Environmental Engineering Introduction Notes

ENVIRONMENTAL SCIENCE AND ENGINEERING Environmental Science Whereas the disciplines of biology, chemistry, and physics (and their sub disciplines of microbiology, organic chemistry, nuclear physics, etc.) are focused on a particular aspect of natural science, environmental science in its broadest sense encompasses all the fields of natural science. The historical focus of study for environmental scientists has been, of course, the natural environment.

Quantitative Environmental Science The scientific method deals with data with recorded observations. When the collection and organization of data reveal certain regularities, it may be possible to formulate a generalization or hypothesis. This is merely a statement that under certain circumstances certain phenomena can generally be observed. What do you do with the hypothesis? If we can use certain assumptions to tie together a set of generalizations, we formulate a theory. Theories that have gained acceptance over a long time are known as laws. Some examples are the laws of motion, which describe the behaviour of moving bodies, and the gas laws, which describe the behaviour of gases. Quantitative environmental science is an organized collection of mathematical theories that may be used to describe and explore environmental relationships.

Environmental Engineering Environmental engineering is the application of science and engineering principles to improve the natural environment to provide healthy water, air and land for human habitation and for other organisms and to remediate polluted sites. Although most environmental engineers are concerned in general with the outdoor environment, their work also may concern the interiors of buildings and other structures. Most environmental engineers perform work in one or a combination of basic specialty areas, namely: 1. Domestic and/or industrial waste collection, treatment and disposal facilities 2. Water supply, purification and distribution systems 3. Refuse and solid waste collection and disposal systems 4. Air pollution control systems 5. Layout of facilities and systems to obtain use of land and resources 6. Radiological health, control of insects and rodents, control of hazardous wastes or toxic substances

How Environmental Engineers and Environmental Scientists Work Together There is an old saying that “Scientists discover things and engineers make them work.”

From an educational point of view, environmental engineering is founded on environmental science. Quantitative environmental science provides the fundamental theories used by environmental engineers to design solutions for environmental problems. Perhaps the best way to explain how environmental scientists and engineers work together is to give some examples:  Early in the 20th century, a dam was built to provide water for cooling in a power plant. The impact of the dam on the oxygen in the river and its ability to support fish life was not considered. The migration of salmon in the river was not considered. To remedy the problem, environmental engineers and scientists designed a fish ladder that not only provided a means for the fish to bypass the dam but also aerated the water to increase the dissolved oxygen. The environmental scientists provided the knowledge of the depth of water and height of the steps the fish could negotiate. The environmental engineers determined the structural requirements of the bypass to allow enough water to flow around the dam to provide the required depth.  Storm water from city streets was carrying metal and organic contaminants from the street into a river and polluting it. Although a treatment plant could have been built, a wet land mitigation system was selected to solve the problem. The slope of the channel through the wet land was designed by the environmental engineers. The provision of limestone along the channel bed to neutralize the pH and remove metals was determined by the joint work of the environmental scientists and engineers. The selection of plant material for the wet land was the job of the environmental scientists.

Occupations Related to Environmental Engineering 1. Pharmacologists and Toxicologists They apply professional and scientific knowledge of the source, chemical and physical properties, action, absorption, distribution, excretion and use of drugs or toxic substances and related chemicals. The environmental engineer typically is concerned with the facilities or systems of industrial or municipal plants to the extent that they affect environmental resources. 2. Sanitarians They plan, develop, administer, evaluate and promote programs concerned with the elimination and prevention of environmental and health hazards. 3. Civil Engineers Formerly, water supply and water pollution control activities of environmental engineering were viewed as a specialized segment of civil engineering, the engineering discipline with major concern for facilities and systems directly related to water resources. 4. Chemical Engineers Environmental engineering, especially the pollution control activities has much in common with chemical engineering. The environmental engineer is normally concerned with the waste treatment systems of industrial plants to the extent that they affect environmental resources. 5. Mechanical Engineers

Mechanical engineering is concerned with (a) heating, ventilating and airconditioning systems (b) mechanical aspects of water supply systems such as pumps, plumbing and boiler water treatment systems and (c) automotive and other power plants affecting air pollution. 6. Biomedical Engineers Biomedical engineering is a specialty field which requires the application of engineering concepts and methodology to investigate problems and phenomena of living systems to advance the understanding of these systems and improve medical practices. 7. Mining Engineers They are concerned with work involving the application of mining engineering principles and practices dealing with mining health and safety, mine water control and drainage and control of mine atmospheres. 8. Health Physicists They detect, monitor and measure the exposure of persons to ionizing radiation and prescribe procedures and precautionary measures for protection of persons working in laboratories, industrial facilities or nuclear power plant.

