Chemical Treatment Technologies for Wastewater Recycling

Chemical Treatment Technologies for Wastewater Recycling

Water is of utmost importance in our daily lives, hence, the need to improve and preserve its quality is growing continuously. Point and non-point sources are contaminating our valuable water resources. The main water pollution sources are from industrial, domestic and agricultural activities and other environmental and global changes. The surface and groundwater in many places around the world is contaminated and not fit for drinking purposes. By 2020, the global population is supposed to reach up to 7.9 billion1 and because of this, the world may experience the great scarcity of freshwater.

Water pollutants

Prior to discussing water treatment and reclamation, one should be aware of the qualitative and quantitative nature of water pollutants. Many pollutants are present in wastewater but toxicity is only observed beyond a certain limit called the permissible limit. The type of pollutants present in the wastewater depends upon the nature of the industrial, agricultural and municipal wastewater releasing activities. The different types of water pollutants may be categorized as inorganic, organic, and biological in nature. The most common inorganic water pollutants are heavy metals, which are highly toxic and carcinogenic in nature. Additionally, nitrates, sulphates, phosphates, fluorides, chlorides and oxalates also have some serious hazardous effects. The toxic organic pollutants are from pesticides which include insecticides, herbicides, fungicides; polynuclear hydrocarbons (PAHs), phenols, polychlorinated biphenyls, halogenated aromatic hydrocarbons, formaldehyde, polybrominated biphenyls, biphenyls, detergents, oils, greases etc. In addition to these, normal hydrocarbons, alcohols, aldehydes, ketones, proteins, lignin, pharmaceuticals etc. are also found in wastewater. Different types of microbes thriving in wastewater may be responsible for a different type of diseases. The harmful microbes include bacteria, fungi, algae, plankton, amoeba, viruses and other worms. These water pollutants remain either in solvated, colloidal or in suspended form.

Wastewater treatment and recycling technologies

Wastewater treatment and reuse is an important issue and scientists are looking for inexpensive and suitable technologies. Water treatment technologies are used for three purposes i.e. water source reduction, wastewater treatment and recycling. At present, unit operations and processes are combined together to provide what is called primary, secondary and tertiary treatment. Primary treatment includes preliminary purification processes of a physical and chemical nature while secondary treatment deals with the biological treatment of wastewater. In tertiary treatment processes, wastewater (treated by primary and secondary processes) is converted into good quality water that can be used for different types of purpose, i.e. drinking, industrial, medicinal etc. supplies. In the tertiary process, up to 99% of the pollutants are removed and the water is converted into the safe quality for a specific use. In a complete water treatment plant, all these three processes are combined together for producing good quality and safe water.


Despite the development of various technologies for water treatment and reclamation, economic, effective and rapid water treatment and reclamation at a commercial level is still a challenging problem. The management of the removed pollutants (sludge) should be kept in mind. The systematic approach of water treatment and recycling technologies involves the understanding of the technology that includes construction and operational cost, along with the maintenance and management of removed pollutants.

Primary water treatment technologies

In this category, water is treated at the primary level using screening, filtration, centrifugation, sedimentation, coagulation, gravity and flotation methods. Normally, these methods are used when water is highly polluted. Brief descriptions of these methods are given below.

Screening, filtration and centrifugal separation

The main idea of screening is to remove the solid waste present in the wastewater and it is used for the removal of pieces of cloth, paper, wood, cork, hair, fibre, kitchen refuse, faecal solids etc. from wastewater. Generally, screening is used as the very first step in a wastewater treatment operation. The screens of various sizes are used for this purpose and the size of the screen is selected as per the requirement i.e. size of the solids present in the wastewater.

Infiltration process, water is passed through a medium having fine pores. Normally, a set-up with a pore size of about 0.1 to 0.5 μm is used for this purpose. It is used for the removal of suspended solids, greases, oils, bacteria etc. Different filters, such as membranes and cartridges can be used. The filtration process can be used to remove solids of size below 100 mg l−1 and to remove oil of 25 mg l−1 which can be reduced by up to 99%. The filtration process is utilized for water treatment. Water produced by filtration is used for adsorption, ion exchange or membrane separation processes. Besides, potable water is produced by filtration systems. The cost of filtration varies from 25 to 450 US$ per million litres of treated water.

