GATE Overflow for GATE ESE - Recent questions in Water & Wastewater Treatment and Management

GATE ES 2026 | Question: 6

Mon, 23 Feb 2026 13:15:38 +0000

In the reversible reaction shown below, $k_{f}$ is the forward reaction rate constant and $k_{r}$ is the reverse reaction rate constant.

The CORRECT expression representing the reaction equilibrium constant is $\_\_\_\_$.

$$A+B \underset{k_{r}}{\stackrel{k_{f}}{\rightleftharpoons}} C+D$$

$k_{f}+k_{r}$
$k_{f} \times k_{r}$
$\dfrac{k_{f}}{k_{r}}$
$\dfrac{k_{r}}{k_{f}}$

GATE ES 2026 | Question: 10

Mon, 23 Feb 2026 13:15:30 +0000

Which ONE of the following options CORRECTLY matches the processes associated with the growth condition given in the table?

$$\begin{array}{|ll|ll|}
\hline & \textbf{Process} & & \textbf{Growth Condition} \\
\hline \text{(P)} & \text{Activated sludge process} & (i) & \text{Anaerobic suspended growth} \\
\hline \text{(Q)} & \text{Anaerobic filter} & (ii) & \text{Anaerobic attached growth} \\
\hline \text{(R)} & \text{Trickling filter} & (iii) & \text{Aerobic suspended growth} \\
\hline \text{(S)} & \text{Anaerobic digestion} & (iv) & \text{Aerobic attached growth} \\
\hline \end{array}$$

$\text{(P) - (ii); (Q) - (iii); (R) - (iv); (S) - (i)}$
$\text{(P) - (iii); (Q) - (ii); (R) - (iv); (S) - (i)}$
$\text{(P) - (iii); (Q) - (i); (R) - (iv); (S) - (ii)}$
$\text{(P) - (iv); (Q) - (iii); (R) - (ii); (S) - (i)}$

GATE ES 2026 | Question: 34

Mon, 23 Feb 2026 13:14:43 +0000

Which of the following options is/are TRUE about reverse osmosis process?

Reverse osmosis is used as a pretreatment for microfiltration process.
Ultrafiltration is used as a pretreatment for reverse osmosis process.
Membrane fouling occurs in microfiltration and reverse osmosis processes.
Concentration polarization does not occur in reverse osmosis process.

GATE ES 2026 | Question: 35

Mon, 23 Feb 2026 13:14:41 +0000

Which of the following options is/are TRUE about coagulation-flocculation process?

Collision of colloidal particles due to their Brownian motion results in orthokinetic flocculation.
Collision of colloidal particles due to the external mixing of bulk medium results only in perikinetic flocculation.
In an electric double layer model of a colloidal particle, Stern layer is more compact than the diffuse layer.
The zeta potential is also called Stern potential.

GATE ES 2026 | Question: 40

Mon, 23 Feb 2026 13:14:34 +0000

Reaction $\mathrm{A} \rightarrow \mathrm{B}$ follows first-order elementary kinetics and the reaction rate constant for $\text{A}$ is $0.01$ per min.

The time taken to reduce the concentration of $\text{A}$ from $100$ $\text{M}$ to $10$ $\text{M}$ in a batch reactor is $\_\_\_\_$ $\min$ (rounded off to one decimal place).

GATE ES 2026 | Question: 48

Mon, 23 Feb 2026 13:14:26 +0000

A sewage treatment plant employing activated sludge process is operated under the following conditions:

Wastewater flow rate into the aeration $\operatorname{tank}=0.150 \mathrm{~m}^{3} / \mathrm{s}$

Soluble $\mathrm{BOD}_{5}$ in the influent $=84 \mathrm{mg} / \mathrm{L}$

Soluble $\mathrm{BOD}_{5}$ in the effluent $=11 \mathrm{mg} / \mathrm{L}$

Mean cell residence time $=5$ days

Total yield co-efficient $=0.5 \mathrm{~kg}$ of $\mathrm{MLVSS} / \mathrm{kg} \mathrm{BOD}_{5}$ removed

Decay rate of microorganism $=0.050 /$ day

The net waste activated sludge produced in the plant is $\_\_\_\_$ $\text{kg}$ of $\mathrm{VSS/day}$ (rounded off to two decimal places).

