to India for recycling or dumping, including 73,000 tons of toxic lead and zinc residues from 49 countries. In 1995, Australia alone exported more than 1,450 tons of hazardous waste such as scrap lead batteries, zinc and copper ash to India. Huge amounts of PVC waste produced in the Netherlands are regularly shipped to Pakistan and the Philippines. Since 1990, more than 100,000 tons have been exported for local recycling into various products of poor quality, which end up in burning dumpsites in a few years. Similarly, other countries of the region, such as Bangladesh, Pakistan, Indonesia, and Thailand have become dumping grounds of huge quantities of hazardous waste for the exporters of industrialized countries both within and outside the Asian region (ESCAP, 2000).
2.6. Environmental Impacts of Waste The adverse environmental impacts of waste have been well documented in the form of water (riverine, coastal and marine, and ground) pollution, air pollution and land pollution (see Chapters 7–9). Many cities like Bangkok, Beijing, Bombay, Calcutta, Dhaka, Hanoi, Jakarta, Kuala Lampur, Manila and Shanghai are plagued by garbage problems. Bangladesh, India, Indonesia, Malaysia, Pakistan, the Philippines and Thailand are faced with growing problems of both domestic and industrial hazardous waste. Rivers are choked by untreated domestic and industrial wastewater in many Asian countries. Some industrial waste, biomedical and nuclear waste constitute the most obvious categories of hazardous products, which are detrimental for human health and environment. Agricultural waste includes residues of fertilizers and pesticides with known toxicity (see Chapter 11). Various incidents of pollution have been reported from industrial waste, effluents from sewage treatment plants, food processing plants along with biocides and toxic effluents from sawmills and timber processing areas. These effects are often enhanced by disposal of hazardous wastes in municipal landfills.
The general scheme of combined impacts of hazardous waste on the human being and environment is shown in Figure 6. Among these are direct transmission of disease and the spread of epidemics, loss of healthy amenable rural and urban environment. The potential spread of AIDS and other infectious diseases through the discharge of biomedical and health-care wastes
into general municipal waste streams is a growing threat in the Asian region. Waste containing oils, pesticides and heavy metals is a major contributor to soil pollution and land contamination in many Asian countries. Other environmental impacts include the adverse effects of windblown and floating debris and plastics on crops, birds and aquatic biota.
3. WASTE TREATMENT IN ASIAN COUNTRIES
Here we will consider municipal wastewater treatment, land disposal and incineration of solid municipal waste.
3.1. Municipal Wastewater Treatment Municipal wastewater collection, treatment and disposal is a basic problem in most Asian countries, especially in urban areas. Wastewater is generally collected by sewer systems from residential, commercial, recreational, and institutional areas and is transported to a central sewage treatment plant, where sewage undergoes physical, chemical and biological treatment. Such systems exist in Seoul, Tokyo, Singapore, Ankara and other cities. However, in many cities of the region, the existing sewarage systems are old. Maintenance has often been poor due to lack of funds and leakages into ground and surface water are common. In a few cases, where sewers and water mains have been placed in close proximity, cracks in both lines have led to sewage contamination of water supplies, particularly, during periods of peak flow. Typical municipal sewage contains oxygen-demanding materials, sediments, grease, oil, scum, pathogenic bacteria, viruses, salts, algae nutrients, pesticides, refractory organic materials, heavy metals, and an astonishing variety of flotsam. Current processes for the treatment of wastewater may be divided into three main categories of primary treatment, secondary treatment, and tertiary or advanced treatment (Figure 7).
Primary treatment The sewage enters a lagoon, or clarifier, whose capacity is large enough to allow a residence time of several hours for wastewater in this lagoon. A coarse screen at the entrance to the clarifier removes large objects, such as pieces of wood, tree branches, and similar debris. A grid tank ahead of the main clarifier allows the deposition of grit, sand, and similar materials. Typically, the sewage enters and leaves at opposite ends of the lagoon, and moves through slowly enough for settling down of any solid substances. Some greasy material may float to the surface, where it is removed by a skimming device. The effluent from the primary settler is almost transparent. However, it has typically very high BOD values, of the order of 500–1500 ppm. The major sources of BOD are the tiny particles, which fail to separate during primary settling. These particles can be coagulated and settled upon the addition of filter alum or ferric chloride. This may reduce BOD significantly, up to the level of 50–150 ppm. In some cities, this coagulation stage is applied as an alternative to the secondary treatment.
Solids removed at the primary stage are collected and deposited at landfill. Secondary treatment The main objective of the wastewater secondary treatment is to reduce the BOD values. The successful reduction may achieve 90% from the initial level. We will consider two common approaches that are in use in the Asian region, the trickling filter and the activated sludge reactor (see Figure 8). The trickling filter is a large round bed of sand and gravel, with coarse gravel on the top and successively finer layers beneath. A rotating boom sprinkles the bed with water from the primary settler. The gravel bed quickly becomes colonized with microorganisms, which use the carbon compounds in the water as the energy materials. A well-maintained trickling bed may remain in operation for several decades (Figure 8).
An alternative method of secondary treatment is the activated sludge reactor. The reactor is a large tank, in which the wastewater is agitated and aerated to provide the oxygen required by the micro-organisms. Wastewater aeration is the principal difference with the trickling filter, where oxygen diffuses naturally from the air. In order to keep the concentration of micro-organisms high and in the maximal growth phase, a portion of the sludge of the micro-organisms is removed from the exit stream of the reactor and recycled into the influent stream (Figure 9). The advantages of the activated sludge reactor in comparison with the trickling filter are related to the maintenance at the optimum temperature for biological activity and relatively small operational space. The first point is important for countries with moderate climate, like South Korea, Japan and partly China, whereas the second one, for many Asian cities (Hong Kong, Singapore, Seoul, etc.) where land is expensive.
