Water as a chemical:

Pure water is a compound of hydrogen and oxygen. It is colorless, odorless and tasteless. It exists as liquid at ambient temperature.

Water - what it contains:

Water has both living and non-living organisms and substances in it. The living oransims can be futher subdivided into macra- and micro- oprganisms. Macro organisms, which are biological, are those that are visible to the naked eye or can be seen through a microscope.

In contrast, microbiologoical micro-organisms are not visible even through a microscope.

Water quality criteria:

The quality of water is a function of several factors. These include its source, location, geological conditions, depth of water level, seasonal changes, domestic activity, agricultural activity, industrial activity, etc.

Excessive exploitation of natural resources and the use of technological advances with no concern for the ecology adversely affect air, water and land, alike.

The substances present in water can be classified as floating matter and suspended matter. Floating matter takes the form of leaves, twigs, dead organisms and algae. Examples of suspended matter present in water are silt, clay, decaying vegetable matter, bacteria, microorganisms, algae, insoluble iron, and manganese.

There are also dissolved impurities which include gases like carbon dioxide, hydrogen sulfide, etc., as well as chemical substances, minerals and salts.

Water sources and water quality:

Water pollution:

Water is essential for living, just like air. One may live without air for a few minutes. But, without water, one is sure to die within a few days. We all know about air pollution. Water pollution is also the gift of modern man to posterity.

How water gets polluted:

Pollution of water sources is caused by sewage and sullage from human settlements, dumping of solid wastes, wastewater from industries, and chemicals in agriculture. When foreign materials harmful to us are added, the water is sure to get polluted. Two readily such foreign materials that come readily to mind are industrial waste and sewage from cities.

Why we need good water:

We need good water for drinking by humans and animals, supporting aquatic life, generating electric power, irrigating crops in fields, and recreation such as water-based sports.

Thus the need for wastewater treatment can never be overemphasized.

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In a previous blog, I have listed some important factors you must take into account before you treat wastewater. These include the presence, in the wastewater, of acidity, alkalinity, hardness, and chloride, as well as the BOD and COD of wastewater. In this blog, I have added substantially to the list. Before wastewater treatment begins, the following factors must also be considered.

Ammonia nitrogen:

This is derived from ammonium compounds and organic compounds in wastewater by aerobic or anaerobic digestion.
Un-ionized ammonia is toxic to fish life. Free ammonia, in concentration above about 0.2 mg/l can cause fatalities to fish. Ammonia toxicity is not a problem in receiving waters with pH below 8.0. This can be estimated by distillation of wastewater at pH above 9. The ammonia liberated is neutralized in sulfuric acid. The excess sulfuric acid is back titrated with alkali. The estimation of ammonia can be done by any other methods like nesslerization or digestion.

Nitrate nitrogen:

Nitrate nitrogen in drinking water with high nitrate content often causes methemoglobinemia (blue-baby disease) in infants. The maximum concentration should not be allowed to exceed 45 mg/l. Nitrate is reduced to nitrite in digestive system which, in turn, attacks the hemoglobin in infants resulting in methemoglobinemia. Nitrate nitrogen can be estimated by measuring the optical density at 220 nm and 275 nm in spectrophotometer.

Nitrite

Nitrite can also interact with amine chemically or enzymatically to form nitrosoamines which are carcinogens. This is measured by colorimetric determination using sulfanilamide.

Sulfate

Sulfate is one of the major anions occurring in natural waters. Sulfates form hard scales in boilers and heat exchangers. Sulfate assumes significance in water and wastewater, as it is associated with odor and sewer-corrosion problems resulting from the reduction of sulfate into hydrogen sulfide under anaerobic conditions. Sulfate in water or wastewater can be estimated by precipitation with barium chloride, acidified with hydrochloric acid.

