Belt Press Cake Solids Drop and Filtrate Quality Fails
The belt filter press at a 1.2 MGD plant dewaters thickened waste activated sludge at 45,000 gal/day and 1.2% feed solids. Polymer is fed at 14 lb active polymer per dry ton, made up to a 0.4% solution. Over one week, cake has fallen from 18% to 14% solids and filtrate TSS has risen to 900 mg/L (the plant targets below 300). Operators noticed the gravity zone standing full of liquid and two spray nozzles plugged. When day shift raised polymer to 20 lb/ton, cake firmed briefly, then filtrate solids got worse and both belts started tracking. A new polymer lot went into service Monday, and WAS feed solids have fallen since the aeration basin MCRT was raised. Hauling costs $60 per wet ton.
1. Calculate the dry solids fed, the wet cake produced at 14% and at 18%, and what the cake dryness is costing per day and per year.
Read the worked answer
Dry solids fed
- 0.045 MG × 12,000 mg/L × 8.34 = 4,504 lb/day = 2.25 dry tons/day
Wet cake mass (assume capture stays roughly constant, ~93–95%, so about 4,200 lb/day of solids reach the cake)
- At 14%: 4,200 ÷ 0.14 = 30,000 lb/day ≈ 15.0 wet tons/day
- At 18%: 4,200 ÷ 0.18 = 23,300 lb/day ≈ 11.7 wet tons/day
Cost of the dryness
- At 14%: 15.0 × $60 = $900/day
- At 18%: 11.7 × $60 = $702/day
- Difference: ~$200/day ≈ $72,000/year
Point to make in the answer: every percent of cake solids is money. The solids did not change - only the water traveling with them did. That water is also returned to the head of the plant as filtrate, so poor dewatering loads the aeration basin with ammonia and solids, which raises MCRT pressure and feeds back into the problem. Note also that filtrate at 900 mg/L over roughly 40,000 gal/day returns about 300 lb/day of solids to the headworks - a real secondary load, not a rounding error.
2. Diagnose the problem and give the order in which you would check and correct things.
Read the worked answer
Diagnosis: multiple simultaneous changes, so separate the variables before adjusting anything.
First - mechanical/hydraulic (cheap, fast, and often the whole answer)
- Gravity zone flooding. Freeboard is where the floc drains and strengthens. Standing liquid means the floc leaves the gravity zone still soupy and the pressure zone cannot make up the difference - this alone explains 2–4 points of cake solids. Check the drainage deck for blinded or torn media, the feed distributor for uneven flow, and the weir/adjustable dam setting. Correct the bed depth and distribution before touching polymer.
- Plugged spray nozzles. Uneven washing blinds the belt, so the pressure zone cannot pull water through. Clean all nozzles, verify spray pattern and pressure, and add a strainer if debris is present.
- Belt tracking and tension. Tracking usually follows uneven loading or unequal tension, not a polymer problem. Verify the tracking rollers, the belt tension, and that both belts are equally loaded.
Second - chemistry
- Verify the polymer make-down. A new lot arrived Monday; check the certificate of analysis, the dilution water (temperature and hardness affect activation), the age of the made-up solution (most polymers degrade within a day or two), mixing energy (over-shearing destroys the molecule), and whether the solution concentration is actually 0.4%.
- Run a bench test (CST, Buchner funnel, or simple gravity drainage jar) on the actual feed sludge using both the old and new polymer lots, at several doses. This is the only way to separate "bad lot" from "changed feed" and it takes 30 minutes.
- Account for the feed change. Higher MCRT means older, more digested, sometimes finer floc that dewaters differently - and lower feed solids means the polymer dose per dry ton rises even if polymer flow is unchanged.
Fix order: restore the gravity zone and sprays → verify polymer make-down → bench-test → then re-tune dose. Adjusting dose while the bed is flooded is guessing.
3. The day operator wants to keep the 20 lb/ton dose because "the cake looked better for an hour." How do you decide the dose, and what is happening at 20 lb/ton?
Read the worked answer
Polymer quantity at the current and proposed dose
- 2.25 dry tons/day × 14 lb/ton = 31.5 lb/day active polymer
- Solution flow at 0.4%: 31.5 ÷ 0.004 = 7,875 lb/day ÷ 8.34 ≈ 945 gal/day of make-up solution
- At 20 lb/ton: 2.25 × 20 = 45 lb/day active ≈ 1,350 gal/day solution
What is happening at 20 lb/ton: over-dosing produces a floc that looks large and firm on the gravity deck but blinds the belt. The excess polymer coats the screen and the floc becomes sticky and shear-sensitive - the pressure zone then squeezes water out sideways or not at all, filtrate solids rise, and the belts load unevenly and start tracking. The brief improvement followed by deterioration is the classic over-dose signature, not evidence that 20 lb/ton works.
How to set the dose properly
- Titrate down, not up: start at 14 lb/ton and step down in 2 lb/ton increments on a stable feed, allowing 2–3 cycles at each step before judging.
- Judge on three numbers simultaneously: cake solids, filtrate TSS, and belt/blinding condition. Optimizing cake solids alone pushes you into over-dose.
- Normalize to dry tons fed, not to gallons - lower feed solids with the same polymer flow is an automatic dose increase, which is likely part of what happened after the MCRT change.
- Hold feed as stable as possible (a consistent thickened sludge beats a variable one); a surge tank ahead of the press is worth more than extra polymer.
- Document the new-lot trial with a side-by-side bench result, and either accept the lot with a revised dose or reject it to the supplier while the sample retention window is open.