The activated sludge process provides two pathways for BOD reducing: oxidation of organic matter to provide energy for the metabolic processes of the micro-organisms; incorporation of the organic matter into cell mass in the process of biosynthesis. These processes are schematically shown in Figure 10. In the first pathway, carbon is removed as the gaseous in the second, as the solid biomass that needs to be landfilled. The disposal of this biomass is a problem, since it is only about 1 % solids and it contains many undesirable hazardous species (see below). Nitrification (see Chapter 13) is a significant process that occurs during biological waste treatment. Ammonium ions are normally the principal inorganic nitrogen species produced at the first stage of biodegradation of nitrogenous organic compounds. Ammonia is oxidized first to nitrite and then to nitrate (reactions (1) and (2))
Both these reactions occur in the aeration tank of the activated sludge plant and are favored in general by long retention time, low organic loading, large amounts of suspended solids, and high temperatures. The subsequent process, denitrification, removes nitrate from wastewater and occurs in the anaerobic condition (reaction (3))
Sewage sludge Both primary and secondary sewage treatments involve settling of particulate matter, and thus produce sludge. The term sludge refers to a material having a high water content (95–99%), even when is has been air-dried. Dewatering is aided by heating
(digesting) the sludge, which causes the small particles to coagulate. Digestion is anaerobic, and is accompanied by the release of reduced gases, such as methane. The dewatered sludge with much coarser particles can be air-dried to a material that is really solid. This material is disposed of by landfilling, by incineration, by ocean dumping, or by spreading it on the land for use as a fertilizer. These methods are applied in the Asian region in various extents depending on the country. Sewage sludge is rich in organic matter, nitrogen and phosphorus and may be used as fertilizer and soil-conditioner. It may be applied for the remediation of disturbed soil cover, for instance, in the areas of mining. The typical rates are of hundreds of tons per hectare. In these areas, artificial landscapes can be made up by planting trees and grasses. The actual restriction for application of sludge is high transportation cost and high content of water (up to 60–65%) even in the dewatered material. A potential drawback to the use of sewage sludge as fertilizer in agricultural fields is the presence of both organic and inorganic toxic compounds. The former compounds are oxidation-resistant organic substances, such as organochlorine species, which become bound in the organic matrix of the sludge. The inorganic toxicants are represented by heavy metals, mainly arsenic, cadmium, lead, mercury and zinc. These metals can be taken up by crops and introduced in the food chains or leached to the groundwater. The following considerations govern the safe use of sewage sludge as a fertilizer in the Asian region: the amount of sludge which may be safely applied varies with the soil type, being higher in clay soils and lower in the sand soils; agricultural soils that are fertilized with sewage sludge should be analyzed regularly for content and accumulation of toxic metals in plant-available forms; sludge should never be applied to a growing crop, otherwise the crop will uptake any toxic species before they have been immobilized by the soil mass. Tertiary wastewater treatment Tertiary wastewater treatment or advanced waste treatment is a term used to describe a variety of processes performed on the effluent from secondary waste treatment. The contaminants removed by tertiary waste treatment typically are suspended solids, dissolved organic compounds and dissolved inorganic materials, including nitrogen and phosphorus as the important algae nutrients. Each of these categories presents its own problems with regards to water quality. Suspended solids are primarily responsible for residual biological oxygen demand in secondary sewage effluent waters. The dissolved organics and heavy metals are the most hazardous toxicants in the effluent. In addition to these chemical pollutants, secondary sewage effluent often contains a number of pathogens (disease-causing micro-organisms), requiring disinfection. Among the bacteria that may be found in the secondary waste water effluent are
organisms causing tuberculosis, dysenteric bacteria (Bacillus dysenteriae, Shigella dysenteriae, Shigella parodysenteriae, Proteus vulgaris), cholera bacteria (Vibrio cholerae), bacteria causing typhoid fever (Salmonella typhosa, Salmonella paratyphi), and bacteria causing mud fever (Leptospira icterohemorrhagiae). Various viruses, causing diarrhea, eye infections, infectious hepatitis, and polio are also encountered in many Asian countries. Tertiary wastewater treatment includes different physical-chemical processes: removal of scum and tiny solid objects; clarification, generally with addition of a coagulant, and frequently with the addition of other chemicals, for example, lime for phosphorus removal; activated carbon absorption; disinfection. Two methods are in use to reduce the BOD below 50 ppm which is typical for secondary sewage effluent, microstraining and coagulation. Microstraining involves forcing the water through very fine screens made from stainless steel. The particles are trapped on the screens, and have to be removed by back-flushing whenever the pressure needed to filter the water becomes excessive. Coagulation process is very similar to that discussed in the advanced primary treatment and drinking water purification (see Chapter 10). Superchlorination of wastewater effluent by high dose of chlorine may be applied to kill any remaining pathogens. Some by-products of such superchlorination, like and organochlorines, are unavoidable and this can restrict the application of superchlorination in the wide scale. However, in the Asian region the threat of microbiological pollution is so much more pronounced that by-product formation and superchlorination must be applied in many treatment plants.
3.2. Traditional Methods of MSW Processing Disposal of municipal, agricultural, industrial and biomedical wastes is given relatively low priority in many countries of the Asian regions despite increasing loads. It has been estimated that only 70% of the municipal waste in the urban areas are collected, of which only 5–10% is properly disposed. There exist limited actions in reducing, reusing and recycling of wastes in the region. Wastes are disposed in a variety of ways. Apart from reuse and recycling, these include animal feeding; indiscriminate dumping on land and in mangroves, creeks, lagoons, coastal waters, and on drains and roadsides; land burying, landfilling, composting, open burning and incineration. In general, the problems of indiscriminate dumping are much more common near major urban centers. In rural areas and suburbs, open burning, composting and land spreading and burying are common practices. Various disposal methods of MSW in the Asian region include animal feeding, composting, vermi-composting, dumping, landfilling, open burning and incineration.
Animal feeding A large portion of the household organic waste in China, India, Indonesia, Malaysia, the Philippines, Thailand and Vietnam is eaten by farm animals or fed to pets. Pig and poultry farmers routinely collect food waste from households, hotel and restaurants for animal feed. Composting Backyard composting of the organic portion of MSW is a longstanding tradition in countries like Japan, China, Indonesia and the Philippines. Organic municipal waste is composted in many countries of the region to reduce waste for landfilling. However, most of the composting plants in the developing countries are not functioning well and are not operating at full capacity owing to
(a) high operating and maintenance cost compared with open dumping or land filling; (b) cost of compost is higher than commercial fertilizers; (c) incomplete separation of materials such as plastic and glass, making the compost poor for agriculture applications; and (d) poor operation and maintenance of the facilities.
