In this article we will discuss about the methods and procedures to measure physical, chemical and biological parameters of water quality. Learn about:- 1. Physical Parameter of Water Quality 2. Chemical Water Quality Parameters 3. Biological Parameter.
Physical Parameter of Water Quality:
The availability of a water supply adequate in terms of both quantity and quality is essential to human existence. Early people recognised the importance of water from a quantity viewpoint. Civilisation developed around water bodies that could support agriculture and transportation as well as provide drinking water.
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Recognition of the importance of water quality developed more slowly. Early humans could judge water quality only through the physical senses of sight, taste and smell. Not until the biological, chemical and medical sciences developed, were methods available to measure water quality and to determine its effects on human health and well-being.
Physical parameters define those characteristics of water that respond to the senses of sight, touch, taste or smell. Suspended solids, turbidity, colour, taste and odour and temperature fall into this category.
The development of the science of water chemistry roughly paralleled that of water microbiology. Many of the chemicals used in industrial processes and agriculture have been identified in water. However, the effort to identify other chemical compounds which may already be found in trace quantities in many water supplies and to determine their effect on human health was only recently begun.
It is likely that new analytical techniques will be developed that will identify compounds not yet known to exist in water and it is conceivable that these materials will also be linked to human health. Thus, the science of water quality will remain a challenge for engineers and scientists for years to come.
Suspended Solids:
Solids can be dispersed in water in both suspended and dissolved forms. Although some dissolved solids may be perceived by the physical senses, they fall more appropriately under the category of chemical parameters.
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Solids suspended in water may consist of inorganic or organic particles or of immiscible liquids. Inorganic solids such as clay, silt and other soil constituents are common in surface water. Organic material such as plant fibers and biological solids (algal cells, bacteria, etc.) are also common constituents of surface waters.
These materials are often natural contaminants resulting from the erosive action of water flowing over surfaces. Because of the filtering capacity of the soil, suspended material is seldom a constituent of groundwater.
Other suspended material may result from human use of the water. Domestic waste-water usually contains large quantities of suspended solids that are mostly organic in nature. Industrial use of water may result in a wide variety of suspended impurities of either organic or inorganic nature. Immiscible liquids such as oils and greases are often constituents of waste-water.
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Suspended material may be objectionable in water for several reasons. It is aesthetically displeasing and provides adsorption sites for chemical and biological agents. Suspended organic solids may be degraded biologically, resulting in objectionable by-products. Biologically active (live) suspended solids may include disease-causing organisms as well as organisms such as toxin-producing strains of algae.
There are several tests available for measuring solids. Most are gravimetric tests involving the mass of residues. The total solids test quantifies all the solids in the water, suspended and dissolved, organic and inorganic. This parameter is measured by evaporating a sample to dryness and weighing the residue.
The total quantity of residue is expressed as milligrams per liter (mg/l) on a dry-mass-of-solids basis. A drying temperature slightly above boiling (104°C) is sufficient to drive off the liquid and the water adsorbed to the surface of the particles, while a temperature of about 180°C is necessary to evaporate the occluded water.
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Most suspended solids can be removed from water by filtration. Thus, the suspended fraction of the solids in a water sample can be approximated by filtering the water, drying the residue and filter to a constant weight at 104°C (± 1°C) and determining the mass of the residue retained on the filter.
The results of this suspended solids test are also expressed as dry mass per volume (milligrams per liter). The amount of dissolved solids passing through the filters, also expressed as milligrams per liter, is the difference between the total-solids and suspended-solids content of a water sample.
It should be emphasised that filtration of a water sample does not exactly divide the solids into suspended and dissolved fractions according to the definitions. Some colloids may pass through the filter and be measured along with the dissolved fraction while some of the dissolved solids adsorb to the filter material.
The extent to which this occurs depends on the size and nature of the solids and on the pore size and surface characteristics of the filter material. For this reason, the terms filterable residues and non-filterable residues are often used.
Filterable residues pass through the filter along with the water and relate more closely to dissolved solids, while non-filterable residues are retained on the filter and relate more closely to suspended solids. ‘Filterable residues’ and ‘non-filterable residues’ are terms more frequently used in laboratory analysis while the ‘dissolved solids’ and ‘suspended solids’ are terms more frequently used in water-quality-management practice. For most practical applications, the distinction between the two is not necessary.
Once samples have been dried and measured, the organic content of both total and suspended solids can be determined by firing the residues at 600°C for 1 hour. The organic fraction of the residues will be converted to carbon dioxide, water vapour and other gases and will escape.
The remaining material will represent the inorganic or fixed residue. When organic suspended solids are being measured, a filter made of glass fiber or some other material that will not decompose at the elevated temperature must be used.
Use:
Suspended solids, where such material is likely to be organic and/or biological in nature, are an important parameter of waste-water. The suspended-solids parameter is used to measure the quality of the wastewater influent, to monitor several treatment processes and to measure the quality of the effluent. EPA has set a maximum suspended-solids standard of 30 mg/l for most treated waste-water discharges.
Turbidity:
A direct measurement of suspended solids is not usually performed on samples from natural bodies of water or on potable (drinkable) water supplies. The nature of the solids in these waters and the secondary effects they produce are more important than the actual quantity. For such waters a test for turbidity is commonly used.
Turbidity is a measure of the extent to which light is either absorbed or scattered by suspended material in water. Because absorption and scattering are influenced by both size and surface characteristics of the suspended material, turbidity is not a direct quantitative measurement of suspended solids.
For example- one small pebble in a glass of water would produce virtually no turbidity. If this pebble were crushed into thousands of particles of colloidal size, a measurable turbidity would result, even though the mass of solids had not changed.
Most turbidity in surface waters results from the erosion of colloidal material such as clay, silt, rock fragments and metal oxides from the soil. Vegetable fibers and micro-organisms may also contribute to turbidity. Household and industrial waste-waters may contain a wide variety of turbidity-producing material.
Soaps, detergents and emulsifying agents produce stable colloids that result in turbidity. Although turbidity measurements are not commonly run on waste-water, discharges of waste-waters may increase the turbidity of natural bodies of water.
When turbid water in a small, transparent container, such as a drinking glass, is held up to the light, an aesthetically displeasing opaqueness or ‘milky’ colouration is apparent. The colloidal material associated with turbidity provides adsorption sites for chemicals that may be harmful or cause undesirable tastes and odours and for biological organisms that may be harmful. Disinfection of turbid waters is difficult because of the adsorptive characteristics of some colloids and because the solids may partially shield organisms from the disinfectant.
