Commonly Used Methods to Calculate Water Quality Indices

Abstract

Water supplied to communities should be of acceptable level in terms of quality. Water quality can be assessed by the use of the water quality index (WQI). The use of indices is crucial in this era where water quality issues have raised health as well as legal concerns, both at local, national, and international levels. Water samples have to be collected, tested, and values for water quality index determined. It was initially proposed by Horton in 1965. There are several ways to calculate WQI, and this chapter gives formulae for different methods. Water quality indices differ from country to country. Some indices use three, six parameters, or even more than six parameters. Earlier methods to calculate the water quality indices did not capture microbial parameters, hence the reason for various methods. A recent method of calculating a WQI is based on fuzzy logic. Therefore, this chapter looks at the methods from all continents.

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Background

Water from surface sources, underground aquifers or atmosphere forming part of the hydrosphere constitutes the main water supply for agricultural, drinking, environmental and industrial use. Keeping the health of these water sources at acceptable levels for the optimum function to support life (Hasan et al. 2015) is critical to the health of living things including human beings. Moreover, knowledge of raw water quality is essential to define the adequate process to treat natural water for human consumption, as well as to assess water treatment plant performance (de Oliveira, et al., 2019). As a consequence, water samples have to be collected, tested and values of various biological, chemical and physical characteristics of water to be measured for the determination of the Water Quality Index. The concept of indexing water with a numerical value to express its quality, based on physical, chemical and biological measurements, was developed in 1965 by National Sanitation Foundation (NSF) in the United States (Lumb et al., 2011) after being proposed by Horton in the same year. Water Quality Indices (WQI) can, and have been used to identify threats to water quality along a stream and contribute to better water resources management (Misaghi et al., 2017). Indeed, WQI is a very useful, efficient, and simple tool for assessing the suitability of water at a certain location and time (Lumb et al., 2011). Use of the indices ensures that water supplied meets the needs of concerned citizens and policy makers thus helping in managing both surface and groundwater quality for different uses (Akoteyon et al. 2011). It uses a mathematical equation to give the health of a waterbody numerically (Yongera & Puttaiah, 2008).

In many papers, book chapters and journal articles, there are several ways to calculate WQI and this chapter gives the formulae for different ways of calculating them. Indices differ from country to country. Some indices use three parameters, some have more than three parameters. One method for six parameters is given based on New Water Quality Index from Malaysia.

In Water Quality Index proposed by the National Sanitation Foundation, the selection of parameters is based on Delphi method and these models were formulated in additive and multiplicative forms. The term Delphi has its origin in the Oracle of Delphi, and this method is based on the belief that group judgements are mature, well-considered and more valid than individual judgements and hence the need for one parameter to stand in for all the other water quality parameters. Delphi is an ancient Greek city where all the men used to consult one woman for major decisions and hence its analogy to Water Quality Index. Another method comes from Europe (Spain) for calculation of the Water Quality Index. There is also Oregon Water Quality Index (OWQI). Another one is the Canadian Water Quality Index also known as the Canadian Council of Ministers of the Environment Water Quality Index (CCME WQI). This model, CCME WQI, was adopted by the United Nations Environmental Programme (UNEP) as a model for Global Drinking Water Quality Index (GDWQI).

Key Terms in this Chapter

Water supplied to communities should be of acceptable level in terms of quality. Water quality can be assessed by the use of the water quality index (WQI). The use of indices is crucial in this era where water quality issues have raised health as well as legal concerns, both at local, national, and international levels. Water samples have to be collected, tested, and values for water quality index determined. It was initially proposed by Horton in 1965. There are several ways to calculate WQI, and this chapter gives formulae for different methods. Water quality indices differ from country to country. Some indices use three, six parameters, or even more than six parameters. Earlier methods to calculate the water quality indices did not capture microbial parameters, hence the reason for various methods. A recent method of calculating a WQI is based on fuzzy logic. Therefore, this chapter looks at the methods from all continents.

pH: Is negative logarithm of hydrogen ion in aqueous solution. It is used to show the measure of equilibrium between acids and bases. It is controlled by carbon dioxide-bicarbonate-carbonate equilibrium system. Some people call it the power of hydrogen. It is measured electrometrically and is affected by temperature ( WHO, 1996 ).

