Environmental Chemistry

Question 1

Why is pH important to consider during water treatment?

Answer 1

Chemistry of constituents, and corrosion in distribution system.

Inorganic coagulants react with the alkalinity of the water and hydrate to form a metal hydroxide, which decreases the alkalinity and pH of the water. Therefore, supplemental alkalinity may be needed.

Question 2

Why is alkalinity important to consider during water treatment?

Answer 2

Important in coagulation process & corrosion control.

Adequate alkalinity levels in water can helpprevent corrosion of metal pipes, fittings, and fixtures. Alkalinity acts as a protective barrier, forming a thin, insoluble layer on the metal surfaces. This layer reduces the contact between the water and the metal, minimizing the risk of metal leaching into the water supply.

Question 3

Why is hardness important to consider during water treatment?

Answer 3

Presence of Ca & Mg bicarbonates increases soap requirement and causes scaling in pipes, favours softening process.

Question 4

Why is turbidity important to consider during water treatment?

Answer 4

Measure of particulate matter in H2O affects choice of clarification methods and need for pretreatment upstream.

Question 5

Why is total dissolved solids important to consider during water treatment?

Answer 5

Measure of salt & mineral content can affect treatment needs and acceptability of the supply source.

TDS is an essential parameter for evaluating water quality. High TDS levels may indicate the presence of contaminants, such as metals, salts, or pollutants, which can affect the taste, odour, and safety of water. If theTDSlevels are high, it can interfere with the effectiveness of watertreatmentprocesses, like reverse osmosis.

Question 6

Why is dissolved oxygen important to consider during water treatment?

Answer 6

Affects oxidation-reduction conditions and determines the chemical speciation of H2O constituents.

Low level of dissolved oxygen in water is a sign of contamination and is an important factor indetermining water quality, pollution control and treatment process. As a critical variable, appropriate DO concentration can stimulate microorganisms to degrade organic matter.

Question 7

What is pH?

Answer 7

pH is a term used universally to express the intensity of the acid or alkaline condition of a solution. It is a way of expressing the hydrogen ion concentration, or more precisely the hydrogen-ion activity

pH is the abbreviation of the potential of Hydrogen and is the measure of the concentration of hydrogen ions in a water-based substance.

pH is arranged in a log acale with acidic solutions at the low end and alkaline solution at the high end. Water is neutral and is in the middle of the scale. pH 8 is ten times the alkalinity of pH 7.

Question 8

What causes higher pH in seawater?

Answer 8

The cause of the unbalanced pH is the soil, bedrock, or other underlying composition from whence comes the water source. High alkaline water is a consequence of calcareous rocky areas. It contains compounds of carbonate, bicarbonate, and hydroxide that dissolve and migrate with the water, increasing its pH.

Question 9

Sum of pH and pOH

Answer 9

pH + pOH = 14.0

Question 10

What is the equation for pH?

Answer 10

pH = -log[H3o+]

Question 11

What is the equation for pOH?

Answer 11

pOH = -log[OH-]

Question 12

What is the difference between Alkalinity and Hardness?

Answer 12

The key difference between alkalinity and hardness is that alkalinity is a measure of the water’s ability to resist changes in pH (acidity or basicity) or to neutralise acids. It is a measure of buffer capacity. Alkalinity is primarily determined by the presence of bicarbonate, carbonate, and hydroxide ions in the water.

Whereas hardness is a measure of the concentration of certain metal ions, primarily calcium and magnesium, in the water. In other words, alkalinity measures the total amount of bases present in water, whereas hardness measures the total amount (concentration) of divalent salts.

Question 13

What is hardness?

Answer 13

Water with high dissolved minerals content.

Percolation through limestone, chalk, gypsum or dolomite deposits. Carbonates of Ca/Mg, bicarbonates and sulphates.

Question 14

What is permanent / non-carbonate hardness?

Answer 14

When natural water flows over rocks containing chlorides (Cl2), sulphates (SO4) and nitrates of Ca and Mg, these salts dissolve in water causing hardness.

Does not produce carbon as a product. Sulfates have stronger bonds and are harder to break. The bonds don’t decompose on heating and therefore it is considered permanent.

Question 15

What is temporary or carbonate hardness?

Answer 15

Rainwater absorbs CO2 and forms a weak solution of carbonic acid - Bicarbonates (HCO3).

Percolates through rocks with calcium carbonates (limestone, chalk, marble) and magnesium carbonates (dolomite, magnesite).

Produces carbon as a product. Removable by boiling.

Question 16

What are the advantages of hardness in water?