In natural water bodies, turbidity may impart a brown or other colour to water, depending on the light-absorbing properties of the solids and may interfere with light penetration and photosynthetic reactions in streams and lakes. Accumulation of turbidity-causing particles in porous streambeds results in sediment deposits that can adversely affect the flora and fauna of the stream.
Turbidity is measured photometrically by determining the percentage of light of a given intensity that is either absorbed or scattered. The original measuring apparatus, called a Jackson turbidimeter, was based on light absorption and employed a long tube and standardised candle.
The candle was placed beneath the glass tube that was then housed in a black metal sheath so that the light from the candle could only be seen from the apparatus. The water sample was then poured slowly into the tube until the lighted candle was no longer visible, i.e. complete absorption had occurred.
The glass tube was calibrated with readings for turbidity produced by suspensions of silica dioxide (SiO2), with one Jackson Turbidity Unit (JTU) being equal to the turbidity produced by 1 mg SiO2 in 1 liter of distilled water.
In recent years this awkward apparatus has been replaced by a turbidity meter in which a standardised electric bulb produces a light that is then directed through a small sample vial. In the absorption mode, a photometer measures the light intensity on the side of the vial opposite from the light source, while in the scattering mode, a photometer measures the light intensity at a 90° angle from the light source.
Although most turbidity meters in use today work on the scattering principle, turbidity caused by dark substances that absorb rather than reflect light should be measured by the absorption technique. Formazin, a chemical compound, provides more reproducible standards than SiO2 and has replaced it as a reference. Turbidity meter readings are now expressed as Formazin Turbidity Units or FTUs. The term Nephelometry Turbidity Units (NTU) is often used to indicate that the test was run according to the scattering principle.
Use:
Turbidity measurements are normally made on ‘clean’ waters as opposed to waste-waters. Natural waters may have turbidities ranging from a few FTUs to several hundred. EPA drinking-water standards specify a maximum of 1 FTU, while the American Water Works Association has set 0.1 FTU as its goal for drinking water.
Colour:
Pure water is colourless, but water in nature is often coloured by foreign substances. Water whose colour is partly due to suspended matter is said to have apparent colour. Colour contributed by dissolved solids that remain after removal of suspended matter is known as true colour.
After contact with organic debris such as leaves, conifer needles, weeds or wood, water picks up tannins, humic acid and humates and takes on yellowish-brown hues. Iron oxides cause reddish water and manganese oxides cause brown or blackish water. Industrial wastes from textile and dyeing operations, pulp and paper production, food processing, chemical production and mining, refining and slaughterhouse operations may add substantial colouration to water in receiving streams.
Coloured water is not aesthetically acceptable to the general public. In fact, given a choice, consumers tend to choose clear, non-coloured water of otherwise poorer quality over treated potable water supplies with an objectionable colour.
Highly coloured water is unsuitable for laundering, dyeing, papermaking, beverage manufacturing, dairy production and other food processing and textile and plastic production. Thus, the colour of water affects its marketability for both domestic and industrial use.
While true colour is not usually considered unsanitary or unsafe, the organic compounds causing true colour may exert a chlorine demand and thereby seriously reduce the effectiveness of chlorine as a disinfectant. Perhaps more important are the products formed by the combination of chlorine with some colour-producing organics.
Phenolic compounds, common constituents of vegetative decay products, produce very objectionable taste and odour compounds with chlorine. Additionally, some compounds of naturally occurring organic acids and chlorine are either known to be or are suspected of being carcinogens (cancer-causing agents).
Although several methods of colour measurement are available, methods involving comparison with standardised coloured materials are most often used. Colour-comparison tubes containing a series of standards may be used for direct comparison of water samples that have been filtered to remove apparent colour.
Results are expressed in True Colour Units (TCUs) where one unit is equivalent to the colour produced by 1 mg/l of platinum in the form of chlorplatinate ions. For colours other than yellowish- brown hues, especially for coloured waters originating from industrial waste effluents, special spectrophotometric techniques are usually employed.
In fieldwork, instruments employing coloured glass disks that are calibrated to the colour standards are often used. Because biological and physical changes occurring during storage may affect colour, samples should be tested within 72 hours of collection.
Use:
Colour is not a parameter usually included in waste-water analysis. In potable water analysis, the common practice is to measure only the true colour produced by organic acid resulting from decaying vegetation in the water. The resulting value can be taken as an indirect measurement of humic substances in the water.
Taste and Odour:
The terms taste and odour are themselves definitive of this parameter. Because the sensations of taste and smell are closely related and often confused, a wide variety of tastes and odours may be attributed to water by consumers. Substances that produce an odour in water will almost invariably impart a taste as well. The converse is not true, as there are many mineral substances that produce taste but no odour.
Many substances with which water comes into contact in nature or during human use may impart perceptible taste and odour. These include minerals, metals and salts from the soil, end products from biological reactions and constituents of waste-water. Inorganic substances are more likely to produce tastes unaccompanied by odour. Alkaline material imparts a bitter taste to water, while metallic salts may give a salty or bitter taste.
Organic material, on the other hand, is likely to produce both taste and odour. A multitude of organic chemicals may cause taste and odour problems in water, with petroleum-based products being prime offenders. Biological decomposition of organics may also result in taste and odour-producing liquids and gases in water.
Principal among these are the reduced products of sulphur that impart a ‘rotten egg’ taste and odour. Also, certain species of algae secrete an oily substance that may result in both taste and odour. The combination of two or more substances, neither of which would produce taste or odour by itself, may sometimes result in taste and odour problems.
Consumers find taste and odour aesthetically displeasing for obvious reasons. Because water is thought of as tasteless and odourless, the consumer associates taste and odour with contamination and may prefer to use a tasteless, odourless water that might actually pose more of a health threat. And odours produced by organic substances may pose more than a problem of simple aesthetics, since some of those substances may be carcinogenic.
Direct measurement of materials that produce tastes and odours can be made if the causative agents are known. Several types of analysis are available for measuring taste-producing inorganics. Measurement of taste and odour-causing organics can be made using gas or liquid chromatography.
Because chromatographic analysis is time-consuming and requires expensive equipment, it is not routinely performed on water samples, but should be done if problem organics are suspected. However, because of the synergism, quantifying the sources does not necessarily quantify the nature or intensity of taste and odour.
Quantitative tests that employ the human senses of taste and smell can be used for this purpose. An example is the test for the threshold odour number (TON). Varying amounts of odorous water are poured into containers and diluted with enough odour-free distilled water to make a 200-ml mixture.
