Question 1

A 2% dilution of a sewage sample is incubated for 5 days at 20°C, and the measured dissolved oxygen depletion is 8 mg/L. Compute the 5-day BOD (BOD₅) of the undiluted sewage.

Accepted Answer

Introduction / Context: The biochemical oxygen demand (BOD) test quantifies biodegradable organic load. Because raw sewage typically depletes oxygen rapidly, dilutions are used so that the final dissolved oxygen (DO) remains above zero after five days. The undiluted BOD is then inferred from the observed depletion and the dilution fraction. Given Data / Assumptions: Dilution fraction f = 2% = 0.02. Observed 5-day DO depletion = 8 mg/L. Incubation temperature: 20°C (standard for BOD₅). No seed correction or blank depletion specified; assume negligible. Concept / Approach: For a simple BOD test without seed corrections, BOD₅ (undiluted) = observed DO depletion / dilution fraction. This rescales the consumption measured in the diluted bottle back to the original strength of the sewage. Step-by-Step Solution: Compute BOD₅ = 8 mg/L / 0.02.BOD₅ = 400 mg/L.Select the matching option. Verification / Alternative check: A 2% dilution is a 50:1 factor. Multiplying the depletion by 50 yields the same result: 8 * 50 = 400 mg/L. Why Other Options Are Wrong: 100–300 mg/L: Underestimate; they do not account for the full 50× dilution. 500 mg/L: Overestimate relative to the measured depletion. Common Pitfalls: Forgetting to adjust for seed oxygen demand (if seed is used), or misreading the dilution percentage as 0.2 instead of 0.02; both lead to substantial errors. Final Answer: 400 mg/L

Question 2

Which historical relation is commonly associated with selecting or relating roughness values (that can be translated to Manning's n ≈ 0.012 for smooth pipes) for use in open-channel flow computations?

Accepted Answer

Introduction / Context: Several hydraulic formulae predate today's widespread use of Manning's equation. Designers historically used relations that provided coefficients convertible to present-day roughness terms. The question points to the tradition of choosing smooth-conduit values that correspond roughly to Manning's n ≈ 0.012. Given Data / Assumptions: Smooth pipe or finished surface representative of low roughness. Open-channel or full-flow conditions requiring empirical coefficients. Concept / Approach: Kutter's formula was widely used to obtain Chezy's C as a function of hydraulic radius and a roughness parameter. From Chezy's C, equivalence to Manning's n can be established since Manning's and Chezy's forms are interrelated. Values that imply n around 0.012 are typical for very smooth pipes (e.g., glazed or drawn tubing), offering a historical pathway to today's roughness selection. Step-by-Step Solution: Recognize that Kutter provided a formula to determine Chezy's C based on roughness.Use the C–n relationship: C = (1/n) * R^(1/6) to convert between systems.Note that a smooth-pipe selection can map to n ≈ 0.012, a commonly cited low roughness value. Verification / Alternative check: Hydraulic texts list tables showing equivalences among Chezy C, Manning n, and Kutter/Bazin parameters; Kutter is most frequently cited for historic selection of C leading to modern n-values. Why Other Options Are Wrong: Bazin: Another relation for Chezy C, but the common exam linkage for the n ≈ 0.012 reference is typically associated with Kutter. Williams–Hazen: Pressurized water distribution loss formula; not an open-channel roughness selection path. Crimp and Burge/Scobey: Less standard in this specific context for municipal sewers. Common Pitfalls: Confusing the role of formulas that produce Chezy C versus Manning n directly; forgetting that conversions depend on hydraulic radius assumptions. Final Answer: Kutter

Question 3

Safe entry into sanitary sewers requires strict precautions. Which of the following statements is clearly incorrect and unsafe with respect to permitting workers to enter a sewer?

