Visualizing Wastewater Coagulation
Knowledge about the stability of colloidal suspensions is of great importance in wastewater treatment, as clarification can only take place in unstable dispersions when coagulation of particles occurs. In this application report, coagulation of bentonite clay, provoked by supplementation with a clarifying agent was investigated using a Litesizer 500. The necessary amount of additive to achieve coagulation of a bentonite suspension was determined by measuring the zeta potential and particle size over added flocculant concentration. The “Dosing over volume” measurement mode in the Kalliope software could be used to set up an automated measurement workflow for this application.
Water treatment is the goal-oriented change in water quality, which embraces removal of unwanted particles from the water as well as the addition of substances to adjust water parameters such as the pH value or conductivity. It is a process for purifying wastewater and returning it to the water cycle.
Wastewater contains particulate contamination, which needs to be removed. Each particle has charges distributed on its surface, either negative, positive or a mixture of both. Zeta potential reflects the magnitude of the particle surface charges in a suspension and, as such, is a key indicator of the stability of colloidal suspensions. The higher the absolute zeta potential is, the stronger the electrostatic repulsion between particles, and the more stable the suspension remains. Experience shows that an absolute zeta potential over 30 mV usually indicates good colloidal suspension stability (Figure 1).
Conversely, a zeta potential magnitude inferior to 30 mV is considered indicative of a less stable colloid. In that case, electrostatic repulsion is weak, meaning that the attractive forces between particles may exceed this repulsion and the particles may aggregate.
This knowledge can be used for optimizing wastewater treatment processes. Particulate matter in wastewater is mainly negatively charged, so adding cationic additives neutralizes the net charge and lowers the zeta potential. This in turn favors the formation of large aggregates, which can then be easily removed by filtration, sedimentation or flotation. This process is known as coagulation, and the cationic additives used are termed coagulants. Zeta potential measurements are commonly used to investigate which amount of coagulant is needed to clarify the wastewater.
An excellent model substance for negatively charged particles in wastewater is bentonite, an aluminum phyllosilicate clay with a plate-shaped nanoparticulate structure. A commonly-used coagulant in wastewater treatment plants is aluminum sulfate, or alum, a cationic additive with the formula Al2(SO4)3∙14H2O. In the present application report we investigated the effect of alum supplementation to a bentonite nanoclay dispersion by performing DLS and ELS measurements.
- The Pure Water Occasional. The Pure Water Occasional. [Online] www.purewateroccasional.net.
- Bratby, John. Coagulation and flocculation in water and wastewater treatment. London : IWA publishing, 2016.
- Luxbacher, Thomas. The ZETA Guide: Principles of the streaming potential technique. s.l. : Anton Paar GmbH, 2014.
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