GATE ES 2026 | Question: 49

Mon, 23 Feb 2026 13:14:25 +0000

A continuous flow stirred tank reactor (CSTR) operates under a steady-state condition with a flow rate of $300$ $\text{L}$ per hour. The influent concentration of a substrate entering the reactor is $150 \mathrm{mg} / \mathrm{L}$.

To give a treatment efficiency of $76 \%$ for the substrate that decays according to halforder kinetics with a rate constant of $0.05(\mathrm{mg} / \mathrm{L})^{1 / 2}$ per hour, the required volume of the reactor is $\_\_\_\_$ $\mathrm{m}^{3}$ (answer in integer).

GATE ES 2025 | Question: 9

Thu, 06 Mar 2025 16:22:36 +0000

Multiple effect evaporator is commonly used, in the zero liquid discharge (ZLD) scheme, for $\_\_\_\_\_\_\_$

oxidation of organic pollutants
precipitation of heavy metals
concentrating reverse osmosis $(\mathrm{RO})$ reject salts
performing selective ion exchange

GATE ES 2025 | Question: 29

Thu, 06 Mar 2025 16:21:40 +0000

Consider the following figure of an activated sludge process (ASP), depicting the flow $(\mathbf{Q})$, substrate $(\mathbf{S})$, and microorganism concentration $(\mathbf{X})$ at various points in the system, where subscripts "$\mathbf{0}$ ", " $\mathbf{r}$ ", " $\mathbf{w}$ ", "$\mathbf{e}$", and " $\mathbf{i}$ " indicate influent, recycle line, wastage, effluent, and flow from aeration tank to settling tank, respectively. Note that influent to the ASP has microbes too. $\mathbf{V}$ is volume of aeration tank.

Choose the correct option for net rate of formation of microorganisms in the system at steady state, from the following

$\frac{\left(Q-Q_{W}\right) X_{e}+Q_{W} X_{r}-Q X_{0}}{V}$
$\frac{\left(Q-Q_{W}\right) X_{e}+Q_{W} X_{r}}{V}$
$\frac{\left(Q-Q_{W}\right) X_{e}+Q_{W} X_{r}}{V X}$
$\frac{\left(\mathrm{Q}+\mathrm{Q}_{\mathrm{r}}\right) \mathrm{X}_{\mathrm{i}}+\mathrm{Q}_{\mathrm{W}} \mathrm{X}_{\mathrm{r}}}{\mathrm{VX}}$

GATE ES 2025 | Question: 35

Thu, 06 Mar 2025 16:21:21 +0000

Choose the correct option(s) for removing solids from water.

chlorination
coagulation-flocculation-sedimentation followed by slow sand filtration
chlorination followed by aeration
slow sand filtration

GATE ES 2025 | Question: 41

Thu, 06 Mar 2025 16:21:11 +0000

Aerobic biomass has yield coefficient value of $0.4$ for glucose (molecular weight $= 180 \mathrm{~g} / \mathrm{mole}$) substrate. Bacteria is represented as $\mathrm{C}_{5} \mathrm{H}_{7} \mathrm{O}_{2} \mathrm{N}$ (molecular weight $=113$ $\mathrm{g} / \mathrm{mole}$ ). Assume that no endogenous metabolism occurs. The percentage of carbon going into $\mathrm{CO}_{2}$ from $1 \mathrm{~mole} / \mathrm{L}$ glucose is $\_\_\_\_\_\_$ (rounded off to two decimal places)

GATE ES 2025 | Question: 47

Thu, 06 Mar 2025 16:21:03 +0000

You conducted a batch experiment in the lab for $10$ minutes to degrade a toxic compound, which follows first order kinetics. The compound degrades from $2 \times 10^{-3} \mathrm{M}$ to $2 \times 10^{-4} \mathrm{M}$. The information from the lab experiment will be used to design a plug flow reactor in field conditions.

Given field conditions:

Flow rate of contaminated water to be treated: $1 \mathrm{~m}^{3} /$ hour
Concentration of toxic compound in contaminated water: $5 \times 10^{-1} \mathrm{M}$
Target concentration of toxic compound in treated water: $1 \times 10^{-4} \mathrm{M}$
Temperature is same in lab and field conditions.

