HSCI 457 Review Exams
WATER SUPPLY AND SEWAGE DISPOSAL
Introduction
This chapter provides a comprehensive review of the main topics that are covered in the HSCI 457 Water and Wastewater Technology class and quiz questions designed to reinforce the acquired knowledge.
Water is the most abundant compound on the surface of the Earth and most common natural resource. It is found not only in the oceans and seas, in lakes, rivers, streams, and glacial ice deposits. It is also found in the atmosphere and cloud formations.
Water is composed of hydrogen and oxygen. Pure water is an insulator, has basically nothing in it and therefore does not conduct electricity. But that is a property that we cannot take an advantage of because the water we use has lots of ions in it.
Both hydrogen and oxygen atoms in water have isotopes. There are at present three known isotopes of hydrogen and six isotopes of oxygen. The isotopes of hydrogen are: 1H (protium), 2H (deuterium), and 3H(tritium), which is radioactive. Whereas the isotopes of oxygen are 14O, 15O, 16O, 17O, 18O, and 19O, three of which are radioactive. There are thirty-three possible combinations of isotopes that can occur in pure water.
A single water molecule consists of two hydrogen atoms and an oxygen atom. Each hydrogen atom is attached to the oxygen atom by a single covalent bond. Water is a highly polar molecule, therefore does not have a linear structure. It forms a V-shaped structure with an angle of about 105( between the two bonds, which is called a bent-chain configuration. Water can exist in three phases: solid, liquid, and vapor. Water vapor contains lots of energy, which is one of the forces that act to keep water molecules apart. With the removal of the heat from the system, water molecules start to arrange themselves in an ordered manner. Attachment of two to three bonds is required for it to be called water, and four bonds – to turn it into ice.
Density is a relationship between the mass and the volume. When a substance is heated or cooled, its volume expands or contracts. This means that the density actually changes with the temperature. So warm water is less dense and thus remains on the surface. While cold water has greater density and sinks to the bottom of a lake or a river.
The water in lakes is stratified in three layers based on the temperature and density: epilimnion – the top layer, which has the warmest temperature, thermocline – middle transition layer, in which a very sharp drop in temperature occurs, and hypolimnion – the bottom zone, where water temperature drops below 12(C.
Water has its greatest density at 4(C. As water cools at the surface of a lake, it becomes more dense and is replaced by warmer water from below. This mixing continues until the temperature reaches 4(C. Water expands as its temperature drops below 4(C. It expands even more as it freezes to ice. At temperature below 4(C water enters pseudoctystalline state in which water molecules rearrange themselves by forming crystals and spread out. Water then freezes first on the surface, and the ice remains on the surface since it is less dense than the water.
Properties of Water
Cohesion is water’s ability to stick to itself. It refers to the force between molecules of the same type. A number of observations suggest that the surface of a liquid can act like a stretched membrane under tension. For example, a drop of water on the end of a dripping faucet forms into a spherical shape. This cohesive property of the water creates something, called surface tension, which arises from attractive forces between the molecules. Mosquitoes can land on the water surface and lay eggs on it. Algae can float on it. Even a steel needle can float on the surface even though it is denser than the water.
Adhesion is the water’s ability to stick to something else. It refers to the force between molecules of different type. For example, water sticks to the walls of a glass tube, which is made of a Silica oxide. One thing that water has a problem sticking to is paraffin wax.
Capillary action is the water’s ability to move up. It occurs in the ground water. There are areas under ground where water will rise up to dryer soil. If there are pollutants in its way, it will raise them with it. The pollutants will stay then in the area called capillary fringe, while the water will go back down.
Thermal Properties
Water has a very high specific heat. It takes one calorie to raise one gram of water 1(C. It absorbs a lot of heat before it changes its temperature. Water has an exceptional ability to store heat. That is the reason water temperature in the ocean does not drop at nights, but stays the same. Water’s ability to store heat is the driving force behind our weather patterns.
