2.1 Natural Biodegradation

Natural biodegradation typically involves the use of molecular oxygen (O₂), where oxygen (the terminal electron acceptor) receives electrons transferred from an organic contaminant:

$\mathrm{Organic}\mathrm{substrate}+{\mathrm{O}}_2\to \mathrm{biomass}+{\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{other}\mathrm{products}$

In the absence of oxygen, some microorganisms obtain energy from fermentation and anaerobic oxidation of organic carbon. Many anaerobic organisms (anaerobes) use nitrate, sulfate, and salts of iron (III) as practical alternates to oxygen acceptor as, for example, in the anaerobic reduction process of nitrates, sulfates, and salts of iron (III):

$\begin{array}{l}2{{\mathrm{NO}}_3}^{-}+10{\mathrm{e}}^{-}+12{\mathrm{H}}^{+}\to {\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}\hfill \\ {{\mathrm{SO}}_4}^{2-}+8{\mathrm{e}}^{-}+10{\mathrm{H}}^{+}\to {\mathrm{H}}_2\mathrm{S}+4{\mathrm{H}}_2\mathrm{O}\hfill \\ \mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_3+{\mathrm{e}}^{-}+3{\mathrm{H}}^{+}\to {\mathrm{F}\mathrm{e}}^{2+}+3{\mathrm{H}}_2\mathrm{O}\hfill \end{array}$

2.2 Traditional Biodegradation Methods

Methods for the cleanup of pollutants have usually involved removal of the polluted materials, and their subsequent disposal by land filling or incineration (so-called dig, haul, bury, or burn methods) (Speight, 1996, 2005; Speight and Lee, 2000). Furthermore, available space for landfills and incinerators is declining. Perhaps one of the greatest limitations to traditional cleanup methods is the fact that in spite of their high costs, they do not always ensure that contaminants are completely destroyed.

Conventional biodegradation methods that have been, and are still, used are (1) composting, (2) land farming, (3) biopiling, and (4) use of a bioslurry reactor (Speight, 1996; Speight and Lee, 2000; Semple et al., 2001).

Composting is a technique that involves combining contaminated soil with nonhazardous organic materials such as manure or agricultural wastes; the presence of the organic materials allows the development of a rich microbial population and elevated temperature characteristic of composting. Land farming is a simple technique in which contaminated soil is excavated and spread over a prepared bed and periodically tilled until pollutants are degraded. Biopiling is a hybrid of land farming and composting, it is essentially engineered cells that are constructed as aerated composted piles. A bioslurry reactor can provide rapid biodegradation of contaminants due to enhanced mass transfer rates and increased contaminant-to-microorganism contact. These units are capable of aerobically biodegrading aqueous slurries created through the mixing of soils or sludge with water. The most common state of bioslurry treatment is batch; however, continuous-flow operation is also possible.

The technology selected for a particular site will depend on the limiting factors present at the location. For example, where there is insufficient dissolved oxygen, bioventing or sparging is applied, and biostimulation or bioaugmentation is suitable for instances where the biological count is low. On the other hand, application of the composting technique, if the operation is unsuccessful, it will result in a greater quantity of contaminated materials. Land farming is only effective if the contamination is near the soil surface or else bed preparation is required. The main drawback with slurry bioreactors is that high-energy input is required to maintain suspension and the potential needed for volatilization.

Other techniques are also being developed to improve the microbe-contaminant interactions at treatment sites so as to use biodegradation technologies at their fullest potential. These biodegradation technologies consist of monitored natural attenuation, bioaugmentation, biosimulation, surfactant addition, anaerobic bioventing, sequential anaerobic/aerobic treatment, soil vapor extraction, air sparging, enhanced anaerobic dechlorination, and bioengineering (Speight, 1996; Speight and Lee, 2000).

2.3 Enhanced Biodegradation Treatment

Enhanced biodegradation is a process in which indigenous or inoculated microorganisms (e.g., fungi, bacteria, and other microbes) degrade (metabolize) organic contaminants found in soil and/or groundwater and convert the contaminants to innocuous end products. The process relies on general availability of naturally occurring microbes to consume contaminants as a food source or as an electron acceptor (chlorinated solvents, which may be waste materials from chemical processes). In addition to microbes being present, in order to be successful, these processes require nutrients such as carbon, nitrogen, and phosphorus.

Enhanced biodegradation involves the addition of microorganisms (e.g., fungi, bacteria, and other microbes) or nutrients (e.g., oxygen, nitrates) to the subsurface environment to accelerate the natural biodegradation process.

2.4 Biostimulation and Bioaugmentation

Biostimulation is the method of adding nutrients such as phosphorus and nitrogen to a contaminated environment to stimulate the growth of the microorganisms that break down chemicals. Additives are usually added to the subsurface through injection wells although injection well technology for biostimulation purposes is still emerging. Limited supplies of these necessary nutrients usually control the growth of native microorganism populations. Thus, addition of nutrients causes rapid growth of the indigenous microorganism population, thereby increasing the rate of biodegradation.

It is to be anticipated that the success of biostimulation is case specific and site specific, depending on the properties of the chemicals, the nature of the nutrient products, and the characteristics of the contaminated environments. When oxygen is not a limiting factor, one of keys for the success of biostimulation is to maintain an optimal nutrient level in the interstitial pore water. Several types of commercial biostimulation agents are available for use in biodegradation (Zhu et al., 2004).

Bioaugmentation is the addition of pregrown microbial cultures to enhance microbial populations at a site to improve contaminant clean up and reduce clean up time and cost. Indigenous or native microbes are usually present in very small quantities and may not be able to prevent the spread of the contaminant. In some cases, native microbes do not have the ability to degrade a particular contaminant. Therefore, bioaugmentation offers a way to provide specific microbes in sufficient numbers to complete the biodegradation (Atlas, 1991).

