a Carbohydrate Enzyme Biotechnology Laboratory, Department of Biosciences and Bioengineering, Indian Institute of Technology Guwahati, Guwahati, Assam 781039, India. E-mail: firstname.lastname@example.org
The increased population has led to an increase in the demand for goods which in turn has caused rapid industrialization. In turn, the increase in industrial set-ups has led to the increased production of industrial wastes. These industrial wastes cause major environmental havoc by polluting the water, air and soil. The quality and quantity of wastewater generated depends on the type of industry: it can contain non-biodegradable waste such as heavy metals, pesticides, plastic etc. and biodegradable compounds such as paper, leather, wool etc. Industrial wastewater can be toxic, reactive, carcinogenic or ignitable. Therefore, without proper treatment and management strategies, the discharging of the waste into water bodies can pose dreadful environmental and health effects. Several waterborne pathogens proliferate in wastewater and produce toxins, affecting the earth's ecosystem and human health. The toxins in industrial wastewater cause acute poisoning, immune system suppression and reproductive failure. According to the WHO, around 80% of diseases are waterborne. To address the environmental and health issues created by industrial wastewater, it is absolutely necessary to obliterate its toxicity by adequate treatment with physical, chemical and biological means so that it can be recycled for water conservation.
The escalating population is causing rapid expansion in agricultural and industrial sectors, and this results in a higher demand for water, which is essential for sustaining every life-form on this blue planet. The major sources of water for irrigation of agricultural fields, industry and human and animal consumption are rivers, groundwater and lakes. Due to climatic changes, the occurrence of floods and droughts has become frequent in many parts of the world. On top of that, increasing water pollution from the waste released from various sectors like industry, agriculture, households, municipalities, etc., has greatly contributed to the decline of the quality and quantity of potable water. Therefore, the proper treatment of wastewater before disseminating it into water bodies has become indispensable to maximize the quality and quantity of potable water. Polluted water can be defined as water that contains excessive hazardous contaminants that make it unsuitable for drinking, cooking, bathing and other uses.1 Water pollution generally results from human activity, and the pollutants released mostly come from industrial dumps, sewage leakages, oil spillages, heavy metals, animal wastes, chemical wastes, eroded sediments, deforestation, littering, fertilizers, herbicides, pesticides, etc. These sectors consume around one-third of renewable freshwater that is available and the pollutants released by them contain various synthetic and natural chemical contaminants.2
Wastewater released from various sectors can be categorized into different types, such as, sewage wastewater, domestic wastewater, storm run-off wastewater, agricultural wastewater and industrial wastewater. In the present study, the focus is more on water pollution due to rapid industrialization and its adverse health effects. As per the AQUASTAT database, 3928 km3 of global freshwater is withdrawn every year, 22% (865 km3) of which is used by industry. Industrial effluents are one of the major causes of irreversible damage to the ecosystem. Improper treatment and direct release of these hazardous effluents in the sewerage drains eventually pollutes the groundwater as well as other major water bodies, causing adverse effects on the health of animals as well as aquatic life. Under-treated effluents can also cause other potential environmental pollution like air, land surface, soil, etc. Casual disposal of industrial wastewater used in irrigating crops can cause serious damage to the quality of the crops produced and can also reach the food chain.3 Waterborne diseases caused by water pollution are diarrhoea, giardiasis, typhoid, cholera, hepatitis,4 jaundice5 and cancer.6 Several countries are now framing policies on water quality control. Logical bases are being set up on the amount of pollutants that can be safely assimilated in specific water bodies like rivers and lakes.7 Some such programs deciding the carrying capacity load and discharge standards of individual pollutants are the total maximum daily load (TMDL) under the US Clean Water Act, Integrated Pollution Prevention and Control (IPPC) in Europe and the Central Pollution Control Board (CPCB) in India, which set minimum acceptable standards (MINAS) for the release of municipal and industrial wastes. Several treatment plants are also being set up which use chemical, electrochemical, biological and physical processes for releasing potable water. With both economic growth as well as the scarcity of clean water in mind, several industrial developers and manufacturers are now adopting technologies to ensure cleaner production, less water consumption and less pollution.
