Topic 2: Environmental Chemistry and Microbiology

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1 Civl 141: Environmental Engineering and Management Topic : Environmental Chemistry and Microbiology Introduction Chemistry and microbiology are fundamental to and the common denominator for all environmental media. The basic concepts of chemistry and biology applied to water, air, solid waste, and industrial hygiene are required in the practice of environmental engineering. Therefore, these concepts have been consolidated in this topic to provide a foundation for the remaining topics. A. Units of Measurement Measurement Systems Engineers need to be familiar with two systems of measurement: American units (also called USCS - US Customary system) - ft, lbs etc. International System of units (SI) - essentially the metric system - metres, kilos etc. Although the SI system is preferred, both systems are still in use in Environmental Engineering and both will be used throughout this course. Definitions Understanding the chemistry required for environmental engineering need comprehension of eight essential terms. These terms and their definitions are: Molecular Weight - The sum of the atomic weights of the atoms that make up a molecule. e.g. the molecular weight of sulfuric acid (H SO 4 ) = (1) + + 4(16) = 98 atomic mass units Mole - The weight of one avogadro's number ( ) of molecules of a chemical. Moles can be in any mass units such as pound-moles or gram-moles. Valence - The number of electrons an atom or molecule accepts or donates in reaction with another molecule. Equivalent weight - The equivalent weight of a molecule is calculated as: equivalent weight = molecular weight/valence. Molarity (M) = the number of moles of solute in 1L of solution (mol/l). Normality (N) = the number of equivalents of solute in 1L of solution (eq/l) Oxidation - An atom or molecule is oxidized when it loses electrons. The substance that receives electrons is called the oxidizing agent. 1

2 Reduction - The gain of electrons by a substance. The substance that gives up the electrons is called the reducing agent. Measuring Concentrations of Pollutants Quite often we are concerned with the concentration of some substance in water or air. In either medium concentrations may be based on weight or volume of a combination of the two. Hence we need to be able to covert concentrations expressed in weight into concentrations expressed in volume and vice versa. The conversion methods for liquids and air are different. 1. Liquid conversions Concentrations of substances dissolved in water are usually expressed in terms of weight of substance per unit volume of mixture. e.g. milligrams per liter (mg/l) or micrograms per liter (µg/l). Alternatively concentrations in liquids are expressed as weight of substance per weight of mixture. e.g. parts per million (ppm) or parts per billion (ppb). To do a conversion from weight/volume from weight/weight, we take account of the fact that 1 liter of mixture weights essentially 1000 g. Hence: 1 mg/l = 1 mg/1000 g = (1g/1000) /1000 g = 1 g/10 6 g = 1 ppm [Note: Occasionally the concentrations of liquid waste may be so high that the specific gravity of the mixture is affected and a correction to the above formula needs to be applied as follows: mg/l = ppm (by weight) x specific gravity] Working in the other direction (i.e. from ppm to mg/l) and measuring the concentration of sodium chloride in water: e.g. 1 ppm NaCl in H O = 1 part NaCl/million parts H O = 1/10 6 = 1 mg NaCl/1 kg H O = 1 mg NaCl/1 L H O ρho ρso ln (dilute solution) = 1000 g/l = 1 kg/l Summary: 1 ppm (by wt.) 1m g/l 1 ppb (by wt.) 1µ g/l