Centrifugal separation is used to remove suspended non-colloidal solids (size up to 1 μm). The wastewater is applied to centrifugal devices and rotated at different speeds and the solids (sludges) are separated and discharged. The extent of separation of suspended solids is directly proportional to their densities. In addition to this, the speed of the centrifugal machine is also responsible for the removal of suspended solids. Applications include the source reduction and separation of oils and greases. The different types of centrifugal machines available and in use are solid-bowl, basket type, counter flow and counter-current flow. The cost of the wastewater treatment ranges 25 to 450 US$ per million litres of treated water.

Sedimentation and gravity separation

In this process, the suspended solids, grits and silts are removed by allowing water to be undisturbed/semi-disturbed for different time intervals in various types of tank. The suspended solids settle under the influence of gravity. The settling time depends upon the size and density of the solids or the velocity if the water is in motion. Sometimes, alums are added to augment the sedimentation process. Gravity separation can reduce the suspended solids by up to 60% only. Generally, sedimentation is carried out prior to conventional treatment processes. It is very useful method for the treatment of effluents obtained from the paper and refinery industries. Water treated in this process is used for industrial water supply, water for ion exchange and membrane processes. The technique is also used for source reduction. The cost of the treated water varies from 5 to 10 US$ per million litres.


Sometimes, the suspended solids do not settle down under the sedimentation and gravity method and, hence, non-settlable solids are allowed to settle by the addition of certain chemicals, this process is called coagulation. Alum, starch, iron materials, activated silica and aluminium salts are available for use. In addition, synthetic cationic, anionic and non-ionic polymers are effective but are usually more costly than natural coagulants. pH, temperature and contact time are most important controlling factors in the coagulation process. In biological treatment units, microbes and any organics floating in the water are removed by the addition of certain coagulants. It is the main component of wastewater treatment units and the applications include wastewater treatment, recycling, and removal of pollutants.


Flotation is a common and essential component of a conventional water treatment plant. The suspended solids, oils, greases, biological solids etc. are removed by adhering them with either air or gas in the flotation process.3,11 The solids get adhered to gas or air and form agglomerates, which in turn accumulate at the surface of the water and which can ultimately be skimmed off easily. Some chemicals, such as alum, activated silica etc. help in the flotation process. Compressed air is allowed to pass through the water, which helps in the flotation process. Electro-flotation (electro-flocculation) has been used as an effective process for water treatment and recycling purposes. Up to 75% of suspended solids are removed while up to 99% of oil and grease are removed by this process. It is an effective method for the treatment of wastewater from the paper and refinery industries. The cost varies from 5 to 25 US$ per million litres of treated water.

Secondary water treatment technologies

Secondary water treatment includes biological routes for the removal of soluble and insoluble pollutants by microbes.3,13,14 Water is circulated in a reactor that maintains a high concentration of microbes. The microbes, usually bacterial and fungal strains, convert the organic matter into water, carbon dioxide and ammonia gas.15–19 Sometimes, the organic matter is converted into other products such as alcohol, glucose, nitrate etc. Additionally, the microbes detoxify toxic inorganic matter. The wastewater should be then free from toxic organics and inorganics. The maximum concentrations of total dissolved solids (TDS), heavy metals, cyanides, phenols and oil should not exceed by 16[thin space (1/6-em)]000, 2.0, 60.0, 140, and 50 mg l−1 respectively. The biological treatment includes the aerobic and anaerobic digestion of wastewater. Depending on the materials used, the cost of biological treatment varies between 20 and 200 US$ per million litres.

Aerobic processes

When air or oxygen is freely available in dissolved form in wastewater than the biodegradable organic matter undergoes aerobic decomposition, caused by aerobic and facultative bacteria.20,21 The extent of the process depends on the availability of oxygen, retention time, temperature and the biological activities of the bacteria. Besides, the rate of the biological oxidation of organic pollutants may be increased by the addition of some chemicals required for bacterial growth. The technique is effective for the removal of biological oxygen demand (BOD), chemical oxygen demand (COD), dissolved and suspended organics, volatile organics, nitrates, phosphates etc. The concentration of biodegradable organics can be reduced by up to 90%. The disadvantage of the method is the production of a large number of bio-solids, which require further costly treatment and management. The aerobic process is carried out by trickling filters or activated sludge processes or oxidation ponds.

A simplified representation of aerobic decomposition is given by the following equation.