An assembled panel of five to ten ‘noses’ is used to determine the mixture in which the odour is just barely detectable to the sense of smell. The TON of that sample is then calculated, using the formula-
Where, A is the volume of odorous water (ml) and B is the volume of odour-free water required to produce a 200 ml mixture. Threshold odour numbers corresponding to various sample volumes are shown in Table 2.1. A similar test can be used to quantify taste or the panel can simply rate the water qualitatively on an ‘acceptability’ scale.
Use:
Although odours can be a problem with waste-water, the taste and odour parameter is only associated with potable water. EPA does not have a maximum standard for TON. A maximum TON of 3 has been recommended by the Public Health Service and serves as a guideline rather than a legal standard.
Temperature:
Temperature is not used to evaluate directly either potable water or waste-water. It is, however, one of the most important parameters in natural surface-water systems. The temperature of surface waters governs to a large extent the biological species present and their rates of activity. Temperature has an effect on most chemical reactions that occur in natural water systems. Temperature also has a pronounced effect on the solubilities of gases in water.
The temperature of natural water systems responds to many factors, the ambient temperature (temperature of the surrounding atmosphere) being the most universal. Generally, shallow bodies of water are more affected by ambient temperatures than are deeper bodies.
The use of water for dissipation of waste heat in industry and the subsequent discharge of the heated water may result in dramatic, though perhaps localised, temperature changes in receiving streams. Removal of forest canopies and irrigation return flows can also result in increased stream temperature.
Cooler waters usually have a wider diversity of biological species. At lower temperatures, the rate of biological activity, i.e. utilisation of food supplies, growth, reproduction, etc. is slower. If the temperature is increased, biological activity increases. An increase of 10°C is usually sufficient to double the biological activity, if essential nutrients are present.
At elevated temperatures and increased metabolic rates, organisms that are more efficient at food utilisation and reproduction flourish, while other species decline and are perhaps eliminated altogether. Accelerated growth of algae often occurs in warm water and can become a problem when cells cluster into algae mats.
Natural secretion of oils by algae in the mats and the decay products of dead algae cells can result in taste and odour problems. Higher-order species, such as fish, are affected dramatically by temperature and by dissolved oxygen levels, which are a function of temperature. Game fish generally require cooler temperatures and higher dissolved-oxygen levels.
Temperature changes affect the reaction rates and solubility levels of chemicals. Most chemical reactions involving dissolution of solids are accelerated by increased temperatures.
The solubility of gases, on the other hand, decreases at elevated temperatures. Because biological oxidation of organics in streams and impoundments is dependent on an adequate supply of dissolved oxygen, decrease in oxygen solubility is undesirable.
Temperature also affects other physical properties of water. The viscosity of water increases with decreasing temperature. The maximum density of water occurs at 4°C and density decreases on either side of that temperature, a unique phenomenon among liquids. Both temperature and density have a subtle effect on planktonic micro-organisms in natural water systems.
Chemical Water Quality Parameters:
Water has been called the universal solvent and chemical parameters are related to the solvent capabilities of water. Total dissolved solids, alkalinity, hardness, fluorides, metals, organics and nutrients are chemical parameters of concern in water quality management.
Total Dissolved Solids:
The material remaining in the water after filtration for the suspended-solids analysis is considered to be dissolved. This material is left as a solid residue upon evaporation of the water and constitutes a part of total solids.
Dissolved material results from the solvent action of water on solids, liquids and gases. Like suspended material, dissolved substances may be organic or inorganic in nature. Inorganic substances which may be dissolved in water include minerals, metals and gases.
Water may come in contact with these substances in the atmosphere, on surfaces and within the soil. Materials from the decay products of vegetation, from organic chemicals and from the organic gases are common organic dissolved constituents of water. The solvent capability of water makes it an ideal means by which waste products can be carried away from industrial sites and homes.
Impacts:
Many dissolved substances are undesirable in water. Dissolved minerals, gases and organic constituents may produce aesthetically displeasing colour, tastes and odours. Some chemicals may be toxic and some of the dissolved organic constituents have been shown to be carcinogenic.
Quite often, two or more dissolved substances—especially organic substances and members of the halogen group—will combine to form a compound whose characteristics are more objectionable than those of either of the original materials.
Not all dissolved substances are undesirable in water. For example, essentially pure, distilled water has a flat taste. Additionally, water has an equilibrium state with respect to dissolved constituents. An under-saturated water will be ‘aggressive’ and will more readily dissolve materials with which it comes in contact. Readily dissolvable material is sometimes added to a relatively pure water to reduce its tendency to dissolve pipes and plumbing.
A direct measurement of total dissolved solids can be made by evaporating to dryness a sample of water which has been filtered to remove the suspended solids. The remaining residue is weighed and represents the Total Dissolved Solids (TDS) in the water. The TDS is expressed as milligrams per liter on a dry-mass basis. The organic and inorganic fractions can be determined by firing the residue at 600°C.
An approximate analysis for TDS is often made by determining the electrical conductivity of the water. The ability of a water to conduct electricity, known as the specific conductance, is a function of its ionic strength.
Specific conductance is measured by a conductivity meter employing the Wheatstone bridge principle. The standard procedure is to measure the conductivity in a cubic-centimeter field at 25°C and express the results in millisiemens per meter (mS/m).
Unfortunately, specific conductance and concentration of TDS are not related on a one-to-one basis. Only ionised substances contribute to specific conductance. Organic molecules and compounds that dissolve without ionising are not measured. Additionally, the magnitude of the specific conductance is influenced by the valence of the ions in solution, their mobility and relative numbers.
The temperature also has an important effect, with specific conductance, increasing as the water temperature increases. Conversion of units to milligrams per liter or milli-equivalents per liter must be made by use of an appropriate constant. A multiplier ranging from 0.055 to 0.09 is used to convert millisiemens to milligrams per liter.
To use specific conductance as a quantitative test, sufficient analysis for filterable residue must be run to determine the conversion factor. For this reason, specific conductance is most often used in a qualitative sense to monitor changes in TDS occurring in natural streams or treatment processes.
Use:
Because no distinction among the constituents is made, the TDS parameter is included in the analysis of water and waste-water only as a gross measurement of the dissolved material. While this is often sufficient for waste-waters, it is frequently desirable to know more about the composition of the solids in water that is intended for use in potable supplies, agriculture and some industrial processes. When this is the case, tests for several of the ionic constituents of TDS are made.
The ions usually accounting for the vast majority of TDS in natural waters are listed in Table 2.2. Those listed under major constituents are often sufficient to characterise the dissolved-solids content of water. These are called common ions and are often measured individually and summed on an equivalent basis to represent the approximate TDS.
As a check, the sum of the anions should equal the sum of the cations because electroneutrality must be preserved. A significant imbalance suggests that additional constituents are present or that an error has been made in the analysis of one or more of the ions.
