An activated-sludge waste treatment system is required to...

An activated-sludge waste treatment system is required to reduce the amount of $\mathrm{BOD}_{5}$ from $1000 \mathrm{mg} / 1$ to $20 \mathrm{mg} / \mathrm{l}$ at the exit. The sedimentation unit concentrates biomass by a factor of 3. Kinetic parameters are $\mu_{m}=0.2 \mathrm{~h}^{-1}, K_{s}=80 \mathrm{mg} / \mathrm{l}, k_{d}=0.01 \mathrm{~h}^{-1}$, and $Y_{X S}^{M}=0.5 \mathrm{~g} \mathrm{MLVSS} / \mathrm{g}$ $\mathrm{BOD}_{5}$. The flow of waste water is $10000 \mathrm{l} / \mathrm{h}$ and the size of the treatment basin is 50,0001 . a. What is the value of the solids residence time (i.e., $\theta_{c}$ )? b. What value of the recycle ratio must be used?

An activated-sludge waste treatment system is required to reduce the amount of $\mathrm{BOD}_{5}$ from $1000 \mathrm{mg} / 1$ to $20 \mathrm{mg} / \mathrm{l}$ at the exit. The sedimentation unit concentrates biomass by a factor of 3. Kinetic parameters are $\mu_{m}=0.2 \mathrm{~h}^{-1}, K_{s}=80 \mathrm{mg} / \mathrm{l}, k_{d}=0.01 \mathrm{~h}^{-1}$, and $Y_{X S}^{M}=0.5 \mathrm{~g} \mathrm{MLVSS} / \mathrm{g}$ $\mathrm{BOD}_{5}$. The flow of waste water is $10000 \mathrm{l} / \mathrm{h}$ and the size of the treatment basin is 50,0001 .
a. What is the value of the solids residence time (i.e., $\theta_{c}$ )?
b. What value of the recycle ratio must be used?

Key Concepts

Activated Sludge Process

The activated sludge process is a biological wastewater treatment method that uses aeration and a suspended growth system to promote the degradation of organic contaminants by microorganisms. This process involves the growth of a microbial community that breaks down pollutants present in the wastewater, followed by separation of the biomass from the treated water via sedimentation.

Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand is a measurement of the oxygen required by microorganisms to decompose organic matter present in water. It is a key parameter for assessing the level of organic pollution and the performance of wastewater treatment processes, indicating how much oxygen will be consumed during subsequent biological degradation.

Solids Residence Time (SRT)

Solids Residence Time, or sludge age, refers to the average time that the microbial biomass remains in the treatment system. SRT is critical in managing the growth and decay of microorganisms, influencing both the efficiency of organic matter breakdown and the balance between biomass production and loss.

Process Kinetics in Wastewater Treatment

Process kinetics in wastewater treatment involves the use of parameters such as maximum specific growth rate (?m), half-saturation constant (Ks), decay rate (kd), and yield coefficient (YX/S). These parameters are essential for modeling and designing the treatment process, as they describe the rate at which microorganisms grow, consume substrate, and decay, thereby affecting overall treatment performance.

Recycle Ratio in Activated Sludge Systems

The recycle ratio is the rate at which settled biomass is returned from the sedimentation unit to the aeration basin relative to the influent flow. This ratio is crucial for maintaining an adequate concentration of active microorganisms in the system, ensuring that the microbial population is sufficient to handle the organic load in the incoming wastewater.

Sedimentation and Biomass Concentration

Sedimentation is the process of separating suspended solids (including microbial biomass) from treated water by allowing them to settle. The concentration of biomass achieved during sedimentation can be represented by a biomass concentration factor, which affects the efficiency of recycling microorganisms back to the aeration basin and helps maintain an effective treatment process.

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