wastewater input. Some of the advantages and disadvantages of existing treatment methods are listed in Table 1.6.
(Rhee et al., 2003; Howe et al., 2005; Nuhoglu and Yalcin, 2005; Juang and Tsai, 2006; Hassenklo¨ver et al., 2006; Yan et al., 2006; Bai et al., 2007; Uhnáková et al., 2009). Currently, phenol is produced at a rate of about 6 million tons/year worldwide, with a significantly increasing trend (Buscaa et al., 2008). Its methylated derivative cresol has been detected not only in leachate from creosote sites, raising the pollution of groundwater (Lin and Juang, 2009), but also in a huge range of industrial effluents (Hussain et al., 2008). The cresols have wide applications in the phenolic resin, explosive, petroleum, photographic as well as paint and agricultural industries. They are ingredients of many household disinfecting solutions. Cresol is also an additive to lubricating oils and a component of degreasing compounds and paintbrush cleaners.
M-cresol is a textile scouring agent. O-cresol is used in tanning, fibre treatment and metal degreasing. P-cresol is a solvent for wire enamels and an agent used in metal cleaning, ore flotation, synthetic flavouring and perfumes. The worldwide production of o-Cresol is approximately 37000-38000 tons/annum. Some of the important applications of phenols and its derivatives are given below:
Plastics, resins and plasticizers: Phenol is used above all in the production of plastics and phenol-formaldehyde resins. In addition, cresols are used in the making of tricresyl phosphate, which is a plasticizer useful for cellulose acetate, nitrocellulose, ethane thiol cellulose and vinyl plastics.
Preserving agent for wood, disinfectants and insecticides: Creosote oil, a distillation obtained from the process of coal-making at high temperatures, is used for preserving wood in addition to being a source of cresylic acid.
Cresols are also used as disinfectants and insecticides.
Vegetable hormones and detergents: Phenol is directly used for the production of these compounds.
Medicines: A clear example of a derivative of phenol is acetylsalicylic acid - a compound from which aspirin is obtained.
Dyes, photography and explosives: These industries have many usages of phenol although the total consumption is not very high. Some aminophenols are used as dyes and photographic developers. Trinitrophenol, for example, is used as a dye and as an explosive.
All these above said industries discharge phenolics in their wastewaters. Hence, it is advocated for efficient treatment methods to reduce phenol concentration in
wastewater to acceptable levels. Following is a short account on the state-of-the-art wastewater treatment methods.
1.4.2 Toxicological Aspects and Guidelines of Phenolic Compounds
Phenol and cresol are considered to be hazardous contaminants due to their toxicity for human being, animals and aquatic life (Shourian et al., 2009; Naas et al., 2009).
Due to their high solubility and toxicity, they can easily contaminate the water sources (Mollaei et al., 2010). These two can fastly penetrate in skin and that may cause irritation and respiratory tract. Therefore, human exposure to phenol and cresol by ingestion or inhalation may cause severe liver and kidney damage as well as cardiac depression. Ingestion of more than 1 mg/l of phenol is reported as harmful to human health (Nuhoglu and Yalcin, 2005).
Phenol and phenolic compounds have detrimental effects on the aquatic micro-flora and fauna at a very low concentration of 5 mg/l (Santos et al., 2009). But the concentration of phenol in wastewater generally varies from 0.5 mg/l to 300 mg/l.
Phenol may exert its toxic effect by reducing enzyme activity or even lethal to fish at relatively low levels of 5-25 mg/l. Phenol and cresols, even at a very low concentrations of 2 µg/l imparts objectionable taste and odour to drinking water (Chung et al., 2003).
Due to their potential toxicity, the United States Environmental Protection Agency (US-EPA) has defined phenols, cresols and its derivatives as priority pollutants and set a water purification level concentration less than 1 µg/l in the inland surface waters (Keith and Telliand, 1979). The WHO has set their guideline of 1 µg/l to regulate the phenol concentration in drinking water. Also, a limit of 0.5 µg/l has been directed by the European Council Directive for regulating phenol concentration in the drinking water (Tziotzios et al., 2005). The Indian Standard specifications have set the maximum allowable limit of phenol 0.001 mg/l for drinking water and 1 mg/l industrial effluent discharge into surface water (Mukherjee et al., 2007; Vasu, 2008), whereas, 0.35 µg/l of effluent phenol concentration is fixed for petroleum oil refineries by Central Pollution Control Board (CPCB), India.
1.4.3 Treatment Techniques of Phenolic Wastewater
The removal of phenolics to sufficiently low levels in wastewater is mandatory.
