CE 326 Principles of Environmental Engineering Introduction and Mass Balances

CE 326 Principles of Environmental Engineering
Introduction and Mass Balances
Dr. S.K. Ong
What is Environmental Engineering?
The American Academy of Environmental Engineering defines environmental engineering as:
The professional application of scientific principles and technical practices to the protection and improvement
of health and well-being of humans and their surroundings. It involves
• the management and optimum use of air, water and land resources,
• and the provision of facilities and the control of conditions for living, working and recreation
that will contribute positively to human health, comfort and well being
American Society of Civil Engineers, Environmental Engineering Division defines environmental engineering
Environmental engineering is manifest by sound engineering thought and practice in the solution of problems
of environmental sanitation, notably
• in the provision of safe, palatable, and ample public water supplies;
• the proper disposal of or recycle of wastewater and solid wastes;
• the adequate drainage of urban and rural areas for proper sanitation;
• the control of water, soil, and atmospheric pollution, and
• the social and environmental impact of these solutions.
Furthermore it is concerned with the engineering problems in the field of
• public health hazards, such as control of anthropod-borne diseases,
• the elimination of industrial health hazards and
• the provision of adequate sanitation in urban, rural, and recreational areas, and
• the effects of technological advances on the environment.
Major environmental engineering activities are:
• Water supply and resources engineering
• Wastewater pollution control
• Water quality management
• Solid waste management, pollution prevention, recycling and reuse
• Hazardous waste management
• Air pollution
• "Green" Technologies, "Green" energy technologies, industrial ecology,
• Noise pollution
• Radioactive waste management
Mass Balances
Environmental engineering deals with the protection of human health and the surroundings. Much of environmental
engineering work is driven by environmental legislation and regulations. Pages 16 – 33 provide a summary of the
different laws.
The environment can be viewed as consisting of many systems. For each system, an imaginary close boundary can be
drawn with both external and internal influences, inputs and outputs.
Each system can consist of one or more medium. The “media” are :
Liquid - water in rivers, ocean, groundwater, water droplets in atmosphere, drinking water, wastewater
- atmosphere, air in a reactor, flue gas
- soils, sediments, sludges, solid waste
In a typical system, the pollutant or chemical of interest can move from one medium to another. When more than one
medium is involved we term the problem as a “multimedia” problem.
Example: Landfill
To quantify the system, mass or volume balances are used. To employ mass balances, a control volume, i.e., the
imaginary close boundary must be defined. Using the control volume, we need to define
- all the inputs into the system
- all outputs out of the system
- all reactions within the system that will increase or decrease the pollutant or chemical of interest.
The following general statement can be made for mass balances:
IMPORTANT - each term is expressed in terms of rate, i.e., mass per unit time or volume per unit time (or flow).
Mass rate can be obtained from the product of ______________________________________________
The statement can be simplified, if nothing is accumulating in the system, or the concentration of X in the system is
constant. This condition is termed as “steady state”. Then:
of X
of X
of X
of X
If X is not produced or depleted in the system, then:
INPUT of X - OUTPUT of X = 0
Example 1: Flow in Sewers
Set up mass balances
Input – Output + Production – Depletion = Accumulation
Assume steady state (no accumulation, also no production or depletion of the waste
l/min mg/l = > (mg/min)
QA (50) + (100) (200,000) - QC (1,000) = 0
Two unknowns – need another equation, use volume balance:
QA CA + 100 = QC
Substitute equation (2) into (1)
QA (50) + (100) (200,000) - (QA + 100) (1,000) = 0
Solving for QA
QA = __________________
QC = __________________
Example 2: As a fuel source, 20 kg/hour of ethylene (C2H4) is burned with 400 kg/hour of air in a furnace. What is
the emission rate of CO2 and O2 from the furnace? Composition of air by volume or by moles: N2 = 79%, O2 = 21%
C = 14, N = 14, H = 1, O = 16
Balance equation
Molecular masses
C2H4 + 3 O2 = 2 CO2 + 2 H2O
C2H4 = 28, H2O = 18, CO2 = 44, O2 = 32, air = 28.84
No. of moles/hour of C2H4 = 20/28 = ___________ moles/hour
No. of moles of O2 needed = 3 x 0.71 moles/hour = ___________ moles/hour
No. of moles of CO2 produced = 2 x 0.71 moles/hour = 1.42 x 44 = ___________ kg/hour
No, of moles of O2 in exhaust = (400/28.84) x 0.21 – 2.13 = 2.91 – 2.13 = ____________ moles/hour.
Basic Chemical Concepts and Concentrations
- number of protons in the atom.
- element with different numbers of neutron in the nucleus.
- relative weights of the atoms as compared to C12 isotope of carbon
- quantity of the element in grams corresponding to the atomic weight, e.g., chlorine - 35.5
______________ (MW) - molecular weight in grams of any particular compound
e.g., CaCO3 = (40 + 12 + 16x3) = 100 gms
- One molecular weight of a species contains 6.02 x 1023 particles of that species.
- This quantity is referred to as a _________________.
- given by ___________ where n is the number of units of reaction capacity and is equal to:
(i) charge of the ion, e.g., Na+ has a charge of one, valency or n for sodium is one.
(ii) In an acid-base reaction, n is the number of positively charged hydrogen ions (H +) or
negatively charged hydroxyl ions (OH-) that the element or ion will react.
