Belt Press Performance: Feed Condition or Equipment Capacity?
Your 0.9 MGD activated sludge plant runs 3,200 mg/L MLSS in 1.2 MG of aeration volume and wastes 32,000 gpd of WAS at 7,500 mg/L. The WAS goes to a 40,000-gal holding tank pumped out once per day to a 1.7 m belt filter press running about 8 hours; design solids loading is 500 lb/(h·m). Last month's logs show cake solids falling from 18% to 14%, capture from 95% to 82%, and polymer dose rising from 12 to 18 lb per dry ton. The filtrate is cloudy and grey with a sulfide odor. The press belts look clean and wash-water pressure is normal. Operators want to raise polymer again and buy a second press. You have to justify spending money with the numbers you already have.
1. Run the solids mass balance and state what it does to the case for a second press.
Read the worked answer
WAS solids production:
- 32,000 gal/day × 8.34 lb/gal × 0.0075 = 2,002 lb dry solids/day
Current performance (82% capture, 14% cake):
- Solids captured: 2,002 × 0.82 = 1,642 lb/day
- Wet cake: 1,642 / 0.14 = 11,729 lb/day (about 1,400 gal/day)
- Solids lost to filtrate: 2,002 − 1,642 = 360 lb/day
Previous performance (95% capture, 18% cake):
- Captured: 1,902 lb/day; wet cake: 1,902 / 0.18 = 10,567 lb/day
- Filtrate solids: 100 lb/day
What it means:
- Hauling increases by about 1,160 lb/day of wet cake, roughly 10% more trucked mass for the same solids removal.
- The 360 lb/day recycled in filtrate to a 0.9 MGD plant is 360 / (0.9 × 8.34) ≈ 48 mg/L of additional headworks solids, versus 13 mg/L before. That recycle loads the aeration system and the clarifiers, so the dewatering problem feeds itself.
Capacity check: 2,002 lb/day ÷ 8 h = 250 lb/h; ÷ 1.7 m = 147 lb/(h·m), only about 29% of the 500 lb/(h·m) design rate. The press is not overloaded. A second press is not justified by these numbers; the binding constraint is feed quality, not machine capacity.
2. The press is running at 29% of design but capture is falling. What is your diagnosis, and what would you check first?
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Diagnosis: the feed sludge has gone septic, and the press is being blamed for it.
The evidence points at the feed, not the machine: a sulfide odor, grey cloudy filtrate, falling capture, and rising polymer demand with clean belts and normal wash water. A 40,000-gal holding tank receiving 32,000 gpd gives roughly 1.25 days of storage, which at warm temperatures is long enough for the sludge to go anaerobic. Anaerobic degradation destroys the extracellular polymers that give floc its structure, generates sulfide and fine colloidal particles, and produces a sludge that polymer cannot condition economically. Filtrate quality collapses and polymer dose climbs.
Checks, in order:
- Holding tank DO, ORP, and sulfide, plus the actual storage time from the transfer log (a tank pumped once daily does not always drain fully, so real residence can be several days).
- Polymer make-up: age of the diluted solution (activity drops noticeably after about a day), dilution water quality, mixing energy, and feed point relative to the flocculation zone.
- Press condition: belt tension and tracking, gravity drainage zone, floc appearance in the mix box. Clean belts and good wash pressure rule out the obvious, but confirm floc size before blaming the sludge.
- Upstream: has the WAS quality changed (higher SRT, lower F/M, more stored substrate)? Compare the current SVI to last month's.
If sulfide is confirmed, additional polymer will be money burned for marginal gain.
3. Compute the SRT, and explain how it constrains your dewatering options heading into winter.
Read the worked answer
Solids inventory in aeration:
- 1.2 MG × 8.34 lb/gal × 0.0032 = 32,026 lb
SRT:
- 32,026 lb ÷ 2,002 lb/day ≈ 16 days
Why this matters:
16 days is a long sludge age for a plant that needs nitrification at 12 °C. Minimum SRT for nitrification at that temperature is roughly 7–9 days, so there is a large safety factor, but the excess inventory has costs: low F/M, older floc that dewaters poorly, and more mass to thicken and haul. The temptation will be to cut wasting to reduce hauling, which raises SRT further and makes the dewatering problem worse - exactly the wrong move.
Better options, in priority order:
- Shorten storage, don't reduce wasting. Dewater more frequently, or split the WAS into shorter, more frequent batches so the tank never sits more than 8 hours. This attacks the actual cause at little capital cost.
- Thicken before dewatering. A gravity thickener or co-thickening with primary sludge takes 7,500 mg/L feed to 2–3% and cuts the hydraulic load on the press while improving cake.
- Then revisit wasting. If SRT can safely drop to 10–12 days at 12 °C, do it with a documented minimum-SRT calculation and daily ammonia monitoring.
Track lb dry solids hauled per day and filtrate TSS after each change; if capture returns above 92% with shorter storage, the second press is off the table permanently.