Appropriate strategies of wastewater treatment have to be employed in order to counterbalance the growing environmental problems. For the last two decades, rigorous pollution control and legislation in many countries have resulted in an intensive search for new and more efficient water treatment technologies. Important wastewater treatment technologies that have come up in recent times, include but are not limited to flocculation, precipitation, adsorption on granular activated carbon, reverse osmosis, combustion and advanced oxidation process (AOP) viz., photo fenton, photocatalysis and sonication. Some of these are briefly discussed below:
1.4.3.1Air Stripping
It involves the transfer of volatile organics from liquid phase to the air phase by greatly increasing the air/water contact area. Typical aeration methods include packed towers, diffusers, trays and spray aeration. It is a well established and more widely understood technology than chemical oxidation (Metcalf and Eddy, 2003).
1.4.3.2 Adsorption
This is a separation method in which the contaminants, dissolved in water phase, are transferred to the surface of active carbon, the most commonly used adsorbent, where it is accumulated for subsequent extraction or destruction of the contaminants. The application of adsorption process includes but is not limited to control of color and odors, removal of organic compounds or trihalomethanes precursors, removal of chlorine etc. Numerous literatures have been reported regarding the treatment of phenolics on activated carbon (Calleja et al., 1993; Dargaville et al., 1996;
Viraraghavan and Alfaro, 1998; Przepiórski, 2006; Vázquez et al., 2007). Phenol was found to be a well adsorbable compound onto activated carbon, but only in low concentrations.
1.4.3.3 Electrochemical Oxidation
The use of electrochemical oxidation for the destruction of organic compounds in water solutions has been tried on bench and pilot plant scale operation (Boudenne et al., 1996; Brillas et al., 1990; Brillas et al., 1998). The electrochemical oxidation of organic compounds is thermodynamically favoured against the competitive reaction
of oxygen production by oxidation of water. However, the kinetics of oxidation of water is much faster than the kinetics of oxidation of the organic compounds, among other reasons because of its higher concentration. The mechanism of the electrochemical processes involves three stages: electrocoagulation, electrofloatation and electrooxidation (Boudenne et al., 1996; Brillas et al., 1998).
1.4.3.4 Advanced Oxidation Processes (AOP)
It refers specifically to processes in which oxidation of organic contaminants occurs primarily through reactions with hydroxyl radicals (Glaze et al., 1995). It involves two stages of oxidation: (1) the formation of strong oxidants (e.g., hydroxyl radicals) and (2) the reaction of these oxidants with organic contaminants in water (Alnaizy and Akgerman, 2000). In water treatment applications, AOPs usually refer to a specific subset of processes that involve O3, H2O2, and/or UV light.
1.4.3.5 Ozonation/UV
The O3 system is one of the AOP for the destruction of organic compounds in wastewater. Basically, aqueous systems saturated with ozone are irradiated with UV light of 253.7 nm. The extinction coefficient of O3 at 253.7 nm is 3300 L mol/cm, much higher than that of H2O2 (18.6 L /mol/cm). The decay rate of ozone is about a factor of 1000 higher than that of H2O2 (Guittonneau et al., 1991). The AOP with UV radiation and ozone is initiated by the photolysis of ozone. The photodecomposition of ozone leads to two hydroxyl radicals, which do not act as they recombine producing hydrogen peroxide (Peyton and Glaze, 1988).
1.4.3.6 Ultrasonication
Implosion of cavity bubbles in sonicated water containing dissolved gases results in formation of hydrogen and hydroxyl radicals by fragmentation of water molecules.
These radicals in turn combine and generate other oxidative species such as peroxy and super oxide radicals (•OH) as well as hydrogen peroxide; the quantities of each depend on the ambient conditions and the operating parameters. Such •OH radicals are used for the degradation of the organic compounds (Kidak and Ince, 2006).
1.4.3.7 Solar Photocatalytic Oxidation
It is based on the use of UV light and a semiconductor. Many catalysts have been tested, although titanium dioxide (TiO2) in the anatase form seems to possess the most
interesting features, such as high stability, good performance and low cost (Fox and Dulay, 1993; Legrini et al., 1993; Hoffmann et al., 1995; Bahnemann., 2004). TiO2
has become the most studied and used photocatalyst, because it is easily available, chemically robust and durable. It can be used to degrade, via photocatalysis, a wide range of organic compounds (Leyva et al., 1998; Robert and Malato, 2002; Hincapié et al., 2005; Herrmann et al., 2007). Photocatalytic degradation of phenolic compounds by employing Degussa P-25® in presence of sunlight has been successfully studied by many researchers (Minero et al., 1994).
1.4.3.8 Biodegradation
The process of biodegradation of exploits the ability of microorganisms (such as bacteria, fungi and algae) to convert the phenolic compounds to water, carbon oxide and biomass under aerobic or anaerobic condition. Phenol and cresol can be degraded either aerobically or anaerobically depending upon the specific growth conditions of the microorganisms. Pseudomonas putida bacteria and other members of Pseudomonas genus are the most widely investigated bacteria with higher removal efficiency of phenolic compounds (Naas et al., 2009).