Water can exist in three phases: liquid, solid, and vapor. For ice to enter the liquid state, or for the liquid to turn into vapor, a certain amount of energy is absorbed. Eighty calories are needed to turn one gram of ice into liquid and five hundred thirty six calories – to turn on gram of liquid into vapor.
Solubility Rules
I. The warmer is the temperature of the water, the lower are the dissolved gases. Cold water has more oxygen than the warm water.
Polar gases are more soluble than non-polar. E.g. H2 and O2 do not dissolve in water. But CO2, which is a polar gas, dissolves in water forming H2CO3, carbonic acid.
Water is a universal solvent, meaning that it dissolves more things than any other solvent. E.g. sodium chloride in the water is always found in its ionic form.
Highly polar solvents will readily dissolve ionic or polar solvents. Water helps to dissociate carbonic acid, as it gets to the water systems. All strong acids and bases reach one hundred percent ionization.
Molal Freezing Point Depression
Each solvent has a characteristic freezing point depression constant. One mole of anything dissolved in one kilogram of water will lower its freezing point, which is 32(C, by 1.86(F. E.g. one mole of Na in one kg of water will depress its freezing point by 1.86(F, while one mole of CaCl2 – by 5.58(F.
Fresh Water Systems
Hydrologic cycle starts with the sun. So incident solar radiation strikes the water in the ocean or land surface, which then evaporates and goes back into the atmosphere. As water vapor rises it cools by coming in contact with cold air. The temperature, as it goes up in the elevation, cools. That is called adiabatic or dry air lapse rate, which means every thousand feet you go up in elevation the temperature will drop by 5.5(F. The environmental lapse rate is 3.5(F.
Hydrologic Cycle
Hydrologic cycle is the movement of water on this planet. There are two types of hydrologic cycle: short cycle and long cycle.
Short cycle is a very simple process. Water, struck by incident solar radiation, absorbs energy and enters the vapor phase. As water gets evaporation, it immediately condenses and drops back down in the form of rain.
Long cycle starts the same way. Water that is evaporated from ocean or land surface goes back into the atmosphere and forms clouds. Clouds and atmospheric vapors then condense under the proper conditions to form precipitation, including rain, snow, hail, fog, or dew. If there is a warm air mass in the atmosphere, in order to start creating rain it needs to cool first. It either can meet a cold air mass and cool, or it can be forced up even higher and cool.
Stages in Hydrologic Cycle:
Formation of precipitation occurs in this stage.
Stage two begins as the precipitation goes down. In falling through the atmosphere rain picks up dust particles, bacteria, dissolved gases, ionizing radiation, and chemical substances such as sulfur, nitrogen, oxygen, carbon dioxide and ammonia. As it strikes the ground, plants and trees will be the first to catch it. This stage of hydrologic cycle is called interception, in which plants catch the moisture and then re-evaporate it back into atmosphere. Part of the water reaching the ground infiltrates and percolates down to form the groundwater. This process is called infiltration. Another part of that water, called run-off, contributes to formation of stream, lakes, swamps, or oceans. When flowing over the ground surface, water picks up anything it can move or dissolve such as plants, dead animals, sediments, etc.
Stage three involves evaporation and transpiration. During this stage water from everywhere will evaporate back into the atmosphere. Transpiration is a process during which water released by vegetation during photosynthesis moves back into the atmosphere. Since there is no way one can distinguish between these two types of water vapor moving up, a term evapotranspiration is used to refer to both processes.
Stage four of the cycle takes a look at the groundwater, which in many cases moves towards the ocean. It can also move up as a spring stream.
Freshwater System
There are two categories: lentic and lotic environments.
Lentic – refers to standing water, such as lakes, ponds and swamps. Lotic – refers to water that is running in one direction, such as rivers and streams.
Common Elements
( Food chain: autotrophs, using either light or oxidation of inorganic compounds to get energy; herbivores – primary consumers of autoptrophs, and predators, feeding on herbivores.
