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Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE) PDF Download

BASIC EQUATION FOR BIOLOGICAL TREATMENT OF WASTEWATER 

According to Monod kinetics, 

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE) (4.2.1)
 Where, s is the substrate concentration, ks is the substrate concentration when µ(=µmax/2), µmax is the maximum µ when substrate is not limiting. Also, solid production rate Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE)  is related to substrate utilization rate Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE) by 
 following relationship: 

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE) (4.2.2) 
 Where, Sr is the mass of soluble substrate (i.e. BOD), Y is the yield coefficient (kg of new cells formed/kg BOD removed). However owing to large treatment time in many of the large treatment units, substantial number of cells may die because of endogenous respiration. Therefore, 

Net production rate Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE) (4.2.3)

Where, Kd is the endogenous respiration decay rate constant. For growth phase only, 

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE) (4.2.3)

Case 1- S>>Ks    

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE)(4.2.4)
 i.e. removal rate is independent of substrate concentration and that the removal rate depends on X only. 

Case 2: S<< KS 

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE) (4.2.5)

Here, removal rate depends both upon X and S. Where, X is the mass of biomass in the system (usually represented by MLSS i.e. Mixed Liquor Suspended Solid), µ is the specific growth rate constant (time-1). 

Major Terms [a] Hydraulic detention time,

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE) (4.2.6)

[b] Sludge age or mean residence time(θC)

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE)(4.2.7)

Where, x (=X/V) the concentration of microbial solution in the system, x’ is the concentration of solids withdrawn. 

For the flow through system, x’ = x and Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE)

For the flow system with recycling, x’ < x and  Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE)

[c] Food to microorganism ratio 

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE) (4.2.8)

 

Problem 4.2.1: An aerated activated sludge tank is being operated under following conditions: Q=4400 m3/d, MLSS=3500 mg/l, Y=0.5, tank volume =770 m3, Endogeneous decay rate constant kd=0.09 d-1.
 (a) Estimate weight of solids produced per day for the conditions in which BOD is reduced from 350 mg/l to 130 mg/l.
 (b) Estimate θc.
 (c) Estimate F/M ratio. 

Solution: 

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE)

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE)

=241.4 kg/d. 

θc  = mass of solid in the system/ mass of solid leaving the system per day = Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE)

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE)

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE)

Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE)= 0.359 (kg BOD/kg MLSS) 

θc = 11.17 day.

The document Anaerobic & Aerobic Treatment Biochemical Kinetics | Environmental Engineering - Civil Engineering (CE) is a part of the Civil Engineering (CE) Course Environmental Engineering.
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FAQs on Anaerobic & Aerobic Treatment Biochemical Kinetics - Environmental Engineering - Civil Engineering (CE)

1. What is the difference between anaerobic and aerobic treatment?
Ans. Anaerobic treatment refers to a process where microorganisms break down organic matter without the presence of oxygen, while aerobic treatment involves the use of oxygen by microorganisms to decompose organic matter. Anaerobic treatment is typically used for high-strength wastewater, while aerobic treatment is suitable for lower-strength wastewater.
2. How do anaerobic and aerobic treatment processes work?
Ans. In anaerobic treatment, microorganisms break down organic matter through a series of complex reactions in the absence of oxygen. This process produces biogas, which is rich in methane and can be used as an energy source. In aerobic treatment, microorganisms use oxygen to break down organic matter into carbon dioxide and water. This process requires constant aeration to provide oxygen to the microorganisms.
3. What are the advantages of anaerobic treatment over aerobic treatment?
Ans. Anaerobic treatment offers several advantages over aerobic treatment. Firstly, it requires less energy input due to the absence of aeration. Secondly, anaerobic treatment produces biogas, which can be used as a renewable energy source. Additionally, anaerobic treatment is more suitable for high-strength wastewater with high organic content. It also produces less sludge compared to aerobic treatment.
4. What are the disadvantages of anaerobic treatment compared to aerobic treatment?
Ans. While anaerobic treatment has its advantages, it also has some limitations. One disadvantage is that it requires longer retention times compared to aerobic treatment. Anaerobic treatment also produces a foul odor due to the production of biogas, which can be a nuisance. Furthermore, it is less efficient in removing nutrients like nitrogen and phosphorus compared to aerobic treatment.
5. What factors should be considered when choosing between anaerobic and aerobic treatment processes?
Ans. Certain factors should be taken into consideration when deciding between anaerobic and aerobic treatment. The organic strength of the wastewater, availability of land, energy requirements, and the desired end-products (such as biogas) should be evaluated. Additionally, the operational and maintenance costs, regulatory requirements, and the presence of toxic compounds in the wastewater should also be considered.
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