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Wastewater refers to water that has been used in various human activities and has become contaminated as a result. It is generated from households, businesses, industries, and other sources where water is utilized. This water often carries a diverse range of impurities, including organic and inorganic substances, pathogens, chemicals, and pollutants, making it unsuitable for direct release into the environment or for immediate reuse.

The generation of wastewater is an inherent part of human civilization, reflecting our activities such as bathing, washing, cooking, industrial production, and sanitation. As water is used in these processes, it becomes laden with various substances, some of which can be harmful to both human health and the environment.

Wastewater can have both positive and negative influences on human life, depending on how it is managed and treated. Here are some key points illustrating the influence of wastewater in human life:

Positive Influences

Water Supply Augmentation

Treated wastewater can be reused for non-potable purposes, such as irrigation, industrial processes, and landscape irrigation. This helps in augmenting water supplies and reduces the demand for freshwater resources.

Nutrient Recycling

Wastewater often contains valuable nutrients, such as nitrogen and phosphorus. Through proper treatment, these nutrients can be recovered and reused in agriculture, contributing to sustainable farming practices.

Energy Generation

Some wastewater treatment processes, particularly anaerobic digestion, can produce biogas (mainly methane), which can be used as a renewable energy source. This contributes to efforts to generate clean energy and reduce reliance on fossil fuels.

Industrial Processes

Industries often use water in their processes, and treated wastewater can be a reliable and sustainable water source for certain industrial activities. This reduces the pressure on freshwater resources and supports industrial sustainability.

Negative Influences

Health Risks

Untreated or inadequately treated wastewater can pose significant health risks. Waterborne diseases and infections may spread if wastewater containing pathogens is released into water bodies without proper treatment.

Environmental Pollution

Discharging untreated wastewater into natural water bodies can lead to pollution, negatively impacting aquatic ecosystems, biodiversity, and water quality. The presence of chemicals, heavy metals, and other pollutants can harm plants, animals, and aquatic life.

Aquifer Contamination

Improper disposal of wastewater, particularly in regions with inadequate sanitation infrastructure, can lead to the contamination of groundwater sources. This poses a threat to drinking water quality and public health.

Eutrophication

Excessive nutrients, such as nitrogen and phosphorus, in wastewater can contribute to eutrophication in water bodies. This process leads to the overgrowth of algae, depleting oxygen levels and harming aquatic ecosystems.

Social and Economic Impact

Inadequate wastewater management can have social and economic consequences, affecting communities that rely on contaminated water sources. Waterborne diseases can result in increased healthcare costs and productivity losses.

In conclusion, the influence of wastewater on human life is multifaceted. Properly managed and treated wastewater can contribute positively to water sustainability, agriculture, and energy generation. However, the improper handling of wastewater poses risks to public health, the environment, and overall well-being. Sustainable wastewater management practices are crucial to maximize the positive impacts while minimizing the negative effects on human life.

Wastewater treatment is a process designed to remove contaminants and pollutants from water generated by human activities before it is released back into the environment or reused. The goal is to protect public health, prevent environmental pollution, and ensure the sustainable use of water resources. Here are the key steps and methods involved in wastewater treatment:

Preliminary Treatment

In this stage, large objects such as sticks, leaves, and other debris are screened out, and grit (sand, gravel) is settled. This step helps protect downstream equipment from damage and ensures the efficiency of subsequent treatment processes.

Primary Treatment

Wastewater undergoes sedimentation, where heavier solids settle to the bottom as sludge, and lighter materials float to the surface as scum. The separated solids are then removed, leaving partially treated wastewater.

Secondary Treatment

Biological processes are employed to further treat the wastewater. Microorganisms, such as bacteria and other microbes, break down organic matter in the water. Common methods include activated sludge, trickling filters, and lagoons. This step reduces the concentration of pollutants and organic material.

Tertiary Treatment

Additional treatment processes may be applied to further improve water quality. These processes can include filtration, chemical treatment, and advanced biological treatment methods. Tertiary treatment aims to remove remaining contaminants and nutrients.

Disinfection

Disinfection is the final step to kill or inactivate harmful pathogens (bacteria, viruses, and parasites) present in the treated wastewater. Common disinfection methods include chlorination, ultraviolet (UV) radiation, and ozonation.