Accepted Answer

Introduction / Context: Confined space entry into sewers is hazardous due to toxic gases (H₂S), oxygen deficiency, and explosion risks (methane). Occupational safety protocols mandate ventilation, testing, and emergency preparedness. Given Data / Assumptions: Entry into an active or recently active sanitary sewer. Presence of potential toxic and flammable atmospheres. Standard confined-space procedures apply. Concept / Approach: Safe practice includes ventilation (opening manholes and forced air if needed), pre-entry atmospheric testing, continuous monitoring, proper PPE, and rescue readiness. Any practice that introduces ignition sources (e.g., smoking) is strictly prohibited because sewer atmospheres may contain methane and volatile organics. Step-by-Step Solution: List accepted precautions: ventilation, gas testing, barricading, trained personnel, rescue gear.Identify the statement that advocates an ignition source (smoking) in a potentially explosive atmosphere.Conclude it is incorrect and unsafe. Verification / Alternative check: Standard safety codes (confined space entry) forbid open flames and smoking; calibration-checked gas detectors are used to test oxygen, H₂S, CO, and combustible gases instead. Why Other Options Are Wrong: Ventilating manholes: Correct and necessary. Gas testing: Mandatory. Warning signs/barricades: Required for site safety. Rescue/ventilation equipment: Essential preparedness. Common Pitfalls: Relying on smell to detect H₂S (olfactory fatigue); entering without retrieval lines; neglecting continuous monitoring during work. Final Answer: Workers should be advised to smoke in the sewer to check for gas presence

Question 4

In the design of a grit chamber for sewage treatment, which set of typical design parameters is appropriate to ensure grit removal without carrying organics?

Accepted Answer

Introduction / Context: Grit chambers remove heavy inorganic particles (sand, silt) to protect downstream pumps and clarifiers. The hydraulic conditions must allow grit to settle while keeping most organics in suspension. Given Data / Assumptions: Municipal wastewater with typical grit characteristics. Preliminary treatment stage before primary sedimentation. Constant approach velocity through the chamber. Concept / Approach: Effective grit removal is achieved by controlling velocity and detention time. Velocities around 0.15–0.30 m/s let sand settle but keep organics moving. Shallow to moderate depths (≈0.9–1.2 m) and detention ≈1 minute are widely used baselines in horizontal-flow grit chambers. Step-by-Step Solution: Select a target horizontal velocity: 0.2–0.3 m/s is common.Set detention time near 1 minute to limit basin size while achieving settling of grit sizes 0.2–0.25 mm.Adopt a practical depth around 1 m to simplify cleaning and maintain stable flow. Verification / Alternative check: Design manuals present similar ranges, with aerated grit chambers using controlled spiral flows but comparable effective velocities. Why Other Options Are Wrong: Very high velocities (≥0.8–1.0 m/s): Carry grit through; cause scouring. Very low velocities (≤0.05 m/s): Settle organics and cause odor. Extreme depths or detention times are unnecessary and uneconomical. Common Pitfalls: Ignoring seasonal flow variation; not providing velocity control (proportional weirs or parshall flumes) leading to variable performance. Final Answer: Maintain flow velocity about 0.15–0.30 m/s, provide 0.9–1.2 m depth, and about 1 minute detention

Question 5

The portion of liquid that enters a sewer system specifically due to rainfall (runoff entering inlets and conduits) is called:

Accepted Answer

Introduction / Context: Understanding wastewater components is vital for selecting separate versus combined sewerage and for sizing conveyance and treatment units. During storms, an additional component beyond sanitary flow enters the system. Given Data / Assumptions: Rainfall-runoff captured by drains and inlets. Terminology consistent with municipal engineering practice. Concept / Approach: Storm sewage (also called stormwater) is the runoff from rainfall or snowmelt conveyed by storm drains. In combined systems, it mixes with sanitary sewage; in separate systems, it is routed to storm drains and often discharged to water bodies after appropriate control. Step-by-Step Solution: Identify the source: precipitation-derived runoff.Match to standard term: storm sewage.Differentiate from sanitary sewage (domestic/industrial wastewaters). Verification / Alternative check: Definitions in textbooks consistently distinguish sanitary sewage (from households/industry) from storm sewage (from rainfall), with infiltration/inflow considered extraneous water. Why Other Options Are Wrong: Industrial wastewater: Process discharges; not rainfall-derived. Sanitary sewage: Domestic wastewater independent of rainfall. Infiltration water: Groundwater entering through defects; not the same as storm runoff by definition. Leachate: Liquid from waste percolation; unrelated to storm drains. Common Pitfalls: Confusing storm sewage with infiltration/inflow; in practice all may co-occur but terminology stays distinct for planning and regulatory reporting. Final Answer: Storm sewage

Question 6

For detecting nitrates in sewage in the laboratory, which reagent combination is used to develop the characteristic yellow colour that is later intensified with alkali?