Mixed cultures have been most commonly used as inocula for seeding because of the relative ease with which microorganisms with different and complementary biodegradative capabilities can be isolated (Atlas, 1977). Different commercial cultures were reported to degrade various hydrocarbons (Compeau et al., 1991; Leavitt and Brown, 1994; Chhatre et al., 1996; Mishra et al., 2001; Vasudevan and Rajaram, 2001).

Microbial inocula (the microbial materials used in an inoculation) are prepared in the laboratory from soil or groundwater either from the site where they are to be used or from another site where the biodegradation of the chemicals of interest is known to be occurring. Microbes from the soil or groundwater are isolated and are added to media containing the chemicals to be degraded. Only microbes capable of metabolizing the chemicals will grow on the media. This process isolates the microbial population of interest. One of the main environmental applications for bioaugmentation is at sites with chlorinated solvents. Microbes (such as Dehalococcoides ethenogenes) usually perform reductive dechlorination of solvents such as perchloroethylene and trichloroethylene.

Bioaugmentation adds highly concentrated and specialized populations of specific microbes to the contaminated area, while biostimulation is dependent on appropriate indigenous microbial population and organic material being present at the site.

2.5 In Situ and Ex Situ Biodegradation Methods

Biodegradation can be used as a cleanup method for both contaminated soil and water. Its applications fall into two broad categories: in situ or ex situ. In situ biodegradation treats the contaminated soil or groundwater in the location in which it was found, while ex situ biodegradation processes require excavation of contaminated soil or pumping of groundwater before they can be treated.

In situ technologies do not require excavation of the contaminated soils so may be less expensive, create less dust, and cause less release of contaminants than ex situ techniques. Also, it is possible to treat a large volume of soil at once. In situ techniques, however, may be slower than ex situ techniques, may be difficult to manage, and are only most effective at sites with permeable soil.

The most effective means of implementing in situ biodegradation depends on the hydrology of the subsurface area, the extent of the contaminated area, and the nature (type) of the contamination. In general, this method is effective only when the subsurface soils are highly permeable, the soil horizon to be treated falls within a depth of 8–10 m, and shallow groundwater is present at 10 m or less below ground surface. The depth of contamination plays an important role in determining whether or not an in situ biodegradation project should be employed. If the contamination is near the groundwater but the groundwater is not yet contaminated, then it would be unwise to set up a hydrostatic system. It would be safer to excavate the contaminated soil and apply an on-site method of treatment away from the groundwater.

The typical time frame for an in situ biodegradation project can be in the order of 12–24 months depending on the levels of contamination and depth of contaminated soil. Due to the poor mixing in this system it becomes necessary to treat for long periods of time to ensure that all the pockets of contamination have been treated.

In situ biodegradation is a very site specific technology that involves establishing a hydrostatic gradient through the contaminated area by flooding it with water carrying nutrients and possibly organisms adapted to the contaminants. Water is continuously circulated through the site until it is determined to be clean.

In situ biodegradation of groundwater speeds the natural biodegradation processes that take place in the water-soaked underground region that lies below the water table. One limitation of this technology is that differences in underground soil layering and density may cause reinjected conditioned groundwater to follow certain preferred flow paths. On the other hand, ex situ techniques can be faster, easier to control, and used to treat a wider range of contaminants and soil types than in situ techniques. However, they require excavation and treatment of the contaminated soil before and, sometimes, after the actual biodegradation step.

In situ biodegradation is the preferred method for large sites and is used when physical and chemical methods of remediation may not completely remove the contaminants, leaving residual concentrations that are above regulatory guidelines. This method has the potential to provide advantages such as complete destruction of the contaminant(s), lower risk to site workers, and lower equipment/operating costs. In situ biodegradation can be used as a cost-effective secondary treatment scheme to decrease the concentration of contaminants to acceptable levels or as a primary treatment method, which is followed by physical or chemical methods for final site closure.

Finally, evidence for the effectiveness of biodegradation should include: (1) faster disappearance of chemicals in treated areas than in untreated areas and (2) a demonstration that biodegradation was the main reason for the increased rate of disappearance of the chemical(s). To obtain such evidence, the analytical procedures must be chosen carefully and careful data interpretation is essential, but there are disadvantages and errors when the method is not applied correctly (Speight, 2005; Speight and Arjoon, 2012).

3.1 In Situ and Ex Situ Biodegradation

Biodegradation applications fall into two broad categories: (1) in situ or (2) ex situ. In situ biodegradation processes treats the contaminated soil or groundwater in the location in which it was found. Ex situ biodegradation processes require excavation of contaminated soil or pumping of groundwater before they can be treated. In situ techniques do not require excavation of the contaminated soils so may be less expensive, create less dust, and cause less release of contaminants than ex situ techniques. Also, it is possible to treat a large volume of soil at once. In situ techniques, however, may be slower than ex situ techniques, difficult to manage, and are most effective at sites with permeable soil.

In situ biodegradation of groundwater speeds the natural biodegradation processes that take place in the water-soaked underground region that lies below the water table. One limitation of this technology is that differences in underground soil layering and density may cause reinjected conditioned groundwater to follow certain preferred flow paths. On the other hand, ex situ techniques can be faster, easier to control, and used to treat a wider range of contaminants and soil types than in situ techniques. However, they require excavation and treatment of the contaminated soil before and, sometimes, after the actual biodegradation step.

In situ biodegradation is used when physical and chemical methods of remediation may not completely remove the contaminants, leaving residual concentrations that are above regulatory guidelines. Biodegradation can be used as a cost-effective secondary treatment scheme to decrease the concentration of contaminants to acceptable levels. In other cases, biodegradation can be the primary treatment method and followed by physical or chemical methods for final site closure. Also, it is the preferred method for very large sites.

3.2 Biostimulation and Bioaugmentation

Biostimulation is the method of adding nutrients such as phosphorus and nitrogen to a contaminated environment to stimulate the growth of the microorganisms that break down chemicals. Additives are usually added to the subsurface through injection wells although injection well technology for biostimulation purposes is still emerging. Limited supplies of these necessary nutrients usually control the growth of native microorganism populations. Thus, when nutrients are added, the indigenous microorganism population grows rapidly, potentially increasing the rate of biodegradation.

