Have you ever wondered why there are so many names for something that is essentially the same?
That's exactly what I did. There are a lot of acronyms and shortened names in our profession. Consider the abbreviations MCRT, SRT, and DSRT.
"Go look it up," "I don't know," and "Why do you ask so many questions?" were by far the most prevalent replies upon asking the above questions.
Let's take a look at these abbreviations, what they imply, and why they're vital in the fight to keep the activated sludge treatment process under control.
The ‘M’ in ‘MCRT’ stands for "mean," which is a synonym for "average."
‘CRT’ stands for ‘cell residence time’, which refers to how long a single bacterial cell, or a pound of bacteria, stays in the activated sludge secondary treatment train. MCRT is usually measured in days.
‘SRT’ (solids retention time) is sometimes used interchangeably with ‘MCRT’. They usually mean the same thing, but they're computed with distinct bits of information.
“The total mass of solids in the treatment system is SRT, whereas the total mass of bacteria in the system is MCRT. Days are also used to convey SRT.”
‘MCRT’ is computed using volatile suspended solids (VSS) data (for example, effluent VSS, and waste sludge VSS, mixed liquor VSS), whereas SRT is calculated using total suspended solids (TSS) values (for example, mixed liquor TSS, effluent TSS, waste sludge TSS).
What is the difference between HRT and MCRT?
Same thing goes for HRT also, where HRT is for hydraulic retention time which signifies the time it takes for wastewater to flow through a tank such an aeration tank in hours.
Difference depicted by calculations involved:
1: Hydraulic retention time (HRT)
The hydraulic retention time, or HRT, is the time it takes for wastewater to flow through a tank such an aeration tank in hours.Biological activity can be affected by changes in the HRT of an activated sludge process. Reduced HRT, for example, inhibits nitrification, whereas increased HRT promotes nitrification and the solubilization of colloidal and particulate BOD.
The HRT of an aeration tank is calculated by multiplying the tank's volume in million gallons by the flow rate through the tank. The aeration tank's flow rate must be given in gallons per hour (gph).
Calculation of hydraulic retention time (HRT)
The hydraulic retention time is calculated as
Where, θ = hydraulic retention time (d)
V = Volume of the aeration basin (m3)
Q0 = Influent flow rate (m3/d)
While treating sewage, it is common practice to keep the detention duration between 5 and 8 hours. The return sludge, which accounts for around 25 to 50 percent of the wastewater volume, is also taken into account when determining the size of the aeration tank.
2: 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).
It is important to know the quantity of solids or suspended solids in the activated sludge system and the amount of suspended solids exiting the activated sludge system, in order to calculate the MCRT of an activated sludge process.
The pounds of mixed liquid suspended solids (MLSS) must be computed to estimate the pounds of suspended solids in the activated sludge system. All solids in the aeration tank(s) and secondary clarifier make up the MLSS (s).
In an activated sludge system, the pounds of MLSS are calculated by multiplying the MLSS concentration (mg/l) by the volume (MG) of the aeration tank(s) and clarifier(s) times the weight constant of 8.34 pounds per gallon of wastewater.
Calculation of mean cell residence time (MCRT)
The amount of time that microorganisms stay in the system is measured by their mean cell residence time (MCRT). This is also known as the sludge age or solids retention time (SRT). This is written as:
θc = Total biomass in the aeration basin biomass wasted per unit time
As the value of Xe is negligible, Eq. reduces to
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