MATERIAL BALANCE Introductory Concepts Dimensions are the general expression of a characteristic of measurement such as length, time, mass, temperature, and so on. Units are the means of explicitly expressing the dimensions, such as feet or centimeters for length, or hours or seconds for time. 2 Types of Units: 1. SI (formally Le Système Internationale d’Unités)

2. AE (American Engineering system of units)

Fundamental or Derived





Fundamental (or basic) dimensions/units are those that can be measured independently and are sufficient to describe most physical quantities such as length, mass, time, and temperature. Derived dimensions/units are those that can be developed in terms of the fundamental dimensions/units.

SI Units

Exercises: 3

1. Change 400 ¿ /day

3

to cm /min .

2. Nanosize materials have become the subject of intensive investigation in the last decade because of their potential use in semiconductors, drugs, protein detectors, and electron transport. Nanotechnology is the generic term that refers to the synthesis and application of such small particles. An example of a semiconductor is ZnS with a particle diameter of 1.8 nm. Convert this value to (a) decimeters (dm) and (b) inches (in.). 3. In biological systems, enzymes are used to accelerate the rates of certain biological reactions. Glucoamylase is an enzyme that aids in the conversion of starch to glucose (a sugar that cells use for energy). Experiments show that 1 μg mol of glucoamylase in a 4% starch solution results in a production rate of glucose of 0.6 μg mol/(mL)(min). Determine the production rate of glucose for this system in units of lb mol/(ft 3)(day). The Mole and Molecular Weight A mole is a certain amount of material corresponding to a specified number of molecules, atoms, electrons, or other specified types of particles. In the SI system a mole (which we will call a gram mole to avoid confusing units) is composed of 6.022 × 1023 (Avogadro’s number) molecules. Molecular weight is the mass per mole.

molecular weight ( MW ) =

mass mole

Exercises: If a bucket holds 2.00 lb of NaOH: 1. How many pound moles of NaOH does it contain? 2. How many gram moles of NaOH does it contain?

Introduction to Material Balance Material balances are nothing more than the application of the conservation law for mass: “Matter is neither created nor destroyed.” General Material Balance

Accumulation =Input −Output +Generation−Consumption Generation and consumption of material are generally included in material balances only for chemical components when chemical reaction occurs in the system. Consider a system composed of a single component for which there are no chemical reactions occurring.

Accumulation =Input −Output

The Control Volume Using the mass balance approach, we begin solving the problem by drawing a flowchart of the process or a conceptual diagram of the environmental subsystem. All of the known and unknown inputs, outputs, and accumulation are converted to the same mass units and placed on a diagram. System boundaries (imaginary blocks around the process or part of the process) are drawn in such a way that calculations are made as simple as possible. The system within the boundaries is called the control volume. Exercises: 1. Mr. and Mrs. Konzzumer have no children. In an average week they purchase and bring into their house approximately 50 kg of consumer goods. Of this amount, 50% is consumed as food. Half of the food is used for biological maintenance and ultimately released as CO2; the remainder is discharged to the sewer system. The Konzzumers recycle approximately 25% of the solid waste that is generated. Approximately 1 kg accumulates in the house. Estimate the amount of solid waste they place at the curb each week.

Time as a Factor Rate of Accumulation=Rate of Input−Rate of Output Rate is used to mean “per unit of time.” In calculus, this may be written as:

dM d (¿) d (out) = − dt dt dt

Where M is the mass accumulated and (in) and (out) refer to the mass flowing in or out of the control volume. Exercise: 1. Truly Clearwater is filling her bathtub but she forgot to put the plug in. If the volume of water for both is 0.350 m3 and the tap is flowing at 1.32 L∙min -1 and the drain is running at 0.32 L∙min -1, how long will it take to fill the tub to bath level? Assuming Truly shuts off the water when the tub is full and does not flood the house, how much water will be wasted? Assume the density of water remains constant throughout the control volume. 2. Consider the storage tank shown in the figure. Over a 3 h period, the accumulation of water in the tank was determined to be 6000 kg. Assuming that the feed and removal rates remain constant during the 3 h period of

´ interest, determine the flow rate of the second feed stream, F2 . 10,000 kg/h and the water removal rate,

F´ 1 is

´ P , is 12,000 kg/h.

