Introduction to Water Quality Engineering

DRINKING WATER STANDARDS

Primary Standards, enforeceable by law are parameters that directly affect human health.

Secondary Standards are related to

aesthetics and are not enforceable by the federal government.

Classification of Water Quality Parameters

Water quality parameters can be classified in a number of ways, but most often are grouped as physical, chemical and biological.

Physical Taste, Odors, Color, Temperature, Turbidity

Chemical Inorganics (Calcium, Magnesium, Iron etc.)

Organics (Pesticides, Herbicides, Petroleum)

Biological Bacteria, Viruses, Algae

Sources of Water

Surface Water - Rivers, Lakes, StreamsHigh Withdrawal RatesPollutedRequire Treatment

Ground Water Sea Water Reclaimed Wastewater

Surface Water Treatment Processes

Intake Structure

Coagulant

Flocculation BasinsSedimentation

Clarifier

Filtration

Disinfection

Clear well

To Distribution System

Sludge

Groundwater Treatment Processes

Clear well

Soda-ash

Aeration

Clarifier

SofteningDisinfection

To Distribution System

Sludge

Sludge

ClarifierPump

Well

Lime

pH- Hydrogen Ion Concentration

pH= log[1/H+]

0 Acidic 7 Alkaline 14

Pure water has a pH of 7. It is neutral since [H+]=[OH-]=10-7 mole/L

• pH affects major water treatment processes like

coagulationsofteningdisinfectioncorrosion control

Conductivity Conductivity is a measure of the ability of a solution to

conduct an electric current. Electric current is transported by ions. Conductivity increases as ion concentration increases. Unit = 1mho = 1/ohm (1 mho = 1Siemen in S.I. units)

Sum of +ve ions (cations) = Sum of -ve ions (anions)

Ionic strength is a quantity used to describe the intensity of the electric field in a solution and is defined as follows:

m= 1

2 C

ix z

i2

i

- +++

-

-

Ca+2

Mg+2

Na+

K+

HCO 3- Cl-

NO 3-

SO 4-2

Turbidity and Color• Turbidity - The tendency of water to scatter light at 90

degrees. Turbidity is a measure of water clarity. Caused by suspended solids (thus, turbidity is an indirect measure of suspended solids).

• Measured in NTUs, using a Turbidimeter. For most people, water with <= 5 NTUs looks clear. The American Water Works Association (AWWA) recommends that water to be disinfected should be <= 0.1 NTU.

• Color - True color is caused by dissolved compounds in water. It can be natural or anthropocentric. Dissolved and suspended solids (together) cause apparent color.

• Color is measured in Platinum-Cobalt units. The AWWA recommends <= 15 Platinum Cobalt units. This is also the U.S. secondary drinking water regulation. Color can be measured using light with a wavelength of 455 nm.

Salinity• Measured mainly as Specific Gravity (SG),

and Conductivity (C) by use of a hydrometer or electronic probe. 

• Saltwater is heavier than freshwater.

Alkalinity

• The alkalinity of water is a measure of its capacity to neutralize acids. Three major classes of materials cause the major portion of alkalinity in natural waters. These are as follows: (1) hydroxide (2) carbonates and (3) bicarbonates. Bicarbonates represent the major form of alkalinity.

Hardness Hardness is caused by multivalent cations, particularly

Calcium (Ca) and Magnesium (Mg). Hardness = Ca + Mg in meq/L or mg/L of CaCO3

Softening is the removal of hardness.

HCO3-

CO3-

Carbonate Hardness

Non-carbonate Hardness

Cl-

SO4-2

Lime (CaO)Quicklime Ca(OH)2

Soda-Ash(Na2CO3)

Insoluble FormsCaCO3

Mg(OH)2

Nitrate and Phosphate• Nitrate is a primary drinking water standard.

Their presence also causes the blue-baby syndrome in infants. Nitrate is also a nutrient for algae and can stimulate growth of algae.

• Phosphate is a major source of pollution in surface waters from human activities such as irrigation and agriculture. It is a nutrient for plants and algae and can cause eutrophication of lakes.

DISINFECTION

Cl2 + H2O = HOCl + H+ + Cl-

HOCl= H+ + OCl-

Free Residual Chlorine = [HOCl] + [OCl-]

Combined Chlorine = Cl2 + NH3 = Chloroamines

Total Chlorine = Free + Combined

Common Disinfectant - CHLORINE GAS

Free and Total Chlorine

TotalChlorine

+ DPD =Intensity of Red Color Free Chlorine

Iodide + =Intensity of Red Color Total Chlorine

Freshman Engineering Clinic

Understanding Probes and Meters

Overview

• Conductivity• pH• DO• Turbidimeter• Spectrophotometer

Conductivity Probe

Electricity and Solutions

• Current Flow – Metal– No Chemical Change– Current carried by electrons– Temperature increase increases resistance

• Current Flow – Solution– Chemical Change– Current carried by

ions– Temperature increase

decreases resistance– Resistance great than

with metals

The Cell Constant

electrode distance/electrode area

Conductance

1 Siemen = 1/ohm

Typical Ohm Meter

Summary

• The Conductivity Meter is made up of a probe which has the two electrodes with a fixed area and a fixed distance apart.

