Question

In a batch thickener, the rate of sedimentation can be artificially increased by

• A. The addition of coagulating agents
• B. Cooling the suspension to room temperature
• C. Promoting the free settling
• D. Controlling the flow rate

Hint:

Coagulation and flocculation are common pretreatment steps in sedimentation processes to enhance the settling of fine or colloidal particles that would otherwise settle very slowly.

Answer 1

The Correct Option is A

Step 1: Understand the process of sedimentation in a batch thickener.
Sedimentation is a solid-liquid separation process where solid particles settle out of a suspension under the influence of gravity. In a batch thickener, a suspension is allowed to stand, and the denser solid particles settle to the bottom, forming a sludge or slurry, while the clarified liquid (supernatant) is left above. The rate of sedimentation depends on factors such as the size, shape, and density of the particles, the viscosity and density of the fluid, and any interactions between the particles.
Step 2: Analyze the effect of each option on the rate of sedimentation.
(1) The addition of coagulating agents: Coagulating agents (or flocculants) are chemicals that promote the aggregation of small, dispersed particles into larger flocs. These larger flocs have a greater effective size and mass, which leads to a significantly higher settling velocity according to Stokes' Law (for laminar settling of spherical particles: \(v_s = \frac{g(\rho_p - \rho_f)d_p^2}{18\mu}\), where \(d_p\) is particle diameter). Thus, adding coagulating agents increases the rate of sedimentation.
(2) Cooling the suspension to room temperature: Cooling generally increases the viscosity of the liquid. According to Stokes' Law, an increase in viscosity (\(\mu\)) leads to a decrease in the settling velocity (\(v_s\)). Therefore, cooling the suspension would likely decrease, not increase, the rate of sedimentation.
(3) Promoting the free settling: Free settling refers to the sedimentation of particles at low concentrations where each particle settles independently without significant hindrance from other particles. While maximizing free settling conditions might be desirable in some contexts, it doesn't artificially increase the fundamental settling rate of individual particles beyond what is dictated by their properties and the fluid. Coagulation, for instance, alters the effective particle size and thus increases the settling rate beyond that of individual particles in free settling.
(4) Controlling the flow rate: A batch thickener operates in a static mode; there is no continuous flow rate to control during the sedimentation process itself. Flow rate is relevant in continuous thickeners, where it affects the residence time and thus the overall separation efficiency, but it doesn't directly increase the rate at which individual particles or flocs settle under gravity in a batch system.
Step 3: Conclude the method to artificially increase the sedimentation rate in a batch thickener.
Based on the analysis, the addition of coagulating agents is the most effective way to artificially increase the rate of sedimentation in a batch thickener by promoting the formation of larger and faster-settling flocs.