The required volume of the plug flow reactor is $\_\_\_\_\_\_\_ \: \mathrm{m}^{3}$. (rounded off to two decimal places)

GATE ES 2025 | Question: 48

Thu, 06 Mar 2025 16:21:01 +0000

A common effluent treatment plant with a capacity of $2$ million litres per day (MLD) employs reverse osmosis (RO) for water reuse. The RO unit removes $95 \%$ of the total dissolved solids (TDS) and the water recovery rate is $70 \%$. If the TDS concentration in the RO feed is $8000$ parts per million (ppm), the TDS in the RO reject is $\_\_\_\_\_\_ \: \mathrm{g} / \mathrm{L}$. (rounded off to one decimal place)

GATE ES 2024 | Question: 8

Mon, 19 Feb 2024 10:24:13 +0000

The processes of removal of particles in a rapid sand filter with their description is given in the table.

Process	Description
$\text{(i)}$ Straining	P:Removes only particles in the water large enough to get caught in the pores of the filter
$\text{(ii)}$ Sedimentation	Q: Larger and heavier particles do not follow the fluid streamline around the sand grain and settle on the grain
$\text{(iii)}$ Interception	R: Particles that do follow the streamline, but are too large and are caught because they brush up against the sand grains
$\text{(iv)}$ Diffusion	S: Very small particles are experiencing Brownian motion and may collide with the sand grains by chance

Select the correct match.

$\text{i- S; ii-P; iii-Q; iv-R}$
$\text{i-Q; ii-R; iii-S; iv-P}$
$\text{i-R; ii- S; iii- P; iv-Q}$
$\text{i-P; ii-Q; iii-R; iv-S}$

GATE ES 2024 | Question: 29

Mon, 19 Feb 2024 10:24:09 +0000

Consider second order kinetics $\left(r_{c}=-k C^{2}\right)$ under steady state condition. The ratio of volume of a complete mixed reactor (CMR) to that of a plug flow reactor (PFR) to achieve $90 \%$ reduction in the concentration is ________.

Inlet concentrations in both the reactors are same.

$10.0$
$1.0$
$0.1$
$2.3$

GATE ES 2024 | Question: 43

Mon, 19 Feb 2024 10:24:06 +0000

One million liters per day $\text{(MLD)}$ of wastewater with a soluble $\text{BOD}$ of $200 \mathrm{mg} / \mathrm{L}$ is treated in an activated sludge process. The $\text{BOD}$ of treated wastewater is $20 \mathrm{mg} / \mathrm{L}$. The observed yield coefficient of the biological system is $0.35$.

The daily biomass generation in the system is _________ $\mathrm{kg}$ $\text{(an integer value)}$.

GATE ES 2024 | Question: 52

Mon, 19 Feb 2024 10:24:05 +0000

For a biodegradable waste with a chemical formula $\mathrm{C}_{50} \mathrm{H}_{100} \mathrm{O}_{40} \mathrm{~N}$, the maximum theoretical methane production per ton of waste is __________ $\mathrm{kg}$ $\text{(rounded off to $2$ decimal places)}$.

Assume $100 \%$ anaerobic conversion. Atomic weights of $\text{C-12; H-1; O-16; N-14}$

GATE ES 2023 | Question: 45

Tue, 23 May 2023 03:11:54 +0000

A biological reactor is getting wastewater containing 1 mole/L acetate ions as carbon source. The following reaction takes place in the bio-reactor:
\[
\begin{array}{l}
0.125 \mathrm{CH}_{3} \mathrm{COO}^{-}+0.0295 \mathrm{NH}_{4}^{+}+0.103 \mathrm{O}_{2} \rightarrow 0.0295 \mathrm{C}_{5} \mathrm{H}_{7} \mathrm{O}_{2} \mathrm{~N}+0.0955 \mathrm{H}_{2} \mathrm{O}+ \\
0.095 \mathrm{HCO}_{3}^{-}+0.007 \mathrm{CO}_{2}
\end{array}
\]
Assume that all acetate ions are consumed and ammonia serves as a nutrient source. Given that $1 \mathrm{~g}$ acetate exerts $1.07 \mathrm{~g}$ COD; 1 mole bacteria $=113 \mathrm{~g}$ VSS; 1 mole acetate ion $=59 \mathrm{~g}$. Value of observed yield is ____________ (in $\mathrm{g} \mathrm{VSS} / \mathrm{g}$ COD, $\text{rounded off to two decimal places).}$

