Unit 4
By : Om Sonawane
Microbial Growth Kinetics Terminology in Biological wastewater treatment
Microbial growth kinetics in biological wastewater treatment refers to the study of how microorganisms grow and reproduce in a treatment system. Several key terms are commonly used in this field, including:
Microbial population: the total number of microorganisms present in a given system.
Specific growth rate (μ): the rate at which a microbial population is increasing per unit of time, usually measured in per hour.
Generation time (g): the amount of time it takes for a microbial population to double.
Yield coefficient (Y): the ratio of biomass produced to the amount of substrate consumed.
Monod equation: a mathematical model that describes the relationship between microbial growth rate and substrate concentration.
Inhibition: a reduction in microbial growth or activity due to the presence of certain compounds or factors in the wastewater.
Kinetics: the study of how microbial population changes over time.
Stoichiometry: the balance of chemical elements in a reaction
Microbial conversion: the conversion of substrate into microbial biomass by microorganisms
Microbial death: the loss of microbial cells from the population due to various causes such as starvation, toxic compounds or other environmental factors
Organic loading rate: the rate at which organic matter is added to a treatment system.
Hydraulic retention time: the amount of time a volume of wastewater stays in a treatment system.
Rate of utilization of soluble substrates in Biological wastewater Treatment
The rate of utilization of soluble substrates in biological wastewater treatment depends on various factors, including the type of microorganisms present, the conditions of the treatment process (such as temperature, pH, and oxygen levels), and the concentration of the substrate in the wastewater. Generally, the higher the concentration of the substrate, the faster it will be utilized.
However, as the substrate is depleted, the rate of utilization will decrease. Additionally, the presence of inhibitory compounds, such as heavy metals or toxic chemicals, can inhibit or slow down the rate of substrate utilization.
Other Rate Expression for the utilization of soluble substrate :
Another way to express the rate of utilization of soluble substrate in biological wastewater treatment is through the Monod Equation. The Monod equation is a mathematical model that describes the relationship between the rate of substrate utilization and the substrate concentration. It is given by:
μ = μ max * S / (Ks + S)
Where:
- μ is the specific growth rate (per hour) of the microorganisms
- μ max is the maximum specific growth rate
- S is the substrate concentration (mg/L)
- Ks is the substrate saturation constant (mg/L)
The Monod equation is a useful tool for predicting the rate of substrate utilization under different conditions. It can also be used to determine the optimal substrate concentration for a given treatment process and to determine the maximum specific growth rate of the microorganisms.
Another expression that can be used to express the rate of substrate utilization is the Haldane equation, which is given as:
μ = μ max * S / (Ks + S + (S^2 / Ki))
Where :
- Ki is the inhibition constant
- Haldane equation is used when inhibition occurs due to the presence of toxic compounds in the substrate.
Rate of Biomass growth with soluble substrate
The rate of biomass growth with a soluble substrate in biological wastewater treatment can be expressed in a number of ways. One common way is to use the Monod equation, which describes the relationship between the specific growth rate of microorganisms (μ) and the substrate concentration (S) as:
μ = μ max * S / (Ks + S)
Where:
- μ is the specific growth rate (per hour) of the microorganisms
- μ max is the maximum specific growth rate
- S is the substrate concentration (mg/L)
- Ks is the substrate saturation constant (mg/L)
The Monod equation is a useful tool for predicting the rate of biomass growth under different conditions. It can also be used to determine the optimal substrate concentration for a given treatment process and to determine the maximum specific growth rate of the microorganisms.
Another way to express the rate of biomass growth with a soluble substrate is to use the Haldane equation, which is given as:
μ = μ max * S / (Ks + S + (S^2 / Ki))
Where:
- Ki is the inhibition constant
This equation is used when inhibition occurs due to the presence of toxic compounds in the substrate.
The biomass growth rate can also be expressed as the change in biomass concentration over time, which can be measured directly by monitoring the increase in the total suspended solids (TSS) or volatile suspended solids (VSS) in the treatment system.
Rate Of Oxygen Uptake :
The rate of oxygen uptake, also known as the oxygen uptake rate (OUR), is a measure of the rate at which microorganisms consume oxygen in a biological wastewater treatment system. The OUR is typically expressed in units of milligrams of oxygen per liter of reactor volume per hour (mg/L-hr).