Accepted Answer

Introduction / Context: Nitrogen species monitoring (ammonia, nitrite, nitrate) is important for wastewater process control and environmental compliance. A classical colorimetric method exists for nitrate determination in sewage. Given Data / Assumptions: Bench-top chemical testing without sophisticated instrumentation. Interferences are minimal or accounted for by standard procedure. Concept / Approach: The phenol-disulphonic acid (PDA) method converts nitrates in a dried residue to a nitro-derivative that produces a yellow colour. After neutralization with a strong alkali such as potassium hydroxide (KOH), the colour intensity becomes proportional to nitrate concentration and is read against standards. Step-by-Step Solution: Evaporate a measured sample to dryness.Add phenol-disulphonic acid reagent to react with nitrates (nitration step).Neutralize with KOH; a yellow colour develops.Compare the colour intensity with calibration standards to quantify nitrate. Verification / Alternative check: This classical method appears in many standard laboratory manuals as a reliable colorimetric technique for nitrate in the presence of minimal nitrite interference. Why Other Options Are Wrong: Potassium permanganate: Not a specific colour reagent for nitrate. Sulphuric acid and naphthylamine: Associated with nitrite (Griess reaction), not nitrate directly. Ammonium molybdate with stannous chloride: Used in the phosphate (molybdenum blue) method. Common Pitfalls: Confusing nitrite and nitrate tests; failing to correct for colour/turbidity; not accounting for chloride interference which may require prior treatment. Final Answer: Phenol-disulphonic acid followed by potassium hydroxide

Question 7

In open-channel hydraulics, Chezy's constant (C) is primarily used in which velocity–discharge relationship or formula? Provide the formulation context where C directly multiplies the hydraulic terms.

Accepted Answer

Introduction / Context: Chezy's constant (C) is a classical roughness/flow coefficient used to compute mean velocity in open channels. Knowing where and how this constant is employed helps students connect historical formulas (Chezy, Bazin, Kutter) to modern practice (Manning), and avoid mixing up which parameter belongs to which equation. Given Data / Assumptions: We are dealing with steady, uniform open-channel flow. Hydraulic radius R and bed slope S are well defined. Chezy's constant C appears as a multiplier in a velocity equation. Concept / Approach: Chezy's formula relates mean velocity V to channel properties as V = C * sqrt(R * S). In this expression, C encapsulates the combined effects of boundary roughness and flow regime. Bazin's and Kutter's formulas were developed mainly to estimate C for given conditions, not to replace Chezy's velocity form. Manning's equation uses a different coefficient n and does not include C explicitly. Step-by-Step Solution: 1) Recall Chezy: V = C * sqrt(R * S).2) Identify where C multiplies sqrt(R * S) directly: Chezy's formula.3) Recognize that Bazin and Kutter give empirical expressions for C itself, which are then substituted back into Chezy's velocity form.4) Note that Manning expresses V = (1/n) * R^(2/3) * S^(1/2), no C present. Verification / Alternative check: Dimensionally, with V proportional to sqrt(RS), C is dimensionless in SI (if R and S are in consistent units), matching the Chezy formulation; Manning instead requires n with different dimensional handling. Why Other Options Are Wrong: Bazin's formula: Used to estimate C, not the velocity directly without Chezy's form. Kutter's formula: Also estimates C; it feeds into Chezy's velocity expression. Manning's formula: Uses n, not C. Darcy–Weisbach equation: Pipe/head loss relation; does not employ Chezy's C. Common Pitfalls: Confusing the empirical correlations for C (Bazin/Kutter) with the velocity equation where C is actually used (Chezy). Final Answer: Chezy's formula

Question 8

In 1000 kg of typical domestic sewage, how much matter is present in suspension and in solution? Identify the correct combination based on standard composition ranges used in sanitation design.

Accepted Answer

Introduction / Context: Understanding typical concentrations of suspended and dissolved solids in sewage is essential for sizing screens, grit chambers, primary sedimentation, and biological treatment. Textbook values provide reasonable design estimates when site-specific data are not yet available. Given Data / Assumptions: Total mass of sewage considered: 1000 kg. Representative domestic sewage (no unusual industrial inputs). We are comparing given standard figures for suspended and dissolved fractions. Concept / Approach: Standard references commonly cite order-of-magnitude values for typical sewage: a small fraction is present as suspended solids (settleable and non-settleable), and another fraction as dissolved solids. Among the given options, 0.112 kg in suspension together with 0.225 kg in solution is the pair that matches widely taught benchmark values for preliminary design exercises. Step-by-Step Solution: 1) Interpret the options: individual masses in suspension or solution versus a combined statement.2) Compare with standard indicative values used in many MCQs and handbooks (suspended ≈ 0.112 kg/1000 kg; dissolved ≈ 0.225 kg/1000 kg).3) Select the option that includes both correct figures: the combined statement. Verification / Alternative check: These numbers are consistent with the notion that total solids in domestic sewage typically fall well below 1% by mass, validating that suspended and dissolved portions are of the order of a few hundred grams per 1000 kg. Why Other Options Are Wrong: 0.112 kg in solution: Misplaces the value; 0.112 kg is cited for suspended solids. 0.450 kg in suspension: Exaggerated for typical domestic sewage; would imply unusually high SS. Single-value options omit the dissolved fraction that is commonly cited together. Common Pitfalls: Assuming the suspended fraction dominates total solids; in domestic sewage, dissolved and colloidal fractions are significant. Final Answer: both 0.112 kg in suspension and 0.225 kg in solution