The primary advantage of biostimulation is that biodegradation will be undertaken by already present native microorganisms that are well suited to the subsurface environment and are well distributed spatially within the subsurface, but the main disadvantage is that the delivery of additives in a manner that allows the additives to be readily available to subsurface microorganisms is based on the local geology of the subsurface.

Bioaugmentation is the addition of pregrown microbial cultures to enhance microbial populations at a site to improve contaminant clean up and reduce clean up time and cost. Indigenous or native microbes are usually present in very small quantities and may not be able to prevent the spread of the contaminant. In some cases, native microbes do not have the ability to degrade a particular contaminant. Therefore, bioaugmentation offers a way to provide specific microbes in sufficient numbers to complete the biodegradation.

Microbial inocula are prepared in the laboratory from soil or groundwater either from the site where they are to be used or from another site where the biodegradation of the chemicals of interest is known to be occurring. Microbes from the soil or groundwater are isolated and are added to media containing the chemicals to be degraded. Only microbes capable of metabolizing the chemicals will grow on the media. This process isolates the microbial population of interest. One of the main environmental applications for bioaugmentation is at sites with chlorinated solvents and specific microbes (D. ethenogenes) usually perform reductive dechlorination of solvents such as perchloroethylene and trichloroethylene.

Bioaugmentation adds highly concentrated and specialized populations of specific microbes to the contaminated area, while biostimulation is dependent on appropriate indigenous microbial population and organic material being present at the site. Therefore, it might be that bioaugmentation is more effective than biostimulation, but most cleanup programs have a site specificity that is not able to be matched form one site to another.

However, results suggest that the success of biostimulation is case specific, depending on (1) chemical properties, (2) the nature of the nutrient products, and (3) the characteristics of the contaminated environments. When oxygen is not a limiting factor, one of keys for the success of the biostimulation process is to maintain an optimal nutrient level in the interstitial pore water.

3.3 Monitored Natural Attenuation

The term monitored natural attenuation refers to the reliance on natural attenuation to achieve site-specific remedial objectives within a time frame that is reasonable compared to that offered by other more active methods.

The natural attenuation processes that are at work in such a remediation approach include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. These in situ processes include biodegradation, dispersion, dilution, sorption, volatilization, and chemical or biological stabilization, transformation, or destruction of contaminants. A study of any contaminated site must first be performed to decide whether natural attenuation would make a positive input, and though it has degraded lighter chain hydrocarbons quite extensively; the heavier chain hydrocarbons are less susceptible.

As with any technique, there are disadvantages—the disadvantages of monitored natural attenuation method are the need for longer time frames to achieve remediation objectives, compared to active remediation, the site characterization may be more complex and costly and long-term monitoring will generally be necessary.

3.4 Soil Vapor Extraction, Air Sparging, and Bioventing

Soil vapor extraction removes harmful chemicals, in the form of vapors, from the soil above the water table. The vapors are extracted from the ground by applying a vacuum to pull it out.

Like Soil vapor extraction, air sparging uses a vacuum to extract the vapors. Air sparging uses air to help remove harmful vapors like the lighter gasoline constituents (i.e., BTEX), because they readily transfer from the dissolved to the gaseous phase, but is less applicable to diesel fuel and kerosene. When air is pumped underground, the chemicals evaporate faster, which makes them easier to remove. Methane can be used as an amendment to the sparged air to enhance cometabolism of chlorinated organics. Soil vapor extraction and air sparging are often used at the same time to clean up both soil and groundwater.

Biosparging is used to increase the biological activity in soil by increasing the oxygen supply via sparging air or oxygen into the soil. In some instances, air injections are replaced by pure oxygen to increase the degradation rates. However, in view of the high costs of this treatment in addition to the limitations in the amount of dissolved oxygen available for microorganisms, hydrogen peroxide (H₂O₂) was introduced as an alternative, and it was used on a number of sites to supply more oxygen (Schlegel, 1977) and more efficient in enhancing microbial activity during the biodegradation of contaminated soil and groundwater (Brown and Norris, 1994; Flathman et al., 1991; Lee et al., 1988; Lu, 1994; Lu and Hwang, 1992; Pardieck et al., 1992), but it can be a disadvantage if the toxicity is sufficiently to microorganisms even at low concentrations (Brown and Norris, 1994; Scragg, 1999).

Soil vapor extraction requires drilling extraction wells within the polluted area. The necessary equipment to create a vacuum is attached to the well, which pulls air and vapors through the soil and up to the surface. Once the extraction wells pull the air and vapors out of the ground, special air pollution control equipment collects them. The equipment separates the harmful vapors from the clean air.

Air sparging works very much like soil vapor extraction. However, the wells that pump air into the ground are drilled into water-soaked soil below the water table. Air pumped into the wells disturbs the groundwater. This helps the pollution change into vapors. The vapors rise into the drier soil above the groundwater and are pulled out of the ground by extraction wells. The harmful vapors are removed in the same way as soil vapor extraction. The air used in soil vapor extraction and air sparging also helps clean up pollution by encouraging the growth of microorganisms. In general, the wells and equipment are simple to install and maintain and can reach greater depths than other methods that involve digging up soil. Soil vapor extraction and air sparging are effective at removing many types of pollution that can evaporate.

Air sparging should not be used if free products are present. Air sparging can create groundwater mounding which could potentially cause free product to migrate and contamination to spread. Also, it is not suitable around basements, sewers, or other subsurface confined spaces are present at the site. Potentially dangerous constituent concentrations could accumulate in basements unless a vapor extraction system is used to control vapor migration. If the contaminated groundwater is located in a confined aquifer system, air sparging is not advisable because the injected air would be trapped by the saturated confining layer and could not escape to the unsaturated zone. Anaerobic sparging, an innovative technique in biodegradation, depends on the delivery of an inert gas (nitrogen or argon) with low (< 2%) levels of hydrogen. Cometabolic air sparging is the delivery of oxygen-containing gas with enzyme-inducing growth substrate (such as methane or propane).

