Industrial filtration practice: What is the principal purpose of adding a filter aid (e.g., diatomaceous earth, perlite) during cake filtration?

Introduction / Context:
Filter aids such as diatomaceous earth and perlite are widely used in beverage, pharmaceutical, and chemical industries. They are pre-coated onto the filter medium or dosed into the slurry to prevent blinding, create a porous initial layer, and maintain permeability throughout cake build-up.

Given Data / Assumptions:

• Cake filtration at near-constant pressure is considered.
• Filter medium tends to blind quickly when fine particles deposit directly on it.
• A small fraction of filter aid is either pre-coated or body-fed during the run.

Concept / Approach:
Filtration rate under a given pressure depends on total resistance (medium + cake). A highly porous, permeable precoat reduces initial resistance and slows blinding. As cake grows with embedded filter aid, average permeability increases, sustaining higher filtrate flow rates over time.

Step-by-Step Solution:
Without filter aid: fine particles plug pores of the septum → rapid rise in resistance → low flow.With filter aid: a porous “skeleton” layer forms → effective permeability k increases.For constant pressure filtration: dV/dt ∝ k/μ / (R_medium + R_cake), so higher k directly raises dV/dt.Net effect: a higher and more stable filtration rate.

Verification / Alternative check:
Precoat/body-feed operating manuals show flatter dV/dt decline curves with filter aids, confirming improved throughput and longer cycles at the same pressure differential.

Why Other Options Are Wrong:

• Decrease pressure drop by reducing viscosity: filter aids do not change liquid viscosity; they change cake permeability.
• Increase porosity/permeability (Option C) describes the mechanism, but the principal purpose asked for is the performance outcome: higher rate.
• Act as the septum: the septum is a fixed support (cloth, screen, cartridge), not the filter aid.
• Reduce feed solids concentration: dosing filter aid slightly raises, not lowers, solids in the slurry.

Common Pitfalls:
Confusing mechanism (more porous cake) with the performance metric (higher rate). Also, overdosing filter aid can waste material and add unnecessary solids load.

Final Answer:
Increase the rate of filtration