GATE ES 2023 | Question: 46

Tue, 23 May 2023 03:11:53 +0000

A flask (100 ml volume) has wastewater, which has $0.12 \mathrm{mg} / \mathrm{L}$ geosmin. Activated carbon is added in this flask for adsorbing geosmin as per the Freundlich isotherm model $\left(\mathrm{Q}=2.6 \times \mathrm{C}^{0.73}\right.$ where $\mathrm{Q}$ is $\mathrm{mg}$ adsorbate $/ \mathrm{mg}$ adsorbent and $\mathrm{C}$ is the equilibrium concentration in $\mathrm{mg} / \mathrm{L}$ ). Activated carbon to be added in this flask for getting final remaining geosmin concentration of $0.05 \mathrm{mg} / \mathrm{L}$ would be __________ (in $\mathrm{mg} / \mathrm{L}$, $\text{rounded off to three decimal places).}$

GATE ES 2023 | Question: 48

Tue, 23 May 2023 03:11:51 +0000

You are doing an experiment to find out $\mathrm{BOD}_5$ of a wastewater. You have taken $25 \mathrm{~mL}$ wastewater having ultimate BOD of $75 \mathrm{mg} / \mathrm{L}$ and placed it in $300 \mathrm{~mL}$ BOD bottle and filled it with dilution water. The initial DO of the diluted sample is $6.5 \mathrm{mg} / \mathrm{L}$. On the $5^{\text {th }}$ day, you were not able to measure the DO due to some unavoidable circumstances. However, the DO at the end of the $7^{\text {th }}$ day is found to be $1.25 \mathrm{mg} / \mathrm{L}$. Assume all the experiments are done at the same temperature, and no biodegradable organics are present in the dilution water. The $\mathrm{BOD}_5$ of the wastewater sample is __________ (in $\mathrm{mg} / \mathrm{L}$, $\text{rounded off to two decimal places).}$

GATE ES 2023 | Question: 55

Tue, 23 May 2023 03:11:44 +0000

An aeration tank needs to be installed for the removal of VOC from water, where the required rate of flow of water through the aeration tank is $180,000 \mathrm{~m}^{3} / \mathrm{d}$. Permissible limit of VOC in the water is $12 \mu \mathrm{g} / \mathrm{L}$. The saturation concentration of $\mathrm{VOC}$ is $5 \mu \mathrm{g} / \mathrm{L}$ and gas transfer rate constant is 0.40 per second at $25^{\circ} \mathrm{C}$. The initial concentration of VOC in the water is $33 \mu \mathrm{g} / \mathrm{L}$. The volume of the aeration tank to satisfy the permissible limit of VOC at $25^{\circ} \mathrm{C}$ is _________ (in $\mathrm{m}^{3}$, $\text{rounded off to two decimal places).}$

GATE ES 2021 | Question: 23

Tue, 14 Mar 2023 03:44:27 +0000

A flocculation tank used for water treatment has a velocity gradient $(G)$ of $800 \mathrm{~s}^{-1}$. The volume of the tank is $40 \mathrm{~m}^{3}$. The dynamic viscosity of water is $9 \times 10^{-4} \mathrm{~N} \cdot \mathrm{s} / \mathrm{m}^{2}$. The theoretical power required to maintain the given velocity gradient is__________$\text{kW}$ $\text{(rounded off to the nearest integer)}$.

GATE ES 2021 | Question: 30

Tue, 14 Mar 2023 03:44:21 +0000

An ideal PFR or an ideal CFSTR may be used to degrade a pollutant with first order reaction kinetics. Both the reactors are fed with the same inlet concentration and the same volumetric flow rate, and are designed to achieve the same outlet concentration. Which of the following statements is true when comparing PFR with CFSTR?
PFR is Plug Flow Reactor.
CFSTR is Continuous Flow Stirred Tank Reactor $\text{(also referred to as CSTR)}$.

PFR will always require less reactor volume than CFSTR.
PFR will require the same reactor volume as CFSTR.
CFSTR will always require less reactor volume than PFR.
CFSTR can sometimes require less reactor volume than PFR.

