Civil Engineering 2025 Paper II 50 marks Calculate

Q7

(a) The data related to the activated sludge process is given below: Population = 500000 Wastewater contribution = 150 lpcd BOD of settled wastewater = 200 mg/L Effluent BOD required = 30 mg/L Using the data, find the design parameters as mentioned below (Take F/M = 0·2, MLSS = 3000 mg/L, SVI = 100, where MLSS = Mixed liquor suspended solids, SVI = Sludge volume index): (i) Volume of aeration (ii) Efficiency (iii) Volumetric loading (iv) Return sludge ratio (v) Hydraulic retention time (HRT) 20 marks (b) An unlined irrigation channel in an alluvium of median size 0·30 mm is of trapezoidal section with bed width = 3·0 m, side slope = 1·5 H : 1 V and longitudinal slope = 0·00035. If this channel carries a discharge of 1·5 m³/s at a depth of 0·8 m, then determine— (i) the average bed shear stress due to flow; (ii) the shear stress due to grains; (iii) the shear stress due to bed forms. Take γ = 9790 N/m³. 15 marks (c) Define waterlogging. Enumerate any four causes of waterlogging. What are the effects of waterlogging? Describe any five control measures for waterlogging. 15 marks

हिंदी में प्रश्न पढ़ें

(a) सक्रिय अवंक प्रक्रिया से सम्बन्धित आँकड़े नीचे दिए गए हैं : जनसंख्या = 500000 अपशिष्ट जल योगदान = 150 lpcd स्थिर अपशिष्ट जल का बी० ओ० डी० = 200 mg/L वांछित बहिःसावी बी० ओ० डी० = 30 mg/L ऑकड़ों का उपयोग करते हुए नीचे उल्लिखित अभिकल्पना प्राचलों को ज्ञात कीजिए (F/M = 0·2, एम० एल० एस० एस० = 3000 mg/L, एस० वी० आई० = 100 लीजिए, जहाँ एम० एल० एस० एस० = मिश्रित तरल निलम्बित ठोस पदार्थ, एस० वी० आई० = अवपंक आयतन सूचकांक) : (i) वातन का आयतन (ii) दक्षता (iii) आयतनी भारण (लोडिंग) (iv) प्रत्यागमन अवपंक अनुपात (v) जलीय प्रतिधारण समय (एच० आर० टी०) 20 अंक (b) 0·30 mm माध्य आमाप के एक जलोढक में एक अन-आस्तरित (अनलाइंड) सिंचाई वाहिका समलम्बाकार परिच्छेद की है, जिसकी तल चौड़ाई = 3·0 m, पार्श्व प्रवणता = 1·5 H : 1 V और अनुदैर्ध्य प्रवणता = 0·00035 है। यदि यह वाहिका 0·8 m की गहराई पर 1·5 m³/s के प्रवाह का निर्वहन करती है, तो निर्धारित कीजिए— (i) प्रवाह के कारण औसत तल अपरूपण प्रतिबल; (ii) कणों के कारण अपरूपण प्रतिबल; (iii) तल आकृति के कारण अपरूपण प्रतिबल। γ = 9790 N/m³ लीजिए। 15 अंक (c) जलभराव को परिभाषित कीजिए। जलभराव के किन्हीं चार कारणों का उल्लेख कीजिए। जलभराव के क्या प्रभाव हैं? जलभराव के किन्हीं पाँच नियंत्रण उपायों का वर्णन कीजिए। 15 अंक

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Directive word: Calculate

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How this answer will be evaluated

Approach

This is primarily a calculation-based question demanding precise numerical solutions for (a) activated sludge design parameters (20 marks) and (b) channel shear stress components (15 marks), followed by a descriptive part (c) on waterlogging (15 marks). Allocate approximately 40% time to part (a) for computing volume, efficiency, volumetric loading, return sludge ratio and HRT using F/M ratio and SVI relationships; 30% to part (b) for applying Einstein-Barbarossa or Engelund-Hansen partitioning of shear stress in alluvial channels; and 30% to part (c) for defining waterlogging with Indian examples like Punjab-Haryana canal commands, listing causes (seepage, poor drainage, heavy rainfall, obstruction), effects (salinity, reduced crop yield) and control measures (bio-drainage, interceptor drains, lining, crop rotation, tubewell drainage). Present calculations with clear formula statements, unit conversions, and final boxed answers.

