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I.2.6. ORGANIC POLLUTANTS
Chemical compounds are regarded as inorganic or organic. This division comes from the ideas that there were differences between compounds originating from living plants and animals and those from non-living sources. The organic compounds contain carbon combined with other elements e.g. oxygen, hydrogen and nitrogen. There is also a difference between synthetic and natural organic compounds. Natural compounds are of course those that can be found in the environment and are produced by bacteria, animals and plants. Synthetic compounds are made by humans. It could be a natural compound, which is produced in large scale or it could also mean a new compound that did not exist previously. There is a very large number of different organic compounds and around 100 000 are in commercial use. Each year a couple of thousand new substances are added to the list.
When a chemical is used it will sooner or later be released into the environment. In order to forecast its effect, it is important to have some knowledge about its characteristics. The impact it could have is a combination of its toxicity, biodegradability and potential for bioaccumulation. The worst scenario would be the release of a persistent (non-biodegradable) chemical, which is also toxic and accumulates in animals and plants.
It is an impossible task to describe every single organic compound. Instead, the most important, from an environmental perspective, groups of organic pollutants will be dealt in this chapter.
Most of the modern human activities generate waste associated to a water phase. In the manufacturing of products there are usually also some unwanted by-products formed, which must be eliminated. In the factory, these are separated from the product and discharged as a wastewater. Also in households, a lot of sewage is formed due to daily activities. Normally these wastes are collected and treated in some way but not all substances are reduced during this procedure. In the end the treated water is released to some sort of recipient and the remaining compounds can then cause damage to the environment. The disposal of solid waste, both by households and industries will eventually also lead to some generation of sewage. This is due to the atmospheric precipitation that is transported through the landfill and leach out different compounds. The effect these different types of sewage will have on the environment and treatment processes is in some way depending on where they originate. Some different types of wastewater are presented in the following text together with general characteristics for these water types.
The major contributors to municipal wastewater are households whereas the share of industrial water may differ significantly from case to case. Besides wastewater, there could also be a significant amount of groundwater that leach into the sewage system and dilutes the water. Municipal wastewater is in general regarded as easily degradable but this could be changed if the share of industrial load is increased. If the water is let out to a recipient without treatment it would lead to eutrophication and lack of oxygen in the receiving waters. To avoid this municipal wastewater is treated in treatment plants. To achieve a far-reaching treatment the plant should consist of three different steps. First particular matter is removed with a screen and sedimentation. Remaining particles and dissolved compounds are reduced in a biological step, which usually is some kind of activated sludge process. The activated sludge is a suspension of microorganisms that nourish on the components in the water while they are reproducing. The suspension is settled in a subsequent step and the water is led to the final stage, chemical treatment. When the microorganisms are growing in the biological step they also require nutrients that also are present in the wastewater. However, the amount of the major nutrients nitrogen and phosphorous are usually larger than what is required. In the chemical step the surplus of phosphorous can be removed by precipitation with metallic salts. A final clarifier removes the particles created in the chemical step and the cleaned water is discharged to the recipient.
The wastewater from the industry is much more diverse compared to the wastewater from households, which have a uniform composition despite origin. However, a classification of industrial wastewater can be done based on the type of industry. The following reasoning is of a more general nature and there exists situations where the conditions are somewhat different.
In the pharmaceutical industry the products are usually produced in batches during different campaigns. In addition to this, there are also always some new products included in the production plan or other products are either removed or produced by a changed process. These continuous changes will be reflected in the wastewater composition. So, the water from a pharmaceutical industry would normally vary a lot, perhaps not on a day-to-day basis but on a week-to-week basis. Although the purpose is to produce the product for sale, small residuals quantities may enter the wastewater due to the cleaning of process equipment. Besides products, the byproducts formed during the production process and impurities retrieved during processing may end up in the sewage. These waters could also contain toxicity since the products are designed to be biochemically active.
In the metallurgic industry several operations such as pressing, drilling and rolling are used during the conversion of metal to the final product. Before the product reaches the final market its surface is also painted, passivated or treated in some other way. In some process steps the object is contaminated with grease or cutting fluid, which has to be removed before any further processing can be done. Normally this is done in some degreasing step using alkaline, acid or neutral cleaning solution, which is followed by several rinsing steps. The cleaning solution, rinsing water and other types of surface conditioning baths have to be renewed with regular intervals and some of it is then purged as a wastewater. This sewage may contain metals, tensides, grease and complexing agents and may either have a low or high pH. Some process streams may also contain cyanide and chromium, which require special treatment before they can be discharged.
Foodstuff is produced both in batch and continuous processes and wastewater is generated above all during washing and rinsing of process equipment. Depending on the actual production type there may be large or small variations in the wastewater concentration. However, the components in the wastewater are usually harmless and easily degradable.
The pulp and paper industry produces a broad range of paper products as e.g. newspapers, journal papers, containerboard and different special paper such as filters and thermo paper. All these products are formed from cellulose fibers, which originate either from virgin fibers or from recycled fibers. The production of paper can be regarded as a two-step process. In the first step fibers are detached from the wood either mechanically or chemically. Depending on what type of product will be made from the free fibers, which are referred to as pulp, they are bleached in order to increase the pulp's whiteness. The pulp is then used in the second step when the paper product is formed. The pulp is mixed with water and several additives to a furnish, which in the paper machine is converted to paper. A lot of water is used both in the production of the pulp and the final paper product. The water consumption is usually higher for special paper and fine printing paper than for board made from recycled fibers. When this water is too contaminated and can not be used any more it is discharged. The composition of the wastewater depends on the production process and what type of raw material that is used. Process streams from debarking and bleaching can be harder to degrade and may also be colored. Other streams are usually more degradable and not so colored.
