GATE Civil Engineering Environmental Engineering Syllabus

Name the four common layouts of a water distribution system.

Dead-end (tree) system, grid-iron (gridded) system, ring (circular) system, and radial system. Grid-iron and ring give good circulation and reliability; dead-end is cheapest but accumulates stagnant water.

Define detention (retention) time for a sedimentation tank.

$$t = \frac{V}{Q}$$ where $V$ is tank volume and $Q$ is the flow rate; it is the theoretical average time water stays in the tank. Typical plain settling: $3$–$4$ h.

Define surface overflow rate (SOR) and its significance in a settling tank.

$$\text{SOR} = \frac{Q}{A_{surface}} = v_s$$ It equals the settling velocity of the smallest particle fully removed. Particles with settling velocity $\geq$ SOR are 100% removed regardless of tank depth (ideal Type-I settling).

State Stokes' law for the settling velocity of a discrete spherical particle.

$$v_s = \frac{g\,(\rho_s - \rho_w)\,d^{2}}{18\,\mu}$$ valid for laminar flow (Reynolds number $< 1$), where $d$ is particle diameter and $\mu$ is dynamic viscosity.

What is the purpose of coagulation, and name the most common coagulant with its reaction.

Coagulation neutralizes the negative charge on colloids so they can agglomerate. The common coagulant is alum: $$\ce{Al2(SO4)3.18H2O + 3Ca(HCO3)2 -> 2Al(OH)3 v + 3CaSO4 + 6CO2 + 18H2O}$$ producing gelatinous $\ce{Al(OH)3}$ floc.

What is the optimum pH range for alum coagulation?

Alum coagulation works best in the pH range $5.5$ to $8.0$, where $\ce{Al(OH)3}$ floc has minimum solubility and maximum formation.

Compare slow sand filters and rapid sand filters on filtration rate and pretreatment.

Slow sand filter: rate $\approx 100$–$200\ \text{L/m}^2/\text{h}$, no coagulation needed, removes bacteria via schmutzdecke. Rapid sand filter: rate $\approx 3000$–$6000\ \text{L/m}^2/\text{h}$, requires prior coagulation–sedimentation, cleaned by backwashing.

What is breakpoint chlorination?

It is the point at which enough chlorine has been added to satisfy all demand (oxidizing ammonia and organics); beyond it, added chlorine appears as free available residual chlorine. The dip-and-rise of the residual curve defines the breakpoint.

What forms of chlorine residual exist and which is the stronger disinfectant?

Free residual ($\ce{HOCl}$ and $\ce{OCl-}$) and combined residual (chloramines). $\ce{HOCl}$ (hypochlorous acid) is the strongest disinfectant; free chlorine is far more effective and faster than combined chloramines.

Why does chlorine disinfection efficiency decrease at higher pH?

Chlorine in water forms $\ce{HOCl <=> H+ + OCl-}$. At higher pH the equilibrium shifts toward the weaker disinfectant $\ce{OCl-}$; the strong germicidal $\ce{HOCl}$ dominates below pH $\approx 7.5$, so low pH gives better disinfection.

Estimate the quantity of domestic wastewater from water supply, and give typical infiltration allowance.

Wastewater (dry weather flow) $\approx 70$–$80\%$ of the water supplied. Additional infiltration/inflow of groundwater into sewers is allowed at about $5000$–$50000$ L/ha/day depending on sewer condition and water table.

What is the difference between dry weather flow (DWF) and the maximum sewer design flow?

DWF is the average daily sanitary sewage flow. Sewers are designed for peak flow $\approx 3 \times \text{DWF}$ (and checked for minimum flow $\approx \tfrac{1}{2}\,\text{DWF}$ for self-cleansing).

What is the self-cleansing velocity in sewers and the typical design value?

It is the minimum velocity that prevents deposition of solids, typically $0.6$ m/s for sewage; design velocities are kept between about $0.6$ and $3.0$ m/s to avoid both silting and scouring.

Which unit operations constitute primary treatment of wastewater?

Screening, grit removal (grit chamber), and primary sedimentation. Primary treatment is mainly physical, removing floating and settleable solids and reducing BOD by about $30$–$40\%$.

Typical BOD and suspended solids removal efficiency of primary sedimentation?

Primary sedimentation removes about $30$–$40\%$ of BOD and $50$–$70\%$ of suspended solids.

What is the purpose of a grit chamber and the velocity maintained in it?

A grit chamber removes inorganic heavy particles (sand, gravel) while keeping organic matter in suspension. A horizontal velocity of about $0.3$ m/s is maintained so that grit settles but organics do not.