Na+ reacts with 1 OH- , n is equal to 1;
Ca2+ needs 2OH- , n is equal to 2;
NO3- needs 1 H+, n is equal to 1.
(iii) the number of electrons transferred in an oxidation-reduction reaction. e.g., in
NO3- + 2 H++ 2e- <==> NO2- + H2O
NO3- + 10 H+ + 8e- <==> NH4+ + 3H2O
n =8
The n for NO3- in the above two reactions are different therefore the equivalent weight of a
compound is dependent on the type of reactions it is going through.
______________ (eq)
- a quantity of the compound in grams corresponding to the equivalent weight
What is the purpose of using equivalent weights or equivalents
e.g., 2NaOH + H2SO4 ==> Na2(SO4) + 2H2O
2 moles of NaOH will react with 1 mole of sulfuric acid to produce one mole of sodium
sulfate and 2 moles of water.
By using equivalents, the reaction can be expressed as 1 equivalent of water will react with
1 equivalent of sulfuric acid to produce 1 equivalent of sodium sulfate and 1 equivalent of
Three ways of expressing concentrations:
____________ (m)
- number of moles of a compound per kilogram of solvent
- one lb (gm) of chemical per million lb (gm) of solution
(others : ppb - parts per billion; ppt - parts per thousand)
- For high concentrations; expressed as % of a given compound on a
mass/mass basis, e.g., 36% by weight of hydrogen chloride in water
- in 100g of concentrated hydrochloric acid, there are36 g of HCl and 64 g
of water.
% of A = _____________________________
__________ (M)
- mass in milligrams of compound in 1 liter of solution
- number of moles of a compound per liter of solution
___________ (N)
- number of equivalents of solute per liter of solution or eq/L
(both M and N can be expressed as g/L when multiplied with the
molecular weight and equivalent weight, respectively)
- a more favorable expression used in environmental engineering is
meq/L, i.e., N/1000 solution or mM)
If water has a density of 1,000 kg/m3 or 1,000 gm/liter or 1x 106 mg/liter, then 1 ppm would be equal to 1
mg/L. For analysis, a known volume is analyzed instead of a mass of the solution. For this reason, "mg/L" is
the preferred concentration unit.
Other variations:
(i) concentrations expressed in terms of the specific component of the compound instead of the compound
itself. For example, nitrate (NO3-) may be expressed in terms of nitrogen (N) such as mg of NO3--N/L.
To convert mg NO3-/L to NO3--N/L:
= __________________________________
Example: ______________________________________________________
Useful when you need to find the total concentration of a particular constituent when a variety of different
components containing that constituent is present.
(ii) mg/L as CaCO3 - the calcium carbonate convention. - concentrations normalized to calcium carbonate
(CaCO3). To convert concentrations in mg/L to mg/L as CaCO3.
e.g., 117 mg/l of NaCl. = ______________________________________ mg/L of NaCl as CaCO3
MW of NaCl = 23 + 35.5 = 58.5
n=1 for NaCl therefore equivalent weight = 58.5
e.g., 20 mg/L of Ca2+ = _______________________ = _____________mg/L of Ca2+ as CaCO3.
Note: 1 meq/L = 50 mg/L as CaCO3.
Volume/volume - more commonly used for concentrations of gases in air - (usefulness - changes in temperature and
pressure do not change the volume of the chemical or gas to the volume of the air that contains it). Examples:
- one liter (gallon) of compound in million liter (gallon) of solution
- % of A = ____________________________________________
Trace Concentrations Units (Ref: W.B. Crumett, December, 1976)
1 ppm
1 min/2 years
1 sq. ft/23 acres
1 ppb
1 inch/16,000 miles
1 sec/32 years
1 sq. ft/36 sq. mi.
1 bogey/3,500,000
Water Distribution
Ice and Snow
Underground water
Rivers and Streams
Mass (g)
13,000 x 1020
250 x 1020
50 x 1020
1.2 x 1020
0.17 x 1020
0.02 x 1020
80% of the earth is covered with water.
97.7% found in the oceans, less than 1% of the earth's water is available for use.
70% of our body mass is water.
% of Total
CE 326 Principles of Environmental Engineering
Water Quality Parameters and Standards
Dr. S.K. Ong
Agents that alter the quality of water may be classified under the following four categories:
Water quality parameters for human consumption are regulated by _______________________ (SDWA).
Regulations apply for providers of water for human consumption of
• at least 15 service connections, or
• regularly serves an average of at least 25 individuals daily at least 60 days out of the year
• Two types of standards
Primary - for the protection of human health
Secondary - aesthetic reasons
• Standards given by _________________________ (MCLs) or
Secondary Maximum Contaminant Levels (SMCLs) (see Table 3-6, page 160)
• specifies the _____________________________ (BAT)
• _________________________________ (SWTR)
• require treatment of surface water supplies or groundwater supplies under direct influence of surface water,
• specifies treatment techniques - filtration and/or disinfection
Taste and Odor
Inorganic Chemicals (IOCs)
Cations (Metals)
Organic Compounds
Synthetic Organic
Chemicals (SOCs)
Volatile Organic
Chemicals (VOCs)
Disinfectants and disinfectant
by products (DBPs)
Trihalomethanes (THMs)
Total Coliforms