27 Water Pollution

Dr. C. P. Singh Chandel

Contents

1. Dissolved Oxygen
2. Oxygen Demand
3. Biochemical Oxygen Demand (BOD)
4. Chemical Oxygen Demand (COD)
5. Total Organic Carbon (TOC)
6. Dissolved organic carbon (DOC)
7. Anaerobic (Without Oxygen) Reaction
8. Aerobic (With Oxygen) Reactions
9. Eutrophication
10. References

Introduction

The greatest necessity of humans is the availability of drinking water and its quality. The available water is polluted by human activities, industrial and manufacturing process, discharge of municipal wastewater in water bodies. The several types of pollutants are in water. Some water pollutants are as follows: trace element, radionuclides, pesticides, detergents, algal nutrients, heavy metals, sediments, trace organic pollutants, inorganic pollutants, asbestos, pathogens, chemical carcinogens, sediments, organically-bound metals, etc.

Dissolved Oxygen

Oxygen like water is necessary for the survival of the earth. O2 dissolved in water is necessary for the aquatic life forms. The depletion in the dissolved oxygen in water is measure of water pollution. Dissolved oxygen is one of the best indicators of water quality. The amount of oxygen dissolved in the water is called dissolved oxygen or DO. The amount of dissolved oxygen in water depends on the temperature and salinity of water. The solubility of oxygen in water decreases on increasing temperature. Cold water can hold more dissolved oxygen than warm water and fresh water can hold more dissolved oxygen than saline water. The unit of DO is percent saturation (%) or mg/L.

The major source of oxygen is photosynthesis, which is done by green plants and some bacteria. It is believed that oxygen, we breathe, is produced by green plants. Seventy five percent of the earth’s total oxygen supply comes from phytoplankton in the oceans.

If oxygen consumption is more than the production of oxygen, DO level would decline and forcing some sensitive animals to move away, weaken, or even die. DO levels are not constants and fluctuate seasonally and daily. They vary with water temperature and altitude. Hot water discharges, such as water used to cool machinery in a manufacturing plant or a power plant, raise the temperature of water and lower its oxygen content.

The most important oxidizing agent in natural waters is dissolved molecular oxygen (O2).Each oxygen atom in O2 is reduced from the zero oxidation state to the -2 state in H2O or OH-.

In acidic solution, the redox half reaction is as in Eq. (1)

Winkler Method for DO Measurement

The sample in which DO is measured, a Mn(II) salt is added and the solution is made strongly alkaline. Mn(II) oxidized to MnO2, which can be reacted with KI in acid medium liberates I2 which is titrated with standard Na2S2O3 solution using starch as an indicator. Addition of NaN3 aluminates interference due to nitrite and sulfite. Addition of KF masks Fe+3 ion.

Oxygen Demand

DO is consumed by the oxidation of several type of materials such as oxidation of organic matter, oxidation of nitrogenous compounds, oxidation of reducing materials such as dissolved SO2 and metal ions in reduced oxidation state. If the oxygen is not replenished than the water becomes oxygen deficient. Among the common substances oxidized by dissolved oxygen in water are organic matter. The later is mostly of biological origin, such as dead plant matter and animal wastes.

The amount of oxygen required by the pollutants for their oxidation is measured in terms of oxygen demand, which is done in two ways, biochemical oxygen demand(BOD) and chemical oxygen demand(COD).

Biochemical Oxygen Demand (BOD)

BOD is the capacity of the organic material in a sample of water to consume the dissolved oxygen. BOD is a measure of the quantity of oxygen used by microorganisms in the oxidation of organic matter.

It is an empirical method to measure the capacity of oxygen in a sample of water and is called BOD. In this method, a sealed water sample is maintained in the dark at a fix temperature 20oC or 25oC. The DO is measured in the beginning and at the end of a period (usually 5 days). The BOD is DO consumed during this period. It is also called BOD5.

In practice, to determine BOD a suitable microorganism is added to a diluted effluent or water sample. The BOD test takes 5 days to complete. A dissolved oxygen test kit available commercially is used.

Two samples of same water are used. In the first, DO level is immediately measured. The second water sample is placed in an incubator in complete darkness at 20oC or 25oC for 5days. If an incubator is not available, the water sample in a bottle wrapped in aluminum foil and stored in a dark place at desired temperature. After 5 days, DO in second sample is measured. The difference in two DO readings of first and second samples is called BOD.

Calculation:

BOD5 (mg/L) = Initial DO (mg/L) – 5 day DO value

Higher the BOD, more polluted is water. For examples, sewage water, which is highly polluted has BOD level of 6-9 ppm. Industrial water is much more highly polluted has BOD of 100 ppm and more.

Chemical Oxygen Demand (COD)

In determination of COD, a strong oxidant like acidify K2Cr2O7 is used. All organic compounds and several inorganic chemicals are simultaneously oxidized. This test is faster than the BOD test. The sample is refluxed with K2Cr2O7 and H2SO4 in presence of mercury sulfate and silver sulfate (catalyst). The excess of potassium dichromate is titrated against ferrous ammonium sulfate using ferroin as an indicator. The amount of K2Cr2O7 used is proportional to the oxidizable matter present in the sample.

Most applications of COD determine the amount of organic pollutants found in surface water (e.g. lakes and rivers) or wastewater.

