Question

Q9. A Waste water treatment units is proposed to be designed for the Wilayat of Sur. Population of Sur is 200,000 and an average sewage generation is 210 liter per capita per day. Design an appropriate waste water treatment plant using the following given data. If needed assume other necessary data.

Approach channel velocity = 42 m/min, d = 1.5 w Design three aerated grit chamber Detention time in grit chamber = 3 min Depth of aerated grit chamber = 2.0 m Depth to width ratio in aerated grit chamber is 1:1.2 Total depth of primary clarifier = 3 m (Mechanical Sludge removal) Overflow rate = 20 m3/m2.d Detention time in the primary clarifier should be greater than 1.5 hours Secondary clarifier with a depth of water equal to 0.5 times the Length (Mechanical Sludge removal) Detention time in secondary clarifier = 2 hours

a. List and explain briefly the important parameters to be analyzed for the waste water before and after treatment. (2)

b. Is waste water treatment is compulsory before disposing it to the land? Justify. (2)

c. What is meant by sludge? Is sludge resource or waste? Explain. (2)

d. Why is the aeration tank followed by primary sedimentation process? (2)

e. Design (size of the unit) the following units of the treatment process: Rectangular Bar screen channel, Rectangular Aerated Grit chamber, Six Circular Primary Clarifier, and Six Square Secondary Sedimentation tank.   

Answer 1

a.) Important parameters to be analyzed for waste water before and after treatment are :-

* Physical parameters like colour, odour, temperature, turbidity etc.

Before treatment water water have dark colour due to presence of chemicals and organic matter and after treatment it should be light coloured before disposal otherwise it may pollute the water bodies.

Temperature for untreated waste water may be high or low depending on type of industries they are getting received but after treatment it should match with the water body temperature in which they are to be disposed otherwise it may pose a serious problems for the aquatic life there.

Waste water may have foul smell before treatment but it should be odourless after treatment.

* Chemical parameters like hardness, alkalinity, toxic metals etc.

Hardness and alkalinity may be in significant quantity before treatment in waste water but should be removed to a permissinle limit after treatment before disposal.

Toxic metals present before treatment should also be removed otherwise exposure of such metals may lead to dangerous diseases.

* Biological parameters like microorganisms are present in every waste water as organic materials are the prime impurity in waste water so organic materials must be digested before disposal otherwise the microorganisms deplete the dissolved oxygen in water bodies leading to problems for aquatic life.

b.) Yes, treatment is compulsory before disposing waste water over land as waste water contains lots of impurities in the form of suspended solids, metal particles, dissolved solids, animal and plant residues etc. Which are disposed on land surface directly without any treatment then it may choke the voids of land surface and percolation of waste water is hindered which may lead to sewage sickness and development of insects, flies, mosquitoes in the vicinity may lead to development of several diseases which puts the life there in danger.

C.) Sludge is a form of waste which is generated as a result of waste water treatment. Raw water or pure water when consists of impurities it becomes waste water. When these impurities are removed it takes the form of a thick residue or slurry termed as sludge. It comprises of animal and human excreta, plant wastes, industrial wastes and dissolved and suspended organic and in organic matter. It is waste not a resource but sometimes can be utilized as a resource for energy generation, gas production etc. because of it's calorific value.

d.) In primary sedimentation process mostly suspended organic matter are removed by settling process and roughly small quantity of dissolved organic matter are also removed. But main and serious impurity in waste water is organic matter which is removed by biological action of aerobic or anaerobic micro organisms in a special treatment units.

So for this purpose aeration tanks are provided post primary sedimentation process for the removal of dissolved organic matter where oxygen can be supplied externally as per the requirement. So it is designed specifically that is why it is provided after primary sedimentation process.

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