GATE Environmental Engineering Water & Wastewater Treatment and Management Flashcards

24 sample cards from the Water & Wastewater Treatment and Management deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. What is the design self-cleansing velocity in a sewer and why is it required?

   It is the minimum flow velocity (typically $0.6$–$0.9\,\text{m/s}$) that must be developed at least once a day to prevent deposition of suspended solids and keep the sewer self-cleansing. It ensures grit and organic solids are kept in suspension and carried along.

2. State the typical permissible velocity range in sewers (minimum self-cleansing to maximum non-scouring).

   Minimum (self-cleansing): about $0.6$–$0.9\,\text{m/s}$; maximum (non-scouring, to avoid eroding the sewer material): about $2.4$–$3.0\,\text{m/s}$. Velocity is kept within this range under varying flows.

3. Write Manning's formula used in sewer design.

   $$V = \frac{1}{n}\,R^{2/3}\,S^{1/2}$$ where $V$ is velocity, $n$ is Manning's roughness coefficient, $R$ is hydraulic radius ($=A/P$), and $S$ is the slope of the energy grade line (bed slope for uniform flow).

4. Why are sanitary sewers designed to flow partially full (e.g. at half or three-quarters depth)?

   To provide a margin (free space) for peak/excess flows and infiltration, to allow ventilation and escape of foul/explosive gases above the sewage surface, and because for a circular section the velocity at partial depth can equal or exceed the full-flow velocity, maintaining self-cleansing.

5. How does the design rationale differ between separate sewers and storm water drains regarding flow?

   Sanitary sewers carry relatively steady, smaller dry-weather flows and are designed for self-cleansing velocity at minimum flow. Storm sewers carry large, intermittent peak runoff designed using the rational method $Q = \frac{1}{360}\,C\,i\,A$ (Q in $\text{m}^3/\text{s}$, i in mm/h, A in ha), and may be designed to flow full.

6. State the Rational Method for storm water (peak) runoff.

   $$Q_p = C\,i\,A$$ (consistent units) — peak runoff equals runoff coefficient $C$ times rainfall intensity $i$ (for duration equal to time of concentration) times catchment area $A$. In metric practical form $Q = \frac{1}{360}\,C\,i\,A$ gives $Q$ in $\text{m}^3/\text{s}$ with $i$ in mm/h and $A$ in hectares.

7. What is the time of concentration in storm drainage design?

   The time required for runoff to travel from the hydraulically most remote point of the catchment to the design (outlet) point. It equals the inlet (overland) time plus the channel/sewer flow time. The design rainfall intensity is taken for a duration equal to the time of concentration.

8. List common sewer appurtenances and their purposes.

   Manholes (access for inspection/cleaning, at junctions and bends); drop manholes (connect sewers at very different levels); lamp holes (lowering a lamp to check obstruction); catch basins/gully traps (intercept storm water and trap grit); inlets (admit storm runoff); flushing tanks (flush low-flow sewers); inverted siphons (carry sewage under obstructions); clean-outs; ventilating columns.

9. What is an inverted siphon (depressed sewer) and when is it used?

   A section of sewer that dips below the hydraulic grade line and runs full under pressure to pass beneath an obstruction such as a stream, valley, road, or utility. Because it flows full, self-cleansing velocity must be maintained to prevent silting; multiple barrels are often used.

10. What unit operations constitute preliminary treatment of sewage and what do they remove?

    Screening (removes large floating solids/rags), grit chambers (remove inorganic grit, sand, gravel by Type I sedimentation), and skimming tanks/grease traps (remove oil and grease). Preliminary treatment protects pumps and downstream units; it removes little BOD.

11. How is a grit chamber designed to remove grit but retain organic matter?

    It is a velocity-controlled (constant-velocity) horizontal-flow channel maintaining a flow velocity of about $0.3\,\text{m/s}$, which is just sufficient to settle dense inorganic grit (specific gravity $\approx 2.65$) while keeping lighter organic particles in suspension to pass on for biological treatment. A proportional (Sutro) weir helps keep velocity constant.

12. What does primary sedimentation (primary treatment) of sewage achieve in terms of removal efficiency?

    Primary clarifiers settle suspended solids (Type II) and remove roughly $50$–$70\%$ of suspended solids and about $25$–$40\%$ of BOD by plain settling. The settled organic solids form primary sludge, and floating scum is skimmed off.

