Application of Green Technology in Water and Wastewater Treatments


School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India

department of Chemistry, St. Joseph's College, Moolamattom, India *Corresponding author. E-mail: This email address is being protected from spam bots, you need Javascript enabled to view it


Wastewater originated from various industries like textile, agriculture, food, petrochemical, polymer, pharmaceutical, etc. and contains a large number of contaminants of oil and salt of inorganic and organic compounds. When this wastewater released into the ecosystem without any appropriate treatments causes major ecological issues with high environmental impacts. Also, the natural fresh water resources are getting depleted because of the increased demand for fresh water supply. There are different physical, chemical, and biological methods are developed for the treatment of water but these methods cannot abolish the contaminants. And also most of these conventional methods are veiy expensive. The development of green technology for water treatments has received enormous interest over recent years due to its significant advantages to the environment, society, and economy, hi this chapter, we discuss the various green technologies for the treatment of water and wastewater.


The world’s population has increased to more than seven billion people. Each year world population continues to increase with a 1.2% growth rate.

This results in the increased demands on water purity. Population increase, climate variations, and fast development of several nations and the subsequent large usage of water and contamination of water resources have raised anxiety regarding the unsustainability of existing water use patterns and supply methods. In the perception of the total hydrologic cycle, the adequate global amount of water is normally accessible for the present population but the concentration of world water resources in particular regions is resulting in the emergence of severe water deficiencies in other places.1

Inappropriately handled freshwater resource systems also cause significant water pollution problems other than water scarcity. Around 90-95% of untreated urban sewage in the developing countries released directly into surface waters without any purification process and water regulations. Consequently, water pollution problems are high in developing countries than in other countries.2 The contamination of water resources dangerously affects the quality and supply of freshwater, mainly for domestic and industrial purposes. To overcome these situations, awareness of environmental responsibilities is needed.3 It is veiy essential to develop sustainable wastewater treatment methods for confirming clean water and energy accessibility for upcoming generations. However, the selection and implementation of suitable cost-effective and environmental friendly treatment procedures are very essential. There are different conventional techniques are developed for the purification of water to a desirable quality.

Water treatment involves the removal of unwanted chemical, physical, and biological pollutants from raw or contaminated water to produce pure water for specific applications like human consumption, medical, industrial, chemical, and pharmacology requirements. It is not possible to recognize a water sample of fine quality by visual observation. The only method to attain the details required for deciding the suitable technique for water treatment is chemical analysis. The chemical analysis is somewhat expensive. The international standards like the World Health Organization (WHO) or governments are usually set the standards for drinking water quality.

The World Health Organization report (WHO 2007) in 2007 says that 1.1 billion people lack access to an unproved chinking water supply. Yearly four billion diarrheal disease cases are reported of which 88% are originated from unsafe water and insufficient sanitation and cleanliness. Each year around 1.8 million people die due to diarrheal diseases, hi many developing countries one of the main public health aims is the reduction of deaths caused by waterborne diseases. According to WHO 2005 reports, the modification of the environment by providing safe drinking water reduces 94% of diarrheal issues. Implementation of green technology at home for treating water like chlorination, filters, and solar disinfection, and keeping of water in good containers could save a large number of lives each year.4

There are different conventional methods like physical, chemical, mechanical, or biological or in some cases, the combination of these methods is used for the treatment of wastewater. The main aim of water treatment is to remove any solids and organic matter from the water. In physical treatment methods, the waste materials are separated or isolated from the mainstream and it does not involve any degradation of waste material. While in biological treatments, the microbes are used to feed on the organic waste and pH and aeration are adjusted to maintain microbial activities. The development of green technology for water treatments has received enormous interest over recent years due to its significant advantages to the environment, society, and economy. In this chapter, we discuss the various green technologies for the tr eatment of water and wastewater.


The main purposes of water and wastewater treatment gr een technologies are:

  • (i) to decrease and preserve the exploitation of water and related nonrenewable energy resources
  • (ii) to avoid pollution and mishandling of water and other natural resources
  • (iii) to keep biodiversity, habitats, and ecologies, and
  • (iv) to make sure that upcoming generations can meet up their own requirements.

An environment friendly approach is required to overcome the consequences of the use of toxic chemicals and solvents in water treatment methods. Some of the green technology methods for water treatments are given below.