3 . Air conversions In contrast to liquid measures, concentrations of substance in air are expressed as volume of pollutant per million volumes of air mixture. (e.g. ppm v or ppb v ) We need to take into account temperature and pressure with these conversions. At standard temperature & pressure (i.e. at 7 o K and 1 atm): 1 mole of ideal gas =.4 L =.4 x 10 - m To go from volume/volume to weight/volume e.g. 1 ppm (by vol.) =? mg/m mg/m ppmv mol. wt. ( g / mol) 10 = ( m / mol) 1 Adjusting volume (m ) for different pressure and temperature: mg g PV 1 1 T 1 PV PV 1 1T = V = =? T PT 1 m where T (temp. expressed in Kelvins) = C + 7 P (pressure expressed in atm.) The concentration of carbon dioxide (CO ) is 60 ppm. Express it in mg/m at 1 atm pressure and 0 o C. V ( )( + $ ) PV T ( 1 atm) m 7 0 K = 111 = PT 1 ( 1 atm)( 7 K) = m CO in mg / m 60 gmol mg = = m mol 1g mg / m. Concentrations in terms of a common constituent The compounds of nitrogen are of great interest to the wastewater chemist because of the importance of nitrogen in life processes. The chemistry of nitrogen is complex. In environmental testing, nitrogen is splitted into three categories: organic nitrogen (C-C-C-NH ), ammonia nitrogen (NH ), and inorganic - nitrogen (nitrites, NO and nitrates, NO - ). The sum of the concentration of organic nitrogen and ammonia nitrogen is called Total Kjeldahl Nitrogen. Nitrogen concentrations are designated in terms of the weight of nitrogen atoms in the sample. Regardless of which form of nitrogen is being considered, the concentration is stated in this way: 10 mg/l NO as N or 10 mg/l NO -N

4 This means that the ion being measured is nitrate, but every liter of this solution has 10 mg/l of N in it. The weight of the oxygen isn't being counted. 14 e.g. 0 mg/l NH = 0 = 4. 7 mg/l NH - N (ammonia-nitrogen) mg/L NO - = = 0.0 mg/l NO - - N (nitrite-nitrogen) mg/l NO = = 0.4 mg/l NO - - N (nitrate-nitrogen) B. Stoichiometric Chemical Equations Stoichiometry The stoichiometric or balanced chemical equation describes the equilibrium ratios of a chemical reaction. From the balanced equation, the same number of each kind of atom appears on each side of the equation. Thus, the stoichiometry chemical equation contains both qualitative and quantitative data. The subsequent calculations which can be used to determine the amount of each compound involved is known as Stoichiometry. Ferric chloride and lime are being used to enhance coagulation/flocculation in a water treatment plant. The stoichiometric reaction is as follows: FeCl + Ca( OH) Fe( OH) () s + CaCl Based on moles, this balanced equation states that moles of ferric chloride react with moles of calcium hydroxide to form moles of ferric hydroxide and moles of calcium chloride. Find the quantity of calcium hydroxide required to remove magnesium from water when the magnesium is in the form of magnesium sulfate. The concentration of magnesium sulfate in the water is 1.0 mg/l. MgSO 4 + CaOH ( ) MgOH ( ) + CaSO ( S) 4 1 mole of Ca(OH) is required to remove 1 mole of MgSO 4 symbol 9 \f "Symbol" \s 10}the quantity of Ca(OH) required = 10. mg L 1 mole MgSO 4 74 g = g 1 mole Ca( OH) mg L 4

5 In fact, this example can be interpreted as follows: 1.0 mg/l of magnesium sulfate requires 0.61mg/L of calcium hydroxiode to complete the reaction and yields 0.48 mg/l of a magnesium hydroxide (precipitate) and 1.1 mg/l of calcium sulfate. C. Chemical Reaction Equilibrium Reactions In any chemical reactions such as one expressed as: aa + bb cc + dd there are some assumptions inherent in expressing the reaction in this form. First, nearly all reactions are reversible to some extent. Second, the above equation depicts a reaction where the reactants and products will reach an equilibrium (i.e. the rate of formation of products will be equal to the reverse reaction). In equilibrium, we can write that [ ] [ ] [ ] [ ] C c D d A a B b = K where the [ ] denotes the concentration of a substance in equilibrium expressed in mole/l. K is called the equilibrium constant. carbonate-bicarbonate equilibrium system HCO HCO + H + K 1 = at 5 C HCO CO + H + K = Where K 1, K are named as dissociation constants Acid-Base Reactions Acid-base reactions, perhaps the most important class of chemical equilibria, are particular important in water chemistry. An acid is a substance having the tendency to lose or donate a proton (H + ). A base is a substance having the tendency to add or accept a proton. For example, HCL + H O H O + + Cl - HCl is an acid because it can donate a proton (H + ) to the base (H O) which can accept the proton to become H O +. H O H + +OH - K w = at 5 C [H + ] [OH - ] = K w =