( In both environments with Nitrogen, Carbon, and Phosphate in presence of sunlight, plants will carry out the process of photosynthesis. Out of the three essential inorganic materials the plants need for photosynthesis, Phosphorus is the most deficient. Oxygen released by plants during photosynthesis is very critical. It will dissolve into the water, if the water is cold. Cold water contains much more oxygen than the cold water. During the daytime oxygen levels increase in the system as plants produce more oxygen. At night both animals and plants use oxygen, so its levels drop a little bit. In both environments oxygen levels depend on sunlight, which is strongest at noon. Oxygen levels rise and pick before noon, drop at noon, then come back up a little bit and drop again. That drop of oxygen levels at noon is called midday depression. The reason why that happens is because at extremely strong sunlight photosynthesis stops of decreases.
When light penetrates through water, it is called a euphotic zone. Penetration depends on the quality of the water: there may be no euphotic zone at all or it may extend all the way to the bottom of the lake. Autotrophs and herbivores are found in the euphotic zone. Dysphotic zone, which is below euphotic zone, is a zone of perpetual darkness. This zone may not exist if the water source is shallow and the light penetrates deep down. On the bottom of the lake or a stream is the benthic zone, where the sediments are.
Plants require the dissolved nutrients in their inorganic form, which they will convert into organic compounds in the presence of sunlight. Death is the other end of the cycle. Everything in the lake, including fish and plants, will eventually die and sink to the benthic zone and sit in the sediments.
What happens next is regeneration, a process during which a group of organisms in the benthic zone attack this dead organic material. Phosphorous cycle is very quick: organic phosphate is converted instantaneously into its inorganic form, o-phosphate, by specific bacteria. Nitrogen cycle is slower and more difficult. Proteins, amino acids are the organic forms of nitrates. Sacropthytic organisms are bacteria that attack dead insects and plants. These organisms rip the NH2 group off the large protein molecule and convert in into NH3, ammonia. Thus, the presence of ammonia in the lake would indicate a new pollution, since it is the first step. The second step of the cycle involves a group of bacteria called Nitrosomonas, which will take that NH3 group and convert it into NO2 in the presence of oxygen. So the presence of NO2 in the water would mean that the pollution is older, it is in progress. In the last step of the cycle NO2 is converted into NO3 in the presence of oxygen by a group of bacteria called Nitrobacters. At this point if NO3 is found in the water during water sampling, it means that it is the last step of the pollution.
Limiting factor refers to the fact that plants use inorganic nutrients in a specific ratio. It will take 106 parts of carbon, 10 parts of nitrogen, and 1 part of phosphorus. It has to have all three of these to take any of them. Phosphorus is the limiting factor in both lentic and lotic environment, and when it is not present nitrogen and carbon are not used and keep building up for years. The reason why phosphorus is missing in the lake is that it gets trapped at the bottom of the lake by soil and Iron, which is found in the form of Fe2(OH)3. Iron in its fully oxidized state (Fe+3) forms a physical barrier to phosphorus in the water and keeps it trapped at the bottom.
Lentic Environment
Oligotrophic refers to a young lake. It is relatively deep, clear, and low in nutrients, thereby supporting little plant and animal life. In eutrophic lake, nutrients are at high enough level to support large populations of plants and animals. This means that it has lots of autotrophs, herbivores, and predators, which will eventually die and go to the bottom of the lake making it shallower and shallower. Senescent refers to an old lake, which is extremely high in nutrients.