Sludge Treatment

The sludge generated during the treatment processes contains solids removed from the wastewater. Sludge is treated separately, often through processes such as anaerobic digestion or aerobic digestion, to reduce its volume and stabilize its organic content. The treated sludge can be used for beneficial purposes, such as soil amendment.

Effluent Disposal or Reuse

The final treated water, known as effluent, is either discharged into natural water bodies or, in the case of water reuse, may be further treated for specific applications such as irrigation, industrial processes, or even direct potable reuse in some advanced systems.

Wastewater treatment is a critical component of modern sanitation and environmental protection. It ensures that water released into the environment is of a quality that minimizes harm to ecosystems and human health. Advances in technology and sustainable practices continue to improve the efficiency and environmental impact of wastewater treatment processes.

Wastewater treatment is of paramount importance in human life for several reasons

Protection of Public Health

Proper wastewater treatment helps prevent the spread of waterborne diseases. Untreated wastewater can contain harmful pathogens (bacteria, viruses, parasites) that pose a significant risk to human health. Effective treatment reduces these risks, ensuring the safety of water supplies.

Environmental Protection

Wastewater often contains pollutants, chemicals, and nutrients that, if released untreated, can harm the environment. Treatment processes remove or reduce these contaminants, protecting aquatic ecosystems, wildlife, and overall biodiversity. This is crucial for maintaining a healthy and balanced environment.

Safe Drinking Water Supply

Wastewater treatment contributes to the production of safe and clean drinking water. Many water treatment facilities receive water from rivers or lakes that may receive discharges of treated or untreated wastewater. Treating this water ensures that it meets quality standards for human consumption.

Sustainable Water Management

Wastewater treatment supports the sustainable use of water resources. Treated wastewater can be recycled and reused for non-potable purposes such as irrigation, industrial processes, and even replenishing aquifers. This reduces the demand for freshwater, especially in regions facing water scarcity.

Prevention of Water Pollution

Untreated wastewater can lead to pollution of rivers, lakes, and coastal areas. This pollution can have long-lasting and far-reaching consequences on water quality and ecosystem health. Wastewater treatment helps mitigate these impacts, contributing to the preservation of natural water bodies.

Support for Agriculture

Treated wastewater, when used for irrigation, provides a reliable and sustainable water source for agriculture. This practice helps optimize water use, reduces the reliance on freshwater resources, and supports food production.

Community and Economic Development

Access to clean water and proper sanitation is a fundamental aspect of community development. Wastewater treatment infrastructure improves the overall quality of life in communities by providing a safe and healthy environment. Additionally, it supports economic activities by ensuring a clean and reliable water supply for industries.

Climate Change Mitigation

Wastewater treatment facilities can contribute to climate change mitigation efforts. By capturing and utilizing methane produced during treatment processes, these facilities can generate renewable energy, reducing greenhouse gas emissions.

Particles in sludge feeds interact strongly one with another to prevent settling and offer significant resistance to filtration and compression. This leads to the need for dewatering forces to be compressive ones applied directly to the networked solid phase; sometimes shear forces can be an assist dewatering. Designs of filtration equipment most suitable for sludge dewatering have evolved to meet the intrinsic characteristics of sludges, the most important of which is their compressibility and fine particle sizes, which lead to cakes with extraordinarily high solids contents close to the filter medium. Hence, the membrane plate press, the belt filter, and the decanter centrifuge have become most widely accepted machines for sludge dewatering. Filter presses tend to yield a drier solids discharge, but the level of dryness depends on the sludge properties. The same feed properties dictate the need for chemical pre-treatment to ensure the highest rates of dewatering and best clarity of filtrate, and the correct choice of filter cloth is also crucial in these respects. Commonplace requirements in many processing plants are to minimize the amount of wastewater generated or to reduce the concentration of contaminants in the wastewater, and there are often underlying problems related to dewatering and handling of the sludge. A number of methods are used to reduce the amounts of wastewater discharged and the concentrations of contaminants in the discharge. These include source reduction technologies that minimize the amount of wastewater generated in the plant and treatment technologies that treat wastewater to reduce contamination levels. Contaminant level reduction is primarily either to make the water available for recycling or to reduce costs of treatment. In-plant treatment of wastewater is often a key strategy as a precursor to recycling, and a wide range of treatment options is available. These include careful consideration of alternative uses for the wastewater before it is sent to the treatment plant, technologies to stabilize the wastes (for example, wet oxidation), and separation/concentration technologies (including screens, settlers, filters, centrifuges, and membrane (bio-)processes), as well as thermal processes (for example, evaporation). For dewatering, economic considerations determine that mechanical processes are preferred over thermal ones. Wet oxidation is used to stabilize municipal and industrial wastewater sludges; at lower temperatures and pressures the sludge is conditioned to improve dewatering, but at higher temperatures and pressures biological sludge can be destroyed (as an alternative to incineration). The oxidation process is able to convert oxidisable constituents in the sludge, but still leaves a slurry that has to be dewatered. Hence, dewatering technologies are often key downstream operations in wet oxidation processes as well as in bioprocesses. we will focus on the separation technologies most suitable for sludge dewatering. These are primarily pressure filters, rotary drum filters, and centrifuges.