Co-composting of organic MSW with agricultural waste and sludge from municipal sewage treatment plants is gaining acceptance in Bangladesh, China, India, Philippines and Thailand, where land is available. High operation cost and maintenance and transportation cost, along with incomplete separation of waste materials are major constraints for the adoption of this process. Vermi-composting Vermi-composting is a process which uses worms and micro-organisms to convert organic materials into nutrients-rich humus to use as fertilizer. Vermi-composting of organic MSW is in practical application in China, India, Indonesia and the Philippines. Open dumping In the Asian region, among various methods open dumping is the most common method of solid waste disposal. Dumping sites are especially uncontrolled disposal sites. In some cities, solid waste disposal combines open dumping and burning and sea disposal. In many coastal cities solid waste is disposed on open dumps and then burnt and some waste is pushed off the cliff into the sea. Very often, the dumping sites are swamp lands and ravines and the waste is used for reclamation. Filling of wetlands and ravines with municipal solid waste is being practiced for land development in South Korea, Japan, India, Sri Lanka and the Philippines.
3.3. Landfilling Landfill historically has been the most common way of disposing of solid waste, including municipal solid waste. Landfill involves disposal that is at least partially underground in excavated cells, quarries, or natural depressions. Usually fill is continued above ground to utilize space most efficiently and provide a grade for drainage of precipitation.
Landfill design The greatest environmental concern with landfilling of municipal solid waste that unavoidably contains hazardous compounds, is the generation of leachate from infiltrating surface water and groundwater with resultant contamination of groundwater suppliers. Modern waste landfills provide elaborate systems to contain, collect, and control such leachate (Figure 11). In modern containment landfills, a liner is constructed at the base to minimize percolation of leachate into the underlying groundwater. Depending on the landforms where a waste is disposed, there are three types of containment landfills: (1) belowgrade landfills (see Figure 11); (2) at-grade landfills; and (3) canyon landfills. Environmental impacts of landfill An improperly designed and operated landfill can cause discharge of leachate to groundwater aquifer and groundwater pollution. Table 4 presents the limits of concentrations for various contaminants in landfill leachate. From a disposal standpoint, the primary item of interest is the chemical composition of the leachate generated from a waste. We can see from Table 4 that leachate quality from waste type is not unique. Both the local climate and operational practices heavily influence leachate chemical composition. This also varies with time. The time-quality curve is somewhat bell-shaped. In addition to groundwater pollution, an improper landfill operation can impact several elements of the physical environment like surface water, air, forest lands, and wetlands. The problems of odor and fire hazard owed to gas migration, breeding and harboring of disease vectors are also of special concern. Landfilling in the Asian region Landfilling operation of solid waste in the Asian region is generally unsatisfactory except in countries such as Japan and Singapore, though it is an attractive disposal option. Landfill sites vary from uncontrolled tipping/dumping, controlled dumping, basic sanitary landfill and full sanitary landfill. Many countries such as China, Japan, South Korea, Malaysia and Thailand have adopted controlled tipping or sanitary landfilling for solid waste disposal. Some cities including Tokyo and Seoul do have well-designed and reasonably operated sanitary landfills. Kuala Lumpur has used old tin mines for MSW landfills around the city. The high percentage of organics, combined with much plastic, which forms layers when compacted, contributes to the
build-up of landfill gas (methane) at landfill sites; fires often break out and workers are made ill by the gases. In large cities in the Asian region, the availability of land for landfill siting is a major issue, especially in urbanized and heavily populated areas. For example, in Hong Kong, Singapore and Taiwan, severe land constraints have led to the selection of landfill sites in coastal areas, offshore islands and mountainous terrain rendering complex engineering infrastructure facilities necessary to ensure proper operational and maintenance control. In Singapore, there are two controlled landfill sites; their capacities are almost exhausted and offshore landfill is now in operation. A similar situation is in the Bangkok metropolitan area.
3.4. Incineration Many chemical species of MSW, including polymeric or carbonaceous solids, are degraded by high temperature. In the presence of oxygen, any carbon, hydrogen, and sulfur are oxidized to and respectively. The range of incineration
increases rapidly with temperature. A range of 700–760°C is generally required for combustion, and most general-purpose incinerators operate between 760 and 1100°C. For an incinerator to operate with auxiliary fuel or air preheating, the waste feed or refuse must contain less than 50% moisture or 60% ash, and have more than 25% combustibles. It is extremely difficult to burn and recover useful energy from unsorted municipal waste because of its heterogeneity in size, shape, chemical composition, and heating value. However, pretreatment of the waste before thermal treatment facilitates burning. Such pretreatment contributes to the front-end cost but reduces furnace cost. The waste is upgraded by separation of the nonorganic fraction and drying, shredding, and densifying solids. The various types of incinerators include the moving grate incinerators and the multichamber incinerators. The rotary kilns are used to incinerate a large variety of liquid and solid industrial wastes. Any liquid capable of being atomized by steam or air can be incinerated, as well as heavy tars, sludges, pallets, and filter cakes. Thermal destruction of solid waste The generalized process flow chart for the thermal treatment of municipal solid waste is shown in Figure 12. The analysis of the evolution and/or destruction of hydrocarbons during the incineration of MSW and hazardous waste involves heat transfer, mass transfer, and reaction kinetics. The key phenomena include the flashing of liquid hydrocarbons; the vaporization, desorption, and stripping of hydrocarbons; the pyrolysis and charring of hydrocarbons; and the oxidation of char. To a certain extent these processes occur in parallel (steps 2, 3,4, and 5) and are common to most thermal treatment processes.