Efficiency The effectiveness of an environmental process in removing a contaminant can be determined using the mass balance technique. The mass flow rate can be expressed as

Mass =( concentration) ( flow rate) Time For example:

Mass =( mg∙ m−3 )( m3 ∙ s−1) =mg ∙ s−1 Time In concentration and flow rate terms, the mass balance equation is

dM =C ¿ Q¿ −C out Qout dt Where: C stands for concentration and Q stands for flow rate The ratio of the mass that is accumulated in the process to the incoming mass is a measure of how effective the process is in removing the contaminant.

dM /dt C¿ Q ¿ −C out Q out = C ¿ Q¿ C ¿ Q¿ The left-hand side is given the efficiency notation η.

mass∈¿(100 ) mass∈−mass out η= ¿ Exercise: The air pollution control equipment on a municipal waste incinerator includes a fabric filter particle collector (known as a baghouse). The baghouse contains 424 cloth bags arranged in parallel, that is 1/424 of the flow goes through each bag. The 3

−1

gas flow rate into and out of the baghouse is 47 m ∙ s −3

particles entering the baghouse is 15 g ∙ m

, and the concentration of

. In normal operation the baghouse

particulate discharge meets the regulatory limit of 24 mg ∙m

−3

. During preventive

maintenance replacement of the bags, one bag is inadvertently not replaced, so only 423 bags are in place. Calculate the fraction of particulate matter removed and the efficiency of particulate removal when all 424 bags are in place and the emissions comply with the regulatory requirements. Estimate the mass emission rate when one of the bags is missing and recalculate the efficiency of the baghouse. Assume the efficiency for each individual bag is the same as the overall efficiency for the baghouse.

The State of Mixing Completely mixed systems are systems in which every drop of fluid is homogenous with every other drop, that is, every drop of fluid contains the same concentration of material or physical property (e.g. temperature). Plug-flow systems are systems which are completely unmixed or approximately so. A system is in steady-state condition when the rate of input and rate of output are constant and equal. Meaning, the rate of accumulation is zero.

Exercise: −1

A storm sewer is carrying snow melt containing 1.200 g ∙ L

of sodium chloride into

a small stream. The stream has a naturally occurring sodium chloride concentration −1

of 20 mg∙ L

3

−1

. If the storm sewer flow rate is 2000 L∙ min −1

rate is 2.0 m ∙ s

and the stream flow

, what is the concentration of salt in the stream after the

discharge point? Assume that the sewer flow and the stream flow are completely mixed, that the salt is a conservative substance (it does not react), and that the system is at steady state.

Including Reactions and Loss Processes In most systems of environmental interest, transformations occur within the system: by-products are formed (e.g. CO2) or compounds are destroyed (e.g. ozone). Because many environmental reactions do not occur instantaneously, the time dependence of the reaction must be taken into account. Time-dependent reactions are called kinetic reactions.

dM d (¿) d (out) = − +r dt dt dt Where r is the reaction rate

r=−k C n Where k is the reaction rate constant, C is the concentration of substance and n is the reaction order For a first-order reaction:

r=−kC=

dC dt

Exercise: 3

A well-mixed sewage lagoon (a shallow pond) is receiving 430 m ∙ d

−1

of untreated

sewage. The lagoon has a surface area of 10 ha and a depth of 1.0 m. The pollutant −1

concentration in the raw sewage discharging into the lagoon is 180 mg∙ L

. The

organic matter in the sewage degrades biologically (decays) in the lagoon according −1

to first-order kinetics. The reaction rate constant (decay coefficient) is 0.70 d

.

Assuming no other water losses or gains (evaporation, seepage, or rainfall) and that

the lagoon is completely mixed, find the steady-state concentration of the pollutant 4

3

3

in the lagoon effluent. (Note: 1 ha=10 m ; 1m =1000 L )

WATER POLLUTANTS AND THEIR SOURCES Point Sources Point sources are generally collected and conveyed to a single point of discharge into the receiving water. Examples: Domestic sewage, industrial wastes Point source pollution can be reduced through waste minimization and proper wastewater treatment prior to discharge to a water body.

Nonpoint Sources Nonpoint sources are characterized by multiple discharge points. Examples: Urban and agricultural runoff Nonpoint pollution sometimes requires major engineering work to correct.

