ENVIS Centre on Control of Pollution Water, Air and Noise

INTRODUCTION

Water pollution is the contamination of natural water bodies (e.g. lakes, rivers, sea, ocean, aquifers, ground water etc.). This form of environmental degradation occurs when pollutants are directly or indirectly added into the water bodies without proper treatment to remove harmful compounds.

 

According to the Water (Prevention and Control of Pollution) Act, 1974, water pollution means such contamination of water or such alteration of the physical, chemical, or biological properties of water or such discharge of any sewage or trade effluent or of any other liquid, gaseous, or solid substance into water (whether directly or indirectly) as may, or is likely to, create a nuisance or render such water harmful or injurious to public health or safety, or to domestic, commercial, industrial, agricultural, or other legitimate uses, or to the life and health of animals or plants or of aquatic organisers.

SOURCES OF WATER POLLUTION

General sources of water pollution are : Direct and Indirect contaminant sources.

Direct sources of water pollution include effluent outfalls from factories, refineries, industries etc. that emit fluids of varying quality directly into water bodies.

Indirect sources of water pollution include contaminants that enter the water supply from soils/groundwater systems and from the atmosphere via rain water. Soils and groundwaters contain the residue of human agricultural practices (fertilizers, pesticides, etc.) and improperly disposed of industrial wastes. Atmospheric contaminants are also derived from human practices (such as gaseous emissions from automobiles, factories and even bakeries).

If pollution comes from a single location, such as a discharge pipe attached to a factory, it is known as point-source pollution.If pollution comes from one single source but from many different scattered sources. This is called nonpoint-source pollution.

WATER POLLUTANTS

The substances which are responsible for causing water pollution are called water pollutants.Based on source, water pollutants can be classified as follows:

1. Domestic wastes and sewage
2. Surface run-off
3. Industrial effluents
4. Thermal pollution
5. Marine pollution

 

1. Domestic wastes and sewage

Domestic waste and sewage is the biggest polluter of surface and ground water sources in India. This is due to the big lacuna between the amount of sewage generation and the facilities to dispose it off. The problem is not only the lack of facilities; rather, the non-functioning of existing facilities / treatment plants is more critical.

 

 

2. Surface run-off

The pollutants present on the surface of land, fertilizers, and pesticides added to the soils are washed down into natural water courses during rains. The flow of fertilizer rich water into streams and lakes gives rise to eutrophication. Excess of pesticides in water also adversely affect the aquatic life.

 

 

3. Industrial effluents

Effluents generated from industries which are directly disposed off into the water streams without any treatment are a very important cause of water pollution. Industrial wastes contain a number of toxic chemicals such as mercury (Hg), lead (Pb), cadmium (Cd), arsenic (As) etc. Several types of liquid effluents having toxic chemicals, acids and bases, etc. are also added into the rivers which kills fish and other aquatic life besides being toxic to human beings. Examples of large scale effluent addition into the rivers are Yamuna (near Okhla, Delhi), Gomti (near Lucknow), Ganga (near Kanpur), and Hoogli (near Kolkata) etc.

 

 

4. Thermal pollution

Temperature above the normal range is called as thermal pollution or Calefaction. Thermal pollution occurs as a result of the entry of heated water from industries and power generation plants. Various processes involved in generating thermal pollution are
• Water for cooling condensers
• Feeding boilers for steam generation
• Auxillary plant cooling
• Ash handling
• Gas washing, etc.

 

The immediate effect of an increase in temperature is a decrease in the oxygen concentration. A temperature rise of 10°C will double the rate of many chemical reactions and so the decay of the organic matter, rusting of iron, and the solution rate of salts are also accelerated by calefaction. All organisms have a range of temperature tolerance beyond which they either die or move to more congenital conditions downstream.

 

5. Marine pollution

Marine pollution is the pollution of the water of oceans and seas through various sources. Industrial wastes, heavy metals, insecticides, mine trailings, urban and rural sewage, farm and forests run offs i.e. wastes of all kinds including those which are not biodegradable and even radioactive wastes are dumped into the sea. Sometimes, during war period, oil wells are damaged and oil flows into the sea due to which a large number of useful aquatic life as well as birds are killed. In marine water, the most serious pollutant is oil, particularly when afloat on sea. Point sources of marine pollution are domestic sewage and industrial effluents. Non-point sources draining to coastal waters include surface run-off from agricultural areas, wash out of agrochemicals, and transport of sediments due to coastal erosion or desertification and deforestation in the hinterland.

