Filtration practice: when a filter aid (e.g., diatomite, perlite, cellulose) is mixed into a slurry prior to filtration, what is the primary effect on the developing filter cake structure?

Introduction / Context:
Filter aids are a classic solution for slow, blinding filtrations. By dispersing rigid, porous particles in the slurry or by precoating the medium, they modify the cake microstructure to improve permeability and shorten cycle times while maintaining clarity.

Given Data / Assumptions:

• Slurry contains fine or gelatinous solids that form dense cakes.
• Filter aid particles are rigid and irregular, creating void pathways.
• Operating at constant ΔP is typical in practice.

Concept / Approach:
Permeability k increases with porosity and open-channel connectivity. Adding aid increases cake porosity and reduces specific resistance, thereby increasing filtrate rate at the same pressure. Many aids also reduce cake compressibility, but the universally cited primary effect is increased porosity/permeability.

Step-by-Step Solution:
Recognize bottleneck: dense, compressible cake gives low flow.Introduce rigid, porous aid → more voids → higher k.Outcome: faster filtration and better clarity at constant ΔP.

Verification / Alternative check:
Bench tests (leaf filters) with and without aid typically show decreased cycle time and pressure rise, confirming improved porosity and permeability.

Why Other Options Are Wrong:
Increase compressibility or decrease porosity would worsen performance, contrary to observed practice.Decreasing compressibility can occur, but the principal, most reliable benefit is an increase in porosity.

Common Pitfalls:
Overdosing filter aid can trap valuable solids, reduce yield, and increase consumable cost; optimization by dose–response testing is recommended.

Final Answer:
Increase the porosity