The key variables affecting the rate of destruction of solid waste are temperature, time, and gas-solid contact. Pollutant emission from solid waste incinerators and emissions. Oxides of nitrogen (NO and NO2) and sulfur are emitted from most combustion systems including MSW and hazardous waste incinerators. The two principle mechanisms are shown in Figure 13. The thermal pathway is important in any high temperature process containing molecular nitrogen and oxygen (see Chapter 3). The fuel NO pathway is also important if the fuel or waste contains nitrogen. The level of nitrogen oxides is difficult to estimate quantitatively since rate of formation of NO by both pathways is determined by complex kinetics and gas-phase mixing (Diemer et al, 1999). Sulfur oxide production in incinerators is generally easy to predict because nearly all organic sulfur species are completely converted to Partitioning of heavy metals. Metals entering a solid waste incinerator can leave the system with the bottom ash, the captured fly ash, or the exhaust gases. The fraction leaving with the exhaust gases can include metal vapors such as mercury and very small particles that escape capture in the stack. Metals entering the incinerators as liquid streams are usually carried out of the reactor in the fly ash, which is typically enriched with heavy metals, especially lead and cadmium, relative to the entering solid waste. The enrichment of small particles results from the condensation of vaporized metals. Metal concentrations of particles also typically increase with decreasing particle diameter. Dioxin and furan emissions. PCDD and PCDF congeners are emitted mostly as absorbates on fly ash, rather than free gaseous species. Although the most toxic PCDD and PCDF congeners are only minor components of the total, their presence raises considerable public opposition to incineration of MSW. Surprisingly, hospital incinerators are among the more significant sources of these pollutants, mainly because
of the presence of chlorinated plastics such as polyvinyl chloride in the feed. The proposed mechanism by which chlorinated dioxins and furans form has shifted from one of incomplete destruction of the waste to one of low temperature, downstream formation on fly ash particles. Two mechanisms are proposed: a de novo synthesis, in which PCDD and PCDF are formed from organic carbon sources and Cl in the presence of metal catalysts, and a more direct synthesis from chlorinated organic precursors, again involving heterogeneous catalysis. Bench-scale tests suggest that the optimum temperature for PCDD and PCDF formation in the presence of fly ash is roughly 300 °C. Chlorine may be formed in reaction (4) at temperatures below 900 °C
Both and HCl have been shown to chlorinate hydrocarbons on fly ash particles. Pilot-scale data involving the injection of fly ash from municipal waste combustion show that intermediate oxygen concentration (4–7%) produce the highest levels of both PCDDs and PCDFs. The data also show significant reductions in emission of these contaminants with the upstream injection of at about 800°C (Diemer et al, 1999). Incineration in the Asian countries Incineration process of MSW disposal is capital-intensive and skilled manpower is required for operation and maintenance. Up-to-date, full-scale incinerators are currently in service only in Japan, Singapore, China, Hong Kong, South Korean, Indonesia, and Taiwan. High capital investment, high operation and maintenance costs, and stringent air pollution regulations have severely limited the application of this technology for the disposal of MSW in the region. We will give some examples from various Asian countries. Singapore operates three incinerator plants, burning more than 75% of the daily 6,700 tons of MSW collected. No sorting of waste is carried out before the MSW is fed to the incinerators, with exception of crushing bulky waste. The waste is mixed and burned using rotating roller grates. Auxiliary oil burners are used to start up the combustion process. Combustion is self-sustaining in some case, while at other times wood is added. In general, the combustion fraction of MSW is high and in some instances has been raised by moisture-reducing compaction at transfer stations. Total electrical energy recovery from the plants is about 60 megawatts or 250–300 kWh per ton of MSW incinerated. This energy is used partially to run the incinerators operations, and the balance is sold to the national electricity grid. The new plant, costing US$700 million and constructed in 2000, is generating 80 megawatts of electricity from the waste heat, of which 20 megawatts will be consumed by the plant and the remainder sold to Singapore Power (ESCAP, 2000). Incineration plants in other Asian countries like Japan, South Korea and Taiwan are of similar design to those of Singapore. In South Korea, the rise in incinerated waste was shown from 3% in 1994 to 20% in 1999.
There are 1900 general waste incinerators across Japan, of which 1584 incinerators are operated by local governments and the other 316, by private companies. China has two incinerators and five are now in construction for MSW disposal with foreign assistance. 4. WASTE RECYCLING 4.1. Recycling as the Best Solution of Waste Problems The disposal of waste is a monumental problem on a global scale, and in particular of the Asian society, because of the enormous amounts of wastes produced in the region. Although industry receives much bad press over the waste disposal issue, a great deal of the waste generated by industry is actually recycled: defective products, manufacturing overruns, machined scrap, etc. There is no particular altruism in this, since it is generally cheaper to recycle scrap than to make new product. For instance, production of new aluminum cans from recycled cans is 5% cheaper than from raw materials. Today, major corporations are very conscious of the adverse corporate image of poor waste disposal practices. As a slogan Recycling sounds excellent, and indeed should be practiced wherever possible. However, this is not the only option for waste minimization. Recycling is one of the 3-Rs: waste Reduction, Reuse, and Recycling. Advantages of recycling are related to saving of raw materials, saving of time and energy, lower treatment and disposal cost, less risk of legal liability, and less employee exposure to hazardous materials. Central to the program is a Waste Audit, to establish the identities and quantities of the wastes being generated. In some cases, Waste Exchanges have been set up to allow companies to make use of another’s wastes. The following example is characteristic for Waste Exchange, Company A, having waste acid to neutralize may be able to use waste alkali from Company B, rather than using virgin base. In order to estimate the role of waste recycling in the waste disposal system, let us consider a hierarchy of options for waste disposal: 1. disposal into environment; 2. placed in a permitted landfill; 3. burned within a permitted waste-to-energy incinerator or energy recycling; 4 . put to a low value use, as in conversion of waste plastic into park benches; 5. 6. 7. put to a high value use, as in recycling aluminum cans by melting them down; rebuilt, as in repair of a car; reused, as in refilling beverage containers.