Bioventing is a technology that stimulates the natural in situ biodegradation of any aerobically degradable compounds in soil by providing oxygen to existing soil microorganisms. In contrast to soil vapor vacuum extraction, bioventing uses low airflow rates to provide only enough oxygen to sustain microbial activity. Two basic criteria have to be satisfied for successful bioventing (1) the air must be able to pass through the soil in sufficient quantities to maintain aerobic conditions and (2) natural hydrocarbon-degrading microorganisms must be present in concentrations large enough to obtain reasonable biodegradation rates.

Bioventing is a medium to long-term technology—cleanup ranges from a few months to several years—and it is applicable to any chemical that can be aerobically biodegraded. The technique has been successfully used to remediate soils contaminated by hydrocarbons, nonchlorinated solvents, some pesticides, wood preservatives, and other organic chemicals. Though there are limitations, this technology does not require expensive equipment and relatively few personnel are involved operation and maintenance; therefore bioventing is receiving increased exposure in the remediation consulting community. Potential improvements on the current bioventing methods that have been taking place are the use of electrochemical oxygen gas sensors, detailed characterization of NAPL distribution, and neutron probe logging. Another bioventing enhancement is the use of this technique with bioslurping and soil vapor extraction (Baker, 1999).

Bioslurping is an in situ remediation technology that combines the two remedial approaches of bioventing and vacuum-enhanced free-product recovery. It is faster than the conventional remedy of product recovery followed by bioventing. The system is made to minimize groundwater recovery and drawdown in the aquifer. Bioslurping was designed and is being tested to address contamination by hydrocarbons with a floating lighter nonaqueous phase liquids layer.

Bioslurping efficiently recovers free product and extracts less groundwater for treatment, which speeds up remediation and reduces water handling and treatment costs. It enhances natural in situ biodegradation of vadose zone (which extends from the top of the ground surface to the water table) soils and may be the only feasible remediation technology at low-permeability sites. But for bioslurping to even be considered at a contamination site, free product must be present and the product must be biodegradable, also the soil must respond to bioventing.

3.5 Use of Biosurfactants

Another common emerging technology is the use of biosurfactants, which are microbially produced surface-active compounds. They are amphiphilic molecules with both hydrophilic and hydrophobic regions, causing them to aggregate at interfaces between fluids with different polarities found in chemical spills. Many of the known biosurfactant producers are hydrocarbon-degrading organisms.

Biosurfactants have comparable solubilization properties to synthetic surfactants but have several additional advantages that make them superior candidates in biodegradation schemes. First, biosurfactants are biodegradable and are not a pollution threat. Furthermore, most studies indicate that they are nontoxic to microorganisms and therefore are unlikely to inhibit biodegradation of nonpolar organic contaminants. Biosurfactant production is less expensive, can be easily achieved ex situ at the contaminated site, and has the potential of occurring in situ.

Biosurfactants are also effective in many diverse geologic formations and are compatible with many existing remedial technologies (such as pump and treat rehabilitation, air sparging, and soil flushing) and significantly accelerate innovative approaches including microbial, natural attenuation-enhanced soil flushing, and bioslurping.

3.6 Rhizosphere Biodegradation

Rhizosphere biodegradation is the interaction between plants and microorganisms and is also known as phytostimulation or plant-assisted biodegradation. The plant root zone (the rhizosphere) has significantly larger numbers of microorganisms than soils which do not have plants growing in them, which appears to enhance the biodegradation of organic compounds (Wenzel, 2009). In the rhizosphere biodegradation process, plants provide oxygen, bacteria, and organic carbon to encourage the degradation of organics in the soil. The microorganisms in the environment created by the plants, together with the roots of the plants, can degrade more contaminants that could occur in a purely microbial system.

Plants release stimulants into the soil environment that help to motivate the degradation of organic chemicals by inducing enzyme systems of existing bacterial populations, stimulating growth of new species that are able to degrade the wastes, and/or increasing soluble substrate concentrations for all microorganisms. Plants help with microbial conversions where certain bacteria that metabolize pollutants are able to encourage degradation of chemicals in the soil, so allowing biodegradation to occur with less retardation.

Evaluation of the current efforts suggests that pollutant bioavailability in the rhizosphere of phytoremediation crops is decisive for designing phytoremediation technologies with improved, predictable remedial success. For phytoextraction, emphasis should be put on improved characterization of the bioavailable metal pools and the kinetics of resupply from less available fractions to support decision-making on the applicability of this technology to a given site. Limited pollutant bioavailability may be overcome by the design of plant-microbial consortia that are capable of mobilizing metals/metalloids by modification of rhizosphere pH and ligand exudation, or enhancing bioavailability of organic pollutants by the release of biosurfactants.

The complexity and heterogeneity polluted soils will require the design of integrated approaches of rhizosphere management such as (1) combining cocropping of phytoextraction and rhizodegradation crops, (2) inoculation of microorganisms, and (3) soil management.

3.7 Bioengineering in Biodegradation

In many cases, after a chemical spill, the natural microbial systems for degrading the chemical are overwhelmed. Therefore, molecular engineers are constructing starvation promoters to express heterologous genes needed in the field for survival and adding additional biodegradation genes that code for enzymes able to degrade a broader range of compounds present in the contaminated environments. Various bacterial strains are also being developed, where each strain is specific for a certain organic compound present in chemical spills. This will help increase the speed of biodegradation and allow detailed cleanup to take place where no organic contamination remains in the environment.

Thus, the decision to bioremediate a site is dependent on cleanup, restoration, and habitat protection objectives, and the factors that are present would have an impact on success. If the circumstances are such that no amount of nutrients will accelerate biodegradation, then the decision should be made on the need to accelerate disappearance of chemicals to protect a vital living resource or simply to speed up restoration of the ecosystem. These decisions are clearly influenced by the circumstances of the spill.