Organic matter + O2 + Bacteria → CO2 + H2O + Bacteria + Byproducts

Anaerobic process. If free dissolved oxygen is not available in the wastewater then anaerobic decomposition, called putrefaction, occurs. Anaerobic and facultative bacteria convert the complex organic matter into simpler organic compounds based on nitrogen, carbon and sulphur. The important gases evolved in this process are nitrogen, ammonia, hydrogen sulphide and methane. This method is used to reduce the biological load of wastewater. The anaerobic process is represented by the following equation.

Organic matter + Bacteria → CO2 + CH4 + Bacteria + Byproducts

Tertiary water treatment technologies

Tertiary water treatment technologies are very important in wastewater treatment strategy as these are used to obtain safe water for human consumption. The techniques used for this purpose are distillation, crystallization, evaporation, solvent extraction, oxidation, coagulation, precipitation, electrolysis, electrodialysis, ion exchange, reverse osmosis and adsorption. These methods are described below.


In the distillation process, water is purified by heating it up to 100 °C (boiling point) at which liquid water is vaporized leaving behind the pollutants.28 The vapours thus generated are cooled into liquid water. The wastewater should be free from volatile impurities and water produced by this technique is about 99% free from impurities. Various types of boilers with multistage and double distillation are used in this process. The size of the boilers depends on the water quantity requirements. The applications of distillation in water treatment and reclamation include water supplies in laboratories and medicinal preparations. In addition, distillation is an effective tool for the preparation of potable water from the sea and brackish water. The cost of water production varies between 15 and 2000 US$ per million litres.


Crystallization is a process in which pollutants are removed by raising their concentrations up to a point where they start to crystallize out. This situation is created either by evaporation, by lowering the temperature of the water or by mixing other solvents. It is useful for the treatment of wastewater with high concentrations of TDS including soluble organics and inorganics. During this process, the other constituents like bicarbonate, ammonia, sulfite etc. break down into various gases and, therefore, crystallization, sometimes, may be used for pH control. Generally, crystallization is used for the wastewater released by cooling towers, coal and gas-fired boilers, and the paper and dying industries. It is also used for source reduction. Forced circulation, draft tube baffle, surface cooled crystallizers and fluidized suspensions are used for crystallization. The treated water in this process is of good quality and its cost ranges from 50 to 150 US$ per million litres.


Evaporation is a natural process and, is generally, used to reduce the waste liquid volume but in modern developments it has been used as water treatment method. Water surface molecules escape from the surface under the natural conditions and the escaped molecules are collected in the form of pure liquid water. Mechanical evaporators have also been used for water recycling process. Sometimes vacuum evaporation has been used for wastewater recycling. Evaporation is effective for the removal of inorganic and organic (except volatile organic) contaminants and it works even at very high concentrations (about 10%) of pollutants. Foaming, scaling and fouling along with the presence of suspended solids and carbonates are the major problems associated with this technique as they create a maintenance problem. Evaporation applications include the treatment of wastewater containing fertilizer, petroleum, and from the pharmaceutical and food processing industries. It is also used for the water supply to ion exchangers and membrane processes. Water from evaporation has been used in cooling in towers and boilers. It can be used as a technique of water source reduction. The cost of water production varies between 15 and 200 US$ per million litres.

 Solvent extraction

Organic solvents, immiscible with water and having the capacity to dissolve pollutants, are added to wastewater for the removal of pollutants; this technique is called solvent extraction. The most commonly used solvents are benzene, hexane, acetone and other hydrocarbons. Sometimes, a small quantity of the solvent remains mixed with the water, which is recovered by using the distillation technique. The technique is only effective in removing organics, oils and greases. However, certain metal ions and actinide chemicals may be removed by this method. It is used for recycling and water treatment. It has been used for water source reduction too. The cost varies between 250 and 2500 US$ per million litres of clean water.


In chemical oxidation, organic compounds are oxidized into water and carbon dioxide or some other products such as alcohols, aldehydes, ketones and carboxylic acids which are easily biodegradable. Chemical oxidation is carried out by potassium permanganate, chlorine, ozone, H2O2, Fenton’s reagent (H2O2 and Fe catalyst) and chlorine dioxides. The rate of chemical oxidation depends on the nature of oxidants and pollutants. Besides, pH, temperature and presence of catalyst etc. also play a crucial role in the rate of chemical oxidation. By this method, pollutants such as ammonia, phenols, dyes, hydrocarbons and other organic pollutants may be removed. The cost of water production ranges from 100 to 2000 US$ million litres of clean water.