Name the two main types of biological secondary treatment processes.

Attached-growth (fixed-film) processes such as trickling filters and RBCs, and suspended-growth processes such as the activated sludge process (ASP). Both use microbes to oxidize dissolved/colloidal organic matter.

Define F/M ratio (food-to-microorganism ratio) in the activated sludge process.

$$\frac{F}{M} = \frac{Q\,S_0}{V\,X}$$ where $Q$ = flow, $S_0$ = influent BOD, $V$ = aeration tank volume, $X$ = MLSS. Typical conventional ASP: $0.2$–$0.5\ \text{kg BOD/kg MLSS/day}$.

Define the sludge volume index (SVI) and a good operating value.

$$\text{SVI} = \frac{\text{settled sludge volume (mL/L) after 30 min}\times 1000}{\text{MLSS (mg/L)}}$$ Good settling sludge has SVI between $50$ and $150$ mL/g; values $>150$ indicate bulking.

What is sludge age (mean cell residence time) in the activated sludge process?

$$\theta_c = \frac{V\,X}{Q_w X_r + Q_e X_e} \approx \frac{\text{mass of MLSS in tank}}{\text{mass of SS wasted per day}}$$ Typical conventional ASP: $5$–$15$ days.

Compare a high-rate trickling filter with a standard-rate trickling filter.

Standard-rate: hydraulic loading $1$–$4\ \text{m}^3/\text{m}^2/\text{day}$, no recirculation, higher efficiency (~$80$–$85\%$ BOD). High-rate: hydraulic loading $10$–$40\ \text{m}^3/\text{m}^2/\text{day}$, uses recirculation, lower per-pass efficiency but handles higher loads.

State the NSF/Velz efficiency formula for a single-stage trickling filter (without recirculation).

$$E = \frac{100}{1 + 0.4432\sqrt{\dfrac{u}{V\,F}}}$$ where $u$ = BOD load (kg/day), $V$ = filter volume ($\text{m}^3$), and $F$ = recirculation factor (= 1 if no recirculation).

What are the CPCB general effluent discharge standards for BOD into inland surface water?

For discharge into inland surface waters: $\text{BOD}_5 \leq 30$ mg/L, suspended solids $\leq 100$ mg/L, pH $5.5$–$9.0$, and COD $\leq 250$ mg/L (CPCB general standards).

What is the prescribed effluent BOD limit for discharge onto land for irrigation and into public sewers (CPCB)?

Discharge on land for irrigation: $\text{BOD}_5 \leq 100$ mg/L. Discharge into public sewers: $\text{BOD}_5 \leq 350$ mg/L.

Name the common methods of ultimate sludge disposal.

Sludge thickening, anaerobic digestion, dewatering (drying beds/centrifuge/filter press), followed by ultimate disposal: land application, sanitary landfilling, incineration, or use as soil conditioner/fertilizer.

What gas is produced in anaerobic sludge digestion and what is its main use?

Anaerobic digestion produces biogas, mainly methane ($\ce{CH4}$, ~$65$–$70\%$) and $\ce{CO2}$ (~$30\%$). The methane is combustible and used as fuel for heating digesters and power generation.

List common applications for reuse of treated sewage.

Agricultural and landscape irrigation, industrial cooling and process water, groundwater recharge, toilet flushing/dual plumbing, construction, and creation of recreational/ornamental water bodies. Higher-contact reuse requires higher treatment (tertiary/disinfection).

Classify air pollutants into primary and secondary, with examples.

Primary pollutants are emitted directly: $\ce{CO}$, $\ce{SO2}$, $\ce{NO}$, particulates, hydrocarbons. Secondary pollutants form by reaction in the atmosphere: ozone ($\ce{O3}$), PAN, and photochemical smog (from $\ce{NOx}$ + hydrocarbons + sunlight).

Name the main sources and a key impact for $\ce{SO2}$ air pollution.

Sources: combustion of sulphur-bearing fossil fuels (coal, oil), smelting. Impact: respiratory irritation and acid rain via $$\ce{SO2 + H2O -> H2SO3},\quad \ce{2SO2 + O2 + 2H2O -> 2H2SO4}$$ damaging vegetation, materials and aquatic life.

Which air pollution control devices are best for fine particulates, and on what principle does an ESP work?

For fine particulates: electrostatic precipitators (ESP) and fabric (bag) filters are most efficient ($>99\%$). An ESP charges particles with a high-voltage corona and collects them on oppositely charged plates by electrostatic attraction; settling chambers/cyclones only handle coarse particles.