In general, wastewater has been categorized into two broad types: sewage wastewater and non-sewage wastewater.8 Sewage wastewater includes discharge from domestic activities. The wastewater produced from places like houses, schools, hospitals, hotels, restaurants, public toilets etc. containing body wastes (urine and faeces) comes under sewage wastewater. All the other types of wastewater produced from commercial activities such as that generated from factories and industrial plants are termed non-sewage wastewater. The non-sewage wastewater also includes stormwater and rainwater generated after rainfall or flood events. Day-to-day human activities are majorly water dependent which makes wastewater management and treatment very important. Thus, for the effective management and targeted treatment, wastewater has been further categorized into well-defined types and sub-types depending upon the sources. The four major types of wastewater are stormwater runoff, domestic, agricultural and industrial9 as shown in Figure 1.1. All these types and their respective sub-types are discussed in brief below.
Stormwater runoff wastewater is the heavy rainfall, storm or flood water that is not soaked into the ground and flows above the street or open surfaces.10 It is one of the leading sources of water pollution as many toxic pollutants like plastics, pesticides, herbicides, oils, chemicals, heavy metals and even various pathogens gets washed off into stormwater runoff from streets, industrial sites, construction sites and various other places. Stormwater runoff usually flows either directly or through channelled drains which eventually discharge into nearby natural waterways such as ponds, rivers, streams and lakes without any treatment. This polluted water not only hurts aquatic life, but is also a threat to the entire environment as all life forms are directly or indirectly connected to the natural waterways for their survival.
The wastewater produced by human household activities is known as domestic wastewater. The main source of this wastewater generally consists of two major waste streams: toilet waste, i.e. the liquid released from sanitary/laundry/bathing facilities, and the wastewater generated due to the other household activities such as cooking etc.11 Based upon the source, domestic wastewater is categorized into three different sub-types: black, grey and yellow wastewater.12
The most contaminated form of domestic wastewater with discharge from toilets, kitchen dishwasher and sinks. The contaminants present in blackwater are urine, faecal matter, toilet paper, soaps, discarded food pieces, various chemicals and a lot of cleaning liquids.13 It is extremely polluted wastewater with a high risk of causing diseases.
A less contaminated form of domestic wastewater discharged from baths, washing machines and bathroom sinks.14 To simplify, greywater or sullage is actually blackwater without faecal matter, urine and bits of food waste i.e. domestic/household wastewater without any contact with toilet water. Though it is not referred to as pathogenic, as it is loaded with detergents, soaps, cleaning liquids and various chemicals it should be treated well before being considered for re-use.
This is specifically urine without any other contaminants of blackwater and greywater. Yellowwater does not have any faecal matter, toilet paper, chemicals or even any food particles and is pure urine water.12 Such categorization of domestic wastewater makes the planning and execution of treatment simpler as specific treatments can be applied to the type of water based upon its characteristics.
Agriculture runoff is considered as a major source of water pollution in many watersheds. Agricultural wastewater is sometimes also referred as irrigation tailwater when excess water runs off the fields during surface irrigation.15 This excess water running through the fields become the primary cause of sediment and nutrient runoff to the nearby water sources. In addition, agrochemicals such as fertilizers, pesticides, herbicides, crop residues, animal wastes, pig, poultry and fish farm effluents and dairy farming waste are the pollutants of agricultural wastewater.16 Many farm management techniques are used for agricultural wastewater treatment to majorly prevent surface runoff.17,18
Water with dissolved and suspended substances discharged from various industrial processes, such as the water released during manufacturing, cleaning and other commercial activities, is termed industrial wastewater.19 The nature of the contaminants present in industrial wastewater depends on the type of the factory and the industry. Examples of industries that produce wastewater are the mining industry, steel/iron production plants, industrial laundries, power plants, oil and gas fracking plants, metal finishers and the food/beverage industry. The various contaminants commonly found in industrial water outlets are chemicals, heavy metals, oils, pesticides, silt, pharmaceuticals and other industrial by-products.20,21 In general, it is difficult to treat industrial wastewater, as individual examination of the set-ups and specific treatment plants are required on an industry-based level. Therefore, to deal with this, on-site filter presses are installed to treat the effluent wastewater.22
Wastewater from various industrial sectors contains many pollutants that are toxic and have hazardous effects on human and aquatic life as well as on agriculture. Such pollutants include heavy metals like chromium (Cr), zinc (Zn), lead (Pb), copper (Cu), iron (Fe), cadmium (Cd), nickel (Ni), arsenic (As) and mercury (Hg).4,23 Most of these heavy metal pollutants are released by paint and dye manufacturing, textile, pharmaceutical, paper and fine chemical industries. Phenol and phenolic compounds are also one of the major pollutants present in industrial wastewater.24 They are mostly released by oil refineries, phenol–formaldehyde resin and bulk drug manufacturing industries. A number of poorly biodegradable refractory pollutants like petroleum hydrocarbons, sulfides, aniline, naphthalenic acid, organochlorines, olefins, nitrobenzene, alkanes and chloroalkanes, generated by the petrochemical industries are present in wastewater.25 The