Answer 16

Some people prefer the flavour of hard water.
Calcium strengthens children’s teeth and bones.
Some studies have shown that it helps reduce cardiovascular disease (although still debatable) - extra Ca it carries could help reduce blood pressure.
Some brewers and bakers prefer hard water – Ca and Mg are important yeast nutrients.
The coating on the inner surface of pipework prevents corrosion and the dissolving of potentially poisonous salts of Cu or Pb into the water supply.

Question 17

What are the domestic disadvantages of hardness in water?

Answer 17

Washing: Hard water, when used for washing purposes, does not produce lather freely with soap - decreases the cleaning quality, and wastes soap.

Bathing: Does not produce lather freely with soap solution, but rather sticky scum on the body and bathtub. Decreases quality and wastes soap.

Cooking: The presence of salts increases the boiling point of water. Hence leads to the use of more fuel and time for cooking.

Drinking: Hard water increases the risk of calcium-citrate and calcium-oxalate buildup in the body, thus increasing the risk of kidney stones.

Question 18

What are the industrial disadvantages of hardness in water?

Answer 18

Textile and Dyeing industry: Hard water causes wastage of soap. Precipitates of Ca and Mg soaps adhere to fabrics and cause problems. Dissolved salts in hard H2O may react with costly dyes forming precipitates.

Sugar Industry: When used in sugar refining, hard water causes difficulties in the crystallization of sugar.

Pharmaceutical Industry: Hard water may lead to some undesirable products during the production of pharmaceutical products.

Paper industry: Ca, Mg and Fe salts in water may affect the quality of paper. Ca and Mg salts react with chemicals used to provide smooth and glossy finish to paper, Fe salts affect the color of the paper.

Question 19

Degree of hardness equation

Answer 19

Degree of hardness = (weight of hardness causing salts / molecular weight of hardness causing salts) * 100

Question 20

What is alkalinity?

Answer 20

The alkalinity of a water (or any material) is a measure of its capacity to neutralize acids.

Bicarbonates represent the major form of alkalinity since it is formed in considerable amounts from the action of CO2 upon basic materials in the soil.

Many materials contribute to the alkalinity of water but the major portion of alkalinity in natural waters is caused by: Hydroxides, Carbonates, Bicarbonates

Alkalinity is mainly due to the salts of weak acids and strong basis and such substances act as buffers to resist pH decrease caused by acid addition. Hence, it is a measure of the buffer capacity.

Question 21

Alkalinity as CaCO3 equation

Answer 21

Alkalinity as CaCO3 = No. of gram equivalent per litre of alkalinity causing species × Equivalent weight of CaCO3.

Question 22

Total hardness equation

Answer 22

Total hardness = Carbonate hardness + Noncarbonate hardness.

Question 23

Non-carbonate hardness equation

Answer 23

Non-carbonate hardness = Total hardness − Alkalinity

Question 24

How is hardness classified?

Answer 24

1 - With respect to the metallic ion (Calcium and magnesium hardness)

2 - With respect to the anions associated with the metallic ions (Carbonate and non-carbonate hardness.)

If calcium hardness is determined, magnesium hardness is obtained by subtracting calcium hardness from total hardness:
Total hardness — calcium hardness = magnesium hardness.

This procedure yields reasonably reliable results because most of the hardness in natural waters is due to these two cations.

Question 25

What is pseudo-hardness?

Answer 25

Pseudo-Hardness: Sea, brackish, and other waters that contain appreciable amounts of Na+ interfere with the normal behavior of soap because of the common ion effect. Sodium is not a hardness-causing cation, and so this action which it exhibits when present in high concentration is termed pseudo-hardness.

Question 26

How is hardness data applied?

Answer 26

Chemical Coagulation: Chemicals used for coagulation of water and wastewater react with water to form insoluble hydroxide precipitates. The H+ released reacts with the alkalinity of H2O. Thus, the alkalinity acts to buffer the H2O in a pH range for the effectiveness of coagulant. Alkalinity must be present in excess of that destroyed by the acid released by the coagulant for effective and complete coagulation.

Water Softening: Alkalinity is a major item that must be considered in calculating the lime and soda-ash requirements in the softening of water by precipitation methods. The alkalinity of softened H2O is usually evaluated in terms of whether such waters meet drinking H2O standards.

Buffer Capacity: Alkalinity measurements are made as a means of evaluating the buffering capacity of wastewaters and sludges. It can also be used to evaluate ability of natural water to resist effects of acid rain.

Industrial wastes: Many regulatory agencies (and municipal authorities) prohibit the discharge of wastes containing caustic (hydroxide) alkalinity to receiving waters & sewers. Alkalinity as well as pH is an important factor in determining the amenability of wastewaters to biological treatment.