Use of filter aids (asbestos, kieselguhr/diatomaceous earth): how are they applied during cake filtration to raise porosity and reduce cake resistance?

Introduction:
Filter aids are inert, highly porous powders that improve filtrate clarity and throughput by preventing fine solids from blinding the medium and by forming an open, permeable cake structure. Understanding application modes is central to troubleshooting slow filtrations.

Given Data / Assumptions:

• Common aids: kieselguhr/diatomaceous earth, perlite, cellulose, asbestos (legacy).
• Goal: higher porosity, lower pressure drop across cake.

Concept / Approach:
There are two primary ways to use filter aids: as a precoat layer that protects the septum and as a body-feed mixed with the slurry so solids deposit interspersed with aid particles. After filtration, the aid is removed with the cake; depending on the product, it may be separated by dissolving product solids or thermally destroying the aid if compatible.

Step-by-Step Solution:
Precoat stage: form a uniform aid layer on the medium.Body feed: dose aid into slurry to keep pores open during run.Post-run: remove cake; choose separation method appropriate to chemistry (dissolution or burnout where safe).

Verification / Alternative check:
Industrial operating procedures (precoat + body feed) are standard for rotary vacuum filters and pressure leaf filters to delay terminal pressure rise and maintain flux.

Why Other Options Are Wrong:

• Any single method alone is incomplete; all three statements are valid in practice.
• “None” contradicts routine filtration practice.

Common Pitfalls:
Overdosing aid (wasting capacity), failing to re-precoat after breakthrough, or choosing incompatible aids that contaminate the product.

Final Answer:
All ‘‘a’’, ‘‘b’’ & ‘‘c’’