1. Dewatering-related properties of wastewater sludges

No two wastewaters are alike although, in summary, the general effects on filtration of variations in their characteristics are:

• feed compositions are complex mixtures of organic and mineral particles, biosolids, and molecular and ionic substances;
• feed composition is significant in controlling cake resistance, rate of filtration, and cake moisture content;
• feeds invariably require flocculation to “reduce” their fines content, and the negative effect of the fines on filtration;
• due to their higher biosolids content secondary sludges tend to form wetter cakes and cake form rates are slower when compared with filtration of primary sludges (under the same conditions of filtration);
• the filtering properties of many types of wastewater feeds are dependent on sludge age;
• formed filter cakes tend to vary from moderately to highly compressible.

During filtration, the compressible nature of a filter cake leads to the formation of a solids concentration variation through the cake that decreases from a maximum at the cake–cloth interface. The existence of compressibility in a cake suggests that further liquid can be removed from the cake by applying a compressive force to its surface—the so-called expression process.

2. Filters

The characteristics noted in above Table lead to a preference for pressure filters for sludge dewatering; in the case of some industrial sludges the rotary drum filter can sometimes be considered an option. To make sludge feeds more amenable to mechanical dewatering the feed is more often than not pretreated by flocculants or coagulants, agglomerating the feed particles to increase their effective size.

3. Filter presses

Plate and frame filter presses, recessed plate presses, and membrane plate presses are all used to dewater sludges. Filter plates are supported on side beams or suspended from an overhead beam; filter plates of 1.5 m × 1.5 m are typical, but 2 m × 2 m plates are increasingly common—and larger plates are being developed. For wastewater applications, 80 chambers in a recessed plate press or 60 chambers in a membrane plate press is not uncommon. The ability of the membrane plate presses to utilize the compressible nature of the sludge makes them particularly useful for sludge dewatering applications. A typical filtration cycle for dewatering is: (i) slurry feeding; (ii) cake squeezing by inflating the membranes; (iii) air blow through the cake; (iv) core wash and/or blow. The cake squeeze is affected by diaphragms that are pressurized up to 16 bar in order to lower cake moisture content (or, increase the volume of liquid recovered from the feed). Cake moisture content reductions are dependent on its compressibility properties but moisture contents of 25% more than can be achieved on a conventional filter press are not uncommon. Some operating data for recessed plate filter presses are given in the next Table:   Developments incorporated into modern filter presses to increase filter capacity, reduce cake discharge times, and reduce labor intensity include: (i) automation and mechanization of plate pack opening and plate shifting; (ii) use of long-travel hydraulic cylinders to move the pressure head to reduce press opening times (very large presses may have two moving pressure heads); (iii) cloth shaking or lifting mechanisms to promote cake discharge; (iv) cloth flushing or washing systems, which range from simple spray nozzles mounted above the plates to moving spray bars that are lowered and raised between plates singly or in groups, to remove adhering or penetrating particles (a limitation of most cloth washing systems is that only one side of the cloth is washed); (v) placement of the filter onto load cells to indicate if the filter fails to reach its tare weight (for filter control and/or throughput measurement); (vi) use of “bomb bay” doors to cover discharge chutes to prevent water entry into the dry cake handling facilities; (vii) light curtains and/or protective screens to prevent operator access. Although membrane presses are significantly more expensive than conventional filter presses, the additional capital and operating costs are often justified by shorter cycle times (and hence greater sludge throughput) and the more easily handled cake that is produced.