5.1 Chemical Wastes and Treatment

The manufacture of organic chemicals is a large source of pollution worldwide, and part of the reason for the expansive reach of chemical manufacturing is the diverse and varied types of sectors and activities that are included in it. The EPA defines chemical manufacturing as “creating products by transforming organic and inorganic raw materials with chemical processes.” These are further broken up into commodity and specialty chemicals. Commodity chemicals are basic singular chemicals in ongoing production at industrial plants. Specialty chemicals are batches of combination chemicals made at the request of certain industries and produced on an as needed basis. New chemicals are introduced, and old chemicals are withdrawn constantly, changing the chemical manufacturing market frequently, making it difficult to monitor and evaluate. The sheer size of the industry makes it difficult to monitor as well since the industry accounts for substantial amounts of global income of international trade.

Organic chemicals can be released through the same pathways as other pollutants, including emissions from heating and processing, accidental release of dust or other particulates, accidental spills, and improper disposal of solid waste and wastewater. Once in the environment exposure media includes air, water, soil, and food. In the Blacksmith Institute's database, which focuses on chemical dumps and abandoned sites, the exposure pathways are evenly split between inhalation of contaminated dust and soil, ingestion of contaminated water and food, and inhalation of contaminated gases or vapor. The chemical manufacturing industry is the largest single consumer of water by sector in all OECD countries. The large amount of process water required provides many opportunities for pollutants to be released through wastewater.

The pollutants found in the largest quantities at chemical manufacturing sites include pesticides and volatile organic compounds. Furthermore, it is important to note that volatile organic compounds (VOCs), exposure to volatile organic compounds released from chemical manufacturing sites potentially puts more human health at risk at the sites. Volatile organic compounds are low-molecular-weight chemicals made from carbon and hydrogen, and often including oxygen, nitrogen, chlorine, and other elements. Because of their low molecular weight, volatile organic compounds convert to vapor easily, and vapors of volatile organic compounds are emitted from certain products and processes. There are thousands of volatile organic compounds, many of which are familiar compounds in everyday life, such as ethyl alcohol, propane, mineral spirits, and the chemicals in gasoline, kerosene, and oil.

While many volatile organic compounds are relatively nonhazardous (aside from their flammability), there are thousands of volatile organic compounds that are toxic, and some can cause eye, nose, and throat irritation and headaches, while others are known carcinogens. Some examples of toxic volatile organic compounds include benzene, formaldehyde, toluene, vinyl chloride, and chloroform. Volatile organic compounds come from a wide variety of products, most of which are used daily by society. The list includes most fuels, paints, stains and lacquers, cleaning supplies, pesticides, plastics, glues, adhesives, and refrigerants. Volatile organic compounds, including many more uncommon and toxic types, are very commonly used in manufacturing processes as solvents or raw materials in the production of plastics, chemicals, pharmaceuticals, and electronic products.

Waste elimination is common sense and provides several obvious benefits (Table 9.5). Yet waste elimination continues to elude many companies in every sector, including the process section, and activity from process waste (i.e., a function of their production system design). It may not matter how a producer categorizes the waste or how the producer chooses to pursue waste elimination, one thing remains constant and that is: once identified, waste can be eliminated. There are models and structures that allow a producer to identify and eliminate waste to increase productivity, and hence cost structures, that have a direct impact on process operations and, more than all else, profitability. Waste elimination though identification (by judicious analysis) and treatment subscribe to the smooth operation of a process.

Generally, process wastes (emissions) are categorized as gaseous, liquid, and solid. This does not usually include waste from accidental spillage of an organic chemical feedstock or from a product. Creating standards for the strategic and sound management of chemicals is essential to reducing the risk of exposure. Nationally and internationally both private and public organizations including the United Nations are working to create globally applied standards for the management of chemicals so that the need for chemicals and the hazardous effects of pollution can be balanced.

5.2 Air Emissions

Air emissions include point and nonpoint sources (Speight and Lee, 2000). Point sources are emissions that exit stacks and flares and, thus, can be monitored and treated. Nonpoint sources are fugitive emissions that are difficult to locate and capture. Fugitive emissions occur throughout production facilities and arise from the thousands of valves, pumps, tanks, pressure relief valves, and flanges. While individual leaks are typically small, the sum of all fugitive leaks from any process can be one of the largest emission sources in that process.

The numerous process heaters used in production facilities to heat process streams or to generate steam (boilers) for heating or steam stripping can be potential sources of SO_x, NO_x, CO, particulates, and hydrocarbons emissions. When operating properly and when burning cleaner fuels such as process fuel gas, fuel oil, or natural gas, these emissions are relatively low. If, however, combustion is not complete, or heaters are fired with process fuel pitch or residuals, emissions can be significant. As a result, there has been an increased interest in the application of control to combustion with the main objective to optimize combustor operation, monitor the process, and alleviate instabilities and their severe consequences. As combustion systems have to meet increasingly more demanding air pollution standards, their design and operation becomes more complex. The trend toward reduced emission levels has led to pollutant conversion (Docquier and Candel, 2002; Tecon and Van der Meer, 2008).

The majority of gas streams exiting each process contain varying amounts of process fuel gas, hydrogen sulfide, and ammonia. These streams are collected and sent to the gas treatment and sulfur recovery units to recover the process fuel gas and sulfur though a variety of add-on technologies (Speight and Lee, 2000; Speight, 2014a, b). Emissions from the sulfur recovery unit typically contain some hydrogen sulfide, sulfur oxides, and nitrogen oxides. Other emissions sources from various processes arise from periodic regeneration of catalysts. These processes generate streams that may contain relatively high levels of carbon monoxide, particulates, and volatile organic compounds. Before being discharged to the atmosphere, such off-gas streams may be treated first through a carbon monoxide boiler to burn carbon monoxide and any volatile organic compounds, and then through an electrostatic precipitator or cyclone separator to remove particulates.