Oxygen-Demanding Material An oxygen-demanding material is anything that can be oxidized in the receiving water resulting in the consumption of dissolved molecular oxygen. This material is usually biodegradable organic matter. The consumption of dissolved oxygen (DO) poses a threat to aquatic life that must have oxygen to live. Examples: Domestic sewage (human waste and food residue), food-processing and paper industries Hypoxia is the condition of a body of water that has DO concentrations lower than 1.0 mg/L.

Nutrient Nitrogen and phosphorus are considered pollutants when they become too much of a good thing. The loss of nutrients in runoff or seepage from croplands is the major source of nutrient release from agricultural operations. The most widely used fertilizers are line (to maintain a proper soil pH), nitrogen (N), phosphorus (P), and potassium (K).

Pathogenic Organisms These include bacteria, viruses and protozoa excreted by diseased persons or animals.

Certain shellfish can be toxic because they concentrate pathogenic organisms in their tissues, making the toxicity levels in the shellfish much greater than the levels in the surrounding water.

Suspended Solids Suspended solids are organic and inorganic particles that are carried by wastewater into receiving water. When the speed of the water is reduced by flow into a pool or a lake, many of these particles settle to the bottom as sediment. Sediment also includes eroded soil particles that are being carried by water even if they have not yet settled. Colloidal particles, which do not settle readily, cause the turbidity found in many surface waters. Organic suspended solids may also exert an oxygen demand. Result: increase turbidity, decrease light penetration, increase bacterial population and solids deposit on the bottom of the water body destroying the habitat of benthic organisms.

Salts Total dissolved solids (TDS) are the salts and other matter that do not evaporate from a filtered water sample. Salt accumulation can lead to a reduction in crop yield, particularly crops that are sensitive to salinity (e.g. corn, soybeans, rice, lettuce, squash). Saline soils can be reclaimed by applying sufficient water to the soil to leach the solutes from the root zone.

Pesticides Pesticides are chemicals used by farmers, households, or industry to regulate and control various types of pests or weeds. The major types are herbicides, insecticides, and fungicides. Herbicides are used to kill unwanted plants (i.e. weeds). Insecticides are used to kill insects that would otherwise destroy crops. Fungicides are used to control the growth of fungi, many of which cause plant diseases.

Pharmaceuticals and Personal Care Products These are compounds that are applied externally or ingested by humans, pets, and other domesticated animals. These are released by 1. disposal of expired, unwanted or excess medication 2. metabolic excretion: excretion of the chemically unaltered parent compound and metabolized by-products in urine and feces 3. PCP (deodorants and sunscreens) can be washed during washing, bathing and swimming

Endocrine-Disrupting Chemicals

These include the polychlorinated biphenyls, commonly used pesticides such as atrazine, and the phthalates. EDCs can mimic estrogens, androgens, or thyroid hormones. They interfere with the regulation of reproductive and developmental processes. They alter the normal physiological function of the endocrine system and can affect the synthesis of hormones in the body.

Toxic Metals Most commonly occurring heavy metals are arsenic, cadmium, chromium, copper, nickel, lead, and mercury. They persist in the environment; they tend to accumulate in soils, sediments and biota. They can also bioaccumulate and biomagnify.

Heat Increased temperature results in a decrease in the solubility of oxygen and carbon dioxide.

WATER QUALITY MANAGEMENT IN RIVERS The objective of water quality management is to control the discharge of pollutants so that water quality is not degraded to an unacceptable extent below the natural background level.

Biological Oxygen Demand Theoretical oxygen demand (ThOD) is the amount of oxygen required to oxidize a substance to carbon dioxide and water. This is calculated by stoichiometry. Exercise: Compute the ThOD of 108.75 mg/L of glucose (C 6H12O6). Chemical oxygen demand (COD) is a measured quantity that does not depend on one’s knowledge of the chemical composition of the substances in the water. In a COD test, a strong chemical oxidizing agent (chromic acid) is mixed with a water sample and then refluxed. The difference between the amount of oxidizing agent at the beginning of the test and that remaining at the end is used to calculate COD. Biological oxygen demand (BOD) is the oxygen consumed if the oxidation of an organic compound is carried by out microorganisms using the organic matter as food. The test is a bioassay that uses microorganisms in conditions similar to those in natural water to measure indirectly the amount of biodegradable organic matter present. Bioassay means to measure by biological means. The actual BOD is almost always less than the ThOD due to the incorporation of some of the carbon into new bacterial cells. Thus, a portion of soluble carbon is removed but will not be measured in the BOD test.