Key points expected

  • Part (a): Correct computation of wastewater flow (75 MLD), aeration tank volume using F/M = Q×S₀/(V×X), efficiency via BOD removal, volumetric loading as Q×S₀/V, return sludge ratio from SVI and mass balance, and HRT as V/Q
  • Part (b): Calculation of total bed shear stress τ₀ = γRS, grain shear stress τ' using Einstein's approach or Engelund's τ' = γR'S where R' = 11d₆₅, and bed form shear stress τ'' = τ₀ − τ' for d₅₀ = 0.30 mm
  • Part (c): Precise definition of waterlogging (water table within root zone), four causes including canal seepage in Indo-Gangetic plains, effects on soil aeration and crop productivity
  • Part (c): Five control measures with specificity—surface drainage, subsurface drainage (mole drains in black cotton soils), interceptor drains, canal lining with CC/HDPE, and bio-drainage using Eucalyptus in Rajasthan
  • Appropriate unit handling throughout: m³ for volume, hours for HRT, N/m² for shear stress, dimensionless ratios for efficiency and return sludge ratio

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10Correctly identifies and applies all fundamental concepts: activated sludge kinetics (F/M ratio, SVI-MLSS relationships), shear stress partitioning in alluvial channels (Einstein/Engelund methods), and waterlogging hydrology with specific reference to Indian irrigation commands like Sriram Sagar or Bhakra canal systemsIdentifies most concepts correctly but shows minor errors in formula selection (e.g., using wrong SVI-sludge volume relationship) or oversimplifies waterlogging causes without regional contextFundamental conceptual errors such as confusing MLSS with MLVSS, treating total shear as grain shear only, or defining waterlogging vaguely as mere flooding
Numerical accuracy25%12.5All calculations precise with correct unit conversions: aeration volume ~12,500 m³, HRT ~4 hours, return sludge ratio ~0.25-0.33, shear stress values τ₀ ≈ 2.45 N/m², τ' ≈ 1.8 N/m², τ'' ≈ 0.65 N/m²; final answers boxed with 2-3 significant figuresCorrect methodology but arithmetic slips in unit conversion (lpcd to m³/s) or minor errors in hydraulic radius calculation for trapezoidal section; answers within 10% toleranceMajor calculation errors, wrong order of magnitude (e.g., HRT in days instead of hours), or missing critical steps like settling volume computation from SVI
Diagram quality10%5Clear schematic of activated sludge process (aeration tank, settling tank, return sludge line, waste sludge) and trapezoidal channel cross-section with shear stress distribution; properly labeled with dimensions and flow directionsBasic diagrams present but lacking detail (e.g., no indication of sludge blanket level or velocity distribution in channel)No diagrams or irrelevant sketches; confusing hand-drawn figures without labels
Step-by-step derivation25%12.5Systematic presentation: states given data, writes formula with symbols, substitutes values with units, shows intermediate steps, and derives final answer; for part (b) explicitly shows R = A/P computation and τ partitioning methodologyShows main steps but skips intermediate calculations or assumes certain values without derivation (e.g., direct use of return sludge ratio formula without SVI-based sludge volume calculation)Disorganized jumble of numbers, no formula statements, or jumps directly to answers without showing working
Practical interpretation20%10Interprets design values practically: comments on adequacy of HRT for domestic sewage, compares computed efficiency with typical ASP performance (85-95%), relates shear stress values to sediment transport regime (dunes/ripples), and connects waterlogging control to NABARD/Command Area Development Programme experiences in IndiaBrief mention of practical significance without depth, or generic statements about environmental protection without specific Indian irrigation contextPurely mathematical treatment with no interpretation, or irrelevant discussion unrelated to computed parameters

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