13. What characterizes secondary (biological) treatment of sewage and what BOD removal is expected?

    Secondary treatment uses microorganisms to oxidize dissolved and colloidal biodegradable organic matter (BOD) aerobically. Examples: activated sludge process, trickling filters, oxidation ponds, rotating biological contactors. Combined with primary, it removes about $85$–$95\%$ of BOD and suspended solids.

14. Compare attached-growth and suspended-growth secondary treatment processes with examples.

    Suspended-growth: microorganisms remain suspended in the liquid (flocs), e.g. activated sludge process and oxidation/aerated lagoons. Attached-growth (fixed-film): biomass grows as a biofilm on a fixed medium, e.g. trickling filters and rotating biological contactors (RBCs).

15. Define F/M ratio and SRT (sludge age) in the activated sludge process.

    Food-to-microorganism ratio $F/M = \dfrac{Q\,S_0}{V\,X}$ (mass of BOD applied per unit mass of MLVSS per day). Sludge age / mean cell residence time $\theta_c = \dfrac{\text{mass of MLSS in reactor}}{\text{mass of solids wasted per day}}$, controlling the balance between growth and decay.

16. What is the role of the secondary clarifier and sludge recirculation in the activated sludge process?

    The secondary (final) clarifier separates the biological flocs (MLSS) from treated effluent by Type III settling. A portion of settled sludge is returned (Return Activated Sludge, RAS) to the aeration tank to maintain the required microorganism concentration (MLSS), and the excess is wasted (WAS) to control sludge age.

17. What is the purpose of tertiary (advanced) treatment, and name typical processes?

    Tertiary treatment polishes secondary effluent to remove residual suspended solids, nutrients (N and P), pathogens, and specific dissolved constituents before discharge or reuse. Processes: filtration, nutrient removal (nitrification–denitrification for N, chemical/biological P removal), activated carbon adsorption, membrane processes (RO), and disinfection.

18. Describe biological nitrogen removal via nitrification and denitrification.

    Nitrification (aerobic, autotrophic) oxidizes ammonia: $\ce{NH4+ ->[Nitrosomonas] NO2- ->[Nitrobacter] NO3-}$. Denitrification (anoxic, heterotrophic) then reduces nitrate to nitrogen gas: $\ce{NO3- -> N2(^)}$, using organic carbon as electron donor and removing nitrogen from the wastewater.

19. List the typical sequence of sludge processing/treatment steps before disposal.

    Thickening (concentrate solids, e.g. gravity thickener), stabilization/digestion (anaerobic or aerobic to reduce volatile solids, pathogens and odour), conditioning (chemical), dewatering (drying beds, vacuum/belt/centrifuge filters), and final disposal (landfill, incineration, or land application).

20. What occurs in anaerobic sludge digestion and what useful by-product is generated?

    In the absence of oxygen, facultative and anaerobic bacteria convert organic solids through hydrolysis, acidogenesis/acetogenesis, and methanogenesis, reducing volatile solids and pathogens and stabilizing the sludge. The by-product is biogas, rich in methane ($\ce{CH4}$) and carbon dioxide ($\ce{CO2}$), usable as fuel.

21. After dewatering, what is the relation between sludge volume and moisture/solids content?

    For the same dry solids, sludge volume is inversely proportional to solids content (or $V \propto \frac{1}{100 - p}$ where $p$ is % moisture): $$\frac{V_1}{V_2} = \frac{100 - p_2}{100 - p_1} = \frac{P_{s2}}{P_{s1}}$$ Reducing moisture (increasing solids fraction) sharply decreases sludge volume.

22. What are the common methods of final sludge disposal?

    Sanitary landfilling, incineration (thermal destruction, with energy recovery), land application/agricultural use as a soil conditioner or fertilizer (if pathogen and heavy-metal limits are met), composting, and (historically, now largely banned) ocean dumping.

23. What is sewage farming (land treatment / sewage irrigation)?

    The application of treated or settled sewage onto land for crop irrigation, where the soil and crops utilize the water and nutrients (N, P, K) while soil microorganisms and filtration provide additional purification of the wastewater. It serves both as a disposal method and as a means of beneficial nutrient/water reuse.

24. Define sewage sickness of land in the context of sewage farming.

    The condition in which soil pores become clogged with organic matter and solids from continuous over-application of sewage, so the land cannot oxidize or absorb further sewage; it turns anaerobic and foul-smelling. It is prevented by giving the land rest periods, underdrainage, shallow dosing, and choosing porous soils.

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