To measure BOD, a water sample is inoculated with bacteria that consume the biodegradable organic matter to obtain energy for their life processes. −kt

BOD t=L o−Lt =Lo−Lo e

Where L is the oxygen equivalent of organics at a time, mg/L Lo is the ultimate BOD, that is, the maximum oxygen consumption possible when the waste has been completely degraded. Exercise: If the 3-day BOD (BOD3) of a waste is 75 mg/L and the BOD decay constant, k, is 0.345/day, what is the ultimate BOD?

WATER SUPPLY AND TREATMENT Sources of Water Groundwater Groundwater is water that has percolated downward from the ground surface through the soil pores. Aquifers (groundwater reservoirs) are formations of soil and rock that have become saturated with water. Wells are used to withdraw water. Factors that limit the speed at which water can move through the soil to replenish the well (Flux: 1 m/day to 1m/yr): 1. Soil pore size 2. Water viscosity 3. Others Advantage of groundwater: 1. No need for expensive pipelines and purification. 2. Filtering of pathogenic organisms by the soil particles. 3. Less treatment/expense. 4. Uniform quality and free of turbidity Disadvantages: 1. Polluted groundwater restoration is difficult and long term. 2. May require softening. 3. Water quality is difficult to monitor 4. Siting of septic tanks in relation to wells is critical Surface Water Advantages: 1. Can sustain high withdrawal rates Disadvantages: 1. Open to pollution of all kinds (sources: industrial and municipal wastes, runoff, soil erosion) 2. May necessitate extensive treatment Seawater

Converting seawater to freshwater is costly (2-5 times higher than treating fresh water). Desalination is the general term for the removal of dissolved salts from water. Examples: 1. Distillation depends on the evaporation and condensation of water. But this is energy intensive. 2. Freezing. Lowering of water temperature until pure ice crystals can be separated. 3. Electrodialysis. Forced migration of charged ions through cation-permeable or anion-permeable membrans applying an electric potential across a cell containing mineralized water. 4. Reverse osmosis. Uses membranes that are permeable only to water (but driving force is pressure from pumps). Costs are well below than the other methods. This is normally seen in the Middle East. Reclaimed Wastewater It is water that has been treated sufficiently for direct reuse in industry and agriculture and for limited municipal applications. This can be the only option for those areas without freshwater. This can remove suspended solids, biodegradable organics and bacteria. But color, inorganic salts of Mg, Na, Ca, synthetic organics like pesticides can only be removed by advanced techniques. Activated carbon can remove many organic pollutants due to its large surface area (1000 square meter per gram). It can trap and adsorb water impurities. This can normally be seen in the Middle East, South Africa and arid parts of the US.

Water Treatment Processes Water Treatment Plants Main considerations: 1. Source selection 2. Protection of water quality 3. Treatment methods to be used 4. Prevention of recontamination Precautions to prevent groundwater and surface water pollution: 1. Prohibiting the discharge of sanitary and storm sewers close to the water reservoir 2. Installing fences to prevent pollution from recreational uses of water 3. Restrict fertilizers and pesticides in areas that drain to the reservoir Main unit operations: 1. Screening: remove large items (fish, sticks, leaves) 2. Coagulation/flocculation: addition of chemical to coagulate/flocculate the small particles for easy settling in sedimentation tanks or removed by filtration

3. Sedimentation 4. Filtration: (no prior sedimentation is effective for 5-20 turbidity units) 5. Disinfection: usually with chlorine. Fluoride may also be added to retard tooth decay Key tasks in water treatment: 1. Removal of particulate substances (e.g. sand and clay, organic matter, bacteria) 2. Removal of dissolved substances (causing color and harness) 3. Killing of pathogenic bacteria or viruses Removal of Particulate Matter Screening is used to remove large solids such as logs, branches, rags, and small fish in the first stage of treatment. These could damage pumps and clog pipes and channels. Coarse screens are vertical bars spaces 25 mm (1 in) or more apart. These are placed at the intake point. Fine screens with 6 mm (0.25 in) spacing are placed just ahead the low-lift pumps to raise water to the plant level. Sedimentation uses gravity settling to remove particles from water. It is simple and inexpensive. This may also follow coagulation and flocculation or omitted entirely (for moderately turbid water). Turbidity is caused by particles larger than 10 -4 mm, while particles smaller than this contribute to water and taste. Particles that settled to the bottom is removed manually or by mechanical scrapers and discharged to the sewers, returned to the water source or stored in store for disposal. Detention time is 3 h in tanks 3-5 m deep.