GATE ES 2021 | Question: 32

Tue, 14 Mar 2023 03:44:17 +0000

A sewage treatment plant $\text{(STP)}$ receives sewage at a flow rate of $20000 \mathrm{~m}^3$ per day. The sewage has $200 \mathrm{mg} / \mathrm{L}$ of suspended solids. Assume $60 \%$ suspended solids are removed in the primary clarifier. The underflow $\text{(i.e. sludge)}$ removed from the clarifier contains $5 \%$ solids $\text{(by weight)}$.
The daily volume of the sludge generated will be $\mathrm{m}^3$ per day.

Assume sludge density $=1000 \mathrm{~kg} / \mathrm{m}^3$.

$4$
$8$
$48$
$800$

GATE ES 2021 | Question: 55

Tue, 14 Mar 2023 03:43:48 +0000

A facultative pond system for sewage treatment consists of two ponds in series. The hydraulic retention time $\text{(HRT)}$ of each pond is $6$ days. The total $\text{BOD 5}$ reduction through the entire pond system is $90 \%$. If the ponds are considered to be completely mixed, then the rate constant describing the $\mathrm{BOD}_5$ removal is _______ day $^{-1}$ $\text{(rounded off to two decimal points)}$. Assume that the rate constant is same for both the ponds.

GATE ES 2022 | Question: 12

Fri, 17 Feb 2023 07:06:06 +0000

A river water sample has $\mathrm{pH}$ of $4$ and suspended solids concentration of $100 \; \mathrm{mg} / \mathrm{L}$. If, alum is chosen as a coagulant, what will be the coagulation mechanism?

Ionic layer compression only
Sweep coagulation and Polymer bridging
Polymer bridging only
Charge neutralization, surface adsorption, ionic layer compression

GATE ES 2022 | Question: 30

Fri, 17 Feb 2023 07:05:50 +0000

Which one of the following statements is NOT correct with respect to a batch reactor?

No reactant is added after the reactor has started operation.
No product is withdrawn during the course of the reactor operation.
The reactor operates under steady state.
The reactor operation is carried out for a pre-specified duration.

GATE ES 2022 | Question: 35

Fri, 17 Feb 2023 07:05:46 +0000

Average values of the re-aeration rate constant for river $\mathrm{X}$ and river $\mathrm{Y}$ are $0.92$ per day and $1.12$ per day, respectively. Average de-oxygenation rate constants for the same rivers are $0.23$ per day and $0.35$ per day respectively. Then, which of the following statement(s) is/are correct?

Self-purification capacity of river $\mathrm{Y}$ is more than that of river $\mathrm{X}$.
Self-purification capacity of river $\mathrm{X}$ is more than that of river $\mathrm{Y}$.
River Y has higher turbulence and/or higher velocity and/or higher algal growth compared to river $\mathrm{X}$.
Pollutants in river $\mathrm{X}$ are more biodegradable than that in river $\mathrm{Y}$.

GATE ES 2022 | Question: 36

Fri, 17 Feb 2023 07:05:46 +0000

Which of the following statement(s) is/are NOT correct while comparing continuously stirred tank reactor (CSTR) and plug flow reactor (PFR)?

CSTR and PFR are normally operated under steady state condition.
There is complete homogeneity in the CSTR while there are concentration variations within the PFR.
The overall reaction carried out in a PFR is always higher than that in a CSTR for same total volume.
Reaction kinetics do not play any role in choosing between a CSTR and PFR.

GATE ES 2022 | Question: 52

Fri, 17 Feb 2023 07:05:34 +0000

A sewage treatment plant with a capacity of 10 million litres per day (MLD) is treating the sewage through an aerobic biological process. The inlet and outlet BOD are measured as 100 and $30 \mathrm{mg} / \mathrm{l}$, respectively. If the mixed microbial culture has an observed biomass yield of $0.5 \mathrm{~g}$ Volatile Suspended Solids (VSS)/g BOD, the sludge production rate (in kg per day, rounded off to one decimal place) when the plant is operating at $80 \%$ capacity will be ____________.

GATE ES 2022 | Question: 53

Fri, 17 Feb 2023 07:05:33 +0000

A centrifuge is processing 1000 litres per hour of water containing 2 g per litre of suspended solids. The thickened slurry coming out of the centrifuge has solids concentration of $20 \mathrm{~g}$ per $100 \mathrm{ml}$. If the centrifuge has $99 \%$ efficiency based on solids separation, the flow rate (in litres per hour, rounded off to one decimal place) of the supernatant stream will be___________