 

 

EFFECTS OF WATER POLLUTANTS

Water pollution is very serious problem as it affects all spheres of life of human, animals, and plants.

1. Effects on human health

2. Effects on ecosystems

3. Effects on fauna

4. Economic loss

1. Effect on human health
In countries where there are scanty facilities for treating waste water, people often get exposed to various water-borne diseases such as cholera, diarrhoea, etc. In severe cases, there may be the outbreak of diseases like Hepatitis, tuberculosis, malaria, encephalitis, filariasis etc. As per the report of World Health Organization (WHO), approximately 3.4 million people die of water-borne diseases every year in the world. In developed countries, even where there are better purification practices, people still suffer from the health effects of water pollution. Excess nitrogen in drinking water also poses serious risks to infants.

2. Effect on ecosystems
Nutrient pollution from upstream of land flows down and contaminates the larger water body. It inturns, promotes the algae growth and subsequently growth of many more water organisms takes place. The excess growth of algae affects the fish and other aquatic organisms by absorbing and reducing their oxygen supply. Fishes also die off due to clogging of their gills as a result of algal growth. Thus, the aquatic ecosystems are affected very adversely disturbing all the food chains.

3. Effects on fauna
Animals, both aquatic and terrestrial, are prone to risks generated from the waste water. In a classic case of marine pollution in recent time, 16000 miles of a US coastline was affected by an oil spill. In this case, death of many animals has been reported and there was great loss to aquatic fauna. Animals are also affected by the solid waste thrown into the water bodies, as it harm them in many ways.

4. Economic loss
It is quite obvious that increasing water pollution will lead to the excessive pressure on the existing treatment plants as well as the establishment of new treatment plants. Fishing industry is also affected badly as the fishes die due to depletion of oxygen. Recreational and tourism sectors are also affected negatively as lots of money needs to be spent to clean up the water from algae blooms etc.

CONTROLLING MEASURES OF WATER POLLUTION

Water pollution in natural water bodies can be identified and quantified on the basis of various parameters, such as, dissolved oxygen (DO), biochemical oxygen demand (BOD), coliform organisms, pH etc. As per the water quality criteria, the DO levels in drinking water should be ≥ 6 mg/L and BOD levels should be < 2 mg/L. Moreover coliforms level should not exceed 50 MPN/100 mL in water which is safe for drinking purpose. If the water quality of any source is not complying with these criteria, the water can not be used for drinking purpose without undergoing complete treatment.

Water pollution can be controlled by diluting the water pollutants in a reservoir. The various methods for the control of water pollution can be summarized as follows:

1. The sewage pollutants are required to be treated in sewage treatment plants before their discharge in natural water bodies.

2. Water pollution due to organic insecticides and pesticides can be reduced by the use of very specific and less stable chemicals in the manufacture of insecticides/pesticides. Moreover, use of bio-fertilizers needs to be promoted.

3. Oxidation ponds can be useful in removing low level of radioactive wastes.

4. Hot water should not be disposed directly into the river, as it adversely affects the life of aquatic organisms. Thermal pollution can be reduced by employing techniques such as cooling, cooling ponds, evaporative or wet cooling towers and dry cooling towers.

5. Domestic and industrial waste waters should be treated properly in waste water treatment plants, before discharge in the natural aquatic systems.

6. Strict implementation of legislations for water treatment should be done.

7. No solid waste should be dumped into water bodies.

8. Dead bodies of animals/human should not be floated in water sources.

9. Bathing, washing of clothes, and idol immersion should be strictly restricted in natural water bodies.

WATER TREATMENT TECHNOLOGIES

I. Drinking water treatment technology

II. Waste water treatment technology

I. Drinking water treatment technology

The various methods or the techniques which may be adopted for purifying the public water supplies are:

1. Screening
2. Plain sedimentation
3. Sedimentation aided with coagulation
4. Filtration
5. Disinfection
6. Softening
7. Miscellaneous treatments such as fluoridation, recarbonation, liming, desalination etc.
The necessity of a few or all of these treatments depends solely upon the quality of the available raw water.

