8.3 Activated Sludge Process Control (F/M, MLSS, SVI, RAS/WAS)

Process control is a balance, not a single knob

Activated sludge control means keeping food, bugs, oxygen, and settling in balance so the secondary clarifier can produce clear effluent while the aeration tank meets BOD (and often ammonia) limits. The classic metrics are F/M, MLSS/MLVSS, MCRT/SRT (sludge age), SVI, RAS, WAS, and DO. None of them alone tells the whole story—exam questions often give two readings that conflict until you think through solids inventory and clarifier behavior.

Quick Answer: F/M compares daily BOD load to biomass. MCRT/SRT is solids inventory ÷ solids leaving per day. SVI measures settleability. Raise WAS to lower sludge age/inventory; adjust RAS to control clarifier blankets. In Texas, Class D is not renewable at any activated sludge facility (30 TAC §30.342(c)).

F/M ratio with worked example

The food-to-microorganism ratio (F/M) compares organic loading to biomass:

[ F/M = \frac{\text{lb BOD applied per day}}{\text{lb MLVSS under aeration}} ]

Use the pounds formula for both numerator and denominator:

• lb BOD/day = MGD × mg/L BOD × 8.34
• lb MLVSS = aeration MG × mg/L MLVSS × 8.34

Worked example — F/M. Primary effluent to aeration is 1.5 MGD at 160 mg/L BOD. Aeration volume is 0.60 MG at 2,200 mg/L MLVSS.

1. Food = 1.5 × 160 × 8.34 = 2,001.6 lb BOD/day
2. Microorganisms = 0.60 × 2,200 × 8.34 = 11,008.8 lb MLVSS
3. F/M = 2,001.6 ÷ 11,008.8 ≈ 0.18 lb BOD/lb MLVSS·day

Interpretation (direction matters more than memorizing one magic number):

Target ranges depend on mode (extended aeration runs lower F/M than high-rate conventional). Always compare F/M to your plant’s historical good-performance window.

MCRT / SRT (sludge age)

Mean cell residence time (MCRT) and solids retention time (SRT) describe how long solids stay in the system—often called sludge age:

[ \text{MCRT (days)} \approx \frac{\text{lb solids in system}}{\text{lb solids leaving per day}} ]

Solids leaving ≈ WAS solids + effluent solids (effluent solids are often small but matter at long SRT). Many plants approximate inventory as aeration MLSS pounds (sometimes plus clarifier inventory when significant).

Worked example — sludge age. System solids ≈ 12,000 lb. WAS removes 800 lb/day and effluent solids are 50 lb/day. MCRT ≈ 12,000 ÷ (800 + 50) = 14.1 days.

If you increase WAS to 1,200 lb/day (effluent still 50), MCRT falls to 12,000 ÷ 1,250 ≈ 9.6 days—younger sludge, higher F/M for the same BOD load if MLSS drops.

SVI and settleability

The sludge volume index (SVI) links how far sludge settles in 30 minutes to MLSS concentration:

[ SVI\ (\mathrm{mL/g}) = \frac{\text{30-min settled volume (mL/L)} \times 1{,}000}{\text{MLSS (mg/L)}} ]

Worked example — SVI. A 1-liter settleometer reads 250 mL after 30 minutes. MLSS is 2,500 mg/L.
SVI = (250 × 1,000) ÷ 2,500 = 100 mL/g (generally good settling for many plants).

Bulking = sludge settles slowly and occupies large volume → high SVI → clarifier problems.
Pin floc = tiny weak particles that do not flocculate well → supernatant cloudy even if the bulk sludge sinks → often older sludge / low F/M conditions.

RAS and WAS control moves

Think in cause-and-effect:

• Blanket rising, clear supernatant still OK: often increase RAS temporarily and/or reduce solids inventory (more WAS) after checking for bulking (SVI).
• MLSS climbing for weeks: increase WAS (verify digester/thickener capacity).
• MLSS falling, treatment worsening: reduce WAS; confirm RAS is actually returning concentrated sludge.
• Young sludge symptoms after storm I/I: flow diluted MLSS and cut detention—restore inventory carefully; do not panic-waste.

DO targets

Typical aerobic zone targets for carbonaceous BOD removal are often about 1.5–3.0 mg/L DO (plant-specific). Nitrifying plants may need reliable residual DO in aerobic zones (commonly around 2 mg/L as a teaching benchmark) because nitrifiers are sensitive to low DO. Too little DO invites filaments and odor; too much wastes power and can shear floc in some systems. Always verify probe calibration before changing setpoints.

Texas licensing hard rule — Class D and activated sludge

Under 30 TAC §30.342(c), Class D wastewater licenses are not renewable for operators of:

1. Any activated sludge-type facilities (any size),
2. Certain larger trickling filter / RBC facilities (≥ 100,000 gpd permitted daily average flow), and
3. Facilities using subsurface area drip dispersal for effluent disposal.

For exam purposes, memorize the clean statement: A Class D license is NOT renewable at any activated sludge facility in Texas. Operators must upgrade to Class C or higher before the Class D expires. TCEQ even uses a Class D wastewater renewal questionnaire that asks whether you operate a permitted activated sludge facility of any size—answering yes means do not renew Class D; upgrade instead.

Control dashboard (how to think on exam day)

1. Is oxygen real? (calibrated DO, diffuser/blower health)
2. Is inventory matched to food? (MLSS/MLVSS, F/M, MCRT)
3. Will it settle? (SVI, settleometer, clarifier blanket)
4. Are RAS/WAS valves telling the truth? (flows, concentrations, wasting records)
5. For career/compliance questions: Class D ≠ renewable on activated sludge in Texas

If you can calculate F/M, SVI, and a simple MCRT—and explain bulking vs pin floc plus the Class D rule—you have the process-control core of this chapter.