Activated sludge process control

# Activated sludge process control

> Activated sludge is a population you manage, not a machine you run. Five numbers tell you whether that population is healthy, and the exam expects you to read them cold.

Why the exam tests this

Activated sludge sits inside Treatment Process Evaluation and Adjustment, the single largest duty on the Class I blueprint at 38 percent. Most small-plant operators run one tank every day and develop a feel for it. The exam asks you to explain the numbers behind that feel: MLSS, dissolved oxygen, MCRT, F/M, and SVI, and to know what each one is telling you when it moves.

At Class I and II you mainly need to define each term and read a number against its normal range. At Class III and IV the exam expects you to connect two or three of those numbers and diagnose an upset, filamentous bulking, foaming, or rising sludge, from the pattern they make together, not from any one reading alone.

What you need to know

Activated sludge mixes a living population of bacteria and other organisms into the wastewater in an aeration tank, feeds them oxygen, and lets them eat the dissolved and colloidal matter that primary settling missed. The mix in the tank is the mixed liquor. A secondary clarifier settles the floc out; most of it returns to the aeration tank as return activated sludge (RAS) to keep the population up, and the excess leaves as waste activated sludge (WAS).

You control three things: how many organisms are in the tank, how much oxygen they get, and how long they stay before you waste them out. The levers are wasting, return, and air. Five readings tell you where each of those stands.

MLSS, mixed liquor suspended solids, is the total solids in the aeration tank. Conventional activated sludge typically runs 2,000 to 4,000 mg/L. MLVSS, the living fraction of that total, should sit at 70 to 80 percent of MLSS. Wasting is what lowers MLSS; waste more and it drops.

Dissolved oxygen, DO, usually needs 2 to 4 mg/L, with a conventional target of 2.0 mg/L at the end of the aeration tank. Too little DO favors filaments and facultative organisms that settle poorly. Too much air can shear the floc into fine particles that will not settle either.

MCRT, mean cell residence time, also called sludge age or SRT, is how many days the organisms stay in the system before they are wasted out. It equals the solids in the aeration system in pounds, divided by the pounds leaving per day. Conventional plants typically run 3 to 15 days; under 3 days gives young, slow-settling, turbid sludge.

F/M, the food-to-microorganism ratio, is the influent BOD in pounds per day divided by the MLVSS in the aeration tank in pounds. Too high and the bugs grow dispersed instead of clumping into floc. Too low starves the tank and breeds filaments. Wasting controls F/M the same way it controls MLSS.

SVI, sludge volume index, tells you how well the sludge settles: the milliliters that settle in 30 minutes per gram of MLSS. A range of 50 to 150 mL/g is a fair working target, but your own plant's best SVI comes from your own experience, not a number borrowed from another plant.

pH and alkalinity round out the picture. Bacteria do best at pH 6.5 to 7.5, and nitrification needs a narrower 7.8 to 8.2. Low pH inhibits the nitrifying organisms and favors filaments. The bugs also need nitrogen and phosphorus along with the carbon in BOD, roughly a 100:5:1 ratio; domestic sewage usually carries enough of both.

The organisms carry their own information. Aerobic bacteria are the dominant, most efficient population and produce little odor; facultative organisms are less efficient and make foul products when oxygen runs short. Filaments are a normal part of a healthy floc in small numbers, they hold the floc together, but when they take over they cause bulking, sludge that will not settle. Protozoa and rotifers read the sludge's age: a well-run plant shows mostly free-swimming and stalked ciliates, while heavy rotifers point to old sludge and a lot of tiny flagellates points to a young, troubled population.

At higher grades, the exam also expects you to place a plant by its sludge age. High rate runs an F/M of 0.4 to 1.5 with a sludge age under 5 days and lower effluent quality. Conventional runs F/M 0.2 to 0.4 with a sludge age around 3.5 to 10 days. Extended aeration runs F/M 0.05 to 0.15, sludge age over 10 days, and usually nitrifies, at the cost of a much bigger tank.

Four upsets show up again and again, and the exam tests them by symptom.

• Bulking: sludge settles poorly and solids escape over the weir. Filaments are the usual cause, driven by too little wasting and a high F/M. Raise MCRT and hold DO at 2 to 4 mg/L to recover.
• Septic sludge: the sludge goes anaerobic and floats up with a foul smell, usually from poor mixing or long detention. Keep sludge lines flowing and confirm the draw-off pumps work.
• Rising sludge: floc that settled fine floats back up on fine gas bubbles from denitrification in the clarifier, nitrate turning to nitrogen gas. Raising RAS or lowering MCRT moves the denitrification earlier, where you want it.
• Foaming: light white foam usually means young sludge and low MLSS. Thick, dark, greasy foam points to filaments such as Nocardia, tied to a low F/M and high MCRT.

