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Water quality assessment FAQ Frequently Asked Questions

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Which qualitative analyses define water quality?

In order to assess the need for treatment and the right treatment technology, specific contaminants in water must be identified and measured. Water contaminants can be divided into two groups: dissolved contaminants and suspended solids. Suspended solids, such as silt, sand and viruses, are usually responsible for visible impurities. Suspended matter consists of very small particles, which cannot be removed by means of settling. They can be identified through description of visible characteristics of water, including turbidity and clarity, taste, colour and odour of the water:

- Suspended matter in the water absorbs light, causing the water to have a cloudy appearance. This is called turbidity. Turbidity can be measured with several different techniques, that show the resistance to light transmission of the water.
- Taste sense is able to detect concentrations of a few tenths to several hundreds of ppm. Taste can indicate that contaminants are present, but it cannot identify specific contaminants.
- Colour can suggest that organic impurities are present. In some cases colouring of water can even be caused by metal ions. Colour is measured by comparison of different samples visually or with a spectrometer. This is a device that measures light transmission in a substance, in order to calculate concentrations of certain contaminants. When water has an unusual colour this usually does not mean a health concern.
- Odour detection can be useful, because smelling can usually detect even low levels of contaminants. However, in most countries detecting contaminants through odour is bound to strict regulations, as it can be a danger to ones health when dangerous contaminants are present in a sample.

The total amount of suspended matter can be measured by filtering samples through a membrane and then drying and weighing the residue. Suspended matter is expressed in ppm (parts per million), usually mg/L.

Identification and quantification of dissolved contaminants is done by means of very specific methods in laboratories, because these are the contaminants that are associated with health risks.

Which quantitative analyses define water quality?

Water quality can also be assessed by a number of quantitative laboratory analyses, such as pH, Total Solids (TS), Conductivity and microbial contamination.
The pH is the value that determines if a substance is acid, neutral or basic, calculated from the number of hydrogen ions present. It is measured on a scale from 0 to 14, on which 7 means the substance is neutral. pH values below 7 indicate that a substance is acidic and pH values above 7 indicate that it is basic. When a substance is neutral the number of hydrogen and hydroxyl atoms are equal. When the number of hydrogen (H+) atoms exceeds the number of hydroxyl (OH-) atoms, a substance is acidic.
This is what the pH-scale looks like:

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
acidic neutral basic

Image 1

A pH-meter

The pH level has an effect on many phases of the water treatment process and it affects the scaling-potential of water sources. The pH level can be determined through various analyses methods, such as colour indicators, pH-paper or pH-meters.

Total Solids (TS) is the sum of all dissolved and suspended solids in water. When water is analysed for TS a sample is dried and after that the residue is weighed. TS can be organic and inorganic substances, microorganisms and larger particles such as sand and clay.

Conductivity means the conduction of energy by ions. The measurement of the water's conductivity can provide a clear view of the concentration of ions in the water, as the water is naturally resistant to conduction of energy. Conduction is expressed in Siemens and it is measured with a conductivity meter or cell.

Microbial contamination is divided up in contamination by organisms that have the ability to reproduce and multiply and organisms that cannot do so. Microbial contamination can be contamination by bacteria, which is expressed in Colony Forming Units (CFU), a measure of the bacterial population. Another microbial contamination is contamination by pyrogens. Pyrogens are bacterial products that can induce fever in warm-blooded animals. Next to bacteria and pyrogens waters can also be contaminated by viruses.

Analyses can also be done by measurements of the Total Organic Carbon (TOC) and the Biological and Chemical Oxygen Demand (BOD and COD). BOD is a measure of organic matter in the water, expressed in mg/L. It is the amount of dissolved oxygen that is required for the decomposition of organic matter. The test for BOD takes a five-day period. COD is a measure of organic and inorganic matter in the water, expressed in mg/L. it is the amount of dissolved oxygen required for full chemical oxidation of contaminants.

For water terminology check out our Water Glossary or go back to water FAQ overview

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