Advanced oxidation process

A single oxidation system as stated above is sometimes not sufficient for the total decomposition of organic pollutants present in wastewater. Advanced Oxidation Processes (AOPs) are processes involving the simultaneous use of more than one oxidation process and involve the accelerated production of the highly reactive hydroxyl free radical. These processes include techniques like Fenton’s reagent oxidation, ultraviolet (UV) photolysis and sonolysis, and are capable of degrading the organic pollutants at ambient temperature and pressure. The main advantage of the advanced oxidation process is that organic contaminants are commonly oxidized to CO2. A wide variety of advanced oxidation processes are available like chemical oxidation processes using ozone,45 combined ozone and peroxide,46 ultraviolet enhanced oxidation such as UV/Fenton or photo-Fenton, UV/hydrogen peroxide, UV/ozone,49 UV/air wet air oxidation and catalytic wet air oxidation (where air is used as the oxidant).

Photocatalysis is also one of a series of advanced oxidation processes for organic pollutant degradation. In photocatalysis, light energy from a light source (UV or solar) excites an electron from the valence band of the photocatalyst to the conduction band with a series of reactions which results in the formation of hydroxyl radicals. The hydroxyl radicals have high oxidizing potential and therefore can attack most organic pollutants causing oxidation. Various chalcogenides (oxides such as TiO2, ZnO, ZrO2, CeO2, etc. or sulfides such as CdS, ZnS, etc.) have been used as photocatalysts in the photocatalytic process and the process is found suitable for a wide range of organic pollutants. Sonolysis, i.e., use of ultrasonic waves has been used for the decolourization and degradation of organic pollutants. The mechanism proposed for the sonochemical process is usually based on the formation of short-lived radical species generated in violent cavitation events.59


In precipitation, the dissolved contaminants are converted into solid precipitates by reducing their solubilities and the precipitates are skimmed off easily from the surface of the water. It is effective for the removal of metal ions and organics but the presence of oil and grease may cause a problem in precipitation. The solubility of the dissolved pollutants is decreased either by adding some chemicals or by lowering the temperature of the water. Adding some organic solvents to the water may also reduce the solubility of the contaminant but this technique is costly at a commercial level. These chemicals react with the soluble pollutants to form precipitates. The most commonly used chemicals for this purpose are alum, sodium bicarbonates, ferric chloride, ferrous sulphate and lime. pH and temperature are the main controlling factors for the precipitation process. The removal of about 60% of the pollutants can be achieved by the precipitation. The applications of this method include wastewater treatment from the nickel and chromium plating industries and water recycling. The specific applications include water softening and removal of heavy metals and phosphate from water. The major problem associated with precipitation is the management of the large volume of sludge produced. The cost varies from 20 to 500 US$ per million litres of treated water.

Ion exchange

Toxic ions present in wastewater are exchanged with the non-toxic ions from a solid material called an ion exchanger. Ion exchangers are of two types i.e. cation and anion exchangers which have the capacity to exchange cations and anions respectively. Ion exchangers are resins of natural or synthetic origin having active sites on their surfaces. The most commonly used ion exchangers are sodium silicates, zeolites, polystyrene sulfonic acid, and acrylic and metha-acrylic resins. It is a reversible process and requires low energy contents. Ion exchange is used for the removal of low concentrations of inorganics and organics (up to 250 mg l−1). The concentration of organics and inorganics can be reduced by up to 95%. Applications include the production of potable water, water for industries, pharmacy, softening, fossil fuels, different industries. It has also been used for source reduction purposes. Sometimes, the pre-treatment of the water is required; if oil, grease and high concentrations of organics and inorganics are present. One million litres of wastewater is treated by investing 50 to 200 US$.

Micro- and ultra-filtration

Micro-filtration is required for the removal of particles of 0.04 to 1 μm in size (Fig. 2) provided the total suspended solids do not exceed 100 mg l−1. The filters used in this process are made of cotton, wool, rayon, cellulose, fibreglass, polypropylene, acrylics, nylon, asbestos and fluorated hydrocarbon polymers. These are arranged in different fashions such as tubular, disc, plates, spiral, and hollow fibres. The life of cartridges varies from 5 to 8 years depending upon the concentration of the dissolved solids. The pre-removal of suspended solids is an important factor for promoting the long life of filters. In this method, the operating pressure is about 1–3 bar. The cost of the treated water varies from 15 to 400 US$ per million litres.


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