6 ph = -log[h + ] Precipitation Reactions solid aa + bb A a B b = Ksp ( solubility product) solid [ ] [ ] [ ] where [solid]=1 Forming insoluble precipitates by the addition of chemicals is the most common method of shifting equilibrium. Examples are the use of coagulants such as sluminum sulfate to remove turbidity and the use of lime to precipitate the multivalent ions of hardness. e.g. Al (SO 4 ) 18 H O + Ca(HCO ) Al(OH) (s) + CaSO H O + 6 CO e.g. Ca(OH) + Mg(HCO ) Mg(OH) (s) + CaCO (s) + H O Gas-Liquid Transfer Many situations encountered in environmental science and engineering involve the transfer of gases into and out of liquids. For example, the aeration of rivers and lakes involves the transfer of oxygen from the air to the water, thus supplying the dissolved oxygen essential for fish and many forms of aquatic life. The degree of solubility of a gas in a liquid depends on the kind of gas it is, the nature of the liquid, the pressure, and the temperature. Henry's law states that the weight of any gas that will dissolve in a given volume of a liquid at constant temperature is directly proportional to the pressure that the gas exerts above the liquid. There are three kind of expressions of Henry's equations: expression (1): C g = H C L expression (): X = K H P g mass vol where H gas / & K H (atm -1 ) are Henry law constant but expressed in different units. mass / vol Liq C g : concentration of a substance in gas phase (mg/l) C L : concentration of a substance in liquid phase (mg/l) X: mole fraction of the gas in liquid (dimensionless) P g : partial pressure of the gas in air (atm) 6

7 D. Materials Balance and Reaction Kinetics When chemical reactions take place, matter is neither created nor destroyed. This concept allows us to track pollutants from one place to another with mass balance equations. These are fundamental to understanding the flow of materials. Mass Balance Analysis: Accumulation = Input rate - Output rate ± Change If the change of a substance in a system is due to a decay, then "Change" = - "Decay rate" Any system can be classified as "Steady state or non-steady state" and "Conservative or nonconservative". Thus, the following four cases will usually be used to define a system: Case (a): Steady state, conservative system Case (b): Steady state, non- conservative system Case (c): Non-steady state, conservative system Case (d): Non-steady state, non-conservative system Reaction Kinetics: The rate at which reactant A plus reactant B to yield product C is of major concern in all environmental applications since the time required for a reaction determines the detention time, which dictates the size of the reactor and ancillary equipment. A reaction that occurs at a rate that unrelated to the concentration of any of the products or reactants is referred to as a zero-order reaction. That is dc = k (kinetic rate constant) dt However, the form of kinetics most often seen in environmental engineering, for both chemical and biological reactions, is a first-order reaction. That is dc = kc. dt 7

