Environmental Engineering - Water

CHAPTER 2. WATER QUALITY: DEFINITIONS, CHARACTERISTICS

AND PERSPECTIVES

DR. MUNIRA SHAHBUDDIN

Introduction to Environmental Engineering

IIUM, Malaysia.

WORLD WATER DISTRIBUTION

The earth is covered by 70% water

Only 3% (fresh water) of the total water

are consumable for drink, household,

industrial, domestic purposes.

From the 3%, less than 0.003% is

accessible to human for use (which is

unpolluted).

WATER SUSTAINABILITY

Water sustainability is essential to human existence. Civilization developed around water bodies that could support agriculture, domestic, transportation and as source for food.

Water is in a constant state of motion, as depicted in the hydrologic cycle. Human activities contribute to contamination from industrial, agriculture, domestic use, waste.

The impurities accumulated by water throughout the hydrologic cycle may be in both suspended and dissolved form – causing pollution in water reservoir.

NUTRIENT LOADING INTO THE WATER RESERVOIR

Abundance of nutrient –eutrophication – good, bad or should be balanced?

FORMATION OF ACID RAINPRESENCE OF VOLATILE ORGANIC COMPOUND IN THE ATMOSPHERE

EUTROPHICATION

BRIEF INTRODUCTION TO ENVIRONMENTAL POLLUTION

WATER CRISIS – FLINT, MICHIGAN.http://www.nytimes.com/2016/02/09/us/regulatory-gaps-leave-unsafe-lead-levels-in-water-nationwide.html?_r=0

LOVE CANAL, NEW YORK.

The tragedy caused by chemical waste/contaminations sipped into groundwater reservoir.

YELLOW SEA, CHINA TURNED GREEN DUE TO ALGAE BLOOM

BAOTOU, INNER MONGOLIA

RARE EARTH EXTRACTION AND PROCESS

OWENS LAKE, CALIFORNIA, UNITED STATES

- Abandon chemical

plant

- Presence of

bacterias

- Weather

- Geographical

condition

- Human factor

UNDERSTANDING THE CAUSE

AZURE BLUE LAKES

Tasik Gelugor, Penang, Malaysia.

Buxton, Derbyshire, United Kingdom.

CHEMICAL WATER – QUALITY PARAMETERSWater is an essential, universal solvent.

Chemical parameters are related to the solvent capabilities of water.

Chemical parameters:

- Total suspended solid

- Alkalinity

- Hardness

- Flouride

- Metal, organics and nutrients.

CHEMISTRY OF SOLUTIONS

Atom is the smallest unit of each of the elements. Atoms are building blocks from which molecules of elements and compounds are constructed.

Two hydrogen atoms combine to form a molecule of hydrogen gas

H + H → H2

A mole of an element or compound is its molecular mass expressed in common mass per unit, usually grams.

One mole of a substance dissolved in sufficient water to make one liter of solution is called one molar solution.

Bonding of elements into compounds sometimes accomplished by electrical forces resulting from transferred electrons. Dissociation in water produces species with opposite charges. E.g. NaCl

NaCl ↔ Na + + Cl -

The charged species are called ions, positively charged ions are called cations and negatively charged ions are called anions.

When ions or radicals react with each other to form new compounds, the reaction may not always proceed on a one to one basis as was the case with sodium chloride. They do, however, proceed on an equivalence basis that can be related to electroneutrality.

The equivalence of an element or radical is its gram molecular mass divided by its equivalence. A miliequivalence us the molecular mass expressed in miligrams divided by its equivalence.

CALCULATING EQUIVALENCEHOW MANY GRAMS OF CALCIUM WILL BE REQUIRED TO COMBINE WITH 90G OF CARBONATE TO FORM CALCIUM CARBONATE

SOLUTION

1. Carbonate (CO32-) is a radical composed of carbon and oxygen. In this particular

combination, carbon has an atomic mass of 12 and a valance of 4+, while oxygen has an atomic mass of 16 and valance of -2. Therefore, the radical has a total valance of -2 and an equivalence of 2. One equivalence of carbonate is

12+3 (16)

2= 30 g/equiv

2. The calcium ion has an atomic mass of 40 and a valance of +2; therefore, one equivalent of calcium is

40

2= 20 g/equiv

3. The number of equivalents of calcium must be equal the number of equivalents of carbonate, therefore

90𝑔

30 𝑔/𝑒𝑞𝑢𝑖𝑣= 3 equiv of carbonate

Therefore, 3 equiv x 20 g/equiv = 60g of calcium, and that amount will be required to react with 90 g of carbonate.

Equivalents are very important in water chemistry to calculate the measurement of chemical quantities for desired reaction in water and waste water treatment.

The concentration of A can be expressed as an equivalent concentration of substrate B by the following method.