Stagnation is a result of either ice or warm water on top of the lake. Densities of water make the conditions stagnant. When warm water is on the top and prevents the lake from mixing, it is called summer stagnation. As water is getting cold with the change of seasons, it eventually becomes homothermous, meaning that water has the same temperature throughout the lake. During homothermous conditions a little wind may create currents and the lake will mix. That is called fall-overturn. During winter months, water gets even colder and ice covers the surface of the lake. When conditions become stagnant because of the ice on top of the lake, it is referred to as winter stagnation. In spring it is getting warmer, the temperature of the water goes up and again homothermous conditions are created. This is now called a spring-overturn. During stagnation oxygen is produced by autotrophs up on the top of the lake and it cannot go down to the bottom, where oxygen levels are dropping. While nutrients plants need for growth are trapped at the bottom and cannot move up. Homothermous conditions are important for the lake renewal. If the lake is in overturn, plenty of nitrogen, carbon, and phosphorus will be released and thus fertilize the top of the lake. This will lead to massive plant growth in the epilimnion, which is referred to as eutrophication. As lake goes through this cycle every year, the depth gets more and more shallow.
Waters in both environments are natural. Half of all known chemical elements are found in natural waters. It refers to the mineral content, salinity, which is characterized by dissolved salts and minerals, caused by cations and anions found in water. The most common element found in water is bicarbonate (HCO3(). Saline water is broken down to the following categories: brackish – 1,000 to 4,000ppm, salted – 4,000 to 18,000ppm, and sea – 18,000 to 35,000ppm. Any water with over 5,000ppm salinity is classified as relatively unusable. Sources of minerals in the water are soil, deicing of roads, injection wells, and splitting of rocks.
Salts in the water, MgSO4, CaCl2 and NaCl may create some problems. Salt with saltiest taste is CaCl2, but the most common salt in the water is MgSO4, which may have laxative effects on humans. Minerals causing domestic problems are Ca, Mg, and HCO3(.
When boiling the water, HCO3 always boils out and precipitates. Therefore it is called temporary hardness. Water hardness in general is expressed in terms of CaCO3 regardless of which minerals (Ca, Mg, or HCO3) make it hard. The hardest water is in Arizona, while the softest – in Carolina.
Alkalinity is an important concept in water pollution. It is the measure of water’s capacity to neutralize acids. Water is considered corrosive if its alkalinity is small, and it is scale forming if its alkalinity is high. Water is stable at an alkalinity between 80-100 mg/l. Stability varies depending on the type of water and may be tested using the marble chips. The more hydrogen ions (H+) are in the water, the more acidic it is so pH is low. There are three forms of alkalinity: hydroxide ion (OH-), which can absorb one hydrogen ion, carbonate ion (CO3-)– two hydrogen ions, and bicarbonate (HCO3-) – one hydrogen ion. Hydroxide appears in water only at a pH (10, which does not exist in nature. When carbonic acid (H2CO3) is in water, it dissociated into H+ and HCO3- at a pH around 4.3-4.5. Furthermore, HCO3- dissociates into H+ and CO3-, which occurs at pH of 8.3. To measure alkalinity two types of pH change indicators are used: phenolphthalein and bromocresol green-methyl orange. If after addition of phenolphthalein the water turns pink, it means the pH is (8.3 and there is CO3- or CO3- and HCO3- in the water. And the pH is (8.3 if the water stays colorless and it needs the second titration.
Stream Pollution
Same events that occur in a lake will happen in a stream. Recovery of the stream pollution depends on the stream flow, time or passage, water temperature, and re-aeration. The most critical times of the year when streams are not able to clean themselves are periods of very low flow, high temperature, and high altitude.
Zones of Water Pollution
Discharge or mixing zone is where pollutants enter the stream or the river. If the waste is dark, it will make the water dark, increase its temperature and subsequently oxygen levels will drop. The rate of discharge should not be stronger than flow of the stream.
In zone of degradation oxygen levels drop from BOD (biological/biochemical oxygen demand).
Zone of active decomposition is characterized by the following changes:1)dissolved oxygen (DO) will be at its absolute minimum; 2) odors and gases bubbling up.
I n the zone of recovery the river will slowly restore its oxygen through aeration process. The anaerobic conditions will disappear. Nitrites will be converted into nitrates and stimulate growth of algae, which will produce oxygen through process of photosynthesis.
