In wastewater treatment process, several quality measuring parameters are used and there are given standards by government or other regulation institute to keep treated water in a quality level. Drinking water and industrial wastewater treatment plants should follow theses standards in their treatment processes parameters should be checked in timely manner to check the quality. In this tutorial, we will learn about these characteristics, quality parameters and standards..
Written by: Pavani Nethma Sandaruwani, (undergraduate), Department of Chemical and Process Engineering, Faculty of Engineering, University of Peradeniya, last modified: 03/06/2021
Water is one of the essential elements for the survival of life on earth. considering the geometric shape of the water molecule, it takes on an angular shape. Due to this shape, the water molecule has a polarity. This value is about 1.85D. So, a water molecule can be considered as a high polar molecule.
Because of the high polarity, most polar compounds are soluble in water. Water also has the disadvantage of acting as a good solvent for polar compounds. That is, various harmful compounds dissolve well in water and then water is rapidly polluted. Therefore, when various compounds are dissolved in water and the water is not suitable for the uses, it is called water pollution.
Water pollution is largely caused by human processes.
Atmospheric water is the lowest polluted water in the water cycle. However, volatile polar compounds dissolve in water in the atmosphere ( Hydrogen sulfide (H2S), ammonia (NH3), nitrogen dioxide (NO2) ). As Well as, due to optical chemical reactions, various chemical compounds (NO2, SO2) are produced in the atmosphere. They dissolves in atmospheric water and form different compounds. ( H2SO4, HNO3).
Both polar and nonpolar compounds are more or less water-soluble. Some compounds dissolve heavily but are not harmful (such as NaCl). Some compounds dissolve a little bit but they may be harmful. (such as chemistry heavy metals)
Because of this, the maximum and minimum level range of soluble compounds and dissoluble compounds in water are determined for water suitability for purposes. These are called the water quality parameters. When these parameters go away beyond the limit, water is not considered suitable for the relevant purposes.
Amount of oxygen required to oxidize soluble oxidizing compounds in water is called chemical oxygen demand. This is measured by mg dm-3. Determining the chemical oxygen demand of water is done by titration.
Here, the water sample is reacted with a strong oxidizer (K2Cr2O7 or KIO3) in an acidic medium. In this case, Ag2SO4 is used as a catalyst. Here, all elements in organic compounds are oxidized. (Ex: C-CO2, S-SO3, P-P2O4, H-H2O).
Under the above condition dichromate ions can oxidize the chloride contained in the water into chlorine. But this is prevented by converting chloride ion to non-decomposable mercuric chloride. So, non-reacted extra dichromate ions are measured by titration with Iron Ammonium sulphate.
If the wastewater contains only organic particles and knowing the compositions of those pollutants and the amounts they contain, the COD value can be calculated by stoichiometry equations.
Biological oxygen demand value is the amount of O2 required for biochemical reactions before the nitrification reactions of a water sample begin. Compounds that use dissolved water affect the BOD value.
These biochemical reactions are caused by microbial activities. These microorganisms synthesize the required energy and materials to their existence by circulating the compounds by metabolic processes.
COD value is always higher than the BOD value. The reason for this is all pollutants are not mineralized by biochemical reactions and in addition to the metabolic results of non-mineralized microorganisms.
The pH scale is a scale that expresses the acidity or alkalinity of the solution. pH is defined as the negative log of the hydrogen ion concentration in the solution.
pH = - log[H+]
There are several methods to determine the pH value
The conductivity of water solvent is a measure of the capability of water to pass electrical flow through its aqueous medium. For this, conductive ions should be present in the water.
The quantity of salts and other inorganic chemicals dissolved in water is caused by conductivity and it increases with salinity increases in water.
When salts are dissolved in water, the salts separate into cations and anions and remain as hydrate ions. When a potential difference is applied, those ions run to the electrodes, causing conductivity. The conductivity meter is used to measure the conductivity of water. Its unit is Siemens (S/cm).
Pure water is completely transparent but wastewater is not transparent due to the presence of suspended particulates. Decrease in the transparency of the water is called the turbidity of the water.
The susceptible particles that cause water turbidity is fine clays, small organic or inorganic particles and very fine polymeric particles.
Turbidity can be measured by using a turbidity meter or turbidity tube.it is measured by measuring how much a beam of light is scattered or transmittance through the water.
Its unit is nephelometric turbidity unit (NTU)