AOPs have been defined by Glaze et al.5 and it involves several oxidation steps and is used to removing organic compounds in water by a set of the chemical treatment process. These methods are better substitutes for the removal of dissolved recalcitrant organic substances, which would not be completely eradicated by conventional methods. In this process, highly reactive intermediates like OH radicals are produced by the following routes.6

• Oxidation with O, Here the reaction is carried out in a temperature range between ambient conditions and those found in incinerators.

For example, wet air oxidation (WAO) processes (1-20 MPa and 200-300°C)

• Use of ozone and H,0, and/or photons (high energy oxidants) for the generation of OH radicals

The OH radicals are very reactive and non-selective chemical oxidants. Once produced, it attacks almost all organic compounds. It can degrade the noxious substances present in the wastewater. Due to the high oxidation potential of OH radicals (E°= 2.8 V), it can react with all types of organic compounds results in complete mineralization of these compounds by the formation of water, carbon dioxide, inorganic salts or transfer it into less aggressive products.7'9 AOPs reduce the concentration of pollutants from a few hundred ppm to less than 5ppb.10 Destruction of pollutant substances and the prevention of subsequent formation of toxic residue are some of the important benefits of AOPs. But in conventional nondestructive physical separation processes like flotation, filtration, and adsorption with active coal only eliminate the pollutants and transferring it into other products.11 The AOP uses strong oxidizing agents like hydrogen peroxide, Fenton’s reagents, and ozone to generate hydroxyl radicals. Sometimes these reagents are used with ultraviolet (UV) light which enhances the reaction.

Types of advanced oxidation processes

FIGURE 4.1 Types of advanced oxidation processes.

The AOP can be divided into two categories on the basis of whether UV light is used for the reaction.12 These are given in Table 4.1.

TABLE 4.1 Classification of the Advanced Oxidation Process.






Ozonation with hydrogen peroxide (03/H,0,)


Fenton (Fe2+orFe37H,0,)

Photo-Fenton (Fe37H,0,/UY)

Wet oxidation (WO)

Photocatalytic oxidation (Ш'/catalyst)

Electrochemical oxidation

Of these different AOPs, the most used methods for the removal of toxic organic contents in the wastewater are ozonation, photocatalytic degradation, Fenton’s reagent (H,0,/Fe2+), Photo-Fenton, and WAO. These methods are also very effective in the removal of organic dyes from the water media. The combinations of these different methods are also used for effective removal.


Iii the nonpliotochemical AOP, the hydroxyl radicals are generated via different methods without using light radiation. The important nonphoto- chemical AOPs are given below.


Ozone has a high reduction potential (2.07 V), and it can react with organic compounds directly. Nowadays, the ozonation is extensively used for the water treatment because this reaction does not produce any chlorinated products that may generate in the chlorine disinfection process.13 Ozone is not harmful to any organisms and ozone-based AOPs are very ecofriendly. The ozonation methods are broadly applied for the removal of organic pollutants particularly the colored compounds. The conjugated double bond present in the chromophore groups of colored compounds is easily cleaved by ozone to smaller molecules, either directly, or indirectly.1415

The ozone is decomposed to give hydroxyl radicals according to the following equation. This reaction is catalyzed by hydroxyl ions (OH )

The ozonation processes take place in the following steps:

  • • Formation of ozone
  • • Dissolution of ozone in wastewater
  • • Oxidation of organic compounds


Hydrogen peroxide (H,0,) is a readily obtainable oxidant and is less expensive. When H,0, is added to the zone, the decomposition cycle of ozone initiated and OH radicals are formed.16

H,0, reacts with ozone when present as an anion, HO,

The reaction is continuous by the indirect pathway as illustrated above and OH radicals are generated.17 From the analysis of different reaction steps, it is concluded that two ozone molecules produce two OH radicals:

The study about the removal of the atrazine in filtered Seine River water showed that the degradation rate of pesticide is higher when water was treated with ozone-hydrogen peroxide mixture as a contrast to ozone only. The optimum H,0;/03 mass ratio was from 0.35 to 0.45. The rates of degradation are affected by factors like ozone dosage, contact time, and alkalinity of water.18


Fenton’s reagent oxidation is a catalytic oxidation process, which involves a mixture of strong chemical oxidizer (hydrogen peroxide), ferrous ions as a catalyst, and an acid as an optimum pH adjuster. The Fenton process was reported by Fenton for maleic acid oxidation.19 Fenton’s process is an easy way to generate hydroxyl radicals without any special apparatus and chemicals and takes place at ambient temperature and pressure. This is a simple method for oxidation, as hydrogen peroxide and iron salts are readily obtainable, easy to handle, and environmentally benign.20 The organic compounds are destmcted by reacting with OH radicals.