At Class III and IV, expect a question built around a toxic load. A pale, washed-out basin with high DO and no oxygen demand means the biomass is dead, not thriving, and it will not recover on its own. Recovery there means reseeding with healthy biomass from another working plant, then rebuilding the population slowly.

Worked examples

These three problems use the same loading relation, concentration times volume times 8.34, applied to F/M, MCRT, and SVI. For the full formula set behind that constant, see the math guide at /guides/wastewater-treatment-math.

Example 1: F/M ratio. Your aeration tank holds 0.018 MG of mixed liquor at an MLSS of 3,000 mg/L, with MLVSS at 75 percent of that. Influent BOD is 220 mg/L at a flow of 0.05 MGD.

MLVSS (lbs) = 3,000 mg/L x 0.75 x 0.018 MG x 8.34 = 338 lbs. BOD load (lbs/day) = 220 mg/L x 0.05 MGD x 8.34 = 92 lbs/day. F/M = 92 / 338 = 0.27.

That lands inside the conventional range of 0.2 to 0.4, so the tank is neither starved nor overloaded. Check your own arithmetic at /tools/practice.

Example 2: MCRT. The aeration tank and clarifier together hold 2,400 lbs of MLSS. WAS wasted plus the solids lost in the effluent total 240 lbs/day.

MCRT = solids in the system (lbs) / solids leaving per day (lbs/day) = 2,400 / 240 = 10 days.

Ten days sits inside the conventional range of 3 to 15 days. If that number kept dropping toward 3, expect the sludge to turn young, turbid, and slow to settle. Rework this one at /tools/practice until the division is automatic.

Example 3: SVI. A 1-liter sample from the aeration tank settles to 220 mL after 30 minutes in the settleometer, at an MLSS of 3,000 mg/L.

Percent settleable = 220 / 1,000 = 22 percent. SVI = (percent settleable x 10,000) / MLSS = (22 x 10,000) / 3,000 = 73 mL/g.

That falls inside the 50 to 150 mL/g range associated with fair settling. A rising SVI over several days, even while still inside that range, is worth watching before it turns into bulking.

Common traps

• Confusing MLSS and MLVSS. MLSS is everything suspended in the mix; MLVSS is only the living fraction, 70 to 80 percent of MLSS. Read the question closely for which one it wants.
• Mixing up F/M and MCRT. F/M answers how much food per bug; MCRT answers how many days a bug stays in the system. Both move with wasting, which makes it easy to reach for the wrong one under time pressure.
• Reading DO by itself as the whole story. A DO crash from a slug load and a DO crash from a broken blower look the same on the meter, but the fix is different for each, so the exam wants you to name the cause, not just the symptom.
• Treating every foam the same. Light white foam calls for raising MLSS; thick, dark, greasy Nocardia foam calls for lowering MCRT. Applying the wrong fix to the wrong foam makes the real problem worse.
• Dropping the 8.34 conversion, or mixing flow in gallons per minute with flow in MGD. Either mistake throws off every loading and F/M answer even when the rest of the setup was right.

Practice

Process-control questions built to this module's competency and grade range live in the wwt-1 practice test, at /tools/practice. Drill the MLVSS, F/M, MCRT, and SVI conversions until the 8.34 math is automatic, then work through the filament and foam scenarios until you can name the fix from the symptom alone. Retake the set after a week away from the material; the exam rewards recall you can produce cold, not recall you can only reconstruct with the book open.

Quick reference

• Aeration tank feeds oxygen to the mixed liquor; RAS returns settled floc, WAS removes the excess.
• MLSS: 2,000 to 4,000 mg/L typical for conventional activated sludge; MLVSS is 70 to 80 percent of MLSS.
• DO: 2 to 4 mg/L typical, target 2.0 mg/L at the end of the aeration tank.
• MCRT (sludge age): 3 to 15 days conventional; under 3 days gives young, slow-settling sludge.
• F/M: 0.2 to 0.4 conventional, 0.05 to 0.15 extended aeration, 0.4 to 1.5 high rate.
• SVI: 50 to 150 mL/g is a fair settling range; find your own plant's optimal number by experience.
• Bacterial pH optimum 6.5 to 7.5; nitrification optimum pH 7.8 to 8.2; nutrient ratio about 100:5:1 BOD to nitrogen to phosphorus.
• Bulking: filaments plus high F/M; recover with higher MCRT and DO at 2 to 4 mg/L.
• Rising sludge: denitrification gas in the clarifier; recover with higher RAS or lower MCRT.
• Foaming: white foam means young sludge; dark greasy foam points to Nocardia and a low F/M.
• A washed-out, pale basin with high DO and no demand means dead biomass, not a healthy plant; recovery means reseeding.
• Make one process change at a time and give the plant about a week to respond.