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Water is essential to life; therefore, providing access to safe and clean water is a major concern for health development at the local, national, and regional levels. Enhancing water quality by improving sanitation systems needs a profound understanding of water chemistry and microbiology.
This chapter will give an overview of water chemistry and microbiology. The chapter will be divided into two main parts: the first one will discuss water chemistry in terms of its physical properties and the main chemical processes that takes place within water bodies, the main parameters that govern the water quality, and water quality management. This section will finally mark out the basics of water treatment processes. The second part will be devoted to water microbiology where the main concepts that are encountered when discussing water microbiology will be outlined. An overview will be given of the different types of microorganisms and their classification as well as the main processes they carry out in living cell such as metabolism, respiration, and microbial growth. A brief synopsis will be depicted about the main diseases caused by microorganisms that could be present in water bodies, the main microbial indicator, as well as the main analytical tests for microbial indicators. This section will also give a short overview about microbial control and the terms related to it such as disinfection. The final part of this section will delineate the role of microorganisms in wastewater treatment plants.
Keywords: Water chemistry, Water quality, Quality management, Microbiology, Metabolism, Bacterial growth, Pathogens, Microbial control, Microbial indicators, Biological wastewater treatment
Water is a simple molecule that has only three atoms, but its unique properties make it the most important molecule in life. This extraordinary importance of water molecule came basically from the arrangement of atoms in the molecule and the bonds involved.
All life processes involve water. Water is an excellent solvent for many ionic compounds, as well as for other substances capable of forming hydrogen bonds with water.
Water molecule with molecular formula H2O has two hydrogen atoms and one oxygen atom; the oxygen atom is connected to the two hydrogen atoms by two covalent bonds ( Fig. 1 ). This bond is a polar covalent bond with concentration of the electron density around oxygen which is one of the most electronegative atoms. This arrangement gives water molecule a dipolar character where oxygen atom is the negative pole and hydrogen atoms are the positive poles. This dipole character enables water molecules to be connected among themselves by intermolecular forces called hydrogen bonds. Hydrogen bonds are a relatively strong type of intermolecular interaction in which hydrogen atom from a certain molecule is connected to an electronegative atom (such as oxygen, nitrogen, or fluorine) of another molecule.
Water molecule and hydrogen bonds shown by the dashed lines.
Although many compounds can form hydrogen bonds (such as NH3 and HF), the difference between H2O and other polar molecules is that each oxygen atom can form two hydrogen bonds, the same as the number of lone electron pairs on the oxygen atom. Thus, water molecules are joined together in an extensive three-dimensional network in which each oxygen atom is approximately tetrahedrally bonded to four hydrogen atoms, two by covalent bonds and two by hydrogen bonds. This confers on water its unique physical and chemical properties that will be discussed in the next sections (Chang, 2010; Ebbing and Gammon, 2009; Manahan, 2000).
Table 1 summarizes some of the basic physical properties of water. Most of these properties are highly affected by temperature. Therefore, some values are given at different temperatures. Also the effect of these properties on water behavior is highlighted briefly (Chang, 2010; Ebbing and Gammon, 2009; Manahan, 2000; Venkateswariu, 2003).
Physical Properties of Water
| Property | Value | Temperature (°C) | Effect and Significance |
|---|---|---|---|
| Density | 0.9999 | 0 | Solid form is less dense than its liquid form; ice floats at the surface of liquid water |
| 1.000 g/mL | 3.98 | ||
| 0.9971 g/mL | 25 | ||
| 0.9584 | 100 | ||
| Viscosity | I. 79 cP | 0 | Hot molasses flows much faster than cold molasses |
| 0.89 cP | 25 | ||
| 0.28 cP | 100 | ||
| Surface tension | 75.6 dyn/cm | 0 | Water bugs (striders) seem to skitter across this skin as if ice-skating. You can actually float a pin on water, if you carefully lay it across the surface |
| 72.5 dyn/cm | 25 | ||
| 58.9 dyn/cm | 100 | ||
| Dipole moment | 1.8546 D | Reflects the polarity of water molecule | |
| Specific heat | 1 cal/g/°C | Water can absorb a substantial amount of heat while its temperature rises only slightly | |
| Latent heat of fusion | 80 cal | 0 | |
| Latent heat of vaporization | 540 cal | 100 | |
| Boiling point | 100°C at 1 atm |
Before discussing the chemical properties of water we shall introduce some terms that will be encountered a lot in the coming sections.
A solution is a homogeneous mixture of two or more substances, consisting of ions or molecules. Solute is the component present in smaller amount and solvent is the component in greater amount.
The general term concentration refers to the quantity of solute in a standard quantity of solution.
Concentration might have different expressions depending on the unit of the amount of solute and the unit the amount of solution or solvent. When the solvent is water, it is called aqueous solution.
The molar concentration or molarity (abbreviated M) of a solution is the number of moles of the solute species that is contained in 1 L of the solution. It has the dimensions of mol/L. Percent concentration is another way of expressing concentration. Percent composition can be expressed as mass percent (w/w %) which is the percentage of the mass of solute over the mass of solution. Or simply concentration could be expressed in units of mass per volume (i.e., g/L, mg/L).