In acidic medium, the reaction between the hydrogen peroxide (H,0,) and ferrous ions Fe(II) leads to the formation of a hydroxyl ion and a hydroxyl radical by decomposition of H,02, and the oxidation of Fe(II) to Fe(III). This can be represented by the equation as follows:

In the presence of excess H,0, the Fe (II) oxidizes to Fe (III) within a few seconds and the rate constant for the reaction between ferrous ion and НлО, is veiy high. Then, Fe (III) catalyzes the decomposition of H,02 to hydroxyl radicals.

Hence, the Fenton’s reagent catalyzed waste destruction is simply a Fe(III)-H,0, system catalyzed process. Fenton’s reagent with excess НлО, is fundamentally a Fe (HI)-H202 process (known as a Fenton-like reagent). Therefore, the ferrous ion in Fenton’s reagent is able to change with the ferric ion. The iron salt is used as a catalyst and it is regenerated. Fenton’s reagent can destroy phenols, nitrobenzene, and herbicides present in water, in addition, to decrease chemical oxygen demand (COD) in wastewater.21-24 4.2.2A WET OXIDATION (WO)

WO is a destructive and environmentally secure technology for treating water containing organic pollutants.25 The WAO process was first patented by Zimmerman. By this technique, pollutants present in the water are removed by oxidation with an oxidant such as oxygen or air under high- pressure conditions (10-220 bar) and high temperatures (150-370°C), leading to the formation of hydroxyl radicals.26 This technology is generally named WO when pure oxygen is used and WAO when air is supplied to the system. When hydrogen peroxide is used as an oxidant instead of oxygen it is known as w'et peroxidation (WPO).

By this process, the organic pollutants are not fully removed but are converted into intermediate end products with a considerable decrease in COD and the total organic carbon. The last aqueous effluent will contain a considerable quantity of low' molecular weight organics, ammonia, inorganic acids, and inorganic salts which are highly biodegradable than the untreated effluents. The efficiency of WAO can be enhanced with the presence of carbon materials, noble metals (Ru, Rh, Pd, Ir, Pt, etc.), and oxides of Cr, Mn, Fe, Co, Ni, Cu, Zn, and Mo.27,28 This type of reaction is known as catalytic WAO. Agro-food streams, pulp and paper mill effluents, and leachates from solid waste have been treated by WO. During the WO process, the organic compounds are reduced to CO, or other harmless components; nitrogen is converted into NH3, N03, or elementary nitrogen. The halogen compounds and sulfurs are changed into halides and sulfates. Also, dioxides or other harmful products like NOx, SO,, HC1 are not formed during this process.29


There are different types of electrochemical advanced oxidation methods developed to degrade or mineralize the organic pollutants present in water. These are very environmental friendly methods and can produce electrogenerated in situ hydroxyl radicals (OH). These radicals are very reactive and are powerful oxidizing agents (E°(0H/H20) = 2.8 V/SHE at 25°C).

Anodic oxidation is the simplest and well-known electrochemical oxidation method for water purification. In this method, organic pollutants present in the contaminated water are oxidized by heterogeneous hydroxyl

radicals M('OH) formed by direct charge transfer at the anode (M) as intermediate of O, evolution reaction from the oxidation of water as follows:

The use of active lower O, evolution overpotential anode (e. g„ Pt/ IrO,) allows the conversion of organics into carboxylic acids since M( OH) is chemisorbed on the anode surface with less oxidizing power. While the use of non-active high O, evolution overpotential anode (e. g., PbOv boron-doped diamond (BDD)) resulted in the formation of more reactive physisorbed M(.OH) with high oxidation power. These radicals can oxidize hardly oxidizable compounds like short-chain carboxylic acids.30-32


The oxidation of organic pollutants by conventional methods like ozone or hydrogen peroxide oxidation does not completely oxidize them to CO, and H,0 in several circumstances.33 Sometimes more toxic intermediate oxidation products remain in the solution. The termination of these oxidative reductions is attained by supplementing the reaction with UY radiation. Some of these methods are given below.


