Wastewater is classified based on its physical, chemical, and biological composition.
Physical, chemical, and biological treatment is used depending on the level of pollution and local regulations. To achieve the best water quality, the three treatments are usually combined.
What are the Physical parameters of wastewater?
Color: Fresh domestic sewage is grey, resembling a weak soap solution. With the passage of time, as putrefaction begins, it turns black. The color of septic sewage is more or less black or dark.
The color of industrial wastewater is determined by the chemical processes employed in the industries. When industrial wastewater is mixed with domestic sewage, it can color it.
Odour: Normal fresh sewage has a musty odour that is normally not offensive, but as it ages, it develops an offensive odour. Within 3 or 4 hours, all of the oxygen in the sewage is depleted, and the sewage begins to emit an offensive odour of hydrogen sulphide, gas, and other sulphur compounds, produced by anaerobic microorganisms.
Offensive odours can be harmful in a variety of ways, including:
(i) Decreased appetite for food,
(ii) Decreased water consumption,
(iii) Impaired respiration, nausea, and vomiting, and
(iv) Mental disturbance.
As a result, odour elimination has become a critical consideration in the design and operation of wastewater collection, treatment, and disposal facilities.
Temperature: Due to the addition of warm water from households and industries, the temperature of wastewater is generally higher than that of the water supply. The temperature of the wastewater rises, even more, when it flows in closed circuits.
This causes an increase in the viscosity of water as well as an increase in its bacterial activity. In India, the average temperature of wastewater is around 20°C, which is very close to the ideal temperature for biological activities.
Temperature changes have the following effects on wastewater:
· As the temperature rises, the viscosity of the fluid increases, as does its tendency to precipitate. Extremely low temperatures reduce sedimentation efficiency.
· Bacterial activity increases with increasing temperature, up to about 60°C. It then falls once more. This temperature characteristic thus influences the design and efficiency of wastewater units.
· The solubility of gases in wastewater decreases as temperature rises. This causes the separation of dissolved oxygen and other gases from wastewater, resulting in a reduction in stream self-purification and an increase in bacterial growth.
· The increase in the rate of biochemical reactions that occurs with an increase in temperature, combined with a decrease in the quantity of oxygen present in surface waters, can result in serious depletion of dissolved oxygen concentrations, particularly when large amounts of relatively warm wastewater are discharged into the receiving water.
· When wastewater is discharged into receiving waters, the temperature rises, affecting aquatic life.
· Abnormally high temperatures can promote the growth of undesirable water plants, and wastewater fungus in the receiving waters.
Turbidity: The turbidity of wastewater is determined by the amount of solid matter in suspension. Turbidity is a measure of wastewater's light-emitting properties, and turbidity tests are used to determine the quality of waste discharges, in terms of colloidal matter.
The turbidity of sewage or wastewater is determined by its strength. The higher the turbidity, the stronger or more concentrated the sewage. Turbidity can be measured using a turbidity rod or a Jackson's turbidimeter.
Total Solids: Sewage typically contains 99.9% water and 0.1 percent solids. The total solids content (TS) of a wastewater is defined analytically, as all the matter that remains as residue after evaporation from 103 to 105°C.
Total solids in wastewater can be classified into three types:
· Suspended solids,
· Colloidal solids, and
· Dissolved solids.
Suspended solids are those that can be filtered out using an asbestos mat or filter paper, whereas non-filterable solids are those that cannot be filtered.
Suspended solids are further classified into two types as settleable solids and non-settleable solids.
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