We can divide the substances that dissolve in water into two broad classes, electrolytes and nonelectrolytes. An electrolyte is a substance that dissolves in water to give ions that make the solution an electrically conducting solution such as sodium chloride. A nonelectrolyte is a substance that dissolves in water to give a nonconducting solution because it does not form ions such as sucrose, table sugar (Chang, 2010; Ebbing and Gammon, 2009).
There are principally three types of chemical reactions that could take place in water: precipitation, acid/base, and oxidation–reduction reaction.
In precipitation reaction, dissolved ions react with each other and form a solid compound or precipitate. A typical precipitation reaction that occurs in water is the formation of calcium carbonate solid when solution of calcium is mixed with solution of carbonate (Davis and Cornwell, 2012).
Ca 2 + aq + CO 3 2 − aq → CaCO 3 sMany definitions are available for acids and bases. However, for the purpose of simplicity here, acids can be defined as proton donor and bases as proton acceptor. A neutralization reaction is a reaction of an acid and a base that results in an ionic compound and possibly water. When a base is added to an acid solution, the acid is said to be neutralized. The ionic compound that is produced in a neutralization reaction is called a salt (Chang, 2010; Ebbing and Gammon, 2009). The net equation for most acid base reaction is:
H + aq + OH − aq → H 2 O lOxidation–reduction reaction is a reaction in which electrons are transferred between species or in which atoms change oxidation number. The substance that loses the electron is said to be oxidized and it is called a reducing agent. While the substance that accept the electron is said to be reduced and is called oxidizing agent. When one species release electron, another one should be available to accept the electron. Iron pipe corrosion is an example of oxidation–reduction reaction in which iron metal oxidizes and loses two electrons, while hydrogen ions accepts those electrons and reduces to hydrogen gas (Chang, 2010; Davis and Cornwell, 2012; Ebbing and Gammon, 2009)
Fe → Fe 2 + + 2e − 2H + + 2e − → H 2 gThe pH of a solution is a measure of hydronium ion (H3O + ) concentration, which is, in turn, a measure of acidity. In acidic solutions the pH is smaller than 7, while it is greater than 7 in basic solutions. The pH range in water samples is rarely below 4 or above 10. Determining pH of water is essential as it affects many of the chemical and biological processes that take place in water (Davis and Cornwell, 2012; Venkateswariu, 2003).
Alkalinity refers to the capability of water to neutralize acid. It is defined as the sum total of all titratable bases down to about pH 4.5. It is found experimentally by determining how much acid it takes to lower the pH of water to 4. In most waters the only significant contribution to alkalinity is the carbonate species and any free H + or OH − . The total H + that can be taken up by water containing primarily carbonate species is
alkalinity = HCO 3 − + 2 CO 3 2 + + OH − – H +In most natural water situation (pH 6–8) the OH − and H + are negligible; therefore,
alkalinity = HCO 3 − + 2 CO 3 2 +Note that CO 3 2 − is multiplied by 2 because it can accept two protons
H 2 CO 3 → H + + HCO 3 − → p K a1 = 6.35 HCO 3 − → H + + CO 3 2 − → p K a2 = 10.33From this we can see that:
below pH 4.5 all carbonates are H2CO3 and the alkalinity is negative pH 7.5–8.3 all carbonates are HCO 3 − and alkalinity equals HCO 3 − above pH 11.5 all carbonates are CO 3 2 − and alkalinity equals 2 CO 3 2 + + OH −By convention alkalinity is not expressed in molarity units as shown in the above equations, but rather as mg/L as CaCO3 (Davis and Cornwell, 2012; Manahan, 2000).
Hardness is a term usually used to characterize water. Technically, it is the sum total of all polyvalent cations. Practically it is the sum of calcium and magnesium ions which are predominant cations in natural waters. Hard water requires more soap and synthetic detergent for laundry and contributes to scaling in boilers and industrial equipment.
Hardness is expressed in mg/L as CaCO3, as in alkalinity. Both calcium and magnesium have a valence of two when converting to CaCO3. The sum of calcium and magnesium is the total hardness (TH), which is subdivided to carbonate and noncarbonated hardness.
Carbonate hardness is often called temporary hardness because heating the water will remove it. When water is heated, the insoluble carbonates precipitate and tend to form bottom deposits in hot water heaters. Carbonate hardness is equal to the total hardness or alkalinity, whichever is less.
Noncarbonated hardness is correspondingly called permanent hardness because it is not removed when water is heated. Noncarbonated hardness is the total hardness in excess of the alkalinity. If the alkalinity is equal to or greater than the total hardness, there is no noncarbonated hardness (Davis and Cornwell, 2012; Manahan, 2000).
The cyclic movement of water through the environment is called the hydrologic cycle. The global hydrological cycle is powered by solar energy. It is a closed system, in continual circulation.
