Wastewater Treatment MCQ with answer and solution

Concept

Hydraulic sensitivity and biomass retention in UASB reactors

Formula

Upflow velocity v_up < Settling velocity v_s of sludge granules

Step-by-step solution

1. In UASB, biomass is retained purely by gravity settling of anaerobic granules against upward flow. 2. In trickling filters, biomass is attached to fixed media and cannot wash out easily. 3. SBR operates in batch cycles where settling is decoupled from flow surges. 4. A hydraulic surge in UASB creates excessive upflow velocity, washing the unattached sludge blanket out of the reactor. Answer: C

Explanation

An Upflow Anaerobic Sludge Blanket (UASB) reactor relies on self-immobilized anaerobic granular sludge that remains suspended as a blanket by balancing upward liquid velocity and gravity settling. A sudden hydraulic shock load increases upward velocity beyond the critical settling velocity of sludge granules, causing massive biomass washout and system upset.

Why the other options are wrong

A) SBR is a batch process with discrete fill and react steps that absorb hydraulic shocks easily. B) Trickling filters have attached biofilm resistant to biomass washout. D) Primary clarifiers do not retain active biological biomass.

Shortcut

UASB = Granular sludge blanket suspended by velocity balance; hydraulic shocks cause washout.

Common mistake

Selecting Trickling Filter, overlooking that attached-growth biofilms are very resilient to hydraulic shock.