composition of petrochemical wastes is chemically very complex and their treatment by biological methods is slow and not very effective. Even after the primary biological treatment, the organic pollutants are retained in the secondary effluents. They require chemical oxidants for the formation of inorganic end products and thus exhibit a low ratio of biological oxygen demand (BOD) to chemical oxygen demand (COD).25 Suspended solids and highly organic materials are the major water pollutants released by the paper and pulp industry. Depending on the quality of paper produced and pulp processing, the characteristics of the effluent changes. The constituents of the effluents can be adsorbable organic halogens (AOX), phenolic compounds, biocides, colours, resin acids, non-biodegradable organic materials, tannins, sterols, lignin-derived compounds etc.26,27 Urea, ammonium nitrogen (NH4–N) and other nitrogenous and phosphorus wastes come from various textile printing and dyeing industries, which use water in many steps while processing. Various textile industries generate compounds ranging from heavy metals like chromium to surfactants, bleaching agents including hydrogen peroxide and chlorine, AOX, sodium silicate and alkaline bases.28 Perfluoroalkyl acids (PFAAs) are used as surface protectors for their excellent high surface activity, stability and oil–water repellence. However, two PFAAs with potential health risks are perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA). PFOS is released mainly by textile treatment, metal plating and semi-conductor industries, while PFOA is released by the fluoropolymer production and processing industries.29 They are mainly circulated through the wastewater released from these industrial facilities. Besides all these pollutants, the high salinity of wastewater also has many adverse effects on life forms. Salt removal from wastewater has become as important as removal of organic matter and other pollutants in many countries. High salinity (mainly NaCl) wastewater is generated by petroleum, leather, food processing and agro-based industries.30 Some major industrial sectors and the water pollutants released by them are summarized in Table 1.1.
|Industry||Major water pollutants||Reference|
|Dye manufacturing||Copper, colour, salt, sulfides, formaldehydes||23 and 28|
|Paint manufacturing||Chromium, zinc, lead, volatile organic compounds (VOCs)||23 and 31|
|Textile||Iron, chromium, chlorinated compounds, urea, salts, hydrogen peroxide, high pH NaOH, surfactants||28|
|Pharmaceutical||Cadmium, nickel, phenolic compounds||24 and 23|
|Petrochemical||Petroleum hydrocarbons, phenolic compounds, nitrobenzene, alkanes, chloro alkanes, high salt, etc.||24|
|Paper and pulp||Organic and chlorophenolic compounds, suspended solids, AOX, lignin, tannins, sterols, colours, biocides, etc.||26 and 27|
|Metal working||Perfluorooctane sulfonate (PFOS), ammonium nitrogen, cyanide, phenol, oil and grease||29 and 32|
|Plastic||Perfluorooctanoic acid (PFOA), lead, mercury, cadmium, diethylhexyl phthalate||29,33 and 34|
|Agriculture||Fertilizers, pesticides, insecticides||35|
Rapid industrialization during the last few decades has significantly increased the amount of pollutants in the environment. Improper treatment of some hazardous industrial wastes released into water bodies has been creating toxic effects on all type of life forms directly or indirectly. Heavy metals are one of the major water pollutants that are persistent and non-biodegradable in nature. Intake of some toxic heavy metals by aquatic fauna can cause detrimental health problems in other animals and ultimately humans via the food chain. They can be teratogenic, carcinogenic and can cause oxidative stress, organ damage, nervous system impairments and reduced growth and development.36 Another most prevalent chemical pollutant released by industry are phenolic compounds. They exhibit toxicity by inhibiting normal microbial function, thus affecting biological treatment processes.24 They can also cause reflex loss, sweating, low body temperature, cyanosis, decreased respiration and respiratory failure. The major effluent constituents of the paper and pulp industry like tannins, resins and chlorinated organic compounds can cause genotoxicity and mutagenicity.27 The most common effluents from the paper pulp industry are lignin and its derivatives. They are poorly degradable and during biological treatment may transform into toxic compounds, which can affect the hormonal balance in aquatic animals.37 Reproductive disruption in fish can be caused by binding of some major wood sterols like β-sitosterol and stigmasterol to oestrogen receptors of fish.38 A collective mixture of hazardous constituents in textile effluents make them highly toxic.6 Chromium compounds and oily scum together form a colloidal matter that acts as a barrier to prevent sunlight from entering the water body, thereby decreasing the dissolved oxygen. Many textile industries use chlorine-bound organic colourants that are carcinogenic. According to a case study reported in Tribune, on 7th April 2009, in a village near Bhatinda, Punjab, India, the farmers developed cancer as, due to the lack of canal water, they had to use toxic sludge from factories to irrigate their farms.6 The toxicological effects of PFOA were studied in CD-1 mice that showed the toxic effects in the mother and also in the developmental stages of neonatals.39 The mother was compromised with pregnancy loss, increased liver weight and low weight-gain during pregnancy, while the neonatals suffered reduced postnatal (approximately the first six weeks following birth) survival rates, delayed eye-opening, lower body weight, and reduced growth and development. Animal studies (on cynomolgus monkey) by using PFOS showed decreased body weight, lower triiodothyronine (T3), increased liver weight, and lower cholesterol and oestradiol levels.40 Hypersalanity of water also greatly effects the microbial activity of non-salt-adapted micro-organisms and can also interfere with aerobic treatment processes.30 Efficient removal strategies of these toxic pollutants before releasing them into various water bodies are therefore highly necessary for a hygienic and healthy environment.