The hydrologic cycle, illustrated in Fig. 2 , begins as water moves from the ocean's surface into the air above through evaporation. Water evaporates into the atmosphere and forms clouds above the ocean. Additional water is drawn from the soil by plants, and is then evaporated into the atmosphere from leaves and stems in a process called transpiration. As the air rises and the temperature drops, the moisture-laden air condenses, forming clouds and eventually resulting in precipitation. Precipitation is the term applied to all forms of moisture falling to the ground such as rain, snow, sleet, and hail.
The hydrologic cycle.
In some cases, the raindrops soak into the earth and move slowly into the groundwater. Sometimes it runs off the land surface and moves quickly in a swift-flowing stream. Other times, the raindrop rests in deep river pools or lakes, is taken up by plants and animals, or enters the atmosphere again through evaporation. Other water seeps downward into the soil. This process is called infiltration. If the rock below the soil is permeable, then the water percolates the rock and is stored as groundwater. Ultimately, all water makes its way back to the ocean, which is like a giant reservoir. Water is stored in the ocean until it is delivered to the land as a result of evaporation and precipitation and the cycle continues (Davis and Cornwell, 2012; Manahan, 2000; Vigil, 2003).
According to the United States Geological Survey (Winter et al., 1998), most of the fresh water (84.9%) is locked up as ice in glaciers. Of the balance, 14.16% constitutes groundwater, while that in lakes and reservoirs mounts to 0.55%. Another 0.33% is in form of soil moisture and atmospheric water vapor. Thus, only a very small fraction of fresh water, about 0.004%, flows through rivers and streams. The volume of sea water is 15 times greater than that of fresh water.
Natural waters can be classified into two categories: sea water (inclusive of estuarine water) and fresh water.
Surface waters might possess color, odor, taste, suspended solids, etc. Groundwaters are expected to be free from organic odor and have a relatively less variable composition at the same source.
The United States Geological Survey (Venkateswariu, 2003; Winter et al., 1998) has classified different waters on the basis of their total dissolved solids (TDS) content as given in Table 2 .
Water Quality vs Total Dissolved Solids (Venkateswariu, 2003; Winter et al., 1998)
| Water Quality | TDS (mg/L) |
|---|---|
| Fresh | Less than 1,000 |
| Slightly saline | 1,000–3,000 |
| Moderately saline | 3,000–10,000 |
| Very saline | 10,000–35,000 |
| Briny | Greater than 35,000 |
Many diverse factors have to be taken into account before making comments on water quality. For this reason, the concentrations of inorganic and organic substances dissolved in a body of water and their amount need to be monitored regularly. Agents that alter water quality can be classified under four major categories.
Physical properties of water are related to the appearance of water, namely, the color, temperature, turbidity, taste, and odor. To be suitable for use, water must be free from all impurities that are offensive to the sense of sight, taste, or smell and one very important physical characteristic that should be encountered when discussing water quality is turbidity (Davis and Cornwell, 2012).
The presence of suspended materials such as clay, slit, finely divided organic material, plankton, and other inorganic materials in water is called turbidity. Turbidity is a measure of the clarity of water. Low-turbidity water is clear, while high turbidity water is cloudy or murky. The unit of measuring turbidity is turbidity unit (TU). Turbidity larger than 5 TU is easily detected in a glass of water and is objectionable for aesthetic reasons (Davis, 2010; Davis and Cornwell, 2012).
Those include all the major dissolved constituents in water such as humic substances, in addition to the minor constituents such as heavy metals, detergents, pesticides.
The chemical analysis of a domestic water supply should ordinarily include the determination of water hardness, alkalinity, pH, conductivity, and the presence of chloride, sulfate, and nitrate. Other chemicals of importance are: iron, manganese, fluoride, copper, sodium, and zinc, in addition to some toxic substances such as arsenic, barium, cadmium, chromium, lead, mercury, selenium, silver, and cyanides.
The significant concentrations with respect to several chemicals that might be present in natural waters are given in Table 3 . Above these levels, such chemicals can cause undesirable effects (Davis, 2010; Davis and Cornwell, 2012; Venkateswariu, 2003).
Chemical Constituents of Significance in Natural Waters (Venkateswariu, 2003; Winter et al., 1998)
| Chemical Constituent | mg/L |
|---|---|
| Bicarbonate Carbonate Calcium | 150–200 |
| Magnesium | 25–50 |
| Sodium | 60 (Irrigation) 20–120 (Health) |
| Iron | Less than 3 |
| Manganese | Less than 0.05 |
| Chloride | 250 |
| Fluoride | 0.7–1.2 |
| Sulfate | 300–400 (Taste) 600–1000 (Laxative action) |
The American Water Works Association (AWWA) has issued its set of goals with respect to major physical and chemical agents. These goals are shown in Table 4 .
