Introduction to various aspects of water pollution, water quality standards BOD, COD and Oxygen sag curve

Introduction

The mixing of undesirable substances in water in an uncontrolled amount beyond the tolerable capacity is Water Pollution.

Waste Water Origin

The Core Reason for its pollution is its Incredible Property of Cleaning or washing everything in its path. Almost all the origins mentioned below are directly related to this property.

1. Domestic Cleanings like Laundry, Food Preparation, Body Cleaning and Body Excretions.
2. Agricultural Activities like using inorganic fertilizers, Insecticides and Pesticides.
3. Industrial Processes like Equipment Washing, Discharge of the products and also byproducts.
4. Rain and Flood are the natural processes by which they wash away things like fertilizers, crop fields, river bank, dead body and animals due to turbulency effect, the mixing pollute the water.

In the World Scenario (2)

1. About 90 % pollution load in the river system is due to human waste.
2. At least 2500 million people in the developing countries lack an adequate system for disposing of their feces.
3. About 98% of the people of rural areas use an open field for defecation.

In the Context of Kathmandu (2)

1. Kathmandu produces 150 tons of waste each day, nearly half of which is dumped into the river.
2. More than 40 million liters a day of wastewater is generated in Kathmandu. Water samples from the Dhobikhola show the high concentration of sodium potassium compounds mostly by Bansbari Leather Industry.
3. Excessive nitrate, fluoride, and chlorides in the groundwater are three main problems in the Valley’s water. In the Kathmandu valley tanneries and dye, factories are nastiest felonious.
4. Rivers in the valley, including the Bagmati, Vishnumati, Manohara, and Hanumant are seriously contaminated for this reason. Of the total volume of the flow in the river is 90 percent sewage and the other 10 percent is water.

Types of Pollutants

1. Oxygen-Demanding Wastes: Sewage, food residue, paper, animal droppings, crop residues, leaves
2. Disease-causing Agents: Protozoa, Bacteria
3. Synthetic Organic Compounds: Pesticides, Detergent, Herbicides
4. Inorganic Chemical and Minerals: Arsenic, Salts, Mineral Acids
5. Radioactive Substances: Uranium and Thorium Mining
6. Plant Nutrients: Nitrogen and Phosphorous
7. Sediments
8. Thermal Discharges: Hot water discharge from industries
9. Oil

Effects

1. Nearly 80 % of the world's diseases, particularly in developing world, can be linked with contaminated water.
2. Some 1.1 billion people do not have access to safe water; almost 2.5 billion lack basic sanitation.
3. The World Health Organization (WHO) estimated in 1996 that every eight seconds a child death from the water-related diseases and that every year more than five million people died from illnesses link to unsafe drinking water or inadequate sanitation.
4. WHO also suggest that if sustainable safe water drinking and sanitation services were provided to all?
5. Loss of Aquatic Lives
6. Bad Smell

Waste Water Treatment Methods

1. Primary Treatment like Pretreatment, Sedimentation (Discrete, Flocculent Hindered, and Compressive), Floatation.
2. Secondary (Biological) Treatment like Activated Sludge Process, Trickling Filters.
3. Advanced Treatment like Micro-straining, Coagulation, and Filtration, Adsorption on the Activated Carbon, Solvent of Extraction, Ion Exchange, Reverse Osmosis.
4. Advanced Biological Systems like Aerobic Ponds, Facultative Ponds, Aerated Ponds, and Anaerobic Ponds.

Water Quality Standards

The Quality Parameters Set for Water for its purpose in drinking, cooking food, washing clothes and bathing and finally dumping into the river with no or fewer impacts to living beings as well as natural sources are called the Standards for Quality of Water.

Following are some parameters whose Quality Standards in the context of Nepal is provided below:

1. Appearance ( Turbidity, Color )
2. Potability ( Taste, Odor )
3. Health (Bacteria, Nitrates, Chlorides, etc.)
4. Toxicity ( Metals, Organics )

National Drinking Water Quality Standards, 2062 (5)

* These values show lower and upper-limits

( ) Values in parenthesis refer the acceptable values only when an alternative is not available.

Conclusion

• Water Pollution as a Major Concern
• Best not to Impurify than Cleaning After
• Implementation of Proper Plan
• Effective and Efficient Treatment Methods
• Experts said at a discussion on the present condition of the rivers in the Kathmandu valley organized by the Friends of Bagmati, a group of environment activists who have been advocating cleaner rivers in the city for a better life (2).

Dissolved Oxygen (DO)

All the living organisms depend upon oxygen in one form or another to maintain the metabolic process that produces energy for the reproduction and growth. All the gasses of atmosphere dissolve in water to some degree. Both nitrogen and oxygen are poorly soluble in water. The solubility of atmospheric oxygen in fresh waters ranges from 14.6 mg/l at 0°C to about 7.6 mg/l at 30°C at 1 atmospheric pressure. Dissolved salts of water reduce the solubility of oxygen so also impurities in water.