Sulfur is removed from a number of process off-gas streams (sour gas) in order to meet the sulfur oxide emissions limits of the Clean Air Act and to recover saleable elemental sulfur. Process off-gas streams, or sour gas, from the coker, catalytic cracking unit, hydrotreating units, and hydroprocessing units can contain high concentrations of hydrogen sulfide mixed with light process fuel gases.

Before elemental sulfur can be recovered, the fuel gases (primarily methane and ethane) need to be separated from the hydrogen sulfide. This is typically accomplished by dissolving the hydrogen sulfide in a chemical solvent. Solvents most commonly used are amines, such as diethanolamine (DEA, HOCH₂CH₂NHCH₂CH₂OH). Dry adsorbents such as molecular sieves, activated carbon, iron sponge (Fe₂O₃), and zinc oxide (ZnO) are also used (Speight, 2014a, b). In the amine solvent processes, diethanolamine solution or similar ethanolamine solution is pumped to an absorption tower where the gases are contacted and hydrogen sulfide is dissolved in the solution. The fuel gases are removed for use as fuel in process furnaces in other process operations. The amine-hydrogen sulfide solution is then heated and steam stripped to remove the hydrogen sulfide gas.

Current methods for removing sulfur from the hydrogen sulfide gas streams are typically a combination of two processes in which the primary process is the Claus Process followed by either the Beavon Process or the SCOT Process or the Wellman-Lord Process.

In the Claus process (Speight, 2014a, b, 2016), the hydrogen sulfide, after separation from the gas stream using amine extraction, is fed to the Claus unit, where it is converted in two stages. The first stage is a thermal step: in which the hydrogen sulfide is partially oxidized with air in a reaction furnace at high temperatures (1000–1400°C, 1830–2550°F). Sulfur is formed, but some hydrogen sulfide remains unreacted, and some sulfur dioxide is produced. The second stage is a catalytic stage in which the remaining hydrogen sulfide is reacted with the sulfur dioxide at lower temperatures (200–350°C, 390–660°F) over a catalyst to produce more sulfur. The overall reaction is the conversion of hydrogen sulfide and sulfur dioxide to sulfur and water:

$2{\mathrm{H}}_2\mathrm{S}+{\mathrm{SO}}_2\to 3\mathrm{S}+2{\mathrm{H}}_2\mathrm{O}$

The catalyst is necessary to ensure that the components react with reasonable speed, but, unfortunately, the reaction does not always proceed to completion. For this reason, two or three stages are used, with sulfur being removed between the stages. For the analysts, it is valuable to know that carbon disulfide (CS₂) is a by-product from the reaction in the high-temperature furnace. The carbon disulfide can be destroyed catalytically before it enters the catalytic section proper.

Generally, the Claus process may only remove about 90% of the hydrogen sulfide in the gas stream and, as already noted, other processes such as the Beavon process, the SCOT process, or Wellman-Lord processes are often used to further recover sulfur.

In the Beavon process, the hydrogen sulfide in the relatively low concentration gas stream from the Claus process can be almost completely removed by absorption in a quinone solution. The dissolved hydrogen sulfide is oxidized to form a mixture of elemental sulfur and hydroquinone. The solution is injected with air or oxygen to oxidize the hydroquinone back to quinone. The solution is then filtered or centrifuged to remove the sulfur, and the quinone is then reused. The Beavon process is also effective in removing small amounts of sulfur dioxide, carbonyl sulfide, and carbon disulfide that are not affected by the Claus process. These compounds are first converted to hydrogen sulfide at elevated temperatures in a cobalt molybdate catalyst prior to being fed to the Beavon unit. Air emissions from sulfur recovery units will consist of hydrogen sulfide, sulfur oxides, and nitrogen oxides in the process tail gas as well as fugitive emissions and releases from vents.

The SCOT process is also widely used for removing sulfur from the Claus tail gas. The sulfur compounds in the Claus tail gas are converted to hydrogen sulfide by heating and passing it through a cobalt-molybdenum catalyst with the addition of a reducing gas. The gas is then cooled and contacted with a solution of diisopropanolamine (DIPA) that removes all but trace amounts of hydrogen sulfide. The sulfide-rich DIPA is sent to a stripper where hydrogen sulfide gas is removed and sent to the Claus plant. The DIPA is returned to the absorption column.

The Wellman-Lord process is divided into two main stages: (1) absorption and (2) regeneration. In the absorption section, hot flue gases are passed through a prescrubber where ash, hydrogen chloride, hydrogen fluoride, and sulfur trioxide are removed. The gases are then cooled and fed into the absorption tower. A saturated solution of sodium sulfite is then sprayed into the top of the absorber onto the flue gases; the sodium sulfite reacts with the sulfur dioxide forming sodium bisulfite (NaHSO₃). The concentrated bisulfate solution is collected and passed to an evaporation system for regeneration. In the regeneration section, sodium bisulfite is converted, using steam, to sodium sulfite that is recycled back to the flue gas. The remaining product, the released sulfur dioxide, is converted to elemental sulfur, sulfuric acid, or liquid sulfur dioxide.

Most process units and equipment are sent into a collection unit, called the blowdown system. Blowdown systems provide for the safe handling and disposal of liquid and gases that are either automatically vented from the process units through pressure relief valves or that are manually drawn from units. Recirculated process streams and cooling water streams are often manually purged to prevent the continued buildup of contaminants in the stream. Part or all of the contents of equipment can also be purged to the blowdown system prior to shut down before normal or emergency shutdowns. Blowdown systems utilize a series of flash drums and condensers to separate the blowdown into its vapor and liquid components. The liquid is typically composed of mixtures of water and hydrocarbons containing sulfides, ammonia, and other contaminants, which are sent to the wastewater treatment plant. The gaseous component typically contains hydrocarbons, hydrogen sulfide, ammonia, mercaptans, solvents, and other constituents and is either discharged directly to the atmosphere or is combusted in a flare. The major air emissions from blowdown systems are hydrocarbons in the case of direct discharge to the atmosphere and sulfur oxides when flared.