The OUR can be influenced by a variety of factors, including the type and concentration of microorganisms present, the temperature and pH of the treatment system, and the dissolved oxygen (DO) level in the wastewater. The OUR will generally increase as the DO level decreases, as the microorganisms will consume more oxygen to meet their metabolic demands.
The OUR can be measured using a variety of methods, including the Winkler method, the Azide modification of Winkler method, and the modified Winkler method. These methods involve measuring the amount of dissolved oxygen in a sample before and after the microorganisms have had a chance to consume it.
The rate of oxygen uptake can also be used as an indicator of the health of the microbial population in the treatment system and as a means of controlling the treatment process. For example, if the OUR is too high, it can indicate that the microbial population is overactive and that the treatment process needs to be adjusted. Conversely, if the OUR is too low, it can indicate that the microbial population is not active enough and that the treatment process needs to be optimized.
Effects of Temperature on Microbial Growth Kinetics
Temperature is one of the key environmental factors that can affect microbial growth kinetics. The effects of temperature on microbial growth kinetics can be described by the Arrhenius equation, which states that the specific growth rate (μ) of microorganisms increases with temperature, but at a decreasing rate. The equation is given by:
μ = μ max * e^(-Ea/RT)
Where:
- μ max is the maximum specific growth rate at a specific temperature
- Ea is the activation energy for the growth process
- R is the gas constant
- T is the absolute temperature (in Kelvin)
The Arrhenius equation shows that as the temperature increases, the specific growth rate also increases, but at a decreasing rate. This means that as the temperature increases, the rate at which microorganisms grow becomes less sensitive to further increases in temperature.
Additionally, the equation shows that each microorganism has an optimal temperature range where the growth rate is maximum, called the "Thermophilic" or "Mesophilic" microorganisms. Thermophiles grow best at high temperatures (above 45°C) while mesophiles grow best at moderate temperatures (20-45°C).
It's also important to note that temperature affects not only the growth rate but also the enzymatic activity, substrate utilization, and the overall metabolism of microorganisms. High temperatures can cause thermal stress and denaturation of enzymes, leading to a decrease in microbial activity and growth. Also, low temperatures can inhibit the microbial activity and growth.
Total volatile Suspended solids and active Biomass
Total volatile suspended solids (TVSS) and active biomass are two different measures of the microorganisms present in a biological wastewater treatment system.
Total volatile suspended solids (TVSS) is a measure of the total amount of suspended solids present in a liquid sample that are composed of volatile organic compounds. This includes both living and dead microorganisms. TVSS is typically measured by drying a sample of the liquid at 105-110°C and then weighing the dried solids.
Active biomass, on the other hand, refers to the portion of the microorganisms present in a liquid sample that are actively growing and reproducing. This includes only the living microorganisms, and not the dead ones. Active biomass can be measured through a variety of methods, such as plate counting, microscopy, or by measuring the oxygen uptake rate (OUR).
TVSS is a measure of the total microorganism population in a treatment system while active biomass is the measure of the active and reproducing microorganism population. Therefore, the difference between TVSS and active biomass is that TVSS represents the total amount of microorganisms present, including both living and dead cells, while active biomass represents only the portion of microorganisms that are actively growing and reproducing.
Net Biomass yield and observed yield :
Net biomass yield and observed yield are two different measures of the efficiency of a biological wastewater treatment process.
Net biomass yield refers to the amount of new biomass produced per unit of substrate consumed. It is calculated as the mass of new biomass produced divided by the mass of substrate consumed. This measure is used to evaluate the efficiency of a treatment process in terms of how effectively the microorganisms are converting the substrate into new biomass.
Observed yield, on the other hand, refers to the actual amount of biomass produced per unit of substrate consumed. This measure is affected not only by the efficiency of the microorganisms in converting the substrate into new biomass, but also by other factors such as substrate inhibition, microorganism death, and the presence of other competing microorganisms.
In general, the net biomass yield is considered to be a theoretical value, while the observed yield is a practical value. The observed yield is usually lower than the net biomass yield because of the factors that affect the overall efficiency of the treatment process.
In summary, net biomass yield is the theoretical value of the ratio of the biomass produced to the substrate consumed, while observed yield is the practical value of the ratio of the biomass produced to the substrate consumed, taking into account all the factors that might affect the efficiency of the process.
Laboratory procedure for determination of biokinetic constants :
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