It is important to arrange the cations and anions in the order shown for convenience in determining types of hardness and the quantities of chemicals needed for softening.
Several of the constituents of dissolved solids have properties that necessitate special attention. These constituents include alkalinity, hardness, fluoride, metals, organics and nutrients.
Alkalinity:
Alkalinity is defined as the quantity of ions in water that will react to neutralise hydrogen ions. Alkalinity is thus a measure of the ability of water to neutralise acids.
Impacts:
In large quantities, alkalinity imparts a bitter taste to water. The principal objection to alkaline water, however, is the reactions that can occur between alkalinity and certain cations in the water. The resultant precipitate can foul pipes and other water-systems appurtenances.
Alkalinity measurements are made by titrating the water with an acid and determining the hydrogen equivalent. Alkalinity is then expressed as milligrams per liter of CaCO3. If 0.02 N H2S04 is used in the titration, then 1 ml of the acid will neutralise 1 mg of alkalinity as CaCO3.
Hydrogen ions from the acid react with the alkalinity according to the following equations:
If acid is added slowly to water and the pH is recorded for each addition, a titration curve similar to that shown in Fig. 2.2 is obtained. Of particular significance are the inflection points in the curve that occur at approximately pH 8.3 and pH 4.5.
The conversion of carbonate to bicarbonate Eq. 2.10 is essentially complete at pH 8.3. However, because bicarbonate is also an alkalinity species, an equal amount of acid must be added to complete the neutralisation. Thus, the neutralisation of carbonate is only one-half complete at pH 8.3.
Because the conversion of hydroxide to water is virtually complete at pH 8.3 (Fig. 2.1), all of the hydroxide and one-half of the carbonate have been measured at pH 8.3. At pH 4.5 all of the bicarbonate has been converted to carbonic acid Eq. 2.11 including the bicarbonate resulting from the reaction of the acid and carbonate Eq. 2.10. Thus, the amount of acid required to titrate a sample to pH 4.5 is equivalent to the total alkalinity of the water.
If the volume of acid needed to reach the 8.3 end point is known, the species of alkalinity can also be determined. Because all of the hydroxide and one-half of the carbonate have been neutralised at pH 8.3, the acid required to lower the pH from 8.3 to 4.5 must measure the other one-half of the carbonate, plus all of the original bicarbonate.
If P is the amount of acid required to reach pH 8.3 and M is the total quantity of acid required to reach 4.5, the following generalisations concerning the forms of alkalinity can be made:
if P = M, all alkalinity is OH–
P = M/2, all alkalinity is CO2/3-
P = 0 (i.e. initial pH is below 8.3), all alkalinity is HCO–3
P < M/2, predominant species are CO2/3- and HCO–3
P > M/2, predominant species are OH– and CO2/3-
In observing the pH dependency of the species in Fig. 2.1, The quantity of OH– becomes significant at pH less than about 9.0. Without introducing significant error, it can be assumed that the OH– of samples with pH less than 9.0 is insignificant. The CO2/3- would then be measured by 2P and the HCO–3 would be measured by the remainder (M – 2P).
Use:
Alkalinity measurements are often included in the analysis of natural waters to determine their buffering capacity. It is also used frequently as a process control variable in water and waste-water treatment. Maximum levels of alkalinity have not been set by EPA for drinking water or for waste-water discharges.
Hardness:
Hardness is defined as the concentration of multivalent metallic cations in solution. At supersaturated conditions, the hardness cations will react with anions in the water to form a solid precipitate. Hardness is classified as carbonate hardness and noncarbonate hardness, depending upon the anion with which it associates. The hardness that is equivalent to the alkalinity is termed carbonate hardness, with any remaining hardness being called noncarbonate hardness.
Carbonate hardness is sensitive to heat and precipitates readily at high temperatures.
The multivalent metallic ions most abundant in natural waters are calcium and magnesium. Others may include iron and manganese in their reduced states (Fe2+, Mn2+), strontium (Sr2+) and aluminium (Al3+). The latter are usually found in much smaller quantities than calcium and magnesium and for all practical purposes, hardness may be represented by the sum of the calcium and magnesium ions.
Soap consumption by hard waters represents an economic loss to the water user. Sodium soaps react with multivalent metallic cations to form a precipitate, thereby losing their surfactant properties.
A typical divalent cation reaction is:
Lathering does not occur until all of the hardness ions are precipitated, at which point the water has been ‘softened’ by the soap. The precipitate formed by hardness and soap adheres to surfaces of tubs, sinks and dishwashers and may stain clothing, dishes and other items.
Residues of the hardness-soap precipitate may remain in the pores, so that skin may feel rough and uncomfortable. In recent years these problems have been largely alleviated by the development of soaps and detergents that do not react with hardness.
Boiler scale, the result of the carbonate hardness precipitate may cause considerable economic loss through fouling of water heaters and hot-water pipes. Changes in pH in the water distribution systems may also result in deposits of precipitates. Bicarbonates begin to convert to the less soluble carbonates at pH values above 9.0.
Magnesium hardness, particularly associated with the sulphate ion, has a laxative effect on persons unaccustomed to it. Magnesium concentrations of less than 50 mg/l are desirable in potable waters, although many public water supplies exceed this amount. Calcium hardness presents no public health problem. In fact, hard water is apparently beneficial to the human cardiovascular system.
Measurement:
Hardness can be measured by using spectrophotometric techniques or chemical titration to determine the quantity of calcium and magnesium ions in a given sample. Hardness can be measured directly by titration with ethylenediamine tetraacetic acid (EDTA) using eriochrome black T (EBT) as an indicator.
The EBT reacts with the divalent metallic cations, forming a complex that is red in colour. The EDTA replaces the EBT in the complex and when the replacement is complete, the solution changes from red to blue. If 0.01 M EDTA is used, 1.0 ml of the titrant measures 1.0 mg of hardness as CaCO3.
Use:
Analysis for hardness is commonly made on natural waters and on waters intended for potable supplies and for certain industrial uses. Hardness may range from practically zero to several hundred or even several thousand, parts per million.
Although acceptability levels vary according to a consumer’s acclimation to hardness, a generally accepted classification is as follows:
The Public Health Service standards recommend a maximum of 500 mg/l of hardness in drinking water.
Fluoride:
Generally associated in nature with a few types of sedimentary or igneous rocks, fluoride is seldom found in appreciable quantities in surface waters and appears in groundwater in only a few geographical regions. Fluoride is toxic to humans and other animals in large quantities, while small concentrations can be beneficial.