Phosphates

Most of the synthetic detergents designed for the household applications contain large amounts of polyphosphates as builders. Many of them contain 12-13% phosphorous or over 50% poly-phosphates. The organisms involved in the biological processes of wastewater treatment require phosphorous for reproduction and synthesis of new cellular material. Phosphorous in wastewater causes eutrophication, which affects transportation in sea/lakes. The presence of phosphorous in wastewater needs to be controlled before it is discharged into the receiving water bodies. Phosphorous present in wastewater can be estimated through colorimetric technique, by adding acidified ammonium molybdate solution to form a molybdophosphate complex.

Nutrients

Wastewater often contains large amounts of the nutrients like nitrogen and phosphorus in the form of nitrate and phosphate, which promote plant growth. In severe cases, excessive nutrients in receiving waters cause algae and other plants to grow quickly depleting oxygen in the water. Deprived of oxygen, fishes and other aquatic organisms die, emitting foul odors.
Nutrients from wastewater have also been linked to ocean “red tides” that poison fishes and cause illness in humans.

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Acidity: Can water be acidic in taste? Most natural water, domestic wastewater and many industrial wastewater are buffered by a carbon dioxide-bicarbonate system. Acid waters are of concern because of their corrosive characteristics and the expense involved in removing or controlling the corrosion-producing substances. Mineral acids are measured by titration to a pH of about 3.7.

Alkalinity: When will the water be alkaline in taste? The alkalinity of natural water is primarily due to the salts of weak acids. Although, weak or strong bases may also contribute. Natural water contains appreciable amounts of carbonate and hydroxide
alkalinity. Higher alkaline waters are usually unpalatable. Alkalinity is measured volumetrically by titration with N/50 or 0.020 NH2SO4.

Hardness:Water is more often hard. Do you agree? Hardness is caused by metallic ions that are capable of reacting with soap to form a precipitate. Calcium bicarbonate, magnesium sulfate, strontium chloride, ferrous nitrate and manganese silicate are the major sources for hardness in wastewater. Hardness is determined using ethylene-di-amine tetra acetic acid (EDTA) or its sodium salts as the titrating agent.

Chloride: Chloride is a major contributor to the ‘total dissolved solids’ in water/wastewater. The chloride content of water/wastewater increases as its mineral content increases. Chlorides at a concentration above 1000 mg/l give a salty taste, which is objectionable to many people. Chloride concentration of wastewater is estimated by Mohr’s method using silver nitrate with potassium chromate as an indicator.

Biochemical Oxygen Demand (BOD): The strength of wastewater is judged by BOD. This is defined as the amount of oxygen required by bacteria while stabilizing the organics in wastewater under aerobic conditions, at a particular time and temperature. This can be referred as BOD5, which accounts for 70% of the total BOD. The measurement of BOD is based on the principle: determination of dissolved oxygen content of water/wastewater on the first day and dissolved oxygen content on the fifth day (’5′ in BOD5 indicates this). The difference in dissolved oxygen concentrations between first day and fifth day is expressed as BOD of wastewater.

Chemical Oxygen Demand (COD): What does COD of wastewater mean? This reflects the concentration of organic compounds present in wastewater. This measures the total quantity of oxygen required for oxidation of organics into carbon dioxide and water. The oxidation of organics in wastewater is carried out by the action of strong oxidizing agents. Generally, acidified potassium dichromate is used as an oxidizing agent for the determination of COD. Silver sulfate is used as the catalyst for the oxidation of organics in wastewater during the determination of COD. Mercuric sulfate is added to control the interference of chloride in the estimation of COD. The method consists of adding a known concentration of potassium dichromate (added with silver sulfate and mercuric sulfate) into wastewater containing organic compounds to be oxidized in the heating condition. After oxidation, the excess potassium dichromate is back titrated with ferrous ammonium sulfate.

Importance of COD: Estimation of COD expresses the total concentration of organics present in the wastewater. This measures approximately the theoretical oxygen demand of wastewater. The determination accounts for about 95% of the organic concentration in wastewater. This forms about 1.43 times the BOD of wastewater. BOD to COD ratio reveals the treatability of wastewater. If the ratio of BOD/COD is above 0.5, the wastewater is considered to be highly biodegradable. If the ratio is less than 0.3, the wastewater is deemed to undergo a chemical treatment before the routine biological treatment.

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