1. Screening

Screens are generally provided in front of the pumps or the intake works so as to exclude large sized particles, such as debris, animals, trees, branches, bushes etc.

2. Plain Sedimentation

Most of the suspended impurities present in water do have gravity greater than that of water. In still water, these impurities will therefore, tend to settle down under gravity, although in normal raw supplies, they remain in suspension, because of the turbulence in water. Hence, as soon as, turbulence is slowed down by offering storage to the water, these impurities tend to settle down at the bottom of the tank. A plain sedimentation tank under normal conditions may remove as much as 70% of the suspended impurities present in water. Usually, water depth of 3-5 m is provided in sedimentation tanks.

3. Sedimentation aided with coagulation

Very fine suspended mud particles can be removed easily by increasing their size by changing them into flocculated particles. For this, chemical coagulants are added to water. The use of coagulants is generally necessary for clarifying raw waters containing turbidities greater than 30-50 mg/L. Some of the common coagulants used for the water treatment are alum (Aluminium sulphate, Al₂(SO₄)₃·18H₂O), copperas (Ferrous sulphate, FeSO₄·7H₂O), chlorinated copperas (Ferric sulphate + ferric chloride, Fe₂(SO₄)₃ + FeCl₃), sodium aluminate (Na₂Al₂O₄) etc.

4. Filtration

The process of passing the water through the beds of granular materials (called filters) is known as filtration. Filtration may help in removing colour, odour, turbidity, and pathogenic bacteria from the water.
Two types of filters are commonly used for treating municipal water supplies. These are
(a) slow sand gravity filters, and
(b) rapid sand gravity filters. Sand, either fine or coarse, is generally used as filter medium.
The mechanisms through which the filtration helps in treating water are –
a. Mechanical straining
b. Flocculation and sedimentation
c. Biological metabolism
d. Electrolytic changes

5. Disinfection or Sterilisation

The filtered water which is obtained after going through above procedures normally contains some harmful disease causing bacteria in it. These bacteria must be killed in order to make the water safe for drinking. Disinfection means killing of only the diseases producing bacteria, where as, sterilization means killing of bacteria of all types. The chemical which is used as a disinfectant must be able to give protection against re-contamination, thus it must have the residual sterilising effect.

a. Minor methods of disinfection

i. Boiling of water – Effective method, but practically not possible to boil huge amounts of public water supplies. Moreover, it can only kill the existing germs but cannot take care of the future possible contaminations.
ii. Treatment with excess lime – The excess lime when added to water raises the pH value of water making it extremely alkaline. This alkalinity is detrimental to the survival of bacteria, thus killing them partially or completely. When the lime raises the pH of water to about 9.5, 99.9 to 100% bacteria are removed.
iii. Treatment with ozone (O₃) – Ozonation is a process in which water is made disinfected by using ozone. Since O₃ is an unstable molecule, it readily breaks down into normal oxygen (O₂), and releases nascent oxygen (O) as given below:


The nascent oxygen, so produced, is a powerful oxidizing agent and removes the organic matter as well as the bacteria from the water. Treatment with ozone is beneficial because ozonized water becomes tasty and pleasant unlike the chlorinated water which becomes bitter to tongue.
iv. Treatment with iodine (I₂) and bromine (Br₂) – This method may be used for treating any small scale public supplies for army troops, private plants, swimming pools etc.
v. Treatment with UV-rays – These rays are highly effective in killing all types of bacteria, thus yield truly sterilized water. It is very costly method, needs technical know-how, and possesses possibilities of interruption due to failure of electricity.
vi. Treatment with Potassium permanganate (KMnO₄) – This is popular for disinfecting well water supplies in villages. Besides killing bacteria, it also helps in oxidizing the taste producing organic matter. KMnO₄, although cheap, handy, and quite useful, yet cannot guarantee 100% removal of bacteria. It can remove about 98% bacteria.
vii. Treatment with silver or electro-katadyn process – Metallic silver ions are introduced into the water by passing it through a tube containing solid silver electrodes. The so introduced silver ions have a strong germicidal action, and thus act as disinfectant. However, the use of silver is very costly, and hence not adopted for treating public water supplies.

b. Major methods of disinfection

Major method of disinfection is treatment of water with chlorine (Cl₂), also known as Chlorination.