Concentrations of approximately 1.0 mg/l in drinking water help to prevent dental cavities in children. During formation of permanent teeth, fluoride combines chemically with tooth enamel, resulting in harder, stronger teeth that are more resistant to decay. Fluoride is often added to drinking water supplies if sufficient quantities for good dental formation are not naturally present.
Excessive intakes of fluoride can result in discolouration of teeth. Noticeable discolouration, called mottling, is relatively common when fluoride concentrations in drinking water exceed 2.0 mg/l, but is rare when concentrations are less than 1.5 mg/l. Adult teeth are not affected by fluoride, although both the benefits and liabilities of fluoride during tooth-formation years carry over into adulthood.
Excessive dosages of fluoride can also result in bone fluorosis and other skeletal abnormalities. Concentrations of less than 5 mg/l in drinking water are not likely to cause bone fluorosis or related problems and some water supplies are known to have somewhat higher fluoride concentrations with no discernible problem other than severe mottling of teeth. On the assumption that people drink more water in warmer climates, EPA drinking-water standards base upper limits for fluoride on ambient temperatures.
All metals are soluble to some extent in water. While excessive amounts of any metal may present health hazards, only those metals that are harmful in relatively small amounts are commonly labelled toxic; other metals fall into the nontoxic group.
Sources of metals in natural waters include dissolution from natural deposits and discharges of domestic, industrial or agricultural waste-waters. Measurement of metals in water is usually made by atomic absorption spectrophotometry.
In addition to the hardness ions, calcium and magnesium, other nontoxic metals commonly found in water include sodium, iron, manganese, aluminium, copper and zinc. Sodium, by far the most common nontoxic metal found in natural waters, is abundant in the earth’s crust and is highly reactive with other elements.
The salts of sodium are soluble in water. Excessive concentrations cause a bitter taste in water and are a health hazard to cardiac and kidney patients. Sodium is also corrosive to metal surfaces and, in large concentrations, is toxic to plants.
Iron and manganese quite frequently occur together and present no health hazards at concentrations normally found in natural waters. Iron and manganese in very small quantities may cause colour problems.
Iron concentrations of 0.3 mg/l and manganese concentrations as low as 0.05 mg/l can cause colour problems. Additionally, some bacteria use iron and manganese compounds for an energy source and the resulting slime growth may produce taste and odour problems.
When significant quantities of iron are encountered in natural water systems, it is usually associated with chloride (FeCl2), bicarbonate [Fe(HCO3)2] or sulphate [Fe(SO4)] anions and exists in a reduced state. In the presence of oxygen, the ferrous (Fe2+) ion is oxidised to the ferric (Fe3+) ion and forms an insoluble compound with hydroxide [Fe(OH)3].
Thus, significant quantities of iron will usually be found only in systems devoid of oxygen such as groundwaters or perhaps the bottom layers of stratified lakes. Similarly, manganese ions (Mn2+ and Mn4+) associated with chloride, nitrates and sulphates are soluble, while oxidised compounds (Mn3+ and Mn5+) are virtually insoluble. It is possible, however, for organic acids derived from decomposing vegetation to chelate iron and manganese and prevent their oxidation and subsequent precipitation in natural waters.
The other nontoxic metals are generally found in very small quantities in natural water systems and most would cause taste problems long before toxic levels were reached. However, copper and zinc are synergetic and when both are present, even in small quantities, may be toxic to many biological species.
Toxic metals are harmful to humans and other organisms in small quantities. Toxic metals that may be dissolved in water include arsenic, barium, cadmium, chromium, lead, mercury and silver. Cumulative toxins such as arsenic, cadmium, lead and mercury are particularly hazardous.
These metals are concentrated by the food chain, thereby posing the greatest danger to organisms near the top of the chain. Fortunately, toxic metals are present in only minute quantities in most natural water systems. Although natural sources of all the toxic metals exist, significant concentration in water can usually be traced to mining, industrial or agricultural sources.
Organics:
Many organic materials are soluble in water. Organics in natural water systems may come from natural sources or may result from human activities. Most natural organics consist of the decay products of organic solids, while synthetic organics are usually the result of waste-water discharges or agricultural practices. Dissolved organics in water are usually divided into two broad categories: biodegradable and nonbiodegradable (refractory).
Biodegradable material consists of organics that can be utilised for food by naturally occurring microorganisms within a reasonable length of time. In dissolved form, these materials usually consist of starches, fats, proteins, alcohols, acids, aldehydes and esters.
They may be the end product of the initial microbial decomposition of plant or animal tissue or they may result from domestic or industrial wastewater discharges. Although some of these materials can cause colour, taste and odour problems, the principal problem associated with biodegradable organics is a secondary effect resulting from the action of micro-organisms on these substances.
Microbial utilisation of dissolved organics can be accompanied by oxidation (addition of oxygen to or the deletion of hydrogen from elements of the organic molecule) or by reduction (addition of hydrogen to or deletion of oxygen from elements of the organic molecule). Although it is possible for the two processes to occur simultaneously, the oxidation process is by far more efficient and is predominant when oxygen is available.
In aerobic (oxygen-present) environments, the end products of microbial decomposition of organics are stable and acceptable compounds. Anaerobic (oxygen-absent) decomposition results in unstable and objectionable end products. Should oxygen later become available, anaerobic end products will be oxidised to aerobic end products.
The oxygen-demanding nature of biodegradable organics is of utmost importance in natural water systems. When oxygen utilisation occurs more rapidly than oxygen can be replenished by transfer from the atmosphere, anaerobic conditions that severely affect the ecology of the system will result.
The amount of oxygen consumed during microbial utilisation of organics is called the Biochemical Oxygen Demand (BOD). The BOD is measured by determining the oxygen consumed from a sample placed in an air-tight container and kept in a controlled environment for a pre-selected period of time.
In the standard test, a 300 ml BOD bottle is used and the sample is incubated at 20°C for 5 days. Light must be excluded from the incubator to prevent algal growth that may produce oxygen in the bottle. Because the saturation concentration for oxygen in water at 20°C is approximately 9 mg/l, dilution of the sample with BOD-free, oxygen-saturated water is necessary to measure BOD values greater than just a few milligrams per liter.
The BOD of a diluted sample is calculated by-
Where DOI and DOF are the initial and final dissolved-oxygen concentrations (mg/l) and P is the decimal fraction of the sample in the 300 ml bottle.
Ranges of BOD covered by various dilutions are shown in Table 2.3. These values assume an initial dissolved-oxygen concentration of 9 mg/l in the mixture, with a minimum of 2 and a maximum of 7 mg/l of O2 being consumed. Calculations of BOD5 from this testing procedure are illustrated in the following example.
