Water found in nature is never pure and contains impurities. The quality of water depends on the source of water such water found on the surface is more turbid than the groundwater. Rainwater, although initially pure, absorbs impurities while falling. As it moves on the ground, it collects further impurities. Therefore, it is essential for water engineers to thoroughly analyze and treat the raw water before distributing it to the public, ensuring public health and adhering to standards.
This article aims to expose the reader or water engineer to different water quality parameters, impurities in water, the need for analysis, methods used to determine the nature and extent of various constituents reflecting the quality of water, along with their possible sources and effects.
Water Quality Parameters:
Water quality refers to the chemical, physical and biological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human needs or purpose.
Physical Characteristics of Water
The physical characteristics of water refer to its observable properties and qualities that can be measured or described. These physical characteristics play essential roles in the behaviour, uses, and functions of water in various natural and human-made systems. It becomes essential to identify and address the physical characteristics of water and water quality before supplying to public consumption. Water can be disturbed by a lot of pollutants, affecting its appearance, taste, odour, and overall aesthetic qualities, which can be unsuitable for human consumption and other applications. Here are some key physical characteristics of water:
- Turbidity
- Colour
- Test and Odour
- Temperature
1. Turbidity:
Turbidity is a physical characteristic of water which is a measure of the cloudiness or haziness of water caused by the presence of suspended particles. These particles include sediment, clay, silt, organic matter, algae, and other substances. High turbidity levels in water can affect its aesthetic quality, but more importantly, they can have implications for various water uses and ecosystems. Turbidity depends upon finesses and the concentration of particles present in water.
The measurement of turbidity is typically done using a turbidimeter, which quantifies the amount of light scattered or absorbed by suspended particles in the water. Turbidity is reported in units such as Nephelometric Turbidity Units (NTU) or Formazin Nephelometric Units (FNU). The IS value of turbidity in drinking water is 10 to 25 NTU.
Another measurement for turbidity is Jackson Turbidity Units (JTU), which is equal to turbidity produced by 1 mg SiO2 in 1 litre of distilled water.
2. Colour of Water
The colour of the water is another physical characteristic of the water quality, which is due to dissolved organic matter from vegetation or some inorganic materials. The excessive growth of algae and aquatic micro-organisms may also sometimes
The colour of the water is measured by comparing the colour of the water sample with a standard glass tube containing the solution of different standard colour intensities. The standard unit of colour is that which is produced by one milligram of platinum cobalt dissolved in 1 litre of distilled water. The permissible colour for domestic water is 20 parts per million (ppm) on the platinum cobalt scale.
3. Taste and Odour
The presence of taste and odour in water can be attributed to dissolved organic materials, inorganic salts, or dissolved gases. These dissolved gases may include H2S, CH4, CO2, O2, and others, along with organic matter and mineral substances like NaCl, iron compounds, carbonates, sulphates of various elements, phenols, and other tarry or oily substances. Chlorination, especially, can contribute to the occurrence of tests and odour in water.
Generally, the test contributed by oxygen and carbon dioxide is considered desirable. However, for drinking water, it is crucial to ensure the absence of any undesirable test and odour. To eliminate any taste or odour from water, it is necessary to first determine the chemical composition of the water. This information helps identify the appropriate treatment, if needed, to make the test and odour of the water acceptable for domestic use.
When evaluating the water quality parameter related to taste and odour, the intensity of the odour can be measured using a term called odour intensity, which is associated with the threshold odour number (TON). A device for conducting the test of odour is called an Osmoscope. The threshold odour indicates the dilution ratio at which the odour becomes barely detectable. To assess the odour of the water to be tested, odour-free water is gradually added and mixed until the water becomes detectable to smell. The number of dilutions with odour-free /odourless water represents the threshold odour number (TON).
This test should be conducted in cold water when it is 20 °C (Punmia, p.209). For general water supply to the public, the water should be free from odour. However, as per the Environmental Protection Agency (EPA) of the United States federal government guidelines, a maximum TON is 3 for public health services.
4. Temperature
Surface water generally maintains a temperature equivalent to atmospheric temperature, whereas groundwater may have a temperature that is either higher or lower than atmospheric temperature. The ideal temperature range for public water supply is typically between 10 to 20 °C. Any temperature exceeding 25 °C is considered undesirable for human consumption for any other reasons. Water temperature measurements can be conducted using standard thermometers calibrated within a range of 0.1 °C, spanning from 0 to 50 °C.
