Why does my belt press cake stay too wet?

The symptom

Dry-solids content falls over several weeks while polymer consumption rises. Slower belt speed may improve dewatering but reduces throughput. More water in the discharged cake increases transport weight and may raise disposal costs.

Root causes

1. 01 A blinded belt can retain fines, grease or polymer residue in its fabric. Drainage falls even when the belt still looks serviceable.
2. 02 Wrong belt permeability for the sludge: an open belt loses fines and stresses the filtrate side, a tight belt drains too slowly for the throughput.
3. 03 Polymer dosing mismatched to the belt: flocs sized for a different drainage profile shear apart in the wedge zone, releasing water the press has already separated.
4. 04 Worn or damaged seam and edges: local leak paths let sludge bypass the pressing zones entirely.

Suitable filter media

• ▸ Choose the belt type from the sludge and drainage profile. These determine strength, openness and retention, for example the choice between psb 15/50 and 29/85.
• ▸ Assess whether professional cleaning can restore drainage. On-site seam repair is an option if the fabric remains sound. Replace the belt if repair is not appropriate.
• ▸ Adjust polymer dosing to the new belt. Floc structure and drainage behaviour affect each other.
• ▸ Have rollers, doctor blades, wash water and tracking assessed as well. They influence belt life alongside the fabric itself.

Prevention

• ⊕ Record dry-solids content and polymer dose per tonne weekly. Changes may become apparent before the cake looks different.
• ⊕ Include wash-water pressure and nozzle condition in the maintenance schedule. A blocked spray bar can lead to belt blinding.
• ⊕ Measure the belt before the shutdown so you can prepare a suitable replacement.