Most natural water and municipal waste-waters will have a population of micro-organisms that will consume the organics. In sterile waters, micro-organisms must be added and the BOD of the material containing the organisms must be determined and subtracted from the total BOD of the mixture. The presence of toxic materials in the water will invalidate the BOD results.
The BOD5 only represents the oxygen consumed in 5 days. The total BOD or BOD for any other time period, can be determined provided additional information is known or obtained. The rate at which organics are utilised by micro-organisms is assumed to be a first-order reaction; that is, the rate at which organics utilised is proportional to the amount available.
Some organic materials are resistant to biological degradation. Tannic and lignic acids, cellulose and phenols are often found in natural water systems. These constituents of woody plants biodegrade so slowly that they are usually considered refractory.
Molecules with exceptionally strong bonds (some of the polysaccharides) and ringed structures (benzene) are essentially nonbiodegradable. An example is the detergent compound Alkyl Benzene Sulphonate (ABS) which, with its benzene ring, does not biodegrade.
Being a surfactant, ABS causes frothing and foaming in waste-water treatment plants and increases turbidity by stabilising colloidal suspensions. This problem was largely alleviated when detergent manufacturers switched to a Linear Alkyl Sulphonate (LAS) compound, which is biodegradable. Many of the organics associated with petroleum and with its refining and processing also contain benzene and are essentially non-biodegradable.
Some organics are non-biodegradable because they are toxic to organisms. These include the organic pesticides, some industrial chemicals and hydrocarbon compounds that have combined with chlorine.
Pesticides, including insecticides and herbicides, have found widespread use in modern society in both urban and agricultural settings. Poor application practices and subsequent washoff by rainfall and runoff may result in contamination of surface streams.
Organic insecticides are usually chlorinated hydrocarbons (i.e. aldrin, dieldrin, endrin and lindane), while herbicides are usually chlorophenoxys (e.g. 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxy-propionic acid).
Many of the pesticides are cumulative toxins and cause severe problems at the higher end of the food chain. An example is the near-extinction of the brown pelican that feeds on fish and other macroaquatic species by the insecticide DDT, the use of which is now banned in the United States.
Measurement of non-biodegradable organics is usually by the Chemical Oxygen Demand (COD) test. Non-biodegradable organics may also be estimated from a Total Organic Carbon (TOC) analysis. Both COD and TOC measure the biodegradable fraction of the organics, so the BOD must be subtracted from the COD or TOC to quantify the non-biodegradable organics. Specific organic compounds can be identified and quantified through analysis by gas chromatography.
Nutrients:
Nutrients are elements essential to the growth and reproduction of plants and animals and aquatic species depend on the surrounding water to provide their nutrients. Although a wide variety of minerals and trace elements can be classified as nutrients, those required in most abundance by aquatic species are carbon, nitrogen and phosphorus.
Carbon is readily available from many sources. Carbon dioxide from the atmosphere, alkalinity and decay products of organic matter all supply carbon to the aquatic system. In most cases, nitrogen and phosphorus are the nutrients that are the limiting factors in aquatic plant growth.
Nitrogen:
Nitrogen gas (N2) is the primary component of the earth’s atmosphere and is extremely stable. It will react with oxygen under high-energy conditions (electrical discharges or flame incineration) to form nitrogen oxides. Although a few biological species are able to oxidise nitrogen gas, nitrogen in the aquatic environment is derived primarily from sources other than atmospheric nitrogen.
Nitrogen is a constituent of proteins, chlorophyll and many other biological compounds. Upon the death of plants or animals, complex organic matter is broken down to simple forms by bacterial decomposition.
Proteins, for instance, are converted to amino acids and further reduced to ammonia (NH3). If oxygen is present, the ammonia is oxidised to nitrite (NO–2) and then to nitrate (NO-3). The nitrate can then be reconstituted into living organic matter by photosynthetic plants.
Other sources of nitrogen in aquatic systems include animal wastes, chemical (particularly chemical fertilisers) and waste-water discharges. Nitrogen from these sources may be discharged directly into streams or may enter waterways through surface runoff or groundwater discharge.
Nitrogen compounds can be oxidised to nitrate by soil bacteria and may be carried into the groundwater by percolating water. Once in the aquifer, nitrates move freely with the groundwater flow. Groundwater contamination by nitrogen from animal feedlots and septic-tank drain fields has been recorded in numerous instances.
In addition to the over-enrichment problems alluded to earlier, nitrogen can have other serious consequences. Ammonia is a gas at temperatures and pressures normally found in natural water systems. The gas (NH3) exists in equilibrium with the aqueous ionic form called ammonium (NH4+).
The hydroxyl ion concentration of the water and thus the pH, controls the relative abundance of each species. Oxidation ofNH3 and NH4 to nitrate and on to nitrate by aquatic microbes results in an additional biochemical oxygen demand.
Nitrate poisoning in infant animals, including humans, can cause serious problems and even death. Apparently, the lower acidity in an infant’s intestinal tract permits growth of nitrate-reducing bacteria that convert the nitrate to nitrite, which is then absorbed into the bloodstream.
Nitrite has a greater affinity for haemoglobin than doe’s oxygen and thus replaces oxygen in the blood complex. The body is denied essential oxygen and, in extreme cases, the victim suffocates. Because oxygen starvation results in a bluish discolouration of the body, nitrate poisoning has been referred to as the ‘blue baby’ syndrome, although the correct term is methemoglobinemia.
Once the flora of the intestinal tract has fully developed, usually after the age of 6 months, nitrate conversion to nitrite and subsequent methemoglobinemia from drinking water is seldom a problem. Fortunately, the natural oxidation of nitrite to nitrate occurs quickly so that significant quantities of nitrites are not found in natural water.
Tests for nitrogen forms in water commonly include analysis for ammonia (including both ammonia and ammonium), nitrate and organic nitrogen. The results of the analyses are usually expressed as milligrams per liter of the particular species as nitrogen. Tests for ammonium and organic nitrogen are more common on waste-water and other polluted waters, while the test for nitrate is the most common on clean-water samples and treated waste-waters.
Phosphorus appears exclusively as phosphate (PO3/4–) in aquatic environments. There are several forms of phosphate, however, including orthophosphate, condensed phosphates (pyro, meta and polyphosphates) and organically bound phosphates. These may be insoluble or particulate form or may be constituents of plant or animal tissue.
Like nitrogen, phosphates pass through the cycles of decomposition and photosynthesis. Phosphate is a constituent of soils and is used extensively in fertiliser to replace and/or supplement natural quantities on agricultural lands. Phosphate is also a constituent of animal waste and may become incorporated into the soil in grazing and feeding areas.