In zone of clean water oxygen levels are really elevated. And BOD is almost satisfied.
Turbidity – suspended particles in water that are responsible for cloudiness. The actual turbidity can be a result of two types of particles: colloidal particles and coarse dispersion. Colloidal particles do not settle due to large surface area. Coarse dispersion particles will eventually settle. Even though turbidity can create some color, water has its own color. There are two types of color: apparent color and true color. Apparent color is a result of suspended material in the water. True color will be seen only if apparent color is removed. The only way to separate these two colors is through centrifuge.
Groundwater
Bodies of water on our planet are surface waters, wetlands, estuaries, and groundwater. The quality of groundwater is based on the climate. Areas with greatest rainfall have the least mineralized water in US, such as coastlines. In dessert areas water is highly mineralized.
Sources of Groundwater Contamination
Groundwater can be contaminated from springs loaded with poisonous minerals. Hot springs water is normally very high in minerals.
Land surface origin contamination is a major source.
Surface impounds. One example is slurry manure, which will break down to NO2’s and NO3’s in water supplies. Nitrites in water are capable of blocking oxidation of hemoglobin thus causing internal suffocation. Nitrates in water are a problem for infants. Another example of surface impounds are fertilizers full of NO3’s. So there are two sources of NO3’s going into the water supplies.
Sewage – organic material, which will break down to NO2’s and NO3’s.
Sources above the water table: cesspools, septic tanks, landfills, underground storage tanks, graveyards, and underground pipe lines.
Sources bellow water table: mines, injection wells.
Contamination Control
Physical and chemical characteristics of the land itself, such as the thickness of unsaturated zone, will help minimize contamination. Processes taking place in that zone are oxidation of contaminants, their biological breakdown, and sorption (chemicals stick to soil particles).
Natural processes refer to the fact that water is moving. Once contaminants get into the water, the following may occur: filtration, ion exchange, sorption, dispersion, dilution, and oxidation.
Hydraulics migration refers to how strong and powerful is the entrance of contaminants.
Each contaminant has its own biological, physical, and chemical characteristics. This has to do with retardation or plume of the chemicals, which refers to how fast they move down the stream. The faster the chemical spreads out, the lower the concentration.
Severity of contamination is based on the "mass-flow rate", which is dictated by three individual criteria: characteristic of the waste, volume of the waste, and the size of area spilled. When there is a spill on the surface of the ground, vertical percolation occurs first. When it finally reaches the saturated zone, it tends to move horizontally. Horizontal movement is based on hydraulic gradient, filtration, sorption, chemical processes, and retardation plume. Groundwater will always flow down the hydraulic gradient.
Water Treatment
Sedimentation is a process during which water sits in huge man-made reservoirs. That process is specifically called impounding. It results in color enhancement, sedimentation of heavy materials, and oxidation of the water. Then water is taken out of sedimentation and run through fish screens to remove debris. There are two types of screens: stationary and traveling. Next water is run through microstrainer, which basically removes microorganisms.
Aeration is exposure of maximum amount of water to the maximum amount of air in the shortest period of time. Methods of aeration include building a water- fall and a nozzle method.
Coagulation is a process during which colloidal particles are brought together by the use of different coagulant aids. Types of coagulant aids utilized are alum, iron, clay, and polymers. Process of coagulation includes a very fast 20-40 sec. mix, initial destabilization of water, and formation of flocs.
Flocculation is a slower mixing process, which allows the flocs to build and get bigger in size. The flow velocity should not be faster than 0.5-1.5 ft/min. Otherwise the floc will have a chance to break up. Then water is moved into the sedimentation tank, where the floc will settle down. The best quality water is collected from the top of the tank and sent to filtration.
