Sludge Age

It was break time during a class I was teaching when an operator came up and asked me, “Ricketts, what does sludge age really mean?” And I had to think for a minute. My response was “It’s a measure of the stability of the sludge and a decent way to know if you have enough of the right kind of organisms in the plant.” Before I could continue, another operator chimed in with, “It’s the number of pounds of total suspended solids (TSS) in the aeration basin divided by pounds of influent total suspended solids per day.” And I screamed, “Halleluiah, someone remembered the math!”

The math used to calculate the sludge age is fairly simple, but it is the interpretation of the data and how we use that data that is important. The sludge age is a tool we use to help determine how stable wastewater is or how much biochemical oxygen demand (BOD) has been removed and how much sludge has been produced. It also tells us what type of organisms should be present to help in stabilizing the sludge, flocculation and settling.

Instead of using a microscope to determine the types of organisms that live in the aeration section of the plant, we use sludge age. These organisms have to be given the correct environmental conditions and time to proliferate.

The sludge age tells us about the number and weight of organisms we have in the basin. We find this information by calculating the pounds of total suspended solids (TSS) in the basin as compared to the pounds of TSS in the influent flow. We use the weight of the TSS because if we tried to count the organisms, the numbers would astronomical and nearly impossible to count.

A sludge age of less than 10-day-old sludge normally indicates that the mixed liquor suspended solids are not concentrated enough, and the bacteria have not developed the slime layer needed for flocculation. Normally, a bright white foam will occur on the aeration section of the plant, looking similar to soap suds. All of the sludge from the clarifier should be returned to the aeration section of the plant to raise the sludge age as soon as possible. Young sludge may occur at the startup of a new facility or after a washout. At this point, the bacteria are not available in great enough numbers to remove much of the BOD or ammonia.

As the sludge is allowed to age, the amount of bacteria increases and the predators of the bacteria also increase. This is good because the predators, ciliates, rotifers and other protozoans, aid in the destruction of the bacteria and help in the process of flocculation. These predators also feed on flocculated and non-flocculated bacteria.

In extended aeration plants, the free-swimming ciliates are normally the first predators observed in 12- to 15-day-old sludge. These organisms normally feed on non-flocculated or planktonic bacteria.

The crawling ciliates start to be observed in 15- to 25-day-old sludge. These organisms help stabilize the flocculated particles by feeding on the bacteria that make up the flocculated particles; as they feed, they release a mucus-like material that helps the flocculated particles hold together.

In 20- and 30-day-old sludge, there should be a thriving ecosystem of bacteria, ciliates (free-swimming, crawling and stalked) and rotifers. The populations of these organisms help us determine how stable the sludge is and whether it is ready to be disposed of or not.

As the sludge continues to age, the ecosystem will stabilize at the sludge age at which the operator has determined will produce the best effluent with the least amount of sludge produced. The optimum sludge age will be different for each facility due to the type of treatment, the size of the facility and the quality of the influent.

 Let’s do a sludge age problem just for practice.

Sludge age
Lbs. of MLSS in aeration section
Lbs. TSS in influent

 A 2.5 MGD oxidation ditch has a MLSS concentration 2940 mg/l with an influent flow of 1.95 MGD with a TSS concentration of 234 mg/l. What is the sludge age in this facility?

 Lbs. MLSS in OD
2.5 MGD X 8.34 lbs./gal. X 2940 mg/l = 61,299 lbs. MLSS in OD

Lbs. TSS influent
1.95 MGD X 8.34 lbs./gal. X 234 mg/l = 3805.5 lbs. TSS influent

Sludge age
61,299 lbs. MLSS
3805.5 lbs. TSS
16.1-day-old sludge

Sludge age is one tool in your toolbox of information that you collect and use to help operate your plant. You use this number to help determine when and how much sludge to waste and how much to return. Never try to operate your plant off one tool. You can’t fix the engine in your car with only one tool, so why try to operate your plant using one number?

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