PHILOSOPHY OF WATER AND WASTEWATER TREATMENT

SOLUTION

1. Discuss the philosophy behind water and wastewater treatment with reference to water/wastewater quality characteristics, standards, unit processes and process train. (3)

Answer:

Water from natural sources, i.e., surface or ground water will contain certain impurities, either in particulate or dissolved form. A listing of the concentration of these impurities is known as water quality characteristics. This water is to be used for beneficial purposes like domestic, agricultural or industrial use. Maximum acceptable quantities of impurities in water for various beneficial uses are specified. A listing of these values are known as water quality standards. A comparison of the water quality characteristics and standards enables one to determine the impurities in the water whose concentration is unacceptably high for a particular beneficial use. The purpose of water treatment is to remove these impurities, such that the water conforms to the standards, and hence is suitable for the designated beneficial use. Unit processes are devices/machines/reactors designated to remove one or more of these impurities during water treatment. Several such unit processes put together constitute a process train, which will remove the required impurities, and thus make water conform to the water quality standards.

During beneficial use of water, impurities are added to it, and water is thus converted to wastewater. Wastewater may be disposed to land or water bodies or reused or recycled. Wastewater disposal or reuse/recycling standards are available. In case the concentrations of impurities in wastewater, i.e., wastewater characteristics are incompatible with the prevailing standards, wastewater treatment is required. As with water treatment, suitable process train must be provided for this purpose to reduce the concentration of the objectionable impurities.

2. Write a short note to clearly bring out the difference between disposal, reuse and recycling of wastewater. (3)

Answer:

Due to the addition of impurities during beneficial use, water is transformed to wastewater.

This wastewater may be discharged to aquatic or terrestrial environment, provided the relevant discharge standards are attained through wastewater treatment. This is known as wastewater disposal.

Alternatively, the wastewater may be used for certain other beneficial purpose, provided adequate treatment is given to the wastewater to conform to the water quality standards for that beneficial use. This procedure of using water for multiple beneficial use is known as reuse.

Alternatively, the wastewater may be used for the same beneficial purpose as before, provided adequate treatment is given to the wastewater such that it again conforms to the standards for the original beneficial use. This procedure for using water for the same beneficial use is known as recycling.

3. Why is equilization necessary in a wastewater treatment plant. With appropriate diagrams describe the difference between on-line and off-line equilization. (1+3)

Answer:

Rapid changes in wastewater quantity and quality make operation of a wastewater treatment plant difficult. This is because of the necessity to constantly adjust the process parameters in the treatment units to compensate for the unsteady conditions under such circumstances. Provision of an equilization tank dampens the rapid changes described above, and hence the treatment plant can operate under steady state conditions.

QS QU Equilization Tank On-Line Equilization: Variable flow, QU is influent to the equilization tank. However, a steady flow QS is effluent from this tank. When QU>QS, excess wastewater is stored in the tank. When QU<QS, the deficit is made up from the wastewater stored in the tank.

Off-line Equilization: When QU > QS, the difference, i.e., QU-QS is stored in the equilization tank. When QU < QS, the difference, i.e., QS-QU is supplied from the equilization tank.