Biochemical Oxygen Demand (BOD): BOD represents the oxygen demanding capacity of organic material both the natural (e.g., decaying plant and animal material) or man-made (like petroleum and other chemicals that are manufactured). BOD is not as site-specific like the other measures of oxygen demand. Organic matter in water can react with chlorine in water treatment plants to form harmful by-products in drinking water ( WHO, 1996 ).

Emerging Pollutants: Emerging pollutants (EMPs) are now found in water bodies and their presence is of great concern globally. EMPs originate from natural as well as synthetic sources. They comprise pharmaceuticals and personal care products (PPCP), soap, hormones, chemical from industries, pesticides, leaks from used information technology gadgets among others. In monitoring water quality, the most authorities have not been measuring these EMPs despite their greatest danger on human health. Indeed, the first water quality index did not capture these EMPs in their equations. EMPs have now been found in most surface and ground-waters in different countries. They reach these water bodies through the surface runoff, effluent from wastewater treatment plants, leakage from sewer lines, septic tank leakage and leachate from landfills. This is because the most of our treatment plants are not designed to treat EMPs which end up short circuiting the system because the pollutants are hydrophilic, with partial degradation, and persistency. If ingested into the biosphere, the EMPs can lead to disruption of our blood cells and antibiotic resistance among other challenges. Little information is documented on the levels of these EMPs in the world. Since they occur in very minute amounts, their measurements cannot be achieved by local tools normally used for other parameters and nowadays people use 2-dimensional gas chromatography coupled to high resolution time-of-flight mass spectrometry (GCxGC-HRTOFNS) to detect them. One of the EMPs called Bisphenol A is found in 62% of water sampled in South Africa ( Wanda et al., 2017 ).

Dissolved Oxygen (DO): As the name suggests it is the amount of oxygen dissolved in water. It is a must for living organisms with other organisms requiring high dissolved oxygen like trout and catfish requiring less dissolved oxygen. It is measured in mg/L by use of DO meter. DO is also reported as percent saturation. Percent saturation tells us what amount of oxygen in dissolved at a given temperature. Increase in temperature reduces the amount of dissolved oxygen. When water holds all the DO it can at a given temperature, it is said to be 100 percent saturated with oxygen (CWT, 2004).

Chemical Oxygen Demand (COD): Measures chemical waste and is related with BOD in water. Organic carbon is changed to carbon dioxide through the oxidation either by burning or by chemical oxidation in water. The carbon dioxide gas is swept out or dissolves in water provoking the pH changes ( WHO, 1996 ).

Nitrate and Nitrite: They occur naturally in the form of ions making part of the nitrogen cycle. Nitrate ion (NO 3 – ) is stable and is chemically unreactive. Its presence is reduced due to microbial action. On the other hand, nitrite (NO 2 – ) is unstable. As with nitrate it is reduced by chemical and biological process. Nitrate is found in inorganic fertilizers used in agriculture. Nitrate is also found in explosives and is a component in making of glass. Sodium nitrate is applied in food preservation. Nitrate are also found in plants as plants absorb it from the soil. Nitrate used in agricultural activities reach the water bodies, both ground and surface water objects ( WHO, 2011 ).

Total Solids: This is alternative for turbidity ( WHO,1996 ). Total solids used in Oregon Water Quality Index includes both suspended and dissolved solids. This means that it represents pollution in terms of physical characteristics as well as dissolved substances. Total solids detect unusually high concentrations of suspended solids (e.g., high flow and erosional conditions that occur during heavy rainfall that results in runoff reaching the stream with elevated levels of suspended solids).

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