Two Primary Clarifiers Carrying a Three-Clarifier Flow

Solids captured = MGD × 8.34 × (influent TSS − effluent TSS) = 4.2 × 8.34 × (240 − 95) = 4.2 × 8.34 × 145 ≈ 5,080 lb/day

(If performance were still at 60 mg/L, the captured mass would be 4.2 × 8.34 × 180 ≈ 6,300 lb/day - the clarifiers are failing, so the lower number understates what they should be removing.)

Solids currently pumped Timer runs 10 min every 2 hours = 5 min/hr = 120 min/day at 100 gpm = 12,000 gal/day Mass = 12,000 × 8.34 × 0.040 ≈ 4,000 lb/day

Deficit ≈ 1,080 lb/day of solids not leaving the clarifiers - which is exactly why the blanket is at 4.5 ft and rising.

Pumping time needed Gallons = 5,080 / (8.34 × 0.040) ≈ 15,200 gal/day → 152 min/day at 100 gpm (about 180 min/day if the sludge should be 6,300 lb/day).

Why the answer is not simply 'set the timer to 155 minutes' A single long draw on a fixed-speed pump pulls a cone of clarified water down to the hopper after the first few minutes, thinning the sludge, wasting pump capacity, and pushing more water to the digesters. Convert to more frequent, shorter draws (for example 15 min every hour, then adjust based on the grab sample concentration at the end of each draw), and verify that the sludge line, valves, and pump are not partly plugged - a discharge pressure or amperage check will show that quickly.

Areas and volumes

• One unit: π × 30² ≈ 2,827 ft²; volume = 2,827 × 10 = 28,270 ft³ ≈ 211,400 gal
• Two units: 5,654 ft², 422,800 gal
• Circumference per unit = π × 60 ≈ 188 ft; two units = 377 ft of weir

With two units in service

• Overflow rate = 4,200,000 / 5,654 ≈ 743 gpd/ft² (typical primary design 600-800 - high end but acceptable)
• Detention = 422,800 / 4,200,000 × 24 ≈ 2.4 hr (typical 1.5-2.5 hr - lower end)
• Weir loading = 4,200,000 / 377 ≈ 11,100 gpd/ft (under the usual 15,000 gpd/ft guideline)

With one unit in service Overflow rate would be 4,200,000 / 2,827 ≈ 1,485 gpd/ft², more than double the guideline - solids washout would be guaranteed. That confirms the third unit must not be brought back until it is full and the flow is balanced between units.

Interpretation Hydraulically the plant is marginal but not failing: the detention time is short and the overflow rate is at the top of the range, which shortens the settling window and lets the sludge blanket creep up. But the blanket depth plus scum buildup plus the 1,080 lb/day solids deficit point to a sludge withdrawal problem first and a hydraulics problem second. Fixing the wasting should recover the blanket within a few days; if the blanket stays at 4.5 ft after that, then look at inlet baffling and short-circuiting (density currents, a broken or missing baffle, or an unbalanced split between the two units).

Hours 0-4

• Confirm both units are splitting flow evenly (measure the weir flow at each; a float or a clamp-on meter is enough).
• Measure blanket depth in each unit, not just once per day - every shift, from a marked rope and sludge judge.
• Change the timer to frequent short draws and grab a sample at the end of each draw to confirm the solids concentration is holding. Check pump suction, sludge line, and discharge pressure.

Days 1-3

• Compute the daily solids balance and compare it to the pumping rate; adjust until the blanket is trending down.
• Increase scum removal; check the scum baffle and the weir for grease buildup that is disturbing the flow split.
• Watch the aeration basin: 35 mg/L extra TSS is an extra 1,220 lb/day of load (4.2 × 8.34 × 35). Track MLSS, DO, and ammonia; the bugs will see it before the permit does.
• If the blanket will not come down, consider a low dose of polymer to the primary clarifier influent as a temporary crutch - but only after confirming the wasting fix.
• Bring the third unit back only after it is filled, hydrostatically checked, and clean; refill slowly.

Documentation and reporting

• Daily blanket depths, sludge concentrations, timer settings, and the mass balance.
• The repainting project is planned maintenance, which usually is not a reportable bypass - but check whether your NPDES permit and state rules require notice when a unit is out of service and effluent limits are at risk. A written note to the regulator before the fact is far cheaper than explaining after the fact.
• If primary effluent TSS causes a secondary upset and a permit exceedance, that is a reportable noncompliance event under the permit's upset/bypass and 24-hour notification provisions. Log the call, the time, the contact, and the corrective action.
• Any visible scum or solids in the effluent or on the ground counts as a spill or discharge - photograph it and report it per your spill plan.