Chlorination is a process in which, water is made disinfected by the action of chlorine. It is a major method of disinfection of public water supplies. Cl₂ is cheap, reliable, easy to handle, easily measurable, and above all, it is capable of providing residual disinfecting effects for long periods, thus affording complete protection against future recontamination of water in the distribution system. Various forms in which chlorine can be applied in water treatment systems are:

a. As free chlorine
i. In the form of liquid chloride ion (Cl¯),
ii. As Cl₂ gas
b. As combined chlorine
i. In the form of hypochlorites or bleaching powder (Ca(OCl)₂),
ii. In the form of chloramines (NH₂Cl) i.e. a mixture of ammonia and chlorine,
iii. In the form of chlorine dioxide gas (ClO₂)

Chlorine tablets are available in the market under various trade names, such as “Halazone tablets”. A single tablet of 0.5 g is sufficient to disinfect about 20 litres of water.

6. Softening

The reduction or removal of hardness from water is known as water softening. It is not essential to soften the water in order to make the water safe for drinking purposes. The advantage of softening lies chiefly in the reduction of soap consumption. The hardness in water can be of two types:

a. Temporary hardness – It is also called as carbonate hardness, since it is caused by carbonates and bicarbonates of calcium (Ca) and magnesium (Mg). It can be removed by boiling of water or by adding lime.
b. Permanent hardness – It is also called as the non-carbonate hardness. It is caused by the sulphate (SO4¯ ¯), chlorides (Cl¯), and nitrates (NO₃¯) of Ca and Mg.

The permissible hardness for public supplies normally ranges between 75 – 115 mg/L.

7. Miscellaneous treatments

a. Removal of colours, odours, and taste from water
The special treatments for the removal of colours, odours, and tastes are
i. Aeration – application of oxygen (O₂)
ii. Activated carbon treatment – Specially treated carbon which possesses the property of absorbing and attracting impurities, such as gases, liquids, and finely dissolved solids.
iii. Treatment with copper sulphate (CuSO₄·7H₂O) – It helps in removing colours, tastes, and odours from water.
iv. Treatment with oxidizing agents – Oxidizing agents used are KMnO₄, Cl₂, O₃ etc.

b. Removal of salt and dissolved solids from water i.e. Desalination
Process of removing the salt content is known as desalination. The various methods used are
i. By evaporation and distillation
ii. Electrodialysis method
iii. Reverse osmosis method
iv. Freezing process
v. Solar distillation method
vi. Other methods

c. Removal of iron (Fe) and manganese (Mn) from water
The Fe and Mn may be present in water either in combination with organic matter or without such combination. When present without combination with organic matter, they can be easily removed by aeration, followed by coagulation, sedimentation, and filtration. On the other hand, when Fe and Mn are present in combination with organic matter, removal becomes difficult. In such cases, addition of lime, Cl₂, or KMnO₄ is useful.

d. Addition to and removal of fluorides from water
Fluoride (F¯) content in water should be about 1 mg/L. To ensure this, fluorides are either added in water (i.e. Fluoridation) or removed from this (i.e. Defluoridation)

e. Arsenic contamination and its removal
The maximum permissible limit of Arsenic (As) specified by World Health Organization (WHO) as well as Bureau of Indian Standards (BIS) is 0.01 mg/L (10 ppb). If there is excess concentration of As in water, then it needs to be removed using any one of the following methods:
i. Coagulation – precipitation technique by using aluminium and ferric salts
ii. Adsorption technique by using activated alumina or ion exchange resins
iii. Membrane technology like reverse osmosis and electrodialysis

II. Waste water treatment technology

Sewage, before being discharged of either in river streams or on land, has to be treated so as to make it safe. The degree of treatment required, however, depends upon the characteristics of the source of disposal. Sewage can be treated in different ways.
Treatment processes are often classified as –
1. Preliminary treatment
2. Primary treatment
3. Secondary (or Biological) treatment

1. Preliminary treatment
Preliminary treatment consists solely in separating the floating materials (like dead animals, tree branches, papers, pieces of rags, wood etc.) and also heavy settleable inorganic solids. It also helps in removing the oils and greases etc. from sewage.

The processes used in preliminary treatment are
a. Screening – for removing floating papers, rags, clothes etc.
b. Grit chambers or Detritus tanks – for removing grit and sand
c. Skimming tanks – for removing oils and greases.