When measuring the temperature of large bodies of water, it is recommended to record the temperature at specific depths using a broken capillary thermometer. For depths greater than 15 meters, a thermo-couple may be employed. Monitoring the temperature of the water is crucial as it helps determine properties such as density, viscosity, vapour pressure, and surface tension. These factors play a significant role in the understanding and assessment of water characteristics.
Chemical Characteristics of Water
The chemical characteristics of water refer to the composition and properties of various chemical substances present in water. These substances can be naturally occurring or the result of human activities, and they play a crucial role in determining water quality and its suitability for various purposes. Understanding the chemical characteristics of water is important for assessing its safety, identifying potential contaminants, and implementing appropriate treatment measures. Pure water should have the following chemical properties:
- pH Value of Water
- Total Solid And Suspended Solids
- Hardness of Water
- Heavy Metals
1. pH Value of Water
The pH value denotes the concentration of hydrogen ions (H+) in the water and serves as a measure of the acidity or alkalinity of a substance. By taking the negative logarithm of the reciprocal of H+, pH allows us to quantify the concentration of these ions. Higher values of pH indicate lower hydrogen ion concentrations, representing alkaline solutions. Conversely, lower pH values signify higher hydrogen ion concentrations, indicating acidic solutions.
In the case of pure water, it maintains a balance between positively charged hydrogen ions (H+) and negatively charged hydroxyl ions (OH-). The self-ionization of water occurs when a small proportion of water molecules naturally dissociate, forming equal amounts of H+ and OH- ions. This dynamic equilibrium results in a pH of 7, indicating a neutral solution. Pure water is essential as a reference point for determining the acidity or alkalinity of other substances. Any deviation from pH 7 in water indicates the presence of additional acidic or alkaline components.
It’s worth noting that while pure water is considered neutral with a pH of 7, the pH of natural water sources can vary due to the presence of dissolved minerals and gases. Factors such as carbon dioxide absorption and geological characteristics of the water source can influence its pH. Additionally, various industries and applications may require water with specific pH ranges to meet their needs, and water treatment processes can be employed to adjust and control the pH accordingly.
2. Total Solid And Suspended Solids
Total solids in water encompass the combined amount of suspended solids, colloidal solids, and dissolved solids. To measure the quantity of suspended solids, a water sample is filtered through a fine filter, dried, and weighed. The quantity of dissolved and colloidal solids is determined by evaporating the filtered water from the suspended solid test and weighing the residue. This process allows for the direct determination of total solids in a water sample.
Further analysis can be conducted by fusing the residue of total solids in a muffle furnace. Organic solids will decompose during this process, while only inorganic solids will remain. By weighing the residue after fusion, it becomes possible to determine the amount of inorganic solids. Subtracting the weight of inorganic solids from the measurement of the total solids enables the calculation of organic solids present in the water sample.
The acceptable level of solids in water is typically restricted to 500 parts per million (ppm). However, in some instances, higher concentrations of up to 1000 ppm might be allowed, although they have the potential to induce specific psychological effects on the human body.
Also, read: Unit Hydrograph: Theory | Application | Limitation
3. Hardness of Water
The hardness of water is the characteristic which prevents the formation of sufficient leather or foam when such hard water is mixed with soap. It is caused by the presence of specific salts such as carbonate, bi-carbonates, chlorides and sulphates of Calcium and Magnesium present in water which forms scum by reaction with soap.
Hard water can be problematic as it leads to increased soap usage, scale formation in boilers, corrosion and deposits in pipes, and imparts an unpleasant taste to food, among other inconveniences.
Bicarbonate or carbonates of calcium and magnesium in water result in temporary hardness. It is referred to as temporary hardness or carbonate hardness. This hardness can be partially removed by boiling the water or completely eliminated by adding lime. Boiling temporary hard water causes the escape of carbon dioxide gas, leading to the precipitation of insoluble calcium carbonate. However, magnesium carbonate, which is more soluble, remains in the water even after boiling. Consequently, temporarily hard water tends to deposit calcium scales in boilers.
The sulphates, chlorides and nitrates of calcium or magnesium are terms as permanent hardness or non-carbonate hardness of the water. Such hardness cannot be removed at all by simple boiling. Therefore, special treatment for softening is required.
There are three methods of determining the total hardness of water: i) Clark’s method, ii) Hener’s method and iii) Versenate method. The measurement of the hardness of water is defined as the calcium carbonate equivalent of calcium and magnesium ions present in water and is expressed in mg/l.