Water Quality Goals According to the AWWA
| Agent (Property) | Goal, mg/L |
|---|---|
| Turbidity | < 0.1 turbidity units (TU) |
| Color | < 3 color units |
| Odor | None |
| Taste | None objectionable |
| Aluminum | < 0.05 |
| Copper | < 0.2 |
| Iron | < 0.05 |
| Manganese | < 0.01 |
| Total dissolved solids (TDS) | 200.0 |
| Zinc | < 1.0 |
| Hardness | 80.0 |
Biological agents are very important due to their relation to public health and may also be significant in modifying the physical and chemical characteristics of water. Water for drinking and cooking should be free from disease-causing organisms (pathogens). These organisms include bacteria, protozoa, viruses, fungi, and helminthes (worms) (Davis and Cornwell, 2012).
This factor must be considered in areas where there is a possibility that water comes in contact with radioactive substances. Radioactive substances could find their way to drinking water from atomic energy power sources and the mining of radioactive materials, as well as naturally occurring radioactive materials (Davis, 2010; Davis and Cornwell, 2012).
Seasonal variations in the quality of some surface waters could be large enough to make the use of such waters more problematic. The self-purification capacity and the water intake structure are also important factors that affect the quality of water. Whatever might be the quality of water available to a user, it can certainly be upgraded by properly designed and executed treatment procedures.
Governments with the aid of organizations such as Environmental Protection Agency (EPA) and World Health Organization (WHO), as well as local water authorities have set specifications for water. These specifications vary according to the intended use of water (for drinking, agriculture, industry, or disposal wastewater into the aquatic system or landfills). Water specifications lay down the maximum contamination levels or maximum permissible level with respect to several chemicals and pathogens that might affect water quality in order to meet minimum requirement for protection of public health (Davis, 2010; Davis and Cornwell, 2012; Weiner and Matthews, 2003).
Water pollutants are categorized as point source or nonpoint source. When water pollution arises from a single source, this is called point source pollution (an example would be chemicals from a single factory). Conversely, when pollution affecting a body of water issues from multiple sources (multiple factories), it is called nonpoint source pollution. Point source pollutants are all dry-weather pollutants that enter watercourses through pipes or channels. Point source pollution comes mainly from industrial facilities and municipal wastewater treatment plants. Storm drainage, agricultural runoff, construction sites, and other land disturbances are considered nonpoint source pollution (Weiner and Matthews, 2003).
The range of pollutants is vast. The major pollutants that could affect water quality are overviewed here.
Oxygen-demanding materials might be discharged from municipal wastewater treatment plants, food-processing plants, breweries, as well as paper mills, compose one of the most important types of pollutants because these materials decompose in the watercourse, and can deplete the water of dissolved oxygen.
Sediments and suspended solids (SS) may also be classified as a pollutant. Sediments consist of mostly inorganic material washed into a stream as a result of land cultivation, construction, demolition, and mining operations. Sediments interfere with fish spawning because they can cover gravel beds and block light penetration, making food harder to find. Sediments can also damage gill structures directly, smothering aquatic insects and fishes. Organic sediments can deplete the water of oxygen, creating anaerobic conditions, and may create unsightly conditions and cause unpleasant odors.
Nutrients, mainly nitrogen and phosphorus, can promote accelerated eutrophication, or the rapid biological aging of lakes, streams, and estuaries. Phosphorus and nitrogen are common pollutants in residential and agricultural runoff, and are usually associated with plant debris, animal wastes, or fertilizer.
Heat may be classified as a water pollutant when it is caused by heated industrial effluents or from alterations of stream bank vegetation that increase the stream temperatures due to solar radiation. Heated discharges may drastically change the ecology of a stream or lake. Although localized heating can have beneficial effects like freeing harbors from ice, the ecological effects are generally harmful. Heated effluents lower the solubility of oxygen in water because gas solubility in water is inversely proportional to temperature, thus reducing the amount of dissolved oxygen available to oxygen-dependent species. Heat also increases the metabolic rate of aquatic organisms (unless the water temperature gets too high and kills the organism), which further reduces the amount of dissolved oxygen because respiration increases.
Synthetic chemicals and pesticides, herbicides, fertilizers, pharmaceutical substances such as antibiotics, cosmetics and personal care products, detergents, toxic chemicals, and heavy metals can adversely affect aquatic ecosystems as well as making the water unusable for human contact or consumption. These compounds may come from municipal wastewater, industrial effluents, or agricultural and urban runoff.
Pathogenic microorganisms are important pollutants that directly affect human health. Water-borne pathogen contamination in ambient water bodies and related diseases are a major water quality concern throughout the world. Water-borne diseases (i.e., diarrhea, gastrointestinal illness) are caused by various bacteria, viruses, protozoa, algae, and fungi.
A major pathogen is fecal coliform bacteria (i.e., Escherichia coli) that is the bacteria that normally live in the intestinal tract of warm-blooded animals and indicate contamination by animal wastes. Other bacterial pathogens include Vibrio cholera which cause cholera, and Shigella and Salmonella that cause dysentery. Other types of microorganisms that could contribute to biological water pollution are: protozoa (such as Cryptosporidium parvum, Giardia lamblia, Entamoeba histolytica that cause diseases such as Cryptosporidiosis, Giardiasis, and Amoebiasis); viruses such as Coronavirus, Hepatitis A virus (HAV) that cause Hepatitis A, and Poliovirus which cause Poliomyelitis; algae such as Desmodesmus armatus that cause desmodesmus infection; and several fungi such as Aspergillus which most frequently affects the lungs. Some higher organisms such as nematodes could be present in water and lead to water-borne disease (Tortora et al., 2010).