• An electric current goes through the solution.• The digital display functions as the signal

converter so the user can see the results.• Conductance is simply the inverse of Resistance.

pH Probe

Chemical Reactions

How pH of a solution is calculatedpH = -log aH+

orH+=10-pH

aH ~= [ H+] (moles/L) for dilute solutions

pH Levels

• pH 0-7 solution is acidic

• pH 7 solution is neutral

• pH 7-14 solution is a base

Probe

Electrodes or sensing membranes

Glass Electrode

Probe Meter

Electrode Behavior

The Electrode behavior is described by the Nernst Equation

Emeasured = E0 + (2.3 RT/nF) * log aH+

Emeasured: The potential across the electrode

E0: The potential across the reference electrode

2.3 RT/nF: Nernst Constant

aH+: The amount of hydrogen ions in the solution

Summary

• A potential develops across the electrode when the probe is inserted in the solution

• Comparison between the unvarying potential and the potential across the electrodes

• The potential is sent to the meter calibrated from 0-14 through the wire that connect them

• The pH is calculated by the meter’s software using Nersnt’s equation.

Dissolved Oxygen Probe

Two Types of DO Probes

• Galvanic

• Polarographic

Components

DO Probe Characteristics

• The pH of the solution does not affect the performance of DO probes.

• Chlorine and hydrogen sulfide cause erroneous readings in DO probes.

• Atmospheric pressure affects the saturation of oxygen. DO probes must be calibrated for the barometric pressure when reading in mg/l.

• Membrane thickness determines the output level of the probe.

• Membrane thickness also determines the speed of response to change in DO levels.

Basic Explanation

Oxygen diffuses through the membrane, because a concentration gradient exists between the environments outside and inside the probe. Once

inside the electrode oxygen is reduced at the cathode:

½ O2 + H2O + 2e- = 2 OH-

Basic Explanation (Cont)

The anode reacts with the product of the depolarization with a corresponding

release of electrons.

Ag + 4 OH- = Ag(OH)42- + 2e-

Summary

• Oxygen diffuses through the membrane on to the cathode.

• Here it has a chemical reaction and combines with the anode.

• This causes an electrical current, which is converted to voltage by flowing across a resistor.

• Then this voltage is sent to a meter where the result is analysed and a corresponding DO value is displayed.

Turbidimeter

Turbidity

• What is it?–Clarity of water–Measured in Nephelometric Turbidity

Units (NTU’s)

Cause of Turbidity

• Presence of clay, silt, organic matter, and soil erosion

• Scattering of light increases with a greater presence of these materials in water

Hach 2100N Laboratory Turbidimeter

Turbidimeter

• Micro-processor-based model• Employs advanced optical and electronic

design• Has two types of sensors

– Scattering Sensor– Transmissive Sensor

Turbidity Test

Summary

• Turbidity is the clarity of water• We measure turbidity in NTU’s• Particles suspended in water cause turbidity• In the lab, we use the Hach 2100N

Laboratory turbidemeter to measure turbidity

• The scattering of light at a 90 degree angle to the light source is measured.

Spectrophotometer

Spectrophotometry• Most widely used method of quantitive and qualitive

analysis in chemical and biological sciences.• Accurate and very sensitive method which can analyze

quantities as small as micrograms.• Method depends on the light absorbing properties of either

substance being analyzed or a derivative.• Basis is simple: The intensity of light which is transmitted

through a solution containing an absorbing substance (chromogen) is decreased by that fraction which is absorbed, and this fraction can be detected photoelectrically.

Beer-Lambert law

• States that the amount of light absorbed is proportional to the number of molecules of absorbing substance in the light path.

• Absorbtion is both proportional to the concentration of the chromogen in a solution and to the length of the light path through the solution. This relationship can be expressed as follows:

-log10(I/I0) = Kcl

-log10(I/I0) = Kcl

• where I and I0 are the intensity of transmitted light in the presence and in the absence of the chromogen respectively.

• c is the concentration of the chromogens.• l Is the length of the light path through the solution• K is a constant, characteristic for each absorbing substance

at a specified wavelength of light and in a specific solvent (often water).

• The ratio I/I0 is called the light transmission and is usually measured in percent.

• The absorbancy (Abs), is the quantity more frequently used

Abs = -log10(I/I0)

Spectrophotometers

• Detects the difference in light through a solution photoelectrically and compares that difference electronically.

• The difference is expressed as percent transmission or absorbance.

• In Beer-Lambert law, proportionally constant K depends on:– wavelength of light– nature of absorbing substance

• Device uses cells or cuvettes– carefully made solution containers– plane, parallel sides to ensure the light path to

be the same through all portions, making it possible to tabulate values of K for various substances in various solvents and wavelengths