At first glance, it would seem that the higher the category, the more environmentally friendly the option. This will not always be true, if total energy and resource cost are computed. For example, at some point the cost of repair of the car (option 6) exceeds
the costs of recycling the components and starting afresh (option 5). An unforeseen problem that has arisen with respect to municipal recycling is that its success is being undermined by insufficient capacity to treat and sell the material collected, especially newsprint. In many countries, virgin product is cheaper than the recycled material, because of collection and transportation costs (Bunce, 1994). As a result, recycling programs in the developed countries tend to be heavily dependent upon government subsidy. However, this is not a case in developing Asian countries with low labor cost. 4.2. Plastic Waste Recycling Plastic wastes are now contributing 7–8% by weight of North American municipal garbage, and even more, up to 10–15% in the Asian countries. About half of this amount represents discarded packaging material (altogether, packaging forms nearly one-third of all municipal garbage). The littering of roadsides, the countryside, and ocean beaches by plastic rubbish calls attention to the near-indestructibility of synthetic plastics such as polyethylene, polystrene, and polyvinyl chloride. As with most other pollutants, many of the virtues of plastics (light weight, inertness to chemical and microbial attack, low cost) become their weaknesses when they are discarded. At present, the proportion of plastic recycling in the various countries is very small, mostly polyethylene terephthalate from soft drink bottles and polyethylene from plastic milk jugs (see details below). This requires manual sorting of the plastics waste stream to produce a homogeneous feed for recycling, since chemically different plastics are not necessary compatible with each other. Plastic life cycle analysis Once the plastic product has been used in its application, it has arrived at the end of its life cycle, as shown in Figure 14. At this stage, there are three general options for plastics: recycling, incineration, and/or dumping. Although incineration is often seen as thermal or energy recycling, it is mentioned here as a separate option because it does not linger on in the life cycle. The material, burned only once, stimulates the greenhouse effect, leads to air pollution, and to a waste of resources with its (contaminated) ash residue. In the case of plastic recycling, energy recycling is often not a permitted option, on account of the negative image of incineration and PCDD/PCDF formation. An intermediate solution, which is under development, is feedstock recycling, in which the plastic is depolymerized and the monomers recycled as chemical feedstocks. A recent development is the recycling of mixed plastic waste into items such as fence posts, park benches and other structural units, which have the advantages over wood and steel that they do not rot. Even incompatible plastics can be blended together to form articles having the strength needed for these application. Degradable plastic materials Two approaches to degrading plastics are now available: photodegradation and biodegradation. However, in most Asian countries, these product forms represent only a very
tiny fraction of all plastics. Furthermore, they do not degrade away literally to nothing. Plastics buried in the landfills do not photodegrade because they are not exposed to sunlight, and even biodegradation is extremely slow under these conditions. Photodegradable plastics are copolymers of conventional polymers such as polyethylene with a small amount of carbon monoxide. The presence of carbon monoxide in the feed affords a polymer containing a small proportion of randomly located carbonyl groups, the key feature of which is their ability to undergo chain scission upon absorption of light in the 300–330 nm range. This is a well-known reaction in organic photochemistry, the mechanism of which creates breaks in the polymer chain to loss of structural integrity (Bunce, 1994), as shown in Figure 15. The structural integrity of polymers is maintained by van der Waals attractions of neighboring chains. Breaks in the polymer chains lead to the plastic disintegrating physically, although in the chemical sense little of the material has already decomposed. The chief advantage of photodegradable plastic is that unsightly litter disappears to powder upon sunlight exposure. We can add one more advantage of this plastic. The carbonyl chromophers present in photodegradable plastics absorb nearly 300 nm, in the UV-B range of tropospheric solar radiation. Artificial lighting, both incandescent tungsten bulbs and fluorescent lamps, produces very little UV-B, and window glass filters out this radiation from sunlight. Thus, the plastics will be degraded only when are discarded to landfill. Biodegradable plastics are also copolymers of conventional polymers such as polyethylene with a small amount of starch molecules. The presence of starch allows micro-organisms to eat this organic component and disintegrate the polymer chains.
Another example of biodegradable plastics is related to the application of polyhydroxyalkanoate (PHAs), which may be biosynthesized from activated sludge of municipal wastewater (Satoh et al, 1998). Recycling in the Asian region The rate of recovery of recyclables from the MSW stream in many countries of the region such as Japan, South Korea and Singapore has increased dramatically in recent years (Hara, 1997, ESCAP, 2000). Overall resource recovery grew from less than 10% of all MSW in 1988 to 30% in 1998 in the region. Most of the increase is attributable to greater rates of recovery of paper and paperboard, plastics, glass and metals. The paper and paperboard category dominates in terms of total tons of materials recovered (almost 60%), which tends to mask the importance of the total mass of recovered materials, but in terms of its total economic value, it far exceeds the paper products category. Among the Asian countries, Japan recycles huge quantities of materials from MSW stream. Almost half of the waste paper is retrieved or recycled in Japan and the retrieval rate increased from about 48% in 1990 to about 56% in 1997. Similarly, recycling rates of aluminum cans and steel cans increased from 40% to 60% and 45% to 70% from 1990 to 1997, respectively. Glass recycling rate also increased from 48% in 1990 to 57% in 1997. South Korea and Singapore recycle huge amounts of paper and cardboard, plastics, glass and metals. The example of Singapore is shown in Table 5. In the Asian region, recycling of plastics derived from MSW is thriving as a booming business. The plastic recycling boom is due to a combination of various factors such as unemployment, low labor cost and a wider market. Plastic recycling