Such species can be introduced into water bodies as the result of municipal and industrial wastewater discharges, or as a result of aquaculture activities. In addition to causing diseases, the presence of these organisms in water could alter the original microbial floral community in those water bodies.
Oil pollution can result from leak out of oil from huge tanker loaded with crude oil and cause water pollution with petroleum compounds.
Acids and bases from industrial and mining activities can alter the water quality in a stream or lake to the extent that it kills the aquatic organisms living there, or prevent them from reproducing. Sulfur-laden water leached from mines, including old and abandoned mines as well as active ones, contain compounds that oxidize to sulfuric acid on contact with air (Davis and Cornwell, 2012; Manahan, 2000; Weiner and Matthews, 2003).
Oxygen-demanding materials and nutrients are pollutants of significant importance; they deserve particular focus and will be the subject of discussion in the following sections.
Dissolved oxygen (DO) refers to the level of free, noncompound oxygen (O2) dissolved in water or other liquids. The bonded oxygen in water (H2O) is in a compound and does not count toward dissolved oxygen levels. DO is an important parameter in assessing water quality because of its influence on the organisms living within a body of water. Oxygen gets into water by diffusion from the surrounding air, by aeration, or as a waste product of photosynthesis. DO is essential to the survival of organisms in a stream. The presence of oxygen is a positive sign and the absence of oxygen is a sign of severe pollution. Waters with consistently high dissolved oxygen are considered to be stable aquatic systems capable of supporting many different kinds of aquatic life (Davis and Cornwell, 2012; Weiner and Matthews, 2003).
Anything that can be oxidized in the receiving water with consumption of molecular oxygen is termed oxygen-demanding material. These materials are usually biodegradable organic compounds but also include some inorganic compounds. The consumption of DO poses a threat to higher forms of aquatic life. The critical level of DO varies greatly among species. For example, brook trout may require 7.5 mg/L of DO, while crab may survive at 3 mg/L. Oxygen-demanding materials in domestic sewage come primarily from human waste and food residue. Almost all naturally organic matters such as animal droppings, crop residue, or leaves, which get into water from nonpoint sources, contribute to the depletion of DO (Davis, 2010; Davis and Cornwell, 2012; Weiner and Matthews, 2003).
Biochemical oxygen demand (BOD) is the most commonly used method for measuring the quantity of organic oxygen-demanding materials. BOD is a measure of the quantity of oxygen used by aerobic microorganisms (need molecular oxygen for living) in the oxidation of organic matter present in a given water sample at certain temperatures over a specific time period. The BOD value is most commonly expressed in milligrams of oxygen consumed per liter of sample during a prescribed period of incubation at 20°C and is often used as an indicator of the degree of organic pollution of water.
Natural sources of organic matter include plant decay and leaf fall. However, plant growth and decay may be unnaturally accelerated when nutrients and sunlight are overly abundant due to human influence. Urban runoff carries pet wastes from streets and sidewalks; nutrients from lawn fertilizers; and leaves, grass clippings, and paper from residential areas, which increase oxygen demand. Oxygen consumed in the decomposition process robs other aquatic organisms of the oxygen they need to live. Organisms that are more tolerant of lower dissolved oxygen levels may thrive in a diversity of natural water systems including aerobic bacteria. Most of them feed on dead algae and other dead organisms and are part of the decomposition cycle. Algae and other organism can produce oxygen through photosynthesis using the energy of sun. At night they use up oxygen in respiration. Therefore, there is a continuous competition between the consumption of oxygen and its production through photosynthesis. The rate of oxygen consumption and therefore BOD is affected by a number of variables: temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water (Davis and Cornwell, 2012; Weiner and Matthews, 2003).
The most commonly used test for BOD is called BOD5.This test provides a measure of the oxygen consumed in the biological oxidation of organic material in a sample at 20°C over a period of 5 days. In the 5-day BOD test (BOD5), the sample of water is suitably diluted (in case of wastewater samples) with well-oxygenated water and an inoculum of microorganisms is introduced. The initial oxygen concentration is measured and the sample stored in darkness (to avoid photosynthetic oxygen generation) at 20°C for 5 days. Over the 5-day period, the oxygen concentration is measured at regular intervals and at the end of the 5-day period. The difference in oxygen concentration between the beginning and the end of the 5-day test period, taking due account of dilution, gives the BOD5 (American Public Health Association (APHA), 2012). The concentration of oxygen could be measured using the traditional Winkler titration method (APHA, 2012) or is done automatically with modern BOD apparatus equipped with manometer and DO sensor. BOD5 method is the most widely used method for assessment of water quality (Weiner and Matthews, 2003).