Biochemical oxygen demand

Biochemical oxygen demand (BOD) is defined as the amount of oxygen required by the microorganisms to decompose organic matter at a particular time and temperature. BOD test is widely used to determine the pollution strength of the domestic as well as industrial wastes in terms of the oxygen that they require to turn to end products as CO2 and H2O. BOD test is most essential a bioassay procedure involving the measurement of oxygen which is consumed by living organisms (mainly bacteria) while utilizing the organic matter present in the waste are carbohydrates, proteins and fats. It is standard at 20°C with usual peak temperature of summer of London where the test originatedOnce the organisms become established, they oxidize nitrogen in the form of ammonia to nitrates and nitric acids in amounts that induce serious error in BOD estimation.

Estimation of BOD

1. Most widely used parameters of the organic pollution applied to both wastewater and surface water are the 5 day BOD i.e. (BOD5) at 20°C.
2. BOD test determination involves the measurement of the dissolved oxygen used by micro-organisms in the bio-chemical oxidation of organic matter.
3. The reason is that BOD test results are now used
   • to determine the approximate quantity of oxygen that will be required to biologically stabilize the organic matter present
   • to determine the extent of waste treatment facilities
   • to measure the efficiency of the biological treatment process.
4. In the standard BOD test we use a small sample of the wastewater to tested is placed along with the dilution of water in a BOD bottle (300 ml). The dissolved oxygen concentration of the mixture in the bottle is measured. The bottle is enclosed for 5 days at 20°C and the finally dissolved oxygen concentration is measured again. The BOD of the sample is the decrease in the dissolved oxygen concentration values, expressed in mg/l; divided by the decimal fraction of the sample used.

Limitations of BOD:

1. A minimum DO depletion of 2 mg/l is desirable.
2. The final DO should never be 0 mg/l. (as it is impossible to know the entire DO content to fully depleted i.e., within 1, 2, 3, 4 or 5 days) and preferably it should not be less than 1 mg/l.

Chemical oxygen demand

COD may be defined as the amount of dissolved oxygen required to decompose the organic and inorganic sample solution. It is used to measure the content of oxidizable organic as well as inorganic matter of the given sample of water. The amount of oxygen equivalent is measured by using a very strong chemical oxidizing agent in the acidic medium. Potassium-dichromate has been found to be excellent for this purpose. The COD test used is advantage to measure the oxidizable matter in industrial and municipal wastes containing compounds that are toxic to the biological life i.e. which is not possible with BOD test. The COD of a waste is higher than the BOD because large compounds are chemical oxidized in a short time. It had benefits of getting completed in 3 hours compared to that of five days of the BOD test. It is possible to correlate BOD and COD. BOD5/COD ratio is called Biodegradability.

Index varies from the 0.4 to 0.8 for domestic wastewaters.

• If BOD/COD is > 0.6 then the waste is fairly biodegradable and can be effectively treated biologically.
• If BOD/COD ratio is between 0.3 and 0.6, then seeding is required to treat biologically.
• If BOD/COD is < 0.3 then it cannot be treated biologically.

Oxygen Sag Analysis

The oxygen sag or oxygen deficit in the stream at any point of time during the self-purification process is the difference between the saturation DO content and the actual DO content at that time.

The normal saturation DO value for fresh water depends on upon the temperature, and its value varies from 14.62 mg/l at 0°C to 7.63 mg/l at 30°C. (at normal atmospheric pressure). At the point where wastewater is discharged into the stream, the DO content of the stream may be equal to the saturation DO or less. If less, it is termed as initial oxygen deficit Do.

Do = Saturated DO – Actual DO

At this stage, when the wastewater with an initial BOD load Lo is discharged into the stream, the DO content of the stream starts depleting and the oxygen deficit D increases initially. The variation of oxygen deficit D along the length of the stream is depicted by the

Oxygen Sag Curve as shown in above figure.

The major point of interest in the oxygen sag analysis is the point of minimum DO or the point of the maximum deficit. The maximum or critical deficit, labeled as Dc occurs at the inflection point of the oxygen sag curve (DO content increase thereafter).

References:
1. Mackenzie L. Davis & David A. Cornwell, “Introduction to Environmental Engineering”, McGraw Hill.
2. Gilbert M. Masters, Standford University, “Introduction to Environmental Engineering and Science”, Printice Hall.
3. Stephan Konz, Kansas State University, “Work design”, Grid Publishing Inc., Colombus, Ohio
4. C. S. Rao, “Environmental Pollution Control Engineering”, New age International (P) Limited, Publishers, India.