𝑔

𝐿𝐴

𝑔

𝑒𝑞𝑢𝑖𝑣𝐴

x (g/equiv)B = (g/L) A expressed as B (2-2)

DETERMINING EQUIVALENT CONCENTRATIONWHAT IS THE EQUIVALENT CaCO3 CONCENTRATION OF (A) 117 mg/L OF NaCl AND (B) 2 X10 -3

MOL OF NaCl

A)

i) One equivalent of CaCO3

40+12+3(16)

2= 50 g/equiv = 50,000 mg/equiv = 50 mg/mequiv

ii) One equivalent of NaCl

23+35.5

1= 58.5 g/equiv = 58.5 mg/mequiv

By equation (2-2)

117𝑚𝑔/𝐿

58.5 𝑚𝑔/𝑒𝑞𝑢𝑖𝑣x 50 mg/mequiv = 100 mg/L of NaCl as CaCO3

B) 2 x10-3 mol of NaCl

One mole of a substance is divided by its valence is one equivalent

2 x10−3 𝑚𝑜𝑙/𝐿

1𝑚𝑜𝑙/𝑒𝑞𝑢𝑖𝑣= 2 x10-3 equiv/L

Thus 2 x10-3 x 50,000 mg/equiv = 100 mg/L

The solid form NaCl may be dissociating into its ionic components (dissolution) or recombine to form solid (precipitation)

Conditions of equilibrium can be expressed by the mass action equation. For generalized reaction

AxBy ↔ xA + yB

Solid compound Ionic compound

The mass action equation is

[𝐴]𝑥[𝐵]𝑦

[𝐴𝑥𝐵𝑦]= K

K value is an equilibrium constant for a given substance in pure water at a given temperature.

[𝐴𝑥𝐵𝑦] = 𝐾𝑠 = constant and [𝐴]𝑥[𝐵]𝑦 = K𝐾𝑠 = 𝐾𝑠𝑝

𝐾𝑠𝑝 is known as solubility product for the ion pair

DETERMINING EQUILIBRIUM CONCENTRATIONSThe solubility product for the dissociation of Mg(OH)2 is shown in Table 2-3 as 9x10-12 Determine the concentration of Mg2+ and OH -at equilibrium, expressed as miligrams per liter of CaCO3

Solution

1. Write the equation for the reaction

Mg(OH)2 ↔ Mg2+ and 2OH-

2. The solubility of the product equation becomes

[Mg2+ ] [OH- ]2 = 9x10-12

If x is the number of moles of Mg2+ resulting from the dissociation, then OH- is equal to 2x, Therefore

[x] [2x]2 = 9x10-12

4x3 = 9x10-12

X = 1.3𝑥10 − 4 𝑚𝑜𝑙/𝐿 = Mg

2X = 2.6x10-4 mol/L = OH

3. 1.3x10−4 mol/L

0.5 𝑚𝑜𝑙/𝑒𝑞𝑢𝑖𝑣x 50,000 mg/equiv = 13.0 mg/L of Mg as CaCO3

4. 2.6x10−4 mol/L

1𝑚𝑜𝑙/𝑒𝑞𝑢𝑖𝑣x 50,000 mg/equiv = 13.0 mg/L of Mg as CaCO3

SOLUBILITY PRODUCTS OF SELECTED ION PAIRS (2-12)

TOTAL DISSOLVED SOLIDS

The material remaining in the water after filtration for the suspended solids analysis is considered to be dissolved.

Many dissolved substances are undesirable in water – aesthetically displeasing color, taste and odor.

It is important to arrange the cation and anions in order to determine hardness and the quantities of chemicals needed for softening.

Several constituents of dissolved solid necessitate special attention such as alkalinity, hardness, fluoride, metals, organic and nutrient.

TESTING FOR ION BALANCETESTS FOR COMMON IONS ARE RUN ON A SAMPLE OF WATER AND THE RESULTS ARE SHOWN BELOW. IF A 10 PERCENT ERROR IN THE BALANCE IS ACCEPTABLE, SHOULD THE ANALYSIS BE CONSIDERED COMPLETE.

Constituents

Ca2+ = 55 mg/L HCO3- = 250 mg/L

Mg2+ = 18 mg/L SO42- = 60 mg/L

Na+ = 98 mg/L Cl- = 89 mg/L

Ion Cations Ions Anions

ConcMg/L

Equivmg/mequiv

Equivcon. Mg/L

ConcMg/L

Equivmg/mequiv

Equivcon. Mg/L

Ca2+ 55 40/2 2.75 HCO3- 250 61/1 4.1

Mg2+ 18 24.3/2 1.48 SO42 60 96/2 1.25

Na+ 98 23/1 4.26 Cl- 89 35.5/1 2.51

Total ions 8.49 7.86

Calculate percent of error

8.49−7.89

7.89X 100 = 8%

Therefore, accept analysis.

ALKALINITY

Constituents of alkalinity in natural water systems include CO32-, HCO3-, OH-, HSiO3, H2BO3, HPO42-, HS- and NH30. These compounds result from the dissolution of mineral substances in the soil and atmosphere.

Phosphates may originate from detergents in wastewater discharges and from fertilizer, insecticides while ammonia and sulphide may be products of microbial decomposition.

By far the most common constituents of alkalinity are bicarbonate (HCO3-), carbonate (CO32-) and hydroxide (OH-)

IS SEAWATER ALKALINE OR ACIDIC

DISSOLVED OXYGEN

HARDNESS

Concentration of multivalent metallic cations in solutions. At supersaturated conditions, the hardness cations will react with anions in the water to form a solid precipitate.

Hardness is classified as carbonate hardness and non carbonate hardness. Depending on the anion with which it associates.

Sources

Multivalent metallic ions – magnesium, calcium, strontium and aluminium.

IMPACTS

Soap consumption by hard waters represent an economic loss to the water user

PROBLEMS WITH HARDWATER

- Excessive use of detergent

- Abundance of nutrient

- Hardly dissolve chemicals

- How to improve chemical reaction with

hardwater?