The amount of dissolved oxygen used by microorganisms in order to oxidize organic material in the absence of nitrification is called carbonaceous BOD or CBOD. However, additional amounts of dissolved oxygen could be depleted from the sample due to nitrification process, that is, the conversion of ammonia (NH3) to nitrate ( NO 3 − ) by nitrogenous bacteria which is called nitrogenous BOD or NBOD.
The forms of nitrogen in urine and proteins (urea, uric acids, ammonia, amino acids, and nitrates) are nutrients for algae and aquatic plant growth. The nitrogenous waste in municipal and industrial sewage is used by autotrophic bacteria, and they use a significant amount of oxygen as an energy source and convert ammonia to nitrates. The described BOD5 test in Section 1.7.5 gives value for both carbonaceous and nitrogenous BOD. Thus, if we wish to determine the value of CBOD only, nitrification suppressor or inhibitor such as ATU (Allylthiourea) should be added (Davis and Cornwell, 2012).
Another closely related test to BOD is the chemical oxygen demand (COD) test which is also considered as an important indicator of water quality. COD is a measurement of the oxygen required to oxidize soluble and particulate organic matter in water. This test gives the electron donating capacity of practically all the organic compounds in the sample, biodegradable or nonbiodegradable and soluble or particulate.
The COD of a wastewater, in general, is greater than the BOD5 because more compounds can be oxidized chemically than can be oxidized biologically.
The method involves using a strong oxidizing chemical, potassium dichromate Cr 2 O 7 2 − , to oxidize the organic matter in solution to carbon dioxide and water under acidic conditions.
The sample is then digested for approximately 2 h at 150°C. The amount of oxygen required is calculated from the quantity of chemical oxidant consumed (APHA, 2012; Davis, 2010).
Nitrogen and phosphorus are important elements that could be present in water and affect water quality fundamentally since they are essential nutrients for biological growth.
Nitrogen occurs in five major forms in aquatic environments: organic nitrogen, ammonia, nitrite, nitrate, and dissolved nitrogen gas.
Ammonia is one of the intermediate compounds formed during biological metabolism and, together with organic nitrogen, is considered an indicator of recent pollution.
Aerobic decomposition of organic nitrogen and ammonia eventually produces nitrite ( NO 2 − ) and finally nitrate ( NO 3 − ). Therefore, high nitrate concentration may indicate that organic nitrogen pollution occurred far enough upstream that the organics have had time to oxidize completely. Similarly, nitrate may be high in groundwater after land application of organic fertilizers if there is sufficient residence time (and available oxygen) in the soils to allow oxidation of the organic nitrogen in the fertilizer (Andrews et al., 2004; Manahan, 2000).
Because ammonia and organic nitrogen are pollution indicators, these two forms of nitrogen are often combined in one measure, called Kjeldahl nitrogen (APHA, 2012). Nitrate and nitrite could be determined separately by spectrophotometric methods (APHA, 2012).
The nitrogen cycle ( Fig. 3 ) is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature.
Although there is an abundance of nitrogen in the atmosphere, most plants cannot use this form of nitrogen. Instead, nitrogen must be in its “fixed” form, or in a compound, in order to be usable by plants. To get to this fixed form, nitrogen must first go through the cycle. The steps of the nitrogen cycle are the following: Nitrogen Fixation, Nitrification, Denitrification, Ammonification, and Assimilation.
Nitrogen Fixation occurs when atmospheric nitrogen is converted to ammonia by a pair of bacterial enzymes called nitrogenase present in a few species of bacteria, including cyanobacteria. Although ammonia (NH3) is the direct product of this reaction, it is quickly ionized to ammonium ( NH 4 + ).
Nitrification is the two-step process in which ammonia is converted to nitrites ( NO 2 − ) and then to nitrates ( NO 3 − ). Two different genera of bacteria that are present in the soil oxidize the ammonia into inorganic forms of nitrogen: these are Nitrosomonas and Nitrobacter.
Denitrification is the process of reducing nitrate and nitrite, highly oxidized forms of nitrogen available for consumption by many groups of organisms, into gaseous nitrogen. It can be thought of as the opposite of nitrogen fixation. Certain types of bacteria are responsible for this transformation.
Assimilation is the process by which living organisms incorporate NO 3 − and NH 4 + (ammonium) formed through nitrogen fixation and nitrification. Plants take in this form of nitrogen through their roots and incorporate them into nucleic acids and plant protein. Animals are then able to receive and utilize the nitrogen from plant tissues through consumption.
Ammonification occurs when a plant or animal dies or excretes waste. Decomposers, such as bacteria and fungi, first break down the proteins in the organic matter. This releases ammonia, which dissolves with water in the soil. Ammonia then combines with a hydrogen ion to create ammonium.
Denitrification is the process in which microorganisms, such as bacteria, break down nitrates to metabolize oxygen. This releases nitrogen gas back into the atmosphere, completing the cycle (Andrews et al., 2004; Manahan, 2000).
