The document discusses continuous stirred-tank reactors (CSTRs), which are commonly used as models for chemical reactors. A CSTR consists of a cylindrical vessel with an agitator that keeps reactants uniformly mixed. It operates continuously with influents added and effluents removed at constant rates. CSTRs can achieve steady state through chemostatic or turbidostatic principles. They find wide application in activated sludge wastewater treatment processes, where part of the sludge is continuously recycled to the reactor.

1. Continuous stirred-tank reactor
[CSTR]
BY
P.S.MOHAMED MOHAIDEEN
2021406030
M.Tech(ENVIROMENTAL SCIENCE
&TECHNOLOGY.
Alagappa College of Technology,

2. INTRODUCTION
• The continuous stirred-tank reactor (CSTR),
 mixed flow reactor (MFR)
 vat or back mix reactor
continuous-flow stirred-tank reactor (CFSTR)
is a common model for a chemical reactor in chemical engineering and environmental
engineering.
• Influents are added and Effluents are removed continuously at a constant
mass flow rate.
• A continuous stirred tank reactor consists of a cylindrical vessel with motor driven
central shaft that supports one or more agitators (impellers).
• The shaft is fitted at the top of the reactor, the number of impellers is variable and
depends on the size of the reactor.
• As it is a continuous reactor the steady state conditions can be achieved by either
CHEMOSTATIC or TURBIDOSTATIC principles

3. • RESIDENCE TIME – average amount of time a discrete quantity of reagents
spend inside the tank
• RESIDENCE TIME = VOLUME(m3)
(d) VOLUMETRIC FLOW RATE(m3/d)
• At steady state, the FLOW RATE IN must be equal to the mass FLOW RATE
OUT

4. • Different types of continuous reactor are
A) Single stage CSTR,
B) Multi stage CSTR (SERIES or PARALLEL).

5. Biological Treatment Using Continuous Stirred-
Tank Reactor Technology
• Stirred-tank reactors may be operated either in batch mode or in continuous
mode.
• In large biological water treatment plants, such bioreactors are normally operated
in continuous mode and are called continuous stirred-tank reactors (CSTRs).
• In a CSTR, the reactants are uniformly distributed instantly on entering the
reactor.
• The extent of biodegradation, however, depends on Microbial population and on
optimum conditions of microbial growth including
• food to microorganism ratio,
• pH,
• temperature,
• mean cell residence time(MCRT), and
• other environmental conditions.
• CSTR technology finds wide application in activated sludge processes where a
part of the sludge is continuously recycled back to the reactor.

7. Modelling Activated Sludge Process
• The cell accumulation rate within reactor = rate of cell inflow + rate new
cell mass growth from substrate consumption - rate of cell decay (or
death) - rate of cell exit with treated effluent - rate of cell exit with
underflow sludge (which is wasted).
• That is
• Depletion rate (substrate) within control volume = Inflow rate –
conversion rate (degradation) – rate of discharge with effluent – rate of
discharge with underflow sludge
• That is

8. • Where,
• X is the microbial cell concentration within the reactor with volume V;
• Xu is the cell concentration in the leaving underflow sludge; and
• Xe is the concentration of cell (escaping from the reactor) dX/dt is the total
accumulation rate within reactor volume V.
• Q0 is the feed inflow rate;
• Qe and Qu are flow rates of treated effluent and the sludge underflow;
• Kd the microbial decay constant;
• μmax is the maximum substrate conversion rate;
• S is the substrate concentration in the reactor;
• S0 is the substrate concentration in the influent; and
• Ks is the Monod half-velocity rate constant .

9. Mean cell residence time(MCRT)
The mean cell residence time, or MCRT, is the length of time solids or bacteria are kept in the activated sludge system in days.
The solids retention time is also known as the MCRT (SRT).

13. Image

15. • Microbes need flow of electrons and energy for their maintenance
• resynthesis,
• repair,
• osmotic regulations, and
• Transport.
• and considering loss of heat to the environment.
• This energy requirement is met by oxidation of decayed microbes,
• Although the total mass of the decayed microbes does not undergo such
oxidation (part remaining as inert mass).
• Cells oxidize themselves to meet this energy demand.
• This decay is called endogenous decay and is expressed mathematically as:

19. Industrial Operations: Practical Considerations
and Troubleshooting
• Bulking Sludge—Problem of High Filamentous Growth
Most of the operational trouble in activated sludge plants is due to microbial flock
characteristics. Unless these flocks are well settleable, clear water at the effluent
outlet cannot be ensured and recycling of sludge from settling unit to the aeration
unit will be difficult.
• Causes of Rising Sludge
Rising sludge may result from low activated sludge return, excessive aeration, low
DO, enhanced denitrification at temperature above 20˚C, low F/M ratio, and high
presence of reduced sulfur (sulfides).
• TROUBLESHOOTING:
• Chlorination treatment reduces the growth of filamentous microbes .
• Regular monitoring of the parameters and the plant can avoid the problem of Rising Sludge.

20. APPLICATIONS

21. References
• https://www.michigan.gov/documents/deq/wrd-ot-activated-sludge-
manual_460007_7.pdf
• https://www.sciencedirect.com/science/article/pii/B9780128103913
000035?via%3Dihub
• https://www.sciencedirect.com/topics/earth-and-planetary-
sciences/activated-sludge
• https://pubmed.ncbi.nlm.nih.gov/25635665/#:~:text=Continuous%20
stirred%20tank%20reactors%20(CSTR,to%20optimize%20the%20proc
ess%20energetically.