8 E. Environmental Microbiology Classical Microbiology vs. Environmental Microbiology Classical Microbiology: Studying different groups of microorganisms and their activities with emphasis on their internal cellular structure, evolutionary relatedness, mode of reproduction, physiology, metabolism and identification. Environmental Microbiology: It is a branch of applied microbiology which deals with the use of microbial principle and technique to achieve a comfortable and healthy environment for human beings. Some common examples in the field of Environmental Microbiology include: application of microorganisms in the purification of wastewater use of right kind of technologies to destroy pathogens in water use of some microorganisms as indicators to determine the safety of a water sample for human consumption use of microorganisms to convert organic matter in wastewater to useful fuel (e.g., methane gas), etc. Therefore, in order to apply Environmental Microbiology effectively, an individual should have a good knowledge in engineering as well as in microbiology. The following Table 1 shows some bacteria of significance in the environment. Table 1: Some Bacteria of Significance in the Environment Group of bacteria Genus Environmental significance Pathogenic bacteria Salmonella Cause typhoid fever Shigella Cause Dysentery Mycobacterium Cause tuberculosis Indicator bacteria Escherichia Fecal pollution Enterobacter Streptococcus Clostridium Decay bacteria Pseudomonas Degrade organics Flavobacterium Degrade proteins Zooglea Floc-forming organism in activated sludge plants Clostridium Produce fatty acids from organics in anaerobic digester Micrococcus Produce fatty acids from organics in anaerobic digester Methanobacterium Produce methane gas from fatty acids in anaerobic digester Methanococcus Produce methane gas from fatty acids in anaerobic digester Methanosarcina Produce methane gas from fatty acids in anaerobic digester Nitrifying bacteria Nitrobacter Oxidize inorganic nitrogenous compounds Nitrosomonas Denitrifying bacteria Bacillus Reduce nitrate and nitrite to nitrogen gas or nitrous oxide Pseudomonas Reduce nitrate and nitrite to nitrogen gas or nitrous oxide Nitrogen-fixing bacteria Azotobacter Capable of fixing atmospheric nitrogen to NH Beijerinckia Sulfur bacteria Thiobacillus Oxidize sulfur and iron Sulfate-reducing bacteria Desulfovibrio Involved in corrosion of iron pipes Photosynthetic bacteria Cholorbium Reduce sulfides to elemental sulfur Chromatium Iron bacteria Filamentous Sphaerotilus Responsible for sludge bulking in activated sludge plants Iron oxidizing Leptothrix Oxidize ferrous iron 8

9 Water Quality Indication Many forms of microbial life can exist in water provided that the appropriate physical and nitritional requirements for growth are met. For example: Dissolved oxygen is necessary for the growth of aerobic bacteria and protozoa Nitrogen and phosphorus as well as light are essential to algae The number and types of microorganisms present give an indication of water quality. For example: In clean water or water with a low nutrient content, the total number of microorganisms is limited, but a great variety of species can exist As the nutrient content increases, the number of microorganisms increases while the number of species is reduced In polluted (low oxygen) stream, a few species of anaerobic or facultative bacteria will predominate. Typical numbers of bacteria for various waters are presented in Table. Table : Typical Bacterial Counts in Water Source Bacteria per 100 ml Coliform bacteria per 100 ml Tap water Clean, natural water Polluted water Raw sewage The Use of Escherichia Coli. as Indicator Organism Water used for drinking and bathing can serve as a vehicle for the transmission of a variety of human enteric pathogens that cause waterborne (illness-causing) diseases. The detection of pathogens in water is difficult, uneconomical, and impractical in routine water analyses. Instead, water is tested using a surrogate that is an indicator of fecal contamination. The reasons to choose fecal organisms as indicator bacteria are because the fecal organisms will survive slightly longer than the pathogens and are present together with any pathogenic organisms. Therefore, (1) if fecal organisms are present, pathogens are likely to be present; () the density of the indicator organisms (i.e., fecal organisms) is related to the probability of the presence of pathogens. Escherichia coliform (E. coli.) is the most frequent and predominant type of fecal organism (or fecal coliform) found in the human intestine, while fecal streptococci is more plentiful in animals than in humans. The ratio of fecal coliforms and fecal streptococci (FC/FS ratio) is used to differentiate the source of pollution. With a ratio of 4.0 or more, the pollution is considered to be from human wastes, whereas ratios below 0.7 indicate pollution from animals wastes. The enumeration of the bacteria indicators is carried out by a method, namely, the multiple-tube fermentation technique, also called the most probable number (MPN) procedure. 9

Aquatic Chemistry (10 hrs)

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