5.3 Wastewater

Wastewaters from the organic chemicals industry and the refining industry consist of process water, cooling water, storm water, and sanitary sewage water (Speight, 2005; Speight and Arjoon, 2012). In fact, water used in processing operations accounts for a significant portion of the total wastewater. Process wastewater arises from desalting crude oil, steam-stripping operations, pump gland cooling, product fractionator reflux drum drains, and boiler blowdown. Because process water often comes into direct contact with oil, it is usually highly contaminated. Most cooling water is recycled over and over. Cooling water typically does not come into direct contact with process oil streams and therefore contains less contaminants than process wastewater. However, it may contain some oil contamination due to leaks in the process equipment. Storm water (i.e., surface water runoff) is intermittent and will contain constituents from spills to the surface, leaks in equipment, and any materials that may have collected in drains. Runoff surface water also includes water coming from crude and product storage tank roof drains. Sewage water needs no further explanation of its origins but must be treated as opposed to discharge on to the land or into ponds.

Wastewater is treated in on-site wastewater treatment facilities and then discharged to publicly owned treatment works (POTWs) or discharged to surfaces waters under National Pollution Discharge Elimination System (NPDES) permits. Organic chemicals production facilities typically utilize primary and secondary wastewater treatment.

Primary wastewater treatment consists of the separation of oil, water, and solids in two stages. During the first stage, an API separator, a corrugated plate interceptor, or other separator design are used. Wastewater moves very slowly through the separator allowing free oil to float to the surface and be skimmed off, and solids to settle to the bottom and be scraped off to a sludge collection hopper. The second stage utilizes physical or chemical methods to separate emulsified oils from the wastewater. Physical methods may include the use of a series of settling ponds with a long retention time, or the use of dissolved air flotation (DAF). In DAF, air is bubbled through the wastewater, and both oil and suspended solids are skimmed off the top. Chemicals, such as ferric hydroxide or aluminum hydroxide, can be used to coagulate impurities into a froth or sludge that can be more easily skimmed off the top. Some wastes associated with the primary treatment of wastewater at organic chemicals production facilities may be considered hazardous and include API separator sludge, primary treatment sludge, sludge from other gravitational separation techniques, float from DAF units, and wastes from settling ponds.

After primary treatment, the wastewater can be discharged to a POTW or undergo secondary treatment before being discharged directly to surface waters under a NPDES permit. In secondary treatment, microorganisms may consume dissolved oil and other organic pollutants biologically. Biological treatment may require the addition of oxygen through a number of different techniques, including activated sludge units, trickling filters, and rotating biological contactors. Secondary treatment generates biomass waste that is typically treated anaerobically and then dewatered.

Some production facilities employ an additional stage of wastewater treatment called polishing to meet discharge limits. The polishing step can involve the use of activated carbon, anthracite coal, or sand to filter out any remaining impurities, such as biomass, silt, trace metals, and other inorganic chemicals, as well as any remaining organic chemicals.

Certain process wastewater streams are treated separately, prior to the wastewater treatment plant, to remove contaminants that would not easily be treated after mixing with other wastewater. One such waste stream is the sour water drained from distillation reflux drums. Sour water contains dissolved hydrogen sulfide and other organic sulfur compounds and ammonia which are stripped in a tower with gas or steam before being discharged to the wastewater treatment plant. Wastewater treatment plants are a significant source of process air emissions and solid wastes. Air releases arise from fugitive emissions from the numerous tanks, ponds, and sewer system drains. Solid wastes are generated in the form of sludge from a number of the treatment units.

Many production facilities unintentionally release, or have unintentionally released in the past, liquid hydrocarbons to groundwater and surface waters. At some production facilities, contaminated groundwater has migrated off-site and resulted in continuous seeps to surface waters. While the actual volume of hydrocarbons released in such a manner are relatively small, there is the potential to contaminate large volumes of groundwater and surface water possibly posing a substantial risk to human health and the environment.

5.4 Other Waste

Solid wastes are generated from many of the organic chemcials production processes and from refining processes, organic chemicals handling operations, as well as wastewater treatment (Chapter 4). Both hazardous and nonhazardous wastes are generated, treated, and disposed. Solid wastes in a process are typically in the form of sludge (including sludge from wastewater treatment), spent process catalysts, filter clay, and incinerator ash. Treatment of these wastes includes incineration, land treating off-site, land filling on-site, land filling off-site, chemical fixation, neutralization, and other treatment methods (Speight, 1996; Woodside, 1999; Speight and Lee, 2000; Speight and Arjoon, 2012).

A significant portion of the nonorganic chemicals product outputs of production facilities is transported off-site and sold as by-products. These outputs include sulfur, acetic acid, phosphoric acid, and recovered metals. Metals from catalysts and from the crude oil that have deposited on the catalyst during the production often are recovered by third-party recovery facilities.

Storage tanks are used throughout the refining process to store crude oil and intermediate process feeds for cooling and further processing. Finished organic chemicals are also kept in storage tanks before transport off-site. Storage tank bottoms are mixtures of iron rust from corrosion, sand, water, and emulsified oil and wax, which accumulate at the bottom of tanks. Liquid tank bottoms (primarily water and oil emulsions) are periodically drawn off to prevent their continued build up. Tank bottom liquids and sludge are also removed during periodic cleaning of tanks for inspection. Tank bottoms may contain amounts of tetraethyl or tetramethyl lead (although this is increasingly rare due to the phase out of leaded products), other metals, and phenols. Solids generated from leaded gasoline storage tank bottoms are listed as a hazardous waste.

5.5 Options

Pollution prevention is the responsibility of everyone and preventing pollution may be a new role for production-oriented managers and workers, but their cooperation is crucial. It will be the workers themselves who must make pollution prevention succeed in the workplace.