Runoff from agricultural areas is a major contributor to phosphate in surface waters. The tendency for phosphate to adsorb to soil particles limits its movement in soil moisture and groundwater, but results in its transport into surface waters by erosion. Municipal waste-water is another major source of phosphate in surface water.
Condensed phosphates are used extensively as builders in detergents and organic phosphates are constituents of body waste and food residue. Other sources include industrial waste in which phosphate compounds are used for such purposes as boiler-water conditioning.
While phosphates are not toxic and do not represent a direct health threat to human or other organisms, they do represent a serious indirect threat to water quality. Phosphate is often the limiting nutrient in surface waters.
When the available supply is increased, rapid growth of aquatic plants usually results, with severe consequences. Phosphate can also interfere with water-treatment processes. Concentrations as low as 0.2 mg/l interfere with the chemical coagulation of turbidity.
Phosphates are measured colorimetrically. Orthophosphates can be measured directly, while condensed forms must be converted to orthophosphate by acid hydrolysation and organic phosphates must be converted to orthophosphates by acid digestion.
Results of the analysis are reported as milligrams per liter of phosphate as phosphorus. Careful handling of samples prior to analysis is crucial. For example, acid-washed glass bottles should be used for sampling, as bottles washed in phosphate detergent may contaminate samples.
Biological Parameter of Water Quality:
Water may serve as a medium in which literally thousands of biological species spend part, if not all, of their life cycles. Aquatic organisms range in size and complexity from the smallest single-cell microorganism to the largest fish.
All members of the biological community are, to some extent, water quality parameters, because their presence or absence may indicate in general terms the characteristics of a given body of water. As an example, the general quality of water in a trout stream would be expected to exceed that of a stream in which the pre-dominant species of fish is carp. Similarly, abundant algal populations are associated with a water rich in nutrients.
Biologists often use a species-diversity index (related to the number of species and the relative abundance of organisms in each species) as a qualitative parameter for streams and lakes. A body of water hosting large numbers of species with well-balanced numbers of individuals is considered to be a healthy system. Based on their known tolerance for a given pollutant, certain organisms can be used as indicators of the presence of pollutants.
Pathogens:
From the perspective of human use and consumption, the most important biological organisms in water are pathogens, those organisms capable of infecting, or of transmitting diseases to humans. These organisms are not native to aquatic systems and usually require an animal host for growth and reproduction.
They can, however, be transported by natural water systems, thus becoming a temporary member of the aquatic community. Many species of pathogens are able to survive in water and maintain their infectious capabilities for significant periods of time. These waterborne pathogens include species of bacteria, viruses, protozoa and helminths (parasitic worms).
Bacteria:
Bacteria are considered to be single-celled plants because of their cell structure and the way they take in food. They utilise soluble food taken in through a rigid cell wall. But unlike green plants that use photosynthesis, bacteria do not produce their own food.
Bacteria are very small, typically about 2 μm in size and can be seen only with the aid of a microscope. They occur in three basic cell shapes: rod shaped or bacillus, sphere shaped or coccus and spiral shaped or spirellus. In some cases, the individual cells grow together in larger groups or chains.
Sphaerotilus natans is an example of a species of bacteria that grows in a chain or filament enclosed within a long sheath or tube. Excessive growth of these filamentous organisms is known to be one of the causes of reduced treatment efficiency in biological sewage treatment plants.
In less than 30 minutes, a single bacterial cell can mature and divide into two new cells. This process of reproduction is called binary fission. Under favourable conditions of food supply, temperature and pH, bacteria can reproduce so rapidly that a bacterial culture may contain as many as 20 million individual cells per milliliter after just one day of growth.
This rapid growth of visible colonies of bacteria on a suitable nutrient medium makes it possible to detect and count the number of bacteria in water.
There are several distinctions among the various species of bacteria. One depends on how they metabolise their food. Bacteria that require oxygen for their metabolism are called aerobic bacteria or aerobes.
Those that live only in an oxygen-free environment are called anaerobic bacteria or anaerobes. The distinction between aerobes and anaerobes is of great significance in water pollution and waste-water treatment. (Some species, called facultative bacteria, can live in either the absence or presence of oxygen.)
Another distinction among species of bacteria is a function of the type of food that they require. Those that utilise simple inorganic compounds for nourishment are called autotrophic bacteria; those that require complex organic substances are called heterotrophic bacteria. The nitrifying bacteria, for example, which use ammonia as food and convert it to nitrate, are among the autotrophs.
Other examples of autotrophs include the iron bacteria and the sulphur bacteria. Iron bacteria thrive in some water pipelines and often cause taste and odour problems in drinking water. The sulphur bacteria, which are also anaerobes, are active in sewers and speed the deterioration of concrete pipes by converting hydrogen sulphide gas to sulphuric acid.
One of the most important factors affecting the growth and reproduction of bacteria is temperature. At low temperatures, bacteria grow and reproduce slowly. As the temperature increases, the rate of growth and reproduction just about doubles for every additional 10°C (up to the optimum temperature for the species).
The majority of species of bacteria are classified as mesophilic, having an optimum temperature of about 35°C. Those that do best at elevated temperatures of about 60°C are called thermophilic bacteria. Bacteria with an optimum growth temperature between 0°C and 20°C are called psycrophilic bacteria.
Algae:
Algae are microscopic plants that contain photosynthetic pigments, such as chlorophyll. They are autotrophic organisms that support themselves by converting inorganic materials into organic matter using energy from the sun. During the process of photosynthesis, they take in carbon dioxide from the air and give off oxygen.
A basic characteristic of these simple plants is their lack of roots, stems and leaves. Free-floating algae are also called phytoplankton. (Plankton are tiny floating plants or animals that live in either fresh or salt waters. Over 90 per cent of atmospheric oxygen is produced by salt water or marine phytoplankton, by the process of photosynthesis.)
Even though most species of algae are microscopic, they can be easily noticed when their numbers proliferate in the water. Excessive growths of algae, called algal blooms, are often unsightly. Some algal species are multicellular, growing as filaments that sometimes appear as a green slime in the water.
Common species include the blue-green algae such as Anabaena, green algae such as Spirogyra, yellow-green algae such as Botrydium and red algae such as Gelidium. Another important group of algae, called diatoms, produce hard shells of silica. Deposits of these shells, from dead diatoms, that have accumulated over many hundreds of years form diatomaceous earth, a material sometimes used for filtering water.
Algae play a role in the ageing or eutrophication of lakes. They also are important in waste-water treatment stabilisation ponds. Algae are generally nuisance organisms in public water supplies because of the taste and odour problems that they cause and because of the extra expense required to filter them out of the water.
