SPSC 2011 Assistant Engineer / Assistant Executive Engineer (Civil) BPS-17, Engineers Cadre CCE (2011) — Sanitary Engineering & Water Supply Paper

Assistant Engineer / Assistant Executive Engineer (Civil) BPS-17, Engineers Cadre CCE (2011) — Sanitary Engineering & Water Supply. Combined Competitive Examination for Engineers Cadre · Monday 28 March 2011, 2:00 pm to 5:00 pm · Max marks 100. In combination with Group-II of Part-I only. Attempt any 05 questions.

1. 1Explain the rational method of design of storm water sewers. Explain the relation between design rainfall intensity and time of concentration. The main sewer is designed to serve an area of 20 km² with an average population of 200 persons per hectare. The average rate of sewage flow is 200 lpcd. The flow is 50% in excess of the average together with rainfall equivalent of 10 mm in 24 hours, all of which are run off. What should be the capacity of sewer in m³/s? (20)

2. 2Write explanatory note on Manholes, Street inlets, Catch basins, Grease and oil traps, Air ejector, Flushing tanks, Proportionate weir inlets, and Method of conveying sewers across obstacles. (20)

3. 3Determine the diameter of a circular sewer to serve an area of 36 hectares (85 acres), having a population of 250 persons per hectare (100 persons per acre). The rate of water supply is 180 litres (40 gals). The sewer is to run 2/3 full. Take velocity of flow as 1.35 m/s (4.5 ft/sec). (20)

4. 4Describe the composition of sewage and also possible methods of measuring the strength of sewage. What is BOD? Deduce an expression for B.O.D. with time. What are the factors on which the de-oxygenation constant rate (k) depends? The BOD of sewage for 7 days at 25 degree Celsius is 250 mg/L. The BOD rate constant is 0.23/day. Calculate the 5 days BOD at 20 degree Celsius. (20)

5. 5Describe the theory of sewage disposal by oxidation ponds. Discuss the suitability of oxidation ponds for sewage treatment in Pakistan. Design a facultative Waste Stabilization Pond based on the following data: (20) (i) Allowable BOD loading = 200 kg/ha/day (ii) Elevation of the locality = 30 m above sea level (iii) Mean monthly temperature = 35 degree Celsius (iv) Sky clearance Factor = 0.90 (v) Population to be served = 40,000 (vi) BOD of sewage = 200 mg/l (vii) BOD removal rate constant = 0.20/day at 25 degree Celsius

6. 6Describe the process of purification of sewage by trickling filters. Sketch and describe the standard rate trickling filter for purification of sewage. What preliminary treatment should sewage undergo before it can be treated by the filter? Describe the biological change that takes place in the filter. (20)

7. 7Describe briefly the various stages that are involved in the construction of 1250 mm (48-in) diameter cast iron sewer, keeping in view that the sewer has to be laid in a waterlogged sandy soil, at a depth of 5.4 m (18 ft) in a busy street. (20) (The paper prints "125 mm"; 48 in ≈ 1220 mm.)

8. 8(a) Draw a flow diagram of line of water treatment plant. (06) (b) Define terminology of Coagulation and briefly determine the types of Coagulants. (07) (c) Determine the quantity of Alum required to treat 13 million litre water/day at treatment plant. Assume reasonable coagulant dosage. (07)