Chlorinating
Because of a threat of possible water contamination as it leaves the plant, some type of disinfectant must be put into the water supply. Chlorine has been proven to be very effective against bacteria entering our water supply. There are few basic types of chlorine: gas (Cl2), liquid (in the form of NaOCl), and powder (in the form of Ca(OCl2)). Reactions of chlorine with water:
1) Cl2( + H2O ( HCl- + HOCl ( H+ + OCl-
In the first step of the reaction, which occurs at pH between 4-7, hydrochloric (HCl-) and hypochlorous (HOCl) acids are formed. As the pH rises >8, the HOCl dissociates into hydrogen ion and hypochloride ion. Both HOCl and OCl- are called free available chlorine, but only HOCl is adequate as a disinfectant.
Ca(OCl2) + H2O ( a) Ca2+ +2OCl- + H2O
b) OCl- +H+ ( HOCl
When liquid or powder forms of chlorine disinfectant are used, OCl- is directly added to the water, which will pull H+ ions out of the water to form HOCl. So if chlorine gas is used, there’ll be tendency for a pH to drop. But if liquid or powder is used, the pH will go up.
The problem with chlorine is that it combines with hydrocarbons to form chlorine substitutions, which make up a group of chemicals referred to as trihalomethanes or THMs. The more oils, hydrocarbons (HCs), and organic compounds are in the water, the higher is the amount of THMs, which are carcinogenic. The standard for THMs in the water required by state is <100ppb. It was found that if chlorine combined with something before it got to HCs, it would not combine with them. Ammonia (NH3) is added to water where it combines with HOCl and forms a compound called chloramine. So instead of adding chlorine, chloramines are formed and added to the drinking water.
Biochemical Oxygen Demand
Biochemical oxygen demand (BOD) is the amount of oxygen required by bacteria while stabilizing decomposable organic material. It is a common parameter used to define the strength of pollution. The following conditions are required for a BOD test: aerobic environment, mixed population of microorganisms, temperature of 20(, and protection of the sample from air and light.
In BOD test the rate of the reaction is proportional to the waste. The test may last as long as twenty days, but that is too long if the attempt is to measure sudden impact of organic compounds in the water supply. BOD measures the breakdown of carbonatious material, which takes places during the first five days. So to eliminate other bacteria that kick in after day eight, BOD5 is measured. It was found that after five days 68-100% of BOD has been satisfied. BODU is an ultimate test that can only last for a maximum of 20 days. So BODU is always equal to BOD20. The size of the sample used for the test depends on the strength of the waste.
The dilution water added to the waste sample in the test bottle consists of ingredients that will create the best environment possible for bacteria. Nutrients, phosphate buffer, and elements controlling osmotic conditions are added to the distilled water. DOI (dissolved oxygen initial) of the dilution water is measured before the test starts. BOD is calculated using the following formula: DOI-DOF divided by ml of waste / ml of bottle sample (300ml). The BOD test determines not only how efficient the treatment is but also the magnitude of the damage that waste will do to the environment.
Types of Pumps
There are three types of water pumps: fast (3600 rpm), medium (1800 rpm), and slow (900 rpm). The type of a pump used for a house is dependent on the number of people living in it. The disadvantage of the fast pumps is that they burn out faster, through they give out a lot of water.
Formula (rpm) 2 allows us to calculate how much quicker a fast pump will wear out than a medium pump, under the exact same conditions. To use this formula, we take the fast pump rpm, divide it by the medium pump rpm and square the answer.
There are a couple of pumps known, but the one that works well is called centrifugal or suction pump. This pump is designed to operate at one atmosphere pressure equal to 14.7 psi. Under perfect conditions centrifugal pump can pump the water as high as 34 feet. But the problem is that it is impossible to create perfect conditions. Most of the centrifugal pumps operate between 60-80% efficiency, which is equivalent to about 20 feet. It cannot be used of a deep-water aquifer. It is only good for shallow aquifers. Deep well turbine pump is another type of pumps. Most wells are deep well turbines.