has become very popular in countries such as Bangladesh, China, India and Thailand. In these countries, paper and paperboard recycling is also enhanced. Recycling in developing countries relies largely on informal collection of materials from waste carried out by human scavengers or wastepickers. In the Asian cities, it has been estimated that up to 2% of the population survives by recovering materials from waste to sell for reuse or recycling or for their own consumption. 5. SITE REMEDIATION AFTER WASTE DISPOSAL We have considered already (see Chapter 6) soil remediation after release and accumulation of some types of pollutant like excessive salt accumulation and accidental release of oil products. In this subchapter we will further discuss the problems of site remediation, including bioremediation, after accumulation and release of heavy metals and persistent organic pollutants. 5.1. Heavy Metal Remediation This term refers to cleaning up the soils and conjugated compartment like undersurface and groundwater at a former manufacturing facility, waste site or site after accidental release. These sites may be strongly contaminated by different heavy metals; lead,
cadmium, chromium, mercury, and arsenic are only a few from the list. Soil remediation after pollution of different heavy metals is typically a very expensive process on account of the large volumes of soil involved and the relevant remediation of groundwater is much more expensive. Here we must refer to the known paradigm that prevention of pollution is cheaper than control and remediation. Let us consider some technologies for soil remediation from heavy metals. Electrokinetic remediation Electrokinetic remediation involves passing a low level DC electric current having potential difference a few volts through the soil, either in situ with electrodes placed into the soils, or in an external reactor. It has been shown that heavy metals such as zinc, copper, cadmium, chromium, lead, arsenic, mercury, nickel and iron can be efficiently removed from soils by electrokinetics. An externally supplied fluid is used as the conducting medium. When the current is applied to a soil-water system, electrolytic reactions generate an acid-front at the anode and a base-front at the cathode. The acid-front moves towards the cathode due to the electrical, chemical, and hydraulic potential gradients that are established. Simultaneously, the base-front moves towards the anode due to electrical and chemical gradients. Consequently, the pH profile changes in the soil. The acid-front eventually flushes across the soil and neutralizes the base-front developed at the cathode. Ionic migration and diffusion, and any electro-osmotic advecton enhance the transport of the acid-front, generated at the anode. After the current is applied for several hours, the pH drops to 2 at the anode and rises to 12 at the anode and pH profile in the soil is slowly changed by the production of and at each electrode. The change of the soil pH affects the removal efficiency of heavy metals by affecting the pore fluid composition in the soil, and the surface charge density of soil particles. The acid-front of the soil facilitates desorption of heavy metals from the soil surface and dissolution of hydroxyl complexes of heavy metals. As a result, this increases the heavy metals fractions present in the liquid phase, their mobilities, and the removal efficiency of heavy metal. In contrast, the base-front in the cathode zone can immobilize heavy metals by forming their hydroxides and heavy metal precipitation occurs in the soils close to cathode, causing low removal efficiency. The movement of acid-front to the cathode is due to migration (electric potentials), diffusion (chemical potentials), and advection (hydraulic potential) and causes desorption of heavy metals from clay surfaces and transports them into the pore fluid. Electro-osmotic flow and its associated phenomena constitute the mechanisms for removing heavy metals from soils. If traces of electrolysable solute exist in the pore fluid, it is then possible to carry out water electrolysis by inserting electrodes in a saturated soil mass in the field site. The primary electrode reactions are as follows:
Secondary reactions may exist depending on the concentration of the available species as reactions (7) and (8)
The production of ions at the anode reduces the pH according to the reaction (5) and the production of ions at the cathode increase the pH according to the reaction (6). As a result of migration of anions and cations in the pore fluid of the soil under the electrical field and reactions (5) and (6), two supplemental ionic species (H+ and OH–) can significantly influence the removal of heavy metals from the soil. For example, we can consider the results of soil remediation in South Korea from lead pollution (Yang, 1999). The initial concentration of lead was 200–205 ppm (Table 6) The content of lead, taken as a ratio between the final and initial concentrations in ten different slides of soil after remediation process, is shown in Figure 16. Lead was partially removed along the anode section, and it was accumulated in the cathode section for all of the experiments. It was found that the lowest lead concentration occurred in the soil slides nearest to the anode because lead in the elecrokinetic process becomes cationic and moves towards the cathode, most probably as free ions in the pore fluid. Generally, the higher removal efficiency of lead occurred in system III at a 1.1 ml/min flow rate.
Soil vitrification Soil vitrification (conversion to glass) is an aggressive method of soil treatment that is useful for metal contaminated soils. Electrodes are placed vertically in the soil, and the soil is heated electrically to melting (1660–2000°C) in a zone, which grows downwards and outwards from the electrodes. Inorganic hazardous metals are incorporated into the vitrified mass, while organic species of heavy metals are pyrolized and volatilized. The advantage is that this technology can be applied in field conditions without preliminary handling and transportation of the soil. Vitrification is particularly useful for remediation of soils contaminated with radioactive heavy metals. The radioactive heavy metals are encapsulated in a highly inert, nonporous matrix, which can be stored permanently in underground secured radioactive waste storage facilities currently under development in some Asian countries. Vitrification can also be used for sediments and sludge polluted by heavy metals. Soil washing This process is used to concentrate the contaminants into a fraction, which is treated further. The technology involves subjecting the soil to high pressure jets of water, thus breaking down the soil structure. Surfactants are often used to assist the solubilization of the contaminants into aqueous phase. One of such technologies with application of soluble chelation agents is shown in Figure 17. Bioleaching processes use microorganisms to solubilize heavy metals or heavymetal oxides from mineral matrices by biological catalyzed oxidation-reduction processes. In both soil washing or bioleaching processes, the eluted heavy metal ions are then removed from the aqueous stream by the various chemical or biological process technologies (see details in Rorrer, 1999).