2. Primary treatment
Primary treatment consists in removing large suspended organic solids. This is usually accomplished by sedimentation in settling basins. The organic solids, which are separated out in the sedimentation tanks (in primary treatment), are often stabilized by anaerobic decomposition in a digestion tank or are incinerated. The residue is used for landfills or soil conditioners. The liquid effluent from primary treatment often contains a large amount of suspended organic material. It also has a high biochemical oxygen demand (BOD).

The processes used in primary treatment are
a. Sedimentation – for removing part of the organic matter from the sewage effluent as in drinking water purification
b. Sedimentation aided with coagulation – similar to drinking water purification

3. Secondary (or Biological) treatment
Secondary treatment involves further treatment of the effluent coming from the primary sedimentation tanks. This is generally accomplished through biological decomposition of organic matter, which can be performed either under aerobic (in presence of oxygen) or anaerobic (in absence of oxygen) conditions. In these biological units, bacteria are used to decompose the fine organic matter to produce clearer effluent.

In treatment reactors, in which the organic matter is decomposed (oxidized) by aerobic bacteria are known as Aerobic Biological Units, and may consists of
a. Filters (intermittent sand filters and trickling filters) – Sewage is kept in contact with filtering medium, so that fine colloidal organic matter gets trapped in the voids (spaces) of filter medium.
b. Aeration tanks – Sewage received from primary sedimentation tanks is mixed with micro-organisms (e.g. bacteria) and large quantity of air, thus causing precipitation of organic and colloidal matter.
c. Oxidation ponds and aerated lagoons – Oxidation ponds are used for the oxidation of original organic matter and the production of algae which are discharged with the effluent in the natural water bodies. It results in the net reduction in BOD, approximately upto 90%, and coliform removal of upto 99% or so.

The treatment reactors in which the organic matter is destroyed and stabilized by anaerobic bacteria, are known as Anaerobic Biological Units, and may consists of
a. Anaerobic lagoons – These are deep stabilization ponds, usually operating under the action of anaerobic bacteria (bacteria which survive only in absence of oxygen). In these ponds, complex organic materials are broken down into short chain acids and alcohols, which are further degraded into gases such as methane and carbon dioxide.
b. Septic tanks – A septic tank is a kind of sedimentation tank which directly receives raw sewage and removes about 60 - 70% of the dissolved matter from it.
Septic tanks are generally provided in areas where sewers have not been laid and for serving to the sanitary disposal of sewage produced from isolated communities, schools, hospitals, other public institutions etc.
c. Imhoff tanks – An Imhoff tank is an improvement over septic tank, in which the incoming sewage is not allowed to get mixed up with the sludge produced, and the outgoing effluent is not allowed to carry with it large amount of organic load, as in the case of septic tank.
These are very economical and do not require skilled supervision during operations. There is 60 – 65% removal of solids and 30 – 40% removal of BOD.

RECENT DEVELOPMENTS

This Section emphasizes the recent developments in the field of "Water Pollution Control".

1. Upflow Anaerobic Sludge Blanket (UASB)

2. Common Effluent Treatment Plant (CETP)

3. Zero Liquid Discharge (ZLD)

1. Upflow Anaerobic Sludge Blanket (UASB)
The anaerobic process is considered to be a slow process, requiring digestors of large hydraulic retention time (HRT). Hence in recent years, high rate anaerobic systems have been designed, constructed to treat concentrated industrial waste waters and for direct treatment of municipal waste waters. Application of anaerobic treatment technology to municipal waste waters is quite significant for a developing country like India, because of high energy savings and low capital, operational, and maintenance costs involved in such technologies. The various high rate anaerobic systems, that have been devised include:

• Anaerobic contact (AC) process;
• Anaerobic filters (AF);
• Anaerobic fixed films (AFF) reactors;
• Fluidized bed (FB) reactors; and
• Upflow anaerobic sludge blanket (UASB) reactor

Among these, UASB reactor needs special mention because of its wide scale applicability.