Question 9

Which pump type is referred to as a disintegrating pump because it macerates or breaks up solids in sewage before discharge, thereby reducing clogging risk?

Accepted Answer

Introduction / Context: In wastewater collection, solids such as rags and fibrous materials can clog pumps. Some pump designs incorporate macerating (disintegrating) mechanisms so solids are shredded before being conveyed through the volute and discharge piping. Given Data / Assumptions: Pumped medium: raw or screened sewage with potential rags and fibrous solids. Objective: minimize clogging by breaking solids before or at the impeller. Disintegrating pump implies an integrated cutter mechanism. Concept / Approach: The term “disintegrating pump” in many sanitation texts refers to a centrifugal pump fitted with cutter teeth or a macerating head at the inlet. The rotating cutter shreds solids so the impeller can pass the slurry without blockage. This differs from positive-displacement pumps whose valves or tight clearances may trap solids, and from pneumatic ejectors that avoid impellers entirely but do not “disintegrate.” Step-by-Step Solution: 1) Identify which option includes an actual disintegration mechanism at pump entry.2) Recognize that the macerator-equipped centrifugal pump is designed to shred solids.3) Conclude that this is the pump described as “disintegrating.” Verification / Alternative check: Manufacturers’ literature for sewage macerator pumps shows integral cutter/impeller assemblies specifically marketed to prevent ragging and clogging in lift stations. Why Other Options Are Wrong: Reciprocating pump: Uses valves with small clearances; prone to clogging by rags. Pneumatic ejector: Moves sewage by compressed air but does not shred solids. Peristaltic or air-lift pumps: May handle solids but do not disintegrate them. Common Pitfalls: Equating “solids-handling” with “solids-disintegrating” — only macerator/cutter-equipped centrifugal pumps actually shred solids. Final Answer: centrifugal pump (with cutter/macerator arrangement)

Question 10

For a combined sewerage system handling both sanitary sewage and storm runoff, which conduit cross-section is considered the most hydraulically efficient and self-cleansing over a wide range of discharges?

Accepted Answer

Introduction / Context: Combined sewerage systems must perform adequately at very low dry-weather flows and very high storm flows. The choice of cross-section strongly affects hydraulic efficiency, self-cleansing behavior, and construction practicality. Given Data / Assumptions: Combined sewers must convey a wide flow range. Self-cleansing at low flow depths is desirable. Structural feasibility and ease of construction are considered. Concept / Approach: The “new egged” (modified egg-shaped) section provides a narrow invert for higher velocities during low flows, aiding self-cleansing, while the upper widening accommodates peak storm discharges. Circular or rectangular sections are simpler but do not offer the same low-flow velocity advantage. Parabolic and horseshoe shapes see more specialized use and are less common for municipal combined sewers. Step-by-Step Solution: 1) Identify low-flow requirement: maintain velocity high enough to prevent deposition.2) Identify high-flow requirement: provide adequate area and freeboard.3) Recognize that the new egged section best satisfies both by geometry: small hydraulic radius at invert for DWF and large area above for storms. Verification / Alternative check: Standard sanitary engineering texts cite egg-shaped sections as the preferred geometry for combined sewers, with “new egged” refinements improving constructability and performance. Why Other Options Are Wrong: Circular: Adequate at full or near-full flow, but poorer self-cleansing at small depths. Rectangular: Structural and deposition issues at low depths. Parabolic / horseshoe: Not standard for combined sewer networks. Common Pitfalls: Assuming the simplest shape (circular) is best for all conditions; combined systems require geometry tuned for extremes. Final Answer: new egged section

Question 11

In the gas produced during anaerobic digestion of sludge in treatment units, which constituent is typically identified as the main combustible component?