Phytoremediation Pytoremediation is an intriguing process for the biologically promoted removal of heavy metals from both polluted soils and wastewater. Phytoremediation is broadly defined as the use of living green plants to remove, contain, or render harmless environmental contaminants. Phytoremediation processes are capable of removing heavy metals directly from soil or water. Phytoextraction is the use of plants to remove heavy metals from aqueous streams. The basic mechanism of phytoremediation for removal of heavy metals involves two steps (Galiulin et al, 2001): active uptake of heavy metal by the root system, and translocation of heavy-metal ions to green shoots. Plants which can sequester high concentrations of heavy metals are called hyperaccumulator metallophytes. For example, the hyper-accumulator Thalaspi caerulescens can accumulate several heavy metals, including Ni, Zn, and Cd. In this plant, over 18,000 mg of zinc and 1000 mg of cadmium per kg of shoot biomass can be accumulated without inhibition of growth (Rorrer, 1999). The mechanisms for active uptake for heavy-metal ions involve the use of biologically expressed phytochelatins that complex with heavy metals and thus protect the plant’s biochemical processes from the toxic heavy metal effects. Genetic engineering offers a means to improve growth rates of hyper-accumulators or introduce heavy metal remediation activity into plants. For example, the bacterial gene for mercury reductase, which reduces to was recently cloned into Arabidopsis. Engineered reed-bed and constructed wetland systems for removal of heavy metals from wastewater using phytoremediation are in use in some developed Asian countries. The root system of the hyper-accumulator plant penetrates a permeable rock bed. The wastewater is introduced into one end of the bed and flows through the permeable rock layer. The rock layer should be inert to heavy metals binding so that it does not unwittingly serve as a sink for heavy metals. These metals are sequestered by the root system and translocated to the shoots. Periodically, the metal-containing shoots are harvested. The biomass can be burned off or composted to yield a low volume of metal-rich ash. For example, this system is used for phytoremediation of arsenic-contaminated cooling wastewater from the Mae Moh power plant burning Thai lignite with high content of As and other heavy metals (see Chapter 12). Other examples are shown in Box 1. Box 1. Heavy metal phytoextraction from contaminated waters (after Sen and Mondal, 1990 and Shrivastava and Rao, 1997) Sen and Mondal (1990) studied the Cu absorption by floating salvinia (Salvinia natans) abundant in ponds and lakes of India. The Cu concentrations were created in the culture medium where adult plants (with a biomass
of 7.5 g/l) were exposed for five days. The plants absorbed the highest amounts of Cu (92.8–97.0%) from the solutions purified within one day at low metal concentrations only 82.5 and 67.4% were absorbed at concentrations of 50 and respectively. When the solutions contained 50 and the plants perished within three days and one day, respectively. At low concentrations, Cu was accumulated mainly in roots; its content in the macrophyte shoots increased with time at the high metal rates. The authors attempted to release the metal accumulated using the ignition of the plant matter and subsequent boiling in a 1–2N solution. Crystalline copper sulfate 94–96% pure was obtained. The recovery of Cu from the plants exposed in different solutions of its salt ran to 96.7–99.3%. The second stage of the study involved the phytoremediation of the wastewater from the Indian Copper Complex in the city of Bihar (Bihar state) with pH 7.1–7.6 and containing Practically total absorption of the metal was reached when 20 g of plants were kept in 1L of waste liquid for a day. Shrivastava and Rao (1997) estimated the effectiveness of the Hg absorption from water by macrophytes of five genera: Eichhornia, Salvinia, Hydrilla, Chara, and Vallisneria. It was found that after the four-day exposure of the plants in the solution to be purified, the maximum decrease in the Hg content (by 80%) was observed when a combination of macrophytes was used. Next in descending order were individual plant species of genera Eichhornia (by 60%), Salvinia (by 51%), Chara (by 40%), Hydrilla (by 30%), and Vallisneria (by 27%). The authors believe that the combined macrophyte system can effectively absorb Hg from all water layers owing to the peculiarities of habitat of the plants studied. Eichhornia and Salvinia plants will purify the surface layers; Chara and Hydrilla will operate in the water bulk, and Vallisneria will purify the bottom layers by its root system. Phytoremediation processes have two major advantages. First, phytoremediation is an environmentally friendly process for on-site remediation of heavy metals. The only potential drawback is that the species of hyper-accumulator plant introduced into the contaminant site may not be endogenous to the local ecosystems. Second, phytoremediation processes are simple in configuration, require minimal chemical or energy inputs, and are inexpensive to operate. However, like many biologically based processes, they are difficult to control.
5.2. Remediation of Soils Polluted by Persistent Organic Compounds Bioremediation Remediation of soils polluted by persistent organic compounds is based mainly on biological processes and thus called bioremediation. The ultimate goal of bioremediation is to biodegrade toxic compounds into nontoxic forms, eliminating the need for further hazardous waste cleanup. Biological treatment has been used in the Asian region for centuries to degrade various organic wastes. Biological degradation exploits the ability of micro-organisms to produce enzymes that metabolize specific compounds. Some micro-organisms break down a contaminant to carbon dioxide, water, methane,
inorganic salts, and/or biomass, from which they derive carbon and energy for growth. Other microorganisms only partially break down a compound, resulting in incomplete degradation. These intermediate compounds accumulate or are further degraded by other micro-organisms. Often, although not always, these intermediate compounds are less toxic than the parent compounds. The exception is DDT metabolites DDD and DDE, which are more toxic than DDT itself (see Chapter 14). A common method of degradation for highly chlorinated, highly recalcitrant compounds is cometabolism. Cometabolism is the degradation of a compound by a micro-organism that derives no nutritional benefit from that compound. The microorganism population grows on other substrates present in the environment, and the broadly specificity enzymes produced to degrade these food sources fortuitously degrade the contaminant compound. The contaminant degradation rate is very slow and often incomplete, because the microbial population does not increase in biomass or numbers relative to the contaminant’s concentration. Two approaches have been explored for the selection of micro-organisms for degradation of persistent organic compounds, such as organochlorines or polycyclic aromatic hydrocarbons: optimize the growth of the consortia of organisms already present at, and already adapted to the site; developing microbial isolates or genetically engineered organisms with a particular ability to degrade the contaminant from remediated soils. There are many biotechnological companies producing the super-organisms, along with nutrients such as nitrogen and phosphorus to optimize microbial growth. However, experience with super-organisms has been generally disappointing, because the newly introduced micro-organisms fail to compete with the populations of microbes already established in the soil and therefore die out. Alternatively, organisms which have been selected in the laboratory to use, for example PAH or 2,4-D pesticide as their sole carbon source, may rapidly lost that ability when placed into soil, where other, more easily available carbon sources exist. Thus, instead of introducing foreign organisms to a site, a better option is to optimize the growth conditions for the consortia of microorganisms already present in the soil. Natural soil micro-organisms degrade a wide variety of contaminants. Micro-organisms, principally aerobic and anaerobic bacteria and fungi play an important role in the natural degradation of various POPs. These can be transformed oxidatively under aerobic conditions as the conversion of penthachlorophenol to 2,3,5,6-tetrachlorohydroquinone, chloranil and ring cleavage product, or reductively under anaerobic conditions, as dechlorination of PCB contaminants in the sediments. Environmental factors, which greatly influence biodegradation rates, include suitable temperature, oxygen, nutrient availability, pH, moisture content, organic matter concentrations, contaminant concentrations, and the presence of other contaminants. Indigenous micro-organisms are usually the most reliable source of contaminant degraders, and only the environmental conditions need to be adjusted for adequate
degradation to occur. An example of the optimized environmental conditions for biodegradation of PCP is shown in Table 7. Bioremediation may be carried out in situ, with suitable engineering systems for pumping air and nutrients in, and if necessary, leached out. Alternatively, the soil may be removed and treated in an enclosed bioreactor, when the outdoor growing season for the micro-organisms is short, like in the north of Japan or China. Physical-chemical remediation Ozonation of polycyclic aromatic hydrocarbons (PAHs) in the unsaturated soils is one of the advanced technologies for soil treatment. The PAHs are resistant to typical in situ remediation technologies including soil flushing and bioremediation. Ozonation of PAHs in soils is used for overcoming these limitations. In situ ozonation is based on the delivery of gaseous ozone molecules to contaminated media to destroy the contaminants by converting them into innocuous compounds commonly found in nature. A Korean study was demonstrated that the three ring condensed phenanthracene has the highest removal efficiency followed by the four ring condensed pyrene and then the three ring condensed anthracene (Kim et al, 1999). The molecular structure of each PAH significantly affects the reaction rate with ozone rather than the molecular weight. Increasing of the moisture content results in the ozone demand because of the dissolution of gaseous ozone into soil pore water and subsequent decomposition of ozone. However, the presence of water increases the removal rate of PAHs as compared to that of dry soils. Surfactant-enhanced soil flushing is also an effective procedure for remediation of PAH-contaminated soils. The removal efficiency of phenanthrene by this treatment was up to 90% after 25 pore volumes. The toxicological test with earthworm Eigenia foetida showed that the response of this test organism was uniform after 15 pore volumes.