Various types of technologies and strategies are being developed and employed for contamination removal from wastewater released from several industries. Some of the strategies developed and used by some major wastewater producing industries for treatment of effluents are mentioned below.
Heavy metals are considered one of the most hazardous contaminants released from chemical-intensive industries. Conventional strategies like ion-exchange methods involving synthetic ion-exchange matrices for cation and anion exchange, chemical precipitation using precipitants like lime and limestone under basic pH conditions and electrochemical deposition methods are being employed to remove heavy metals from inorganic effluents.41 However, these methods are known to have many disadvantages when completely removing heavy metals and they also have high energy requirements.42 Some of the cheaper and effective technologies developed for quality improvement of treated water are adsorption, membrane filtration, electrodialysis and photocatalysis.41 Low-cost adsorbents like natural material, e.g. zeolites and clinoptilolite; industrial by-products such as iron slags, fly ash, hydrous titanium oxide and waste iron; biosorption using biological and agricultural wastes like inactive microbial biomass, orange peel, pecan shells, hazelnut shell, maize husk or cob, etc.; modified biopolymers like chitosan, starch, chitin and hydrogels are effectively used for the removal of heavy metals. The removal of heavy metals from inorganic solutions can be achieved using membrane filtration techniques like ultrafiltration employing permeable membranes of pore size (5–20 nm). Reverse osmosis capable of removing 98% copper and 99% cadmium, nanofiltration and polymer-supported ultrafiltration are some of the other techniques used to remove heavy metals.41 The electrodialysis process involves an ion-exchange membrane through which the ionized solution is passed and membrane separation takes place under an electric potential.43 This method is effective for the removal of heavy metal ions like Ni, Co and Cd. On the other hand, the photocatalysis method uses titanium dioxide semi-conductors capable of reduction or oxidation of species having appropriate redox potential, like Cu2+, Cr3+ and Cr4+ heavy metal ions.41
Phenol and phenolic compounds are one of the most prevalent refractory chemical pollutants present in wastewater released from industries. Phenolic wastes can be treated by a number of methods including chemical, physical, electrochemical and anaerobic biological processes. Of them, the electrochemical process has been reported to be the most effective in the removal of phenolic wastes.24 It uses electrons as the main reagent for destruction of the pollutants by direct or indirect oxidation processes. Ti/Pt and Ti/Pt/Ir, graphite, Ti/SnO2–PdO2–RuO2 and TiO2–RuO2–IrO2 anodes have been reported, and are used in the electrochemical treatment of tannery wastewater,44 landfill leachate,45 resorcinol and cresols.46,47 However, electrochemical treatment increases the AOX concentration in effluents and therefore, these effluents are properly treated with activated carbon before discharge into the environment.