Procedure

Known volume of the sample is diluted with distilled water and mixed well. To it known amount of standard K2Cr2O7 solution is added. A pinch of Ag2SO4 and HgSO4 is added followed by addition of appropriate amount of H2SO4. Contents in the flask are refluxed on a water bath. Finally following appropriate procedure the contents are titrated with 0.01N ferrous ammonium sulfate using ferroin as an indicator.

A blank is refluxed in the same manner using distilled water instead of sample and titrate it with ferrous ammonium sulfate.

Biochemical Oxygen Demand (BOD)/Chemical Oxygen Demand (COD)

Biochemical oxygen demand (BOD) is a measure of the amount of oxygen that bacteria will consume while decomposing organic matter under aerobic conditions. The main focus of wastewater treatment plants is to reduce the BOD in the effluent discharged to natural waters. Chemical oxygen demand (COD) does not differentiate between biologically available and inert organic matter, and it is a measure of the total quantity of oxygen required to oxidize all organic material into carbon dioxide and water. COD values are always greater than BOD values, but COD measurements can be made in a few hours while BOD measurements take five days. If effluent with high BOD levels is discharged into a stream or river, it will accelerate bacterial growth in the river and consume the oxygen levels in the river.

Total Organic Carbon (TOC)

Total organic carbon is a measure of total concentration of all organic carbon atoms covalently bonded in the organic molecules of a given sample of water. The measurement of total organic carbon is the best method of evaluating the organic content of a water sample. The TOC is independent of the oxidation state of the carbon in an organic compound. The instruments are available for the measurement of TOC. They use a variety of methods such as heat and oxygen, UV radiation, toconvert the organic carbon to carbon dioxide. Then carbon dioxide produced can be measured directly by a non-dispersive infrared analyzer.

Dissolved organic carbon (DOC)

Dissolved organic carbon (DOC) is defined as the organic matter that is able to pass through a filter (filters generally range in size between 0.7 and 0.22 um). The DOC measurements can be performed with commercial instruments available in the market.

Anaerobic (Without Oxygen) Reaction

The biodegradation reactions taking place in absence of oxygen are called anaerobic reactions. Anaerobic reaction occurs in presence of appropriate bacteria. In these reactions the reduced form of compounds are formed.

For example methane (CH4) and a small portion of carbon may be respired as carbon dioxide (CO2) in presence of bacteria.

Another example is a formation of H2S and reduction of Fe3+.

The denitrifying bacteria Pseudomonas and Achromobacter convert nitrates ultimately into ammonia.

Aerobic (With Oxygen) Reactions

In presence of oxygen, biodegradation or the breakdown of organic contaminants by microorganisms is called Aerobic biodegradation. The chemistry is characterized by oxidative conditions. Many organic contaminants are rapidly degraded under aerobic conditions by aerobic bacteria.

The examples of aerobic reactions are given below

Eutrophication

Eutrophication is derived from Greek word “eutrophos” which mean well-nourished or enriched. The addition of waste containing nutrients, mainly nitrates and phosphates. Enriches the water body. These nutrients act as fertilizers and cause population explosion of water microscopic plants like algae and others like duck weed, water hyacinth etc. This is called Eutrophication. In the presence of oxygen, the aerobic bacteria in water bodies release nutrients from the organic wastes. The abundant growth of algae is called algal boom.

Type of Eutrophication

Eutrophication in mainly of two types –

(a) Cultural Eutrophication

(b) Natural Eutrophication

Effect of Eutrophication

The algal boom releases toxic chemicals, which kill fish, birds and aquatic animals causing the water to sink.

Decomposition of algal boom leads to oxygen depletion in water.

The presence of high CO2 level and oxygen depilation leads to death of aquatic organisms. The clean water now becomes a stinking drain.

When oxygen level falls to zero, the anaerobic biodegradation occurs. The oxygen is now obtained from the reduction of nitrates and sulfates. The reduction of latter yields foul smelling hydrogen sulfide.

Control of Eutrophication

Some of the methods for control of eutrophication are as follows-

Planting vegetation to slow erosion.

Use of controlled amount of fertilizers.

Only treated wastewater should be allowed to discharged.

Algal blooms should be removed upon their death and decomposition. Algal food web should be disrupted to stimulate bacterial multiplication

References:

• Colin Baird(1998), Environmental Chemistry, W. H. Freeman and Company, New York
• A. K. De(2014), Environmental Chemistry, New Age International Publishers, Delhi
• Mark J. Hammer and Mark J. Hammer, Jr. (2015), Water and Wastewater Technology, PHI Learning Private Limited, Delhi
• O G Palanna(2009), Engineering Chemistry, Tata McGraw Hill Education Private Limited, New Delhi
• James E. Girard(2011), Principles of Environmental Chemistry, Second Edition, Jones and Bartlett India Pvt. Ltd, New Delhi
• Gilbert M. Masters, Wendell P. Ela(2013), Introduction to Environmental Engineering and Science, PHI Learning Private Limited, Delhi
• P. S. Sindhu(2002), Environmental Chemistry, New Age International Publishers, New Delhi
• Stanley E. Manahan, Environmental Chemistry, Seventh Edition, Lewis Publishers, New York