Generally we can note that polychlorinated organic compounds like PCBs are very resistant to various bioremediation procedures and physical-chemical treatments are the most used for heavily contaminated sites. For remediation of PCB-contaminated sites the following technologies can be recommended. Thermal desorption involves volatilization of contaminants from soil by heating the soil in various kinds of drying kilns, and is the solid phase analog of air or stream stripping. Temperatures in range 200-500 °C are used. The off-gases must then be treated before release to the atmosphere. This method can be applied to hydrocarboncontaminated soils and to soils contaminated with organochlorine pesticides (Bunce, 1994). Soil incineration is another method for treating soils that are contaminated with organic compounds. This method is very expensive but the soil can be completely cleaned of organic pollutants. FURTHER READING 1. Bunce N., 1994. Environmental Chemistry. 2nd edition, Wuerz Publishing Ltd, Winnipeg, Canada, Chapter 8. 2. ESCAP, 2000. State of the Environment in Asia and Pacific. United Nations, Chapter 17. 3. Diemer R. B., Ellis T. D., Silcox G. D., Lighty J. D., and Pershing D. W., 1999. Hazardous waste: incineration. In: Meyers R. A. (Ed.), Encyclopedia of Environmental Pollution and Cleanup, John Wiley, NY, 754–761. 4. Van den Broek W., Wienke D., and Buydens L., 1999. Plastic among nonplastics, identification in mixed waste. In: Meyers R. A. (Ed.), Encyclopedia of Environmental Pollution and Cleanup, John Wiley, NY, 1283–1289. 5. Satoh H., Iwamato Y., Mino T. and Matsuo T., 1998. Activated sludge as a possible source of biodegradable plastics. Water Science Technology, 38(2), 103–109. 6. Yang J.-W., 1999. Transport of heavy metal in soils under elektrokinetic field. In: Proceedings of the 2nd International Symposium on Advanced Environmental Monitoring. KJIST, Kwangju, South Korea, 57–62. 7. Rorrer G. I., 1999. Heavy-metal ions, removal from wastewater. In: Meyers R. A. (Ed.), Encyclopedia of Environmental Pollution and Cleanup, John Wiley, NY, 771–779 8. Galiulin R. F., Bashkin V. N., Galiulina R. R. and Birch P., 2001. A Critical Review: Protection from Pollution by Heavy Metals—Phytoremediation of Industrial Wastewater, Land Contamination & Reclamation, 9(4), 349–357
WASTE TREATMENT WEBSITES OF INTEREST 1. Environmentally friendly technologies for the Asian region, http://www.un.org/escap/apctt QUESTIONS AND PROBLEMS
1. Characterize the hot problems related to waste generation in the whole Asian region and in your country in particular. 2. Describe the generation of municipal solid waste in your country and compare these values with those typical for the whole Asian region. 3. Characterize the typical chemical composition of municipal wastewater and point out the possible sources of these contaminants. 4. Estimate the sources of hazardous solid wastes in your locality and compare with the total generation of these wastes in your country. 5. Discuss the typical chemical composition of industrial wastewater and point out the possible sources of these contaminants. 6. Discuss the production of agricultural waste in the Asian region and stress the similarities and differences between these and other types of wastes. 7. Describe the transboundary shipping of hazardous wastes in the Asian region with discussion of the relevant laws in your country regulating this shipping. 8. Characterize different methods of MSW disposal and note the most applicable methods in your locality. 9. Describe the construction of modernlandfills andenvironmental problems related to the landfilling. 10. Discuss the application of incineration to the MSW, hospital and industrial waste treatment in your country. 11. Discuss the environmental problems of the most concern related to incineration and the ways to minimize these problems. 12. Characterize the advantages of recycling and estimate the role of waste recycling in the waste disposal system. 13. Discuss the generation of plastic waste and plastic recycling in your country. Focus attention on chemical and biological recycling treatments. 14. Discuss the problem of waste recycling both in your country and in the whole Asian region.
15. Discuss the general approaches to soil remediation. Draw the similarities and differences between remediation of sites polluted by heavy metals and persistent organic pollutants. 16. Characterize the physical and chemical principles of electrokinetic treatment of polluted soils. Give examples. 17. What chemical and physical technologies can be applied for remediation of HM polluted sites? 18. Discuss the phytoremediation technology for polluted soils and wastewater. Draw advantages and drawbacks. 19. Describe the principles of bioremediation for soils contaminated by POPs and present a relevant example. 20. Characterize physical-chemical treatments of POP-contaminated soils and its applicability for different compounds.