Kinetics and reactor design - Water & Wastewater Treatment and Management - Environmental Science & Engineering Previous Year Questions

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2024 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2024

Consider second order kinetics (\(r_c = -kC^2\)) 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.

A

10.0

B

1.0

C

0.1

D

2.3

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2024 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2024

One million liters per day (MLD) of wastewater with a soluble BOD of 200 mg/L is treated in an activated sludge process. The BOD of treated wastewater is 20 mg/L. The observed yield coefficient of the biological system is 0.35.
The daily biomass generation in the system is __________ kg (an integer value).

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2025 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2025

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 × 10⁻¹ M to 2 × 10⁻⁴ 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 m³/hour
• Concentration of toxic compound in contaminated water: 5 × 10⁻¹ M
• Target concentration of toxic compound in treated water: 1 × 10⁻⁴ M
• Temperature is same in lab and field conditions.

The required volume of the plug flow reactor is __________ m³. (rounded off to two decimal places)

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4

2021 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2021

A flocculation tank used for water treatment has a velocity gradient (G) of \(800\ \text{s}^{-1}\). The volume of the tank is \(40\ \text{m}^3\). The dynamic viscosity of water is \(9\times10^{-4}\ \text{N·s/m}^2\). The theoretical power required to maintain the given velocity gradient is __________ kW (rounded off to the nearest integer).

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5

2021 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2021

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 (also referred to as CSTR).

A

PFR will always require less reactor volume than CFSTR.

B

PFR will require the same reactor volume as CFSTR.

C

CFSTR will always require less reactor volume than PFR.

D

CFSTR can sometimes require less reactor volume than PFR.

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6

2021 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2021

A 10 million litres per day (MLD) sewage treatment plant (STP) is based on the Activated Sludge Process (ASP). First, the sewage undergoes primary treatment and the resulting treated sewage has BOD₅ of 140 mg/L concentration. This is further passed through a 1500 m³ capacity aeration tank (in ASP), where the mixed liquor volatile suspended solids (MLVSS) concentration is maintained at 3000 mg/L. The concentration of BOD₅ of the treated sewage is 5 mg/L.
The Food to Microorganisms ratio (F/M) of the ASP is ______________ day⁻¹ (rounded off to two decimal places).

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7

2021 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2021

A facultative pond system for sewage treatment consists of two ponds in series. The hydraulic retention time (HRT) of each pond is 6 days. The total BOD₅ reduction through the entire pond system is 90 %. If the ponds are considered to be completely mixed, then the rate constant describing the BOD₅ removal is ______________ day⁻¹ (rounded off to two decimal points). Assume that the rate constant is same for both the ponds.

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8

2022 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2022

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

A

No reactant is added after the reactor has started operation.

B

No product is withdrawn during the course of the reactor operation.

C

The reactor operates under steady state.

D

The reactor operation is carried out for a pre-specified duration.

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9

2022 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2022

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

A

CSTR and PFR are normally operated under steady state condition.

B

There is complete homogeneity in the CSTR while there are concentration variations within the PFR.

C

The overall reaction carried out in a PFR is always higher than that in a CSTR for same total volume.

D

Reaction kinetics do not play any role in choosing between a CSTR and PFR.

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10

2022 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2022

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 mg/l, respectively. If the mixed microbial culture has an observed biomass yield of 0.5 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 __________.

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11

2023 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2023

River water containing two types of spherical suspended particles (clay particles, metal particles) is retained in a sedimentation tank. The clay particles having diameter of 75 μm and specific gravity of 2.65 is settling in the tank with a constant velocity. The velocity of clay particles is 2 times of that of metal particles having specific gravity of 8. Assume discrete settling and laminar flow conditions within the sedimentation tank. The estimated diameter of the metal particles is ______________ (in μm, rounded off to integer)

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12

2023 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2023

A biological reactor is getting wastewater containing 1 mole/L acetate ions as carbon source. The following reaction takes place in the bio-reactor:
\[ 0.125\text{CH}_3\text{COO}^- + 0.0295\text{NH}_4^+ + 0.103\text{O}_2 \rightarrow 0.0295\text{C}_5\text{H}_7\text{O}_2\text{N} + 0.0955\text{H}_2\text{O} + 0.095\text{HCO}_3^- + 0.007\text{CO}_2 \]
Assume that all acetate ions are consumed and ammonia serves as a nutrient source. Given that 1 g acetate exerts 1.07 g COD; 1 mole bacteria =113 g VSS; 1 mole acetate ion = 59 g. Value of observed yield is __________ (in g VSS/g COD, rounded off to two decimal places).

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13

2023 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2023

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 m³/d. Permissible limit of VOC in the water is 12 μg/L. The saturation concentration of VOC is 5 μg/L and gas transfer rate constant is 0.40 per second at 25 °C. The initial concentration of VOC in the water is 33 μg/L. The volume of the aeration tank to satisfy the permissible limit of VOC at 25 °C is __________ (in m³, rounded off to two decimal places).

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2026 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2026

A sewage treatment plant employing activated sludge process is operated under the following conditions:
Wastewater flow rate into the aeration tank = 0.150 m³/s
Soluble BOD₅ in the influent = 84 mg/L
Soluble BOD₅ in the effluent = 11 mg/L
Mean cell residence time = 5 days
Total yield co-efficient = 0.5 kg of MLVSS/kg BOD₅ removed
Decay rate of microorganism = 0.050/day
The net waste activated sludge produced in the plant is _____ kg of VSS/day (rounded off to two decimal places).

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15

2026 · Environmental Science & Engineering · Water & Wastewater Treatment and Management · Kinetics and reactor design

Environmental Science & Engineering (ES) 2026

A continuous flow stirred tank reactor (CSTR) operates under a steady-state condition with a flow rate of 300 L per hour. The influent concentration of a substrate entering the reactor is 150 mg/L.
To give a treatment efficiency of 76% for the substrate that decays according to half-order kinetics with a rate constant of 0.05 (mg/L)^1/2 per hour, the required volume of the reactor is _____ m³ (answer in integer).

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