7.1 Complete-Mix Suspended-Growth Process

7.1 Complete-Mix Suspended-Growth Process

2025 WPI alignment: This section teaches complete-mix suspended-growth activated sludge in Treatment Process Evaluation and Adjustment, the 40-question area containing 6 recall, 34 application, and 5 calculation items.

Why this process task matters

A complete-mix basin rapidly disperses influent through the reactor, buffering localized shock but exposing the entire biomass to the resulting mixed concentration. Control centers on loading, aeration, solids inventory, recycle, wasting, and clarification.

Process-control model

Evaluation and adjustment sequence

1. Calculate current BOD loading, biomass inventory, F/M, SRT, and RAS/WAS rates from measured data.
2. Observe mixed-liquor color, odor, foam, floc, DO profile, settleometer, microscopy, and clarifier condition.
3. Verify flow, analyzers, blowers, RAS/WAS pumps, and units in service before diagnosing biology.
4. Change wasting gradually to control solids age; use RAS mainly to manage clarifier return and blanket.
5. Match air to measured process demand without using high DO as the sole sign of health.
6. Confirm changes through trends in ammonia/BOD, MLSS/MLVSS, SVI, blanket, effluent solids, and oxygen use.

Diagnostic evidence

Calculation and mass-balance connection

Use WPI’s F/M = BOD load per day / MLVSS inventory and MCRT = aeration plus clarifier solids inventory / solids wasted plus effluent solids per day. Keep lb with lb/day or kg with kg/day. RAS percentage is return flow divided by influent flow times 100, but changing RAS does not itself remove solids from the plant.

Worked operating scenario

MLSS rises, SRT lengthens, and effluent remains good while WAS pump run time has not changed. The operator checks actual WAS flow and concentration; a partially blocked line has reduced wasted mass. Increasing RAS would only circulate more solids. Restoring measured WAS and returning gradually to target inventory addresses the cause.

Common exam traps

• RAS recycles solids; WAS removes them.
• Complete mix dilutes a shock spatially but does not eliminate its mass.
• A stable pump run time does not prove stable wasted mass when concentration or flow changes.
• High DO can coexist with low biological activity or over-aeration.

Field-to-exam checklist

• Use mass, not only pump settings, to control solids inventory.
• Complete-mix behavior links all basin zones quickly.
• SRT and F/M require measured load and biomass data.
• Clarifier evidence must be interpreted with RAS, WAS, and settleability.

Distinguishing inventory from concentration

MLSS concentration can rise because solids mass increased, basin volume fell, or both. A concentration alone is not the inventory used in MCRT; multiply by the active liquid volume and include clarifier solids when the supplied relationship requires them. Likewise, a lower MLSS after a storm may reflect dilution rather than immediate biomass loss. Review mass, level, effluent solids, and WAS together before changing wasting. This distinction is especially important in complete mix because a hydraulic change rapidly affects the observed basin concentration.

A spatial sampling profile can test the complete-mix assumption: persistent basin-to-basin or inlet-to-outlet differences may reveal dead zones, unequal distribution, or an equipment limitation.

Worked inventory and loading arithmetic

The two control calculations most often tested are best learned as a single worked plant rather than as isolated formulas.

Plant data. Aeration volume 2.0 MG; MLSS 2,800 mg/L; MLVSS 75 percent of MLSS; influent flow 6.0 MGD at 150 mg/L BOD; WAS 0.04 MGD at 7,500 mg/L; effluent 10 mg/L TSS.

Step 1 — biomass inventory. 2.0 MG x 2,800 mg/L x 8.34 = 46,704 lb of mixed-liquor suspended solids in the basin.

Step 2 — solids leaving per day. WAS: 0.04 x 7,500 x 8.34 = 2,502 lb/day. Effluent: 6.0 x 10 x 8.34 = 500 lb/day. Total = 3,002 lb/day.

Step 3 — solids retention time. 46,704 / 3,002 = 15.6 days. Note that the effluent term is 17 percent of the solids leaving; dropping it would overstate SRT by nearly three days.

Step 4 — food-to-microorganism ratio. BOD load = 6.0 x 150 x 8.34 = 7,506 lb/day. MLVSS inventory = 2.0 x 2,100 x 8.34 = 35,028 lb. F/M = 7,506 / 35,028 = 0.21 lb BOD per lb MLVSS per day.

A complete-mix caveat worth remembering. Because influent disperses immediately, every organism in the basin lives at the same low substrate concentration. That evens out shock loads, but a uniformly low-substrate environment also favours filamentous organisms that compete well under starvation conditions. The standard countermeasure is not a chemical but a configuration change: an anoxic or aerobic selector at the head of the basin creates a short, high-substrate contact zone that favours floc-forming organisms before the mixed liquor reaches the completely mixed volume.