Because of the time delay involved in generating the data used to determine the F:M ratio, it is not used to control the secondary treatment process. The mean cell residence time (MCRT) approach to balancing the solids in the activated sludge treatment system offers a simple and effective way to operate the activated sludge process ahead of the curve. What the MCRT approach attempts to do is account for all of the solids that are in the system as well as all of the solids that exit the system everyday. A system’s MCRT is a representation of the average time (in days) that a bacterial cell will remain in the system before being removed as WAS or leaving in the effluent. The calculation is made by dividing the total pounds of MLSS in the aeration basin by the total pounds wasted each day and the total pounds that exit the in the effluent each day.
A system’s MCRT is very similar to its SRT, except that the MCRT looks at what is leaving the system each day and the SRT looks at what is coming into the system each day. Typically, conventional activated sludge systems run at MCRTs of < 15 days, whereas extended aeration systems run at MCRTs of > 15 days. Contact Stabilization systems, due to their high loadings and high wasting rates tend to run at MCRTs of < 5 days.
In order to use the MCRT approach, daily information about a system’s MLSS concentration, WAS concentration, WAS flow, effluent TSS concentration and effluent flow are needed. Not all treatment plants can generate this amount of process control data everyday. If these pieces of information are available, the actual number of pounds of WAS that must be removed from the system in order to maintain the same solids balance (same MCRT) can be calculated.
It is important to understand that the minutes of WAS pumping that are required each day to maintain the desired MCRT will change from day to day, but that does not mean that the operator should change the pump setting everyday to try to adjust the system. This type of over management of the wasting rate tends to destabilize the activated sludge process. To compensate for this, the MCRT approach to process control should be used based upon a seven-day running average. In other words, seven days worth of WAS pump adjustments are averaged, and that average is what the pump is actually set to run each day. The following example explains:
Monday, August 10th (wasted) 114 minutes (calculated)
Tuesday, August 11th (wasted) 118 minutes (calculated)
Wednesday, August 12th (wasted) 113 minutes (calculated)
Thursday, August 13th (wasted) 115 minutes (calculated)
Friday, August 14th (wasted) 107 minutes (calculated)
Saturday, August 15th (wasted) 116 minutes (calculated)
Sunday, August 16th (wasted) 111 minutes (calculated)
Average 113 minutes actual WAS pumping on Sunday, August 16th
The next day:
Tuesday, August 11th (wasted) 118 minutes (calculated)
Wednesday, August 12th (wasted) 113 minutes (calculated)
Thursday, August 13th (wasted)115 minutes (calculated)
Friday, August 14th (wasted) 107 minutes (calculated)
Saturday, August 15th (wasted) 116 minutes (calculated)
Sunday, August 16th (wasted) 111 minutes (calculated)
Monday, August 17th (wasted) 105 minutes (calculated)
Average 112 minutes actual WAS pumping on Monday, August 17th
Using a seven-day running average prevents large changes to the wasting rate at any one time. It is very important that large changes are not made if the system is to operate as a stable process. Even if the seven-day running average is not used, a good rule of thumb to follow is “never change the waste rate by more than 10% a week”.
The MCRT approach offers a valuable method of balancing the solids in an activated sludge system. It is particularly suited to plants that treat 1.0 MGD and more, because these treatment facilities have the necessary in-house laboratory capability to generate the needed data. It must be applied to a treatment plant in a consistent manner and is only as good as the laboratory sampling and analysis. Any error introduced through non-representative sampling or inaccurate flow measurements will be magnified as errors in the MCRT calculation. It takes skilled operators to apply effectively, but treatment systems that use the MCRT approach have fewer upsets and can recognize problems well ahead of time and address them compared to systems that are run by “the seat of the pants”.