The paper and pulp industry is one of most water-exhaustive and highly polluting industrial sectors. Along with water waste, it also generates a large amount of solid and gaseous waste. A number of treatment processes are used to manage these wastes. The black liquor produced is treated by a membrane filtration system by vibration separation enhanced processing (VSEP) and biological treatment, AOX are reduced by oxygen bleaching, heavy metals are treated by biological and sedimentation treatment methods, anaerobic digestion (AD) followed by pyrolysis and incineration is used for primary and secondary bio sludge treatment, etc.48 AD with incineration helps in the production of beneficial by-products like biochar and biogas and also helps in lignin removal. Apart from these techniques, advanced oxidation with Fenton's reagent, ozonation, gasification and biological treatment are also employed to treat wastewater. A composite coagulant, polymeric ferric aluminium sulfate chloride (PFASC) works with polyacrylamide (PAM) as a tertiary treatment of wastewater from paper mills and reduces chroma (intensity of colour released in paper mill wastewater) by 71.2% and COD by 65.3%.49 High content of BOD and COD in wastewater is treated by aerobic granulation,50 which helps in the removal of tannin and lignin. The chroma produced from the paper and pulp industry can also be reduced by use of agroindustry-based residual biosorbents such as agricultural by-products and activated carbon.51 The use of microbial fuel cells is a new approach for the treatment of wastewater generated by industries. It comes with benefits like electrical energy production and exclusion of the aeration process conventionally used for the removal of many dissolved gases.52 The alternative eco-friendly use of enzyme bleaching, (xylanase and laccase) instead of chlorine bleaching reduces production of AOX or organic chlorinated pollutants.53
The textile industry also consumes a large amount of freshwater. Water is required in many steps during processing and, as a result, it generates a large amount of wastewater. Among other wastes generated by the textile industry, dyes (azo dyes) used for colourization contribute greatly towards the wastewater. Physico-chemical-based conventional treatment processes include adsorption, membrane-based separation techniques and ion-exchange methods. Adsorbents like silicon, carbon and kaolin polymers are used for the removal of dyes.54 Membrane separation techniques like nanofiltration and reverse osmosis are used to treat water containing reactive dyes and other chemical compounds and the ion-exchange method is used for the removal of both anionic and cationic dyes from wastewater. Apart from these, other conventional methods use Fenton's reagent, a strong oxidising agent with excess hydrogen peroxide added for decolourization.54 Ozonation is used for toxic non-biodegradable components. Photochemical methods degrade dyes by UV treatment. Treatment with cucurbituril (a polymer of formaldehyde and glycoluril) can bring about complete degradation of basic, acidic, reactive as well as disperse dyes.54 Besides these conventional methods, biological treatment with different micro-organisms, bacteria, fungi and algae as well as plants has shown to be very effective in the degradation of these chemically stable dyes. Bacterial treatment under anaerobic conditions with species like Pseudomonas putida, Staphylococcus hominis and Citrobacter sp. cleaves the azo linkage of the dyes with the help of reductases.55 Under aerobic conditions, oxygen-insensitive azo-reductases produced by species like Geobacillus stearothermophilus, Micrococcus sp, Staphylococcus arlettae, having a high substrate specificity use NADH as a co-factor and cleave the azo linkages.54 For a more advanced biodegradation of these pollutants from wastewater, a treatment system containing a mixed culture of bacteria is also employed. Fungal strains like Phanerochaete chrysosporium decolourizes by producing non-specific enzymes like laccase, lignin and manganese peroxides for the degradation of azo-dyes.56 Algae help in the degradation of azo-dyes by using them as a nitrogen source and thus contribute to preventing eutrophication in water bodies.57 Microalgae are used to treat the effluents released by the textile industry by adopting techniques like bioadsorption and biodegradation. Phytoremediation involving techniques like phytotransformation, phytostimulation, phytovolatilization, phytoaccumulation, rhizofiltration and phytostabilization are used for the treatment of textile effluents.54
Hypersaline effluents released by some industrial sectors are generally treated by physico-chemical methods. These methods involve thermal techniques that use multiple effect evaporators (MEE). This reduces the volume of effluent and leads to the separation of a solid salt.30 A coagulation–flocculation method is used as a pre-treatment for removal of the colloidal COD fraction from hypersaline effluents. Some other effective techniques applied for desalination are ion-exchange methods using both anionic and cationic exchangers and membrane filtration techniques like reverse osmosis and electrodialysis.30
Growing industrial set-ups have increased the release of pollutants, affecting the entire ecosystem. Water pollution is one of the most devastating effects of industrialization. The potability and hygiene of drinking water have been affected by hazardous impurities that are released by industrial sectors, causing detrimental health effects to human, animal and aquatic life. Though health is of great concern, it cannot be denied that a growing economy also requires industrial growth. For overall socio-economic growth and welfare, research is encouraged into the development of such techniques that can reduce the use of freshwater by industrial sectors as well in the development of efficient and effective water treatment methods. New developments and continuous monitoring of the execution strategies of various programmes and interventions related to industrial wastewater treatment are absolutely necessary for the amelioration of any toxic effects.
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