4. Belt presses

Belt filters are characterized by two continuous, tensioned filter cloths. Flocculated sludge is fed to the lower cloth (belt); initial dewatering is under gravity as the belt carries the sludge into a consolidation zone where it is progressively squeezed under pressure by the upper and lower belts moving towards each other to form a closed “envelope”. The cake is then squeezed under increasing pressure as the cloths move over a sequence of successively smaller diameter rollers. As the two belts pass over the rollers there is a relative movement of the belts, causing the liquid to be removed by a combination of expression and shearing to produce a dry, crumbly cake.     A key to successful operation of a belt press is that the feed must be flocculated, to avoid blinding of the filter belt and facilitate gravity drainage when it is initially fed to the belt. Conditioning is carried out by polyelectrolytes immediately before the drainage zone; some results for different sludge types are given in the next table. Special care must be taken with belt washing, carried out on the belt return cycle with rinse water flow rates as high as 50–200% of that of the sludge. For good machine operation, a feed sludge concentration >3–4% has been recommended.

5. Rotary drum filters

Vacuum filters have operational and process limitations that can be most important when choosing a filter for sludge dewatering. By definition, the driving force for dewatering is limited by the vacuum that can be applied; in practice, a vacuum of not more than 0.25 bar absolute (−0.75 bar g) can be applied. For this reason vacuum filters are not usually employed in systems where most of the particle sizes are smaller than about 5 μm; in turn, vacuum filters are rarely used to dewater municipal sludges but are more often suitable for some types of industrial ones. When vacuum filters are used, rotary drum filters are the preferred choice (Fig. 6) and their continuous operation and virtually no operator intervention during the normal operating cycle can be used to advantage.

6. Decanter centrifuges

High solids decanters are used to mechanically dewater environmental and biosolids sludges and are often a preferred choice of equipment due to

• the high forces of 2000–4000 g applied directly to the feed solids, enabling lower solids moisture contents (the “ultimate” cake dryness depends on the given sludge);
• its ability to handle higher solids content feeds;
• its continuous operation, with solids throughputs up to about 90 te h⁻¹;
• the solids handling capabilities intrinsic through its design, with solids conveyed co-currently along the walls of the bowl by a helical screw.

  The centrifuge can be over-torqued due to the flow properties of the thickened solids, or due to plugging by the accumulation of unconveyed solids in the bowl. Wear problems on the screw can also be caused by more abrasive particles. To improve centrate clarity, flocculants or coagulants are frequently added to the feed to agglomerate finer particles. Examples of the expected performance of decanters dewatering different types of municipal sludges are given in the next table:  

8. Filter media developments

Surface coatings applied to filter fabrics can enhance one or more of its filtration properties; microporous polymer coatings are a relatively new development used to provide a smoother and finer aperture size to the fabric surface and to facilitate easier detachment of the cake and prolong the lifetime of the medium. A polyurethane coating on a woven polyester substrate is the basis for Madison’s Primapor fabric for use on process filters such as rotary drums and filter presses. The “second generation” treatment developed by Madison, Azurtex, has the coating pushed farther into the body of the fabric so that the surface finish is less prone to mechanical damage from external forces. Both treatments give better particle retention with improved cake release.

9. Conclusions

Designs of filtration equipment most suitable for sludge dewatering have evolved to meet the intrinsic characteristics of sludges, the most important of which is their compressibility and fine particle sizes, which lead to cakes with extraordinarily high solids contents close to the filter medium. The sludge feed tends to be networked, that is particles interact strongly one with another to prevent settling and offer significant resistance to compression, which requires that the forces applied for dewatering be compressive ones applied directly to the networked solids phase. Hence, the membrane plate press, the belt filter and the decanter centrifuge have become most widely accepted machines for sludge dewatering. However, usually the same feed properties dictate the need for chemical pre-treatment to ensure the highest rates of dewatering and best clarity of filtrate, and correct choice of filter cloth is also crucial in these respects. The filter press tends to yield drier solids, but the choice of equipment depends not only on the cake dryness but also the process duty requirements and costs. [Ref.]

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.

Coagulation

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

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.

Distillation

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

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

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.

Oxidation

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

Precipitation

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.