Ozonation combined with UY radiation is more effective to remove organic pollutants than ozonation only. The UY radiation enhances the decomposition of ozone and hence large numbers of OH radicals are formed thereby increases the rate of ozonation reaction.34

Ozone absorbs UY radiation at 254 run wavelength. Firstly, an intermediate H,0, is formed and finally hydroxyl radical is formed by the dissociation of H,Ov

Hence, the ozone decomposition reaction and photocatalytic ozone decomposition reactions only differ in their initiation step. In photocatalytic ozone decomposition, the starting radical is formed photochemically by an electron transfer from photocatalysts to oxygen and not by the reaction of OH' ion with ozone.35 The complete mineralization of organic compounds with short molecular chains can be achieved by the UY703 system.36 Peyton et al. showed the higher effectiveness of the 03/UV system for the exclusion of tetrachloroethane (C2C14) from water contrast to ozonation and photolysis only.37


In the U17H,0, system, hydrogen peroxide (H,0,) is added in the presence ofUV light to generate hydroxyl (OH) radicals. The H,0, is a strong reducing agent it is used to remove low-level pollutants in wastewater.38 Sometimes H,0, alone is not an efficient oxidant to oxidize complex pollutants. When it is used with other reagents and energy sources which are able to dissociate it into fr ee radical, its activity is increased. Normally, low or medium pressure mercury lamps are used for the photolysis of НлО, When НлО, irradiated with UY radiation of wavelength less than 300 mu, it gives OH radicals as follows.

The H,0, can react with hydroxyl radical and the intermediary product formed. The reactions can be written as simple form as follows39:

The organic compounds are attacked by both hydroxyl radical and hydroperoxy radicals. But the reduction potential of hydroperoxy radicals (1.7 V) is lower than OH radical (2.8 V); hence, the formation of hydroperoxy radicals radical is not interested in these processes.

One of the main advantages of using the UY/H,0, system is that the H:°2 is readily soluble in water, there is no mass transfer limitation, and it is a good source of hydroxyl radicals. Also, there are no separation techniques needed after treatment40-41 UV/H,0, system is very useful for the removal of organic dyes, particularly the azo dyes,42 chlorophenols,43 and other chlorinated compounds.44 The complete mineralization atrazine, desethylatrazine, and simazine to carbon dioxide within reasonable irradiation times is achieved by using the UV/H,0, system.45


The addition of H,0, to the UV/03 system enhances the decomposition of ozone, thus the rate of formation of OH radicals is increased.46

Trapido et al. proved that 0,/H,02/UY was the most effective system for the degradation of nitrophenol than 03, 03/H,02, 03/UV systems.47 This system is also applied for the removal of volatile organic contaminants like benzene, acetone, dichloroethane, tetrachloroethane, etc. from groundwater.48


In Photo-Fenton oxidation, Fe3+ is added to the H,0,/UV process. At acidic condition (pH= 3) Fe(OH):+ is formed.

This complex is decomposed into Fe2+, and OH ions when exposed to UY radiation.

The organic pollutants can be completely removed by the Photo-Fenton process. Pignatello et al. demonstrated the complete mineralization of a number of herbicides and pesticides using the Photo-Fenton process. In another study, this process is used for the mineralization of chlorophenol.49

The increased efficiency of Photo-Fenton reactions can be attributed to the following reasons:

Photo-reduction of ferric ion: The ferrous ions are produced by the irradiation of ferric ion or ferric hydroxide. Then the cycle continues by the reaction of this ferrous ion with H,0, and producing second hydroxyl radical and ferric ion.

Efficient use of light quanta'. The absorption spectrum of ferric ion or hydroxyl ferric ions expands to the near-UV/visible region with a moderately large extinction coefficient. Hence, it is possible to cany out photo-oxidation and mineralization even by visible light.


Pliotocatalytic oxidation is one of the most efficient and promising AOPs in which the organic compounds are fragmented into the water, carbon dioxide, and mineral salts. The activated species (hydroxyl radicals and superoxide radicals) are used for the complete mineralization.50 The pliotocatalytic degradation by using semiconductors (Ti02, ZnO, Fe,03, WOj and CdS) is widely used for the degradation of organic pollutants. The nano-titanium dioxide (TiO„ anatase form) is a commonly used photocatalyst due to its availability, chemical stability, extraordinary pliotocatalytic activity, nontoxicity, low cost, resistance to photocorrossion, optical and electrical properties, etc.51-52 It is possible to use sunlight in the case of TiO, since it absorbs wavelength below 400 nm.

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