Plant Objectives & Role of Chemicals
Material exists in water as something in solution (like sugar in your tea); something suspended in the water (like tea leaves in a tea pot – usually large, visible solids which settle quickly) or something dispersed as minute particles that are not individually visible (like globules of fat and protein in the milk, in your tea). In scientific terms these are colloidal particles/dispersions.
In effluent treatment, we are often wanting to remove dissolved, suspended or colloidal materials which we do not want to remain in the water. Removing dissolved solids needs membrane, ion exchange or evaporation technologies. Removing suspended solids needs filters or relatively static (low flow) areas to allow separation by sinking or floating. Colloidal particles are so small that they do not separate quickly by sinking or floating.
The theory of fine particles in water is that they are all electro-statically charged (usually) with negative charges. Like-charges repel each other and unlike charges attract each other. Positive charges (eg from hydration) complicate the situation but the overall balance is one of repulsion so the negatively charged particles “stay apart”. That is to say they remain as small particles in suspension. See Figure 1.
Figure 1. Small particles in water, remaining apart.
By reducing or eliminating the overall negativity, these particles can be made to come closer to each other and link together – forming larger particles which tend to sink and can then be separated by gravity. This is done by adding chemical with a positive charge. See Figure 2.
Figure 2. Negativity neutralised by adding extra positive charge (“Coagulant”). Particles agglomerate and “sink”.
This process of electro-neutralisation is technically known as “Coagulation” and the chemicals used are termed coagulants. B & V can supply a number of different coagulants which all work in subtly different ways to bring about the desired inter-actions. Your B&V engineer would be happy to demonstrate this and use his or her skills to select the optimum chemical for your application.
The growth of these “neutralised” particles into larger, visible particles which will then lend themselves well to separation techniques is technically “Flocculation”. We can encourage the growth or agglomeration of particles by adding high molecular weight polymers which loosely bind particles together. See Figure 3.
Figure 3. Neutralised particles bound into bigger agglomerations (“Flocculation”).
The choice of polymer (charge; charge density; molecular weight; quantity) can only be finally confirmed by laboratory tests on samples of the plant water and your B & V engineer will gladly carry out this work to enable process optimisation from the wide range of products available to B & V.