Also, read: Different Methods of Water Distribution: System of Water Supply
Removal of temporary Hardness from water
3.1. Boiling of Calcium Carbonate and Magnesium Carbonate
3.2. Adding Lime to Calcium Carbonate and Magnesium Carbonate
4. Metals and Heavy Metals:
Heavy metals like lead, mercury, cadmium, and arsenic can contaminate water sources through industrial activities, mining, or improper waste disposal. These metals are toxic and can have detrimental effects on human health and the environment. Monitoring and controlling heavy metal concentrations in water are important for protecting public health.
4.1. Iron and Manganese:
Iron in water causes hardness, bad taste, discolouration of clothes and plumbing fixtures and incrustation in water mains. Water containing iron is termed Ferruginous. Thiocyanate or Thioglycolic acid (TGA) are the two commonly used compounds to estimate the presence of iron in the water.
The standard maximum contaminant level (SMCL) for iron in drinking water is set at 0.3 mg/L (milligrams per litre), which is equivalent to 0.3 parts per million (ppm). This concentration was established because it represents the saturation limit for ferric iron. When the concentration of ferrous iron in water is lower than this limit, the iron remains in solution. Upon exposure to air, the ferrous iron oxidizes to ferric iron, but since it remains in solution, there is no precipitation or staining of the water. Consequently, the water retains its clarity without any noticeable taste or odour issues.
Manganese lends a brownish or purplish hue to water and can cause discolouration of laundered items and staining of plumbing fixtures when oxidized. To estimate the manganese content, a method involving the comparison of the pink colour produced during oxidation with permanganate is employed. Sodium para periodate or ammonium persulfate is utilized to generate matching colouration in both the water sample and manganese colour standards.
4.2. Lead and Arsenic:
Both Lead and Arsenic are widely recognized as toxins to the human body. Exposure to lead regularly or temporarily can have severe health consequences and even result in death. It is recommended by EPA that the concentration of lead in the public drinking water system delivered to users for the public should be kept below 0.05µg/L (parts per billion) under normal circumstances. The presence of lead in water can be identified by adding six drops of sulphuric acid, which will cause the formation of a white precipitate.
To remove arsenic from water, ion exchange equipment can be employed, utilizing active alumina or bone char as adsorbents. These materials facilitate the exchange of ions, effectively eliminating arsenic from the water supply. For the detection of trace amounts of arsenic, two methods can be used: the heteropoly blue calorimetric method and the Gutzeit method. These methods involve comparing the stains produced on treated paper strips to determine the presence of arsenic.
4.2. Sodium and Potassium:
The human body has a biological regulatory mechanism that maintains a consistent sodium level, even when there are significant variations in sodium intake. Any excess sodium is primarily eliminated through urine. However, individuals with heart, kidney, or liver diseases may have difficulty eliminating sodium, leading to a condition called edema, characterized by the accumulation of sodium-containing fluid. The concentration of sodium can be determined using methods such as flame photometry or gravimetry. Sodium and potassium levels can also be estimated together and expressed as sodium chloride. In specific cases, sodium can be separately determined using sodium zinc uraylacetate. Potassium can be estimated using perchlorate, which has low solubility in organic solvents, or dipotassium cobaltinitrite.
Also, read: Types of Water Demand
Bacterial And Microscopical Characteristics
The biological and microbiological characteristics of water refer to the presence and composition of various living organisms in water bodies. These organisms can include bacteria, viruses, algae, protozoa, and other microscopic organisms.
Water quality testing for bacteria and microscopic characteristics is essential before delivering it to the public in order to mitigate numerous health issues. By conducting these tests, potential risks associated with bacterial contamination and other microscopic organisms can be identified and addressed, ensuring the safety of the water supply and protecting public health.
As the rainwater falls down through the atmosphere, collects bacteria and other parasitic organisms from the dry dust and smog present in the atmosphere. The initial rainwater washes away most of the dust and is likely to be highly contaminated. The number of bacteria in a teaspoon of rainwater varies from about 5 to 225. Water falling as snow is likely to contain more bacteria than water falling as rain because a snowflake has a larger surface than a raindrop (Garg, p.400).
1. Bacteria
Bacteria are minute single-cell organisms possessing no defined nucleus and no chlorophyll to help them manufacture their own food. They are generally present in raw or contaminated waters at various water sources. They are so small in length (of size 1 to 4 microns) that they cannot be seen with the naked eye and have to be examined under a microscope. Most bacteria are harmless, and under certain conditions beneficial to human beings, animals and crops. Such bacteria or microorganisms are called non-pathogenic bacteria or simply non-pathogens. Whereas, certain types of bacteria are known to cause serious water-borne diseases such as cholera, typhoid, infectious hepatitis, etc. Such bacteria or microorganisms are called pathogenic bacteria or pathogens.