Surfactants (surface active agents) are molecules that are acting at the surface between polar and non-polar phases. One part of the molecule is hydrophobic and the other is hydrophilic. The hydrophobic part is made up of a long hydrocarbon chain, which prefers to be dissolved in a non-polar medium. A polar group or a group that can establish hydrogen bonding is situated at hydrophilic end of the molecule. This part prefers to be immersed in the water phase. A schematic view of a surfactant can be seen in Figure I.2.5.
Figure I.2.5. Structure of the surfactant, sodium myristate, with the hydrophobic and hydrophilic parts denoted.
When enough surfactants are added to water they interact with each other under the formation of small micelles consisting of 50 to 100 individual molecules. The micelles look like a sphere with the hydrophobic end towards the centre and the hydrophilic parts on the outside towards the water. Treating a dirty surface with this water solution causes the hydrophobic substances as grease and fat to dissolve into the centre of the micelles. The hydrophobic substances are thus removed from the surface and kept in the water phase as a stable emulsion. This effect could be used for cleaning, creation of emulsion and foams. Surfactants are used in many different products such as soaps, detergents, shampoo and hair conditioners.
Surfactants can be divided into different categories depending on the electrical charge at the hydrophilic end of the molecule:
The first forms of surfactants were not so degradable, which led to the formation of stable foams in wastewater treatment plants and recipients. This started a debate about their negative impact on the environment and a development towards better products. The first surfactant was made up of highly branched alkyl sulfonate, which was persistent towards biological degradation. These were then replaced by linear alkyl sulfonate, which is easier to degrade. Today's surfactants are usually highly biodegradable since they often are composed of functional groups that exist in natural materials. Surfactants are regarded as harmless towards mammals whereas aquatic organisms are usually affected with a LD50 in the mg/L range. It is particular cationic surfactants that are toxic and they are also used as germicides and disinfectants. However, other types of surfactants exist, such as alkylphenol ethoxylates and are toxic. One example of a surfactant belonging to this group is nonylphenol ethoxylate, which is biodegradable but not completely degradable. It is converted to nonylphenol, which is persistent, bio-accumulative and is toxic towards aquatic organisms. However, most surfactants are not accumulating in organisms, probably due to the high degradability, but cationic surfactants may bind to organic material and sediment by sorption.
The impact of surfactants in seawaters can be seen also in the coastal vegetation. Decline of coastal vegetation has been reported in several countries, affecting a variety of species and countries (Badot and Badot, 1995; Bussotti et al. 1995; Garrec and El Ayeb 2001), and can be attributed to the presence of surfactants transported by winds and deposited on the leaves. In this case, surfactants are absorbed via cuticles and stomata, causing direct damage to cell membranes, destroy chloroplasts and other cellular organelles (Bussotti et al. 1997). Indirect action can be observed, the saline components absorption is enhanced by the reduction of the water surface tension. Damage caused by sea spray carrying surfactants is usually observed within 100 meters from the seashore, near the mouth of rivers or harbors. Symptoms consist in leave discoloration and necrosis beginning form the apex of the leaf.
I.2.6.3. Halogenated Carbons
Halogenated carbons are a group of chemicals with one or two carbon where some or all of the hydrogen has been substituted by a halogen. The group of chemicals where either chlorine or fluorine atoms has replaced all hydrogen is referred to as chlorofluorocarbon (CFC). In hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HCF) there are some hydrogen atoms left. The substitution of hydrogen with a halogen gave the molecule characteristics making the chemicals suitable for use as refrigerants, in making plastic foam and as cleaning agent of electrical units. Although the chemical during its use as e.g. refrigerant is in a closed container large amounts have been released into the atmosphere. It has been estimated that 90% of what have been produced as dichlorodifluoromethane have been released to the air and the cumulative production of this chemical up to 1990 was approximately 107 ton. The problem with the released chlorofluorocarbons is that they are inert and they diffuse to the upper atmosphere. There they start to deplete the ozone layer by a number of reactions with the result that ozone is converted to oxygen. The ozone layer functions as a protective fence against ultraviolet radiation and when it is damaged this radiation reaches the earth's surface causing damage such as skin cancer.
I.2.6.4. Polycyclic Aromatic Hydrocarbons
Polycyclic aromatic hydrocarbons (PAH) are a large group of chemicals made up of only carbon and hydrogen. They differ in size but have at least two condensed aromatic rings in one plane. The smallest PAH is naphthalene, which structure can be seen in Figure I.2.6.
Figure I.2.6. Chemical structure for naphthalene.
The number of aromatic rings, in the most environmental important compounds, range from two to seven. Different alkyl groups can be attached to these aromatic rings and thereby increasing the number of possible combinations.
PAHs are formed in imperfect combustion processes when oil, coal, petroleum products or organic matter is incinerated. Examples of such human activities are coal–fired electricity power plants and internal combustion engine in different vehicles. Smoking tobacco is also an activity, which produces PAH. It is not a large contributor on the whole but it is important since it exposes the smoker and the immediate surroundings to large amounts of PAHs. These compounds are also generated naturally during forest fires and volcanic eruptions.
It is found that PAHs are causing cancer and one of the most dangerous compounds is benzo(a)pyrene (Figure I.2.7.). Most PAH are also considered to be persistent, toxic and accumulates in organisms.
Figure I.2.7. Chemical structure of benzo(a)pyrene.
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