The best way to reduce pollution is to prevent it in the first place. Some companies have creatively implemented pollution prevention techniques that improve efficiency and increase profits while at the same time minimizing environmental impacts. This can be done in many ways such as reducing material inputs, reengineering processes to reuse by-products, improving management practices, and substituting benign chemicals for toxic ones. Some smaller facilities are able to actually get below regulatory thresholds just by reducing pollutant releases through aggressive pollution prevention policies. Furthermore, it is critical to emphasize that pollution prevention in the chemical industry is process specific and oftentimes constrained by site-specific considerations. As such, it is difficult to generalize about the relative merits of different pollution prevention strategies. The age, size, and purpose of the plant will influence the choice of the most effective pollution prevention strategy. Commodity chemical manufacturers redesign their processes infrequently so that redesign of the reaction process or equipment is unlikely in the short term. Here operational changes are the most feasible response. Specialty chemical manufacturers are making a greater variety of chemicals and have more process and design flexibility. Incorporating changes at the earlier research and development phases may be possible for them.

Several options have been identified that production facilities can undertake to reduce pollution. These include pollution prevention options, recycling options, and waste treatment options. Furthermore, pollution prevention options are often presented in four different categories, viz. (1) pollution prevention options, (2) waste recycling, and (3) waste treatment. Either one or the other or any combination of the three options may be in operation in any given process.

Pollution prevention options are usually subdivided into four areas: (1) good operating practices, (2) processes modification, (3) feedstock modification, and (4) product reformulation (Lo, 1991). The options described here include only the first three of these categories since product reformulation is not an option that is usually available to the environmental analyst, scientist, or engineer.

5.6 Recycling

Recycling is the use, reuse, or reclamation of a waste after it is generated. At present the organic chemicals industry is focusing on recycling and reuse as the best opportunities for pollution prevention (Table 9.9). Although pollution is reduced more if wastes are prevented in the first place, a next best option for reducing pollution is to treat wastes so that they can be transformed into useful products.

Caustic substances used to absorb and remove hydrogen sulfide and phenol contaminants from intermediate and final product streams can often be recycled. Spent caustics may be saleable to chemical recovery companies if concentrations of phenol or hydrogen sulfide are high enough. Process changes in the process may be needed to raise the concentration of phenols in the caustic to make recovery of the contaminants economical. Caustics containing phenols can also be recycled on-site by reducing the pH of the caustic until the phenols become insoluble thereby allowing physical separation. The caustic can then be treated in the process wastewater system.

Oily sludge can be sent to a coking unit or the crude distillation unit where it becomes part of the process products. Sludge sent to the coker can be injected into the coke drum with the quench water, injected directly into the delayed coker, or injected into the coker blowdown contactor used in separating the quenching products. Use of sludge as a feedstock has increased significantly in recent years and is currently carried out by most production facilities. The quantity of sludge that can be sent to the coker is restricted by coke quality specifications that may limit the amount of sludge solids in the coke. Coking operations can be upgraded, however, to increase the amount of sludge that they can handle.

Significant quantities of catalyst fines are often present around the catalyst hoppers of fluid catalytic cracking reactors and regenerators. Coke fines are often present around the coker unit and coke storage areas. The fines can be collected and recycled before being washed to the sewers or migrating off-site via the wind. Collection techniques include dry sweeping the catalyst and coke fines and sending the solids to be recycled or disposed of as nonhazardous waste. Coke fines can also be recycled for fuel use. Another collection technique involves the use of vacuum ducts in dusty areas (and vacuum hoses for manual collection) that run to a small baghouse for collection.

An issue that always arises relates to the disposal of laboratory sample from any process control or even environmental laboratory that is associated with a process. Samples from such a laboratory can be recycled to the oil recovery system.

5.7 Treatment Options

When pollution prevention and recycling options are not economically viable, pollution can still be reduced by treating wastes so that they are transformed in to less environmentally harmful wastes or can be disposed of in a less environmentally harmful media (Table 9.10). The toxicity and volume of some de-oiled and dewatered sludge can be further reduced through thermal treatment. Thermal sludge treatment units use heat to vaporize the water and volatile components in the feed and leave behind a dry solid residue. The vapors are condensed for separation into the hydrocarbon and water components. Noncondensable vapors are either flared or sent to the amine unit for treatment and use as process fuel gas.

Furthermore, because oily sludge makes up a large portion of process solid wastes, any improvement in the recovery of oil from the sludge can significantly reduce the volume of waste. There are a number of technologies currently in use to mechanically separate oil, water, and solids, including: belt filter presses, recessed chamber pressure filters, rotary vacuum filters, scroll centrifuges, disc centrifuges, shakers, thermal driers, and centrifuge-drier combinations.

Waste material such as tank bottoms from crude oil storage tanks constitute a large percentage of process solid waste and pose a particularly difficult disposal problem due to the presence of heavy metals. Tank bottoms are comprised of heavy hydrocarbons, solids, water, rust, and scale. Minimization of tank bottoms is carried out most cost effectively through careful separation of the oil and water remaining in the tank bottom. Filters and centrifuges can also be used to recover the oil for recycling.

Spent clay from process filters often contains significant amounts of entrained hydrocarbons and, therefore, must be designated as hazardous waste. Back washing spent clay with water or steam can reduce the hydrocarbon content to levels so that it can be reused or handled as a nonhazardous waste. Another method used to regenerate clay is to wash the clay with naphtha, dry it by steam heating and then feed it to a burning kiln for regeneration. In some cases, clay filtration can be replaced entirely with hydrotreating process options.

Decant oil sludge from the fluidized bed catalytic cracking unit can (and often does) contain significant concentrations of catalyst fines. These fines often prevent the use of decant oil as a feedstock or require treatment which generates an oily catalyst sludge. Catalyst fines in the decant oil can be minimized by using a decant oil catalyst removal system. One system incorporates high voltage electric fields to polarize and capture catalyst particles in the oil. The amount of catalyst fines reaching the decant oil can be minimized by installing high efficiency cyclones in the reactor to shift catalyst fines losses from the decant oil to the regenerator where they can be collected in the electrostatic precipitator.