Accepted Answer

Introduction / Context: Sludge digestion stabilizes organic matter under anaerobic conditions. The process produces a biogas mixture whose composition is important for safety, energy recovery, and odor control. Given Data / Assumptions: Process: anaerobic digestion of sewage sludge. Typical digester operation in mesophilic range. Biogas is collected from the digester headspace. Concept / Approach: Biogas from sewage sludge digestion generally contains methane and carbon dioxide with traces of hydrogen sulfide and other gases. Methane is the primary combustible component and key to energy recovery via flaring or cogeneration. Oxygen is absent under anaerobic conditions, nitrogen is minimal, and hydrogen is minor. Step-by-Step Solution: 1) Recognize that anaerobic pathways convert complex organics to volatile fatty acids and ultimately to methane (CH4) and CO2.2) Typical biogas: about 55–65% methane, 35–45% carbon dioxide, traces of H2S and moisture.3) Identify the main combustible gas: methane. Verification / Alternative check: Energy audits of digesters calculate heating value primarily from the methane fraction; CO2 is inert and does not contribute to calorific value. Why Other Options Are Wrong: Oxygen: Not present in anaerobic headspace. Nitrogen: Minor component; not produced by digestion. Hydrogen: Trace amounts; not the principal constituent. Carbon dioxide: Major but non-combustible component. Common Pitfalls: Confusing “major component” with “combustible component”; CO2 is abundant but non-fuel. Final Answer: Methane

Question 12

Which sewage treatment unit relies on anaerobic decomposition of organic matter as the primary treatment mechanism?

Accepted Answer

Introduction / Context: Treatment units are broadly classified by whether they use aerobic or anaerobic microbiological processes. Knowing which units depend on oxygen and which do not is fundamental to flowsheet design. Given Data / Assumptions: Primary clarification with digestion options. Secondary biological treatment options (aerobic vs anaerobic). Focus on unit where anaerobic breakdown occurs within the structure. Concept / Approach: An Imhoff tank is a two-story structure in which sewage undergoes primary sedimentation in the upper chamber, while settled solids slide into a lower digestion chamber where anaerobic decomposition occurs. Trickling filters and activated sludge aeration tanks are aerobic processes requiring oxygen transfer. Aerated grit chambers and rapid sand filters are not anaerobic biological reactors. Step-by-Step Solution: 1) Identify unit combining sedimentation and digestion under anaerobic conditions: Imhoff tank.2) Exclude units that require aeration or percolation through media with air (trickling filters, activated sludge).3) Confirm anaerobic decomposition is the core in the lower chamber of the Imhoff tank. Verification / Alternative check: Design manuals describe gas vents and scum control specific to Imhoff tanks, proving anaerobic action inside the tank. Why Other Options Are Wrong: Trickling filters / Activated sludge: Aerobic processes. Aerated grit chamber / Rapid sand filter: Physical processes with aeration/filtration; not anaerobic digesters. Common Pitfalls: Assuming all primary units are purely physical; Imhoff combines settling with anaerobic digestion. Final Answer: Imhoff tanks

Question 13

For sedimentation tanks operating with coagulant aid, what is the typical design range of overflow rate (surface loading rate) used in practice?

Accepted Answer

Introduction / Context: Overflow rate (surface loading) is a key sizing parameter for sedimentation tanks. With coagulant addition, higher effective settling rates are achievable compared to plain sedimentation, but typical ranges must still be respected to ensure good removal. Given Data / Assumptions: Coagulant-assisted sedimentation. Conventional rectangular or circular clarifiers. Steady-state design with average temperature conditions. Concept / Approach: The overflow rate equals design flow divided by plan surface area and represents the critical settling velocity. Common design references list about 25,000–35,000 litres/m²/day for sedimentation with coagulant. Choosing values much higher risks carryover of floc; much lower increases cost and footprint. Step-by-Step Solution: 1) Define overflow rate = Q / A_surface.2) Consult standard ranges for coagulant-aided sedimentation.3) Select 25,000–35,000 L/m²/day as typical practice. Verification / Alternative check: Field performance and jar tests generally correlate with this range for municipal water/sewage clarification; higher rates are reserved for specialized high-rate units with tube/plate settlers. Why Other Options Are Wrong: 40,000–100,000: Too high for conventional clarifiers; risk of poor capture. 10,000–15,000: Overly conservative, unnecessarily large basins. Common Pitfalls: Confusing plain sedimentation (lower rates) with enhanced settling using inclined plates/tubes which can tolerate higher rates. Final Answer: 25,000 to 35,000 litres/m²/day

Waste Water Engineering Questions