1.1. Escherichia Coli (E. Coli): E. Coli bacteria live in the human or animal intestines. E. Coli is a type of bacteria that is used as an indicator of faecal contamination in water. Detection of E. Coli in drinking water is an indication of possible recent pollution of human or animal faeces.
1.2. Clostridium welchii: It is sometimes known as Clostridium perfringens which is a Gram-positive, spore-forming bacterium. Clostridium perfringens, the most prevalent bacteria responsible for foodborne illnesses, is commonly present in cultivated soil, sewage, and contaminated water. It naturally resides in the intestines where it is typically harmless and even aids in digestion. Clostridium welchii is known to be an opportunistic pathogen, meaning it can cause infections under certain conditions or when the immune system is compromised. It produces toxins that can lead to various diseases and infections,
1.3. Faecal streptococci: Faecal streptococci, also known as faecal streptococci or faecal coliforms, are bacteria found in the intestinal tracts of humans and warm-blooded animals. Their presence in water indicates faecal contamination and serves as an indicator of potential pathogens and overall faecal pollution. Although they are not pathogenic themselves, their detection warns of possible harmful bacteria, viruses, or parasites associated with faecal matter. Monitoring faecal streptococci levels helps assess water quality and the effectiveness of sanitation and treatment processes. Detection methods include culture-based techniques and molecular methods like polymerase chain reaction (PCR) and gene sequencing.
1.4. Nuisance Bacteria: Nuisance bacteria refer to certain types of bacteria that can degrade the water quality if present. Their presence in water can lead to issues such as pitting and tuberculations in pipes, rendering the water unsuitable for various applications such as building purposes, air conditioning, paper manufacturing, food production, and other industries. These bacteria can cause problems such as odours, unusual taste, frothing, discolouration, and increased turbidity in the water. Specifically, iron bacteria have the ability to oxidize ferrous iron to ferric iron, leading to friction loss in water mains.
2. Viruses:
Viruses are small infectious agents that rely on a host organism to replicate. Waterborne viruses can cause hepatitis A, norovirus, or rotavirus. Monitoring and controlling the presence of viruses in water is crucial to prevent outbreaks and ensure public health.
3. Algae:
Algae are a diverse group of photosynthetic organisms that can range from single-celled to multicellular forms. In aquatic environments, excessive algal growth, known as an algal bloom, can occur due to factors like nutrient pollution. Some types of algae produce toxins, called harmful algal blooms (HABs), which can harm aquatic life and pose risks to human health if ingested or exposed to contaminated water.
4. Protozoa:
Protozoa are single-celled organisms that are commonly found in water bodies. While many protozoa are harmless, certain species like Cryptosporidium and Giardia can cause waterborne illnesses. These parasites are resistant to chlorine disinfection and can survive in water sources, making their detection and control important for safe drinking water.
Also, read: Per Capita Demand of Water | Factor Affecting Per Capita Demand
FAQs
Q: According to EPA guidelines, What should the Threshold Odour Number (TON) be for public water supply?
Ans: According to EPA (Environmental Protection Agency) guidelines, the TON for public water supply should be a maximum of 3.
Q: According to EPA guidelines, What should the Threshold Odour Number (TON) be for public water supply?
Ans: According to EPA (Environmental Protection Agency) guidelines, the TON for public water supply should be between 1 and 3.
Q: How can lead be detected in water?
Ans: Lead in water can be detected by adding six drops of sulphuric acid, which will cause the formation of a white precipitate.
References:
- Garg, S.K. (2010). Environmental Engineering. Vol.-I. Water Supply Engineering (35th ed.). Khanna Publishers. Daryaganj, New Delhi-110002.
- Punmia. B.C, Jain. A & Jain. A. (2005)Water Supply Engineering (2nd ed). Laxmi Publications (P) Ltd. New Delhi.
- Varandani, N. S. (2017) Environmental Engineering. Principles and Practices. Vol.–I. Water Supply Engineering. Pearson India Education Services Pvt. Ltd. Uttar Pradesh, India.
- Modi, P.N.(2010). Water Supply Engineering, Vol.I Standard Book House, New Delhi.
- World Health Organization. (1971). (3rd edition) International Standards for Drinking Water. https://apps.who.int/iris/bitstream/handle/10665/39989/9241540249_eng.pdf;sequence=1
- World Health Organization. (2017). Guidelines for Drinking-water Quality (4th ed.). https://www.who.int/publications/i/item/9789241549950
- Iron. (n.d.). Know your H2O. https://www.knowyourh2o.com/indoor-6/iron