Phosphorus is found in the earth and rocks, and it is taken up by plants and animals. Phosphorus is a very important chemical because it is essential in the formation of DNA. In water bodies, phosphorus occurs almost entirely as organic phosphate and inorganic orthophosphate or polyphosphates (APHA, 2012; Davis and Cornwell, 2012).
Phosphorus is usually measured as total phosphorus (all forms combined) or dissolved phosphorus. Dissolved orthophosphate ( PO 3 − ) is an important indicator of water pollution because it is easily and rapidly taken up by bacteria, and therefore is almost never found in high concentrations in unpolluted waters. The various phosphorus forms can be measured analytically by spectrophotometric techniques (APHA, 2012).
Phosphorous cycle could be defined as the biogeochemical cycle that describes the movement of phosphorus through the lithosphere (solid earth), hydrosphere (water), and biosphere (living organisms) (Manahan, 2000; Ruttenberg, 2003). The global phosphorus cycle ( Fig. 4 ) could be summarized in the following steps:
Phosphorus starts out in rocks and then phosphates are washed off the rocks over a period of time by weathering and rain. After that, phosphate is distributed into soil and water.
Plants then take up phosphate from the soil, and animals may consume these plants. When this happens, the phosphate becomes imbedded into organic molecules (DNA). When plants and animals die and decompose, phosphate will return to the soil.
Organic forms of phosphate can be made accessible to plants by bacteria which break down the organic matter. Once this happens, the phosphate is changed to an inorganic form of phosphorus (this is known as mineralization).
Once the phosphorus is in the soil, it can be transferred to waterways and then oceans. After this phosphorus can be integrated into sediments and rock and the cycle starts again.
The cycle is largely affected by the production and use of fertilizers made from phosphorus and animal manure that contains phosphorus. Soil cannot absorb all the fertilizers, so it eventually gets washed away with other surface runoff. This will cause an increased amount of phosphorus in the water bodies. Consequently, eutrophication will occur.
Eutrophication occurs when extra nitrate and phosphate leach into a body of water and cause excessive growth of microorganisms.
When phosphorus and nitrogen are introduced into the lake, either naturally from storm runoff, or from a pollution source, the nutrients promote rapid growth of algae on the surface layer of the lake. When the algae die, they drop to the lake bottom and become a source of carbon for decomposing bacteria. Aerobic bacteria (that use oxygen for living) will use all available dissolved oxygen in the process of decomposing this material, and the dissolved oxygen may be depleted enough to cause the bottom of the lake to become anaerobic. As more and more algae die, and more and more dissolved oxygen is used in their decomposition, the middle layer of the lake may also become anaerobic (do not need molecular oxygen for living). When this happens, aerobic biological activity will be restricted on the surface layer only. The increasing frequency of this condition over the years is called eutrophication (Davis and Cornwell, 2012; Manahan, 2000; Weiner and Matthews, 2003).
Natural eutrophication may take thousands of years. However, eutrophication process may be shortened to as little as a decade as a result of excessive use of chemicals that provide nutrients such as fertilizers, detergents, and pesticides (Davis and Cornwell, 2012).
Water treatment is any process that makes water more suitable for a specific end use. The end use could be drinking or cooking, industrial supply, irrigation, fire fighting, river flow maintenance, water recreation, or just safe discharge of water to the environment.
Generally speaking, the characteristics of water and the final intended use of water determine the degree of treatment and the treatment method (Davis, 2010; Davis and Cornwell, 2012; Weiner and Matthews, 2003).
Mostly, the focus is on water treatment plants that produce safe drinking water and wastewater treatment plants that treat water for safe discharge or future reuse.
Drinking water treatment plant could be classified into:
Disinfection plant which is used for high-quality water source to ensure that water does not contain pathogens
Filtration plant: this is usually used to treat surface water Softening plant which is used to treat groundwaterTypical filtration plant is shown in Fig. 5 which is designed to remove odors, color, and turbidity as well as bacteria and other contaminants. Filtration plant employs the following steps:
Rapid mixing: where chemicals are added and rapidly dispersed through the waterFlocculation: Chemicals like alum (aluminum sulfate) are added to the water both to neutralize the particles electrically and to make them come close to each other and form large particles called flocs that could more readily be settled out
Sedimentation: During sedimentation, floc settles to the bottom of the water supply, due to its weight
Filtration: Once the floc has settled to the bottom of the water supply, the clear water on top will pass through filters of varying compositions (sand, gravel, and charcoal) and pore sizes in order to remove fine particles that were not settled, such as dust, parasites, bacteria, viruses, and chemicals
Disinfection: involves the addition of chemicals in order to kill or reduce the number of pathogenic organisms
Filtration treatment plant.
Softening plants utilize the same operational unit as filtration plants but use different chemicals in order to remove water hardness (Davis, 2010; Davis and Cornwell, 2012; Weiner and Matthews, 2003).
Municipal wastewater treatment may involve three major categories: primary treatment, secondary treatment, and advance treatment. Fig. 6 shows degrees of wastewater treatment.