This reactor maintains a high concentration of biomass through the formation of highly settleable microbial sludge aggregates. The waste water flows upwards through a layer of very active sludge to cause anaerobic digestion of organics of the waste water. The sludge bed develops micro-organisms capable of flourishing in an oxygen deficient environment. The suspended solids trapped in the sludge bed are degraded by the anaerobic and anaerobically working facultative bacteria, producing methane and carbon dioxide. At the top of the reactor, three phase separation between gas-solid-liquid takes place. The biogas produced during the anaerobic decomposition helps in providing gentle mixing and stirring of the biomass, thereby increasing the efficiency of decomposition, reducing the biochemical oxygen demand (BOD), and suspended solids of the waste water.

Advantages:
1. The capital cost investment of such a plant is about Rs. 20 lakh/MLD as compared to about Rs. 35 lakh/MLD for an activated sludge process.
2. The system requires lesser and simpler electromagnetic parts leading to lesser operational and maintenance costs.
3. Electricity consumption is quite low.
4. The system enables quicker sludge digestion, as compared to the conventional digestors.
5. Biogas is produced in the system as a by-product which can be used to produce electricity to run the system.

Disadvantages:
1. The system helps to lower only two parameters of waste water i.e., BOD and suspended solids.
2. It does not remove toxic pollutants like heavy metals.
3. It requires larger quantity of organic matter to support microbial growth and metabolism.
4. Some of the waste waters may contain minerals, which may interfere with the efficiency of the anaerobic microbes.
5. Pertreatment of waste water with screening and grit removal are usually found necessary for direct anaerobic treatment.

Although there are some drawbacks, but UASB system responds well for the treatment of high strength soluble waste waters, like those from municipalities and industries like food processing, edible oils, distallaries, wineries, brewries, dairies, cheese processing, organic chemicals etc.

2. Common Effluent Treatment Plant (CETP)
Common effluent treatment plant (CETP) is the collective treatment of effluents of various industries at a centralized facility. This concept is similar to the concept of Municipal Corporation treating sewage of all the individual houses. The main objective of CETP is to reduce the treatment cost for individual units while protecting the environment.

Advantages:
1. It facilitates ‘economy of scale’ in waste treatment, thereby reducing the cost of pollution abatement for individual small and medium scale enterprises.
2. It addresses ‘lack of space’ issue.
3. It has relatively better hydraulic stability.
4. It facilitates small scale units, which often can not internalize the externalities due to control of pollution.
5. It eliminates multiple discharges in the area and provides opportunity for better enforcement i.e. proper treatment and disposal.
6. It provides opportunity to improve the recycling and reuse possibilities.
7. It also facilitates better organization of treated effluent and sludge disposal.

Disadvantages:
1. It operates on ‘one size fits all’ basis.
2. Improper management of treatment units at common facility
3. There is build-up of varied nature and scale of industries along with the addition of industries in a haphazard manner, without proper planning.
4. There is no provision to tackle the fluctuations in the pollution load and quantities, at individual member industries.
5. No separate treatment units to deal with hazardous and toxic effluents.

3. Zero Liquid Discharge (ZLD)
Zero liquid discharge (ZLD) is a wastewater treatment that completely eliminates liquid discharge from a system. ZLD represents the ultimate cutting-edge treatment system for the total elimination of wastewater effluent into neighbouring waterways. The goal of any well designed ZLD system is to minimize the volume of wastewater that requires further treatment, process wastewater in an economically feasible manner, while also producing a clean stream suitable for re-use elsewhere in the facility.

The first step to achieve ZLD is to look for ways to limit the amount of wastewater that needs to be treated. The equipments needed to achieve ZLD vary depending on the characteristics of the wastewater as well as the wastewater volume. Typical waste streams in an industrial setting include wastewater rejects typically from reverse osmosis (RO) or ion exchange, cooling tower blow down, spent coolants, DI regenerant, metal finishing wastewaters, tank or equipment washing wastewaters, and other miscellaneous industrial wastewaters such as compressor condensate and floor scrubber wash waters. A traditional approach to ZLD is to use some sort of filtration technology, funnel the reject waters to an evaporator, and send the evaporator concentrate to a crystallizer or spray dryer.

Advantages:
1. Better management of wastewater.
2. Less environmental pollution.
3. Treatment and recovery of valuable products from waste streams.
4. It avoids wastage and spurs recycling by conventional and far less expensive solutions.

Disadvantages:
1. Removal of organic and suspended dissolved solids is a prerequisite.
2. High amount of energy utilization for the evaporation process.
3. Management of solid waste needs consideration due to its potentially hazardous nature.
4. Capital cost is high.