Returned Activated Sludge
The amount of RAS to be sent from the clarifier to the aeration basin
should be carefully calculated. The first step is to determine the amount
of microbes needed in the aeration basin based on the F/M ratio. This
calculation was explained in the topic page on the F/M ratio.
Let's assume that you need to add 50.04 lbs of microbes to the aeration
basin. But sludge is a mixture of microbes and organic matter. How
do you know how much of this mixture to return to the aeration basin?
RAS has some typical properties. When dried, 100 grams of RAS
weighs 5 grams. When cooked at 550°C, 100 grams of RAS weighs
3 grams. And, most importantly for our calculations, 2% of RAS is
microbes. So, if you have 100 grams of RAS, 2 grams of this is made
up of microbes.
Using this last property, we can make a formula to determine how much
RAS to feed in order to provide a given amount of microbes:
Using this formula, we can determine how much RAS
we need to feed into the aeration basin to provide 50.04 pounds of microbes:
So, we need to feed 2,502 pounds of returned activated
sludge into the aeration basin each day.
Of course, to set the RAS feed rate, you are likely to need to know the pounds
of RAS to feed per minute, or the grams of RAS to feed per minute rather
than the pounds per day. These feed rates are easily calculated using conversions
you are already familiar with:
So you would set the feed rate to 789 grams per minute.
How do you set the feed rate of the pump? You
will need a container to collect the sludge being pumped into the aeration
First, you weigh this container. The weight
of the container is known as the tare weight.
Next, put this container under the pipe which pumps
sludge into the aeration basin. Turn on the pump and allow the pump
to run for one minute. Then turn off the pump and remove the container.
Third, weigh the container and the sludge filling
it. This is known as the total weight.
Now you are ready to calculate how much sludge was pumped
during the minute you measured.
Total Weight (g) - Tare Weight (g)
= Sludge Weight (g)
1190 g - 400 g = Sludge Weight
790 g = Sludge Weight
So the pump rate is 790 grams per minute, which is
the optimal feed rate we calculated above.
If the feed rate was greater than the calculated rate,
then you would need to lower the volume of sludge being pumped. If
the feed rate was less than the calculated rate, then you would need to
raise the volume of sludge being pumped. Continue weighing the amount
of sludge the pump is pumping until it reaches the optimal feed rate you
To make sure that you have reached the optimum F/M ratio, try increasing
the pump feed rate slightly. Then measure the B.O.D. of the effluent
water from the aeration chamber. If the B.O.D. has raised, then you
should lower the feed rate. If the B.O.D. has lowered, then you should
continue raising the feed rate. You can continue raising or lowering
the pump feed rate until the B.O.D. in the effluent water has reached the
minimum value. Now you have found the optimum F/M ratio and the optimum
amount of RAS to feed into the aeration basin.
It is very easy to calculate how much sludge will be pumped to
waste each day.
Sludge wasted = (Sludge made per day)
So, if the amount of sludge made each day is 5,000
lb and the amount of sludge returned to the aeration basin is 2,502 pounds,
then the amount of sludge wasted is:
Sludge wasted = 5,000 lb - 2,502 lb
Sludge wasted = 2,498 lb