Primary treatment: the goal is to remove pollutants which will either settle or float in a clarifier tank. Primary treatment will typically remove 60% of the raw sewage suspended solids and 35% of the BOD5. Soluble pollutants are not removed in this process.
Secondary treatment: the major goal is to remove BOD5 that escapes the primary process and to provide additional removal of suspended solids. Secondary treatment is typically achieved using biological processes. These processes could occur naturally in wastewater. Nevertheless, secondary treatments are designed to speed up these natural processes. Although secondary treatment may remove 85% of BOD5 and suspended solids, it does not remove significant amount of nitrogen, phosphorous, or heavy metals, nor does it completely remove pathogenic bacteria and viruses.
Advance treatments or tertiary wastewater treatment: these processes are normally applied when secondary treatment is not adequate. Some of these processes may remove up to 99% of BOD5 and phosphorous, all suspended solids and bacteria, and 95% of nitrogen. They can produce sparkling clean, colorless, and odorless effluent indistinguishable in appearance from high-quality drinking water. Some of these processes involve: Filtration or application of adsorbent such as activated carbon in order to remove persistent organic pollutants. Tertiary treatment may involve addition of chemicals such as ferric chloride or alum to enhance the removal of phosphorous. Nitrogen control step could be added to the tertiary treatment plant in order to facilitate the removal of nitrogen. This step could be accomplished biologically or chemically. The biological step is called nitrification/denitrification. Specific types of bacteria must be present to cause these two reactions to occur. The chemical process is called ammonia stripping; in this process nitrogen is removed in the form of ammonia by raising the pH of the solution by addition of base such as lime to covert ammonium ion to ammonia (Davis, 2010; Davis and Cornwell, 2012).
Degrees of wastewater treatment.
Microbiology is the science devoted to the study of organisms that are too small to be seen by the naked eye. These microorganisms constitute a large and diverse group of free-living forms that exist as single cells or cell clusters. Being free living, microbial cells are distinct from the cells of animals and plants as the latter are not able to live alone in nature but only in characteristic groups. A single microbial cell, generally, is able to carry out its life processes of growth, respiration, and reproduction independently of other cells, whether of the same kind or of different kinds. Microorganisms could be bacteria, viruses, algae, fungi, or protozoa (Hogg, 2005; Madigan et al., 2014; Tortora et al., 2010; Willey et al., 2016). In the following sections we will present an overview about microorganisms, their role in water, their classification, the main processes they perform, disease-causing microorganisms, and the role of microorganisms in wastewater treatment. Most of the examples and discussion in the following sections will be assigned to bacteria as they outline the group of the most important microorganisms when encountering water and wastewater treatment; they can be pathogens or can help in wastewater treatment.
Despite their complexity and variety, all living cells can be arranged into two major groups, prokaryotes and eukaryotes, based on certain structural and functional characteristics. In general, prokaryotes are structurally simpler and smaller than eukaryotes. The DNA (genetic material) of prokaryotes is usually in a single, circularly arranged chromosome and is not surrounded by a membrane; therefore, they do not have true nucleolus. They also lack membrane-enclosed organelles (specialized structures that carry on various activities). Their cell walls almost always contain the complex polysaccharide peptidoglycan. They usually divide by binary fission. During this process, the DNA is copied, and the cell splits into two. On the other hand, the DNA of eukaryotes is found in multiple chromosomes in a membrane-enclosed nucleus. Plants and animals are entirely composed of eukaryotic cells (Hogg, 2005; Madigan et al., 2014; Tortora et al., 2010; Willey et al., 2016).
Taxonomy is the science of classifying living organisms. Taxonomy is a basic and necessary tool for scientists, providing a universal language of communication. In a broader sense taxonomy consists of three separate but interrelated parts: classification, nomenclature, and identification.
Classification is the arrangement of organisms into groups (taxa, singular taxon), nomenclature involves assignment of names to taxa, and identification is the experimental determination of taxon to which an isolate belongs.
In the currently accepted classification of life, there are three main domains which are eukarya, bacteria, and archaea. Animals, plants, fungi, and protists are kingdoms in the domain eukarya. The domain bacteria include all of the pathogenic prokaryotes as well as many of the nonpathogenic prokaryotes found in soil and water.
The organisms are classified as species. Species is a group of closely related organisms that breed among themselves. A group of species that differ from each other in certain ways but are related genetically consist a genus. Related genera make up a family. A group of similar families constitutes an order, and a group of similar orders makes up a class. Related classes, in turn, make up a phylum or division. All phyla or divisions that are related to each other make up a kingdom, and related kingdoms are grouped into a domain. The hierarchal classification of organisms showing the eight major taxonomical ranks in addition to the phylogenetic tree of life are presented in Fig. 7 (Hogg, 2005; Madigan et al., 2014; Tortora et al., 2010; Willey et al., 2016). According to this classification, the full taxonomical position of a bacterium (Escherichia coli) can be described as follows (Willey et al., 2016):
The phylogenetic tree of life and taxonomy.