Protozoa:
Protozoa are the simplest of animal species. These single-celled microscopic animals consume solid or ganic particles, bacteria and algae for food. They are, in turn, ingested as food by higher-level multicellular animals. Floating freely in water, these zooplankton, as they are sometimes called, are a vital part of the natural aquatic food chain. They are also of significance in biological waste-water treatment systems.
Amoebae are-protozoa that move by projecting sections of their bodies; this mobile protoplasm of the amoebae is also used to surround and engulf food particles. Amoebae are commonly found in slimes formed in certain types of sewage treatment processes.
A group of protozoa called flagellates move around in water by means of a long threadlike strand, called a flagella, that propels them with its whiplike action. One such organism, Giardia lamblia, is an intestinal parasite that causes a form of dysentery in humans.
Another type of protozoa has hundreds of short hairs called cilia that propel the organism through the water and that serve to direct food particles into its digestive system. The paramecia, for example- are ciliated protozoa commonly found in freshwater ponds and lakes.
A species of protozoa called Cryptosporidium has been found to be the cause of recent waterborne gastrointestinal disease outbreaks in the United States. These pathogens are frequently found in lakes and streams and are very resistant to disinfection by chlorination (although they can be controlled by ozonation).
The 1996 amendments to the Safe Drinking Water Act call for enhanced surface water treatment rules to prevent such outbreaks.
Viruses:
Viruses are the smallest biological structures known to contain all the genetic information necessary for their own reproduction. So small that they can only be ‘seen’ with the aid of an electron microscope, viruses are obligate parasites that require a host in which to live.
Symptoms associated with waterborne viral infections usually involve disorders of the nervous system rather than of the gastrointestinal tract. Waterborne viral pathogens are known to cause poliomyelitis and infectious hepatitis and several other viruses are known to be or suspected of being, waterborne.
Immunisation of individuals has reduced the incidence of polio to a few isolated cases each year in developed nations. Outbreaks of hepatitis are common throughout the world. Most of the hepatitis cases result from persons eating shellfish contaminated by viruses from polluted waters, although an occasional outbreak will occur at campgrounds or other facilities where crowds gather and where water- supply protection and sanitary facilities are poor.
Although standard disinfection practices are known to kill viruses, confirmation of effective viral disinfection is difficult, owing to the small size of the organism and the lack of quick and conclusive tests for viable virus organisms. The uncertainty of viral disinfection is a major obstacle to direct recycling of waste-water and is a cause of concern regarding the increasing practice of land application of wastewater.
Helminths:
The life cycles of helminths, or parasitic worms, often involve two or more animal hosts, one of which can be human and water contamination may result from human or animal waste that contains helminths. Contamination may also be via aquatic species of other hosts, such as snails or insects.
While aquatic systems can be the vehicle for transmitting helminthal pathogens, modern water-treatment methods are very effective in destroying these organisms. Thus, helminths pose hazards primarily to those persons who come into direct contact with untreated water.
Sewage plant operators, swimmers in recreational lakes polluted by sewage or storm water runoff from cattle feedlots and farm labourers employed in agricultural irrigation operations are at particular risk.
Pathogen Indicators:
Analysis of water for all the known pathogens would be a very time-consuming and expensive proposition. Tests for specific pathogens are usually made only when there is a reason to suspect that those particular organisms are present. At other times, the purity of water is checked using indicator organisms.
An indicator organism is one whose presence presumes that contamination has occurred and suggests the nature and extent of the contaminant(s).
The ideal pathogen indicator would:
(i) Be applicable to all types of water,
(ii) Always be present when pathogens are present,
(iii) Always be absent when pathogens are absent,
(iv) Lend itself to routine quantitative testing procedures without interference from or confusion of results because of extraneous organisms, and
(v) For the safety of laboratory personnel, not be a pathogen itself.
Most of the waterborne pathogens are introduced through fecal contamination of water. Thus, any organism native to the intestinal tract of humans and meeting the criteria would be a good indicator organism. The organisms most nearly meeting these requirements belong to the fecal coliform group.
Composed of several strains of bacteria, principal of which is Escherichia coli, these organisms are found exclusively in the intestinal tract of warm-blooded animals and are excreted in large numbers with feces.
Fecal coliform organisms are non-pathogenic and are believed to have a longer survival time outside the animal body than do most pathogens. Because the die-off rate of fecal coliforms is logarithmic, the number of surviving organisms may be an indication of the time lapse since contamination.
There are other coliform groups which flourish outside the intestinal tract of animals. These organisms are native to the soil and decaying vegetation and are often found in water that was in recent contact with these materials. Because the life cycles of some pathogens (particularly helminths) may include periods in the soil, this group of coliform organisms also serves as an indicator of pathogens.
It is the usual practice in the United States to use the total coliform group (those of both fecal and non-fecal origin) as indicators of the sanitary quality of drinking water, while the indicator of choice for waste-water effluents is the fecal coliform group.
Relatively simple tests have been devised to determine the presence of coliform bacteria in water and to enumerate the quantity. The tests for total coliform organisms employ slightly different culture media and lower incubation temperatures than those used to identify fecal coliform organisms.
The membrane-filter technique, a technique popular with environmental engineers, gives a direct count of coliform bacteria. In this test, a portion of the sample is filtered through a membrane, the pores of which do not exceed 0.45 μm.
Bacteria are retained on the filter that is then placed on selective media to promote growth of coliform bacteria while inhibiting growth of other species. The membrane and media are incubated at the appropriate temperature for 24 hours, allowing coliform bacteria to grow into visible colonies that are then counted. The results are reported in number of organisms per 100 ml of water.
An alternative method often preferred by microbiologists is the multiple-tube fermentation test. Coliform organisms are known to ferment lactose, with one of the end products being a gas. A broth containing lactose and other substances which inhibit non-coliform organisms is placed in a series of test tubes which are then inoculated with a decimal fraction of 1 ml (100,10, 1.0,0.1,0.01, etc.).
These tubes are incubated at the appropriate temperature and inspected for development of gas. This first stage of the procedure is called the presumptive test and tubes with gas development are presumed to have coliforms present. A similar test, called the confirmed test is then set up to confirm the presence of coliform organisms.
Sampling techniques and subsequent handling of the samples are extremely important because samples can easily be contaminated.
It should be emphasised again that pathogens are not identified by the coliform test. The presence of coliform organisms in water does, however, indicate that some portion of the water has recently contacted soil or decaying vegetation or has been through the intestinal tract of a warm-blooded animal. The assumption must then be made that pathogens may have accompanied the coliform bacteria.s