Lesson 9: Colloids and Coagulation

Along with the online lecture, read chapter 8 in Basic Chemistry for Water and Wastewater Operators. You should also review Lesson 4 from ENV110 on Coagulation/Flocculation.

Lecture

Properties of Colloids

A colloid is defined as very small, finely divided solids (particles that do not dissolve) that remain dispersed in a liquid for a long time due to their small size and electrical charge. These particles have negligible settling velocity because their small mass has a low gravitational force compared to surface frictional forces.

One property of colloid systems that distinguishes them from true solutions is that colloidal particles scatter light. If a beam of light passes through a colloid, the light is refelcted (scattered) by the colloidal particles and the path of the light can therefore be observed. When a beam of light passes through a true solution (ie salt in water) there is so little scattering of the light that the path of the light cannot be seen and the small amount of scattered light cannot be detected except by very sensitive instruments. The scattering of light by colloids is known as the Tyndall effect. Absorption is another characteristic of colloids, since the finely divided colloidal particles have a large surface area exposed. The presence of colloidal particles has little effect on the boiling and freezing points of a solution.

The particles of a colloid selectively absorb ions and acquire an electric charge. All of the particles of a given colloid take on the same charge (either positive or negative) and thus are repelled by one another.

Negatively charged particles repel each other due to electricity.

Most particles dissolved in water have a negative charge, so they tend to repel each other. As a result, they stay dispersed and dissolved or colloidal in the water, as shown above.

The purpose of most coagulant chemicals is to neutralize the negative charges on the turbidity particles to prevent those particles from repelling each other. The amount of coagulant which should be added to the water will depend on the zeta potential , a measurement of the magnitude of electrical charge surrounding the colloidal particles. You can think of the zeta potential as the amount of repulsive force which keeps the particles in the water. If the zeta potential is large, then more coagulants will be needed.

Coagulants tend to be positively charged. Due to their positive charge, they are attracted to the negative particles in the water, as shown below.

Positively charged coagulants attract to negatively
charged particles due to electricity.

The combination of positive and negative charge results in a neutral , or lack, of charge. As a result, the particles no longer repel each other.

The next force which will affect the particles is known as van der Waal's forces. Van der Waal's forces refer to the tendency of particles in nature to attract each other weakly if they have no charge.

Neutrally charged particles attract due to van der Waal's forces .

Once the particles in water are not repelling each other, van der Waal's forces make the particles drift toward each other and join together into a group. When enough particles have joined together, they become floc and will settle out of the water.

Particles and coagulants join
together into floc .

Zeta Potential

Zeta Potential refers to the electrostatic potential generated by the accumulation of ions at the surface of the colloidal particle. It can help you understand and control colloidal suspensions. We can often tailor the characteristics of a suspension by understanding how individual colloids interact with one another. At times we may want to maximize the repulsive forces between them in order to keep each particle discrete and prevent them from gathering into larger, faster settling agglomeraties. Sometimes we have the opposite goal and want to separate the colloid from the liquid. Removing the repulsive forces allows them to form large flocs that settle fast and filter easily.

Each colloid carries a "like" electrical charge which produces a force of mutual electrostatic repulsion between adjacent particles. Particle charge can be controlled by modifying the suspending liquid. Modifications include changing the liquid's pH or changing the ionic species in solution. Another more direct technique is to use surface active agents which directly adsorb to the surface of the colloid and change its characteristics.

Zeta Potential is a convenient way to optimize coagulant dosage in water and wastewater treatment. The most difficult suspended solids to remove are the colloids. Due to their small size, they easily escape both sedimentation and filtration. The key to effective colloid removal is reduction of their zeta potential with coagulants, such as alum, ferric chloride and/or cationic polymers. Once the charge is reduced or eliminated, then no repulsive forces exist and gentle agitation in a flocculation basin causes numerous successful colloid collisions. Microflocs form and grow into visible floc particles that settle rapidly and filter easily.

Coagulation/Flocculation in the Plant

In wastewater treatment operations, the processes of coagulation and flocculation are employed to separate suspended solids from water. Finely dispersed solids (colloids) suspended in wastewaters are stabilized by negative electric charges on their sufaces, causing them to repel each other. Since this prevents these charged particles from colliding to form larger masses, called flocs, they do not settle. To assist in the removal of colloidal particles from suspension, chemical coagulation and flocculation are required. These processes, usually done in sequence, are a combination of physical and chemical procedures. Chemicals are mixed with wastewater to promote the aggregation of the suspended solids into particles large enough to settle or be removed.

Once suspended particles are flocculated into larger particles, they can usually be removed from the liquid by sedimentation, provided that a sufficient density difference exists between the suspended matter and the liquid. Such particles can also be removed or separated by media filtration, straining or flotation. When a filtering process is used, the addition of a flocculant may not be required since the particles formed by the coagulation reaction may be of sufficient size to allow removal. The flocculation reaction not only increases size of the floc particles to settle them faster, but also affects the physical nature of the floc, making these particles less gelatinous and thereby easier to dewater.

Review

A colloid is defined as very small, finely divided solids (particles that do not dissolve) that remain dispersed in a liquid for a long time due to their small size and electrical charge. One property of colloid systems that distinguishes them from true solutions is that colloidal particles scatter light. The particles of a colloid selectively absorb ions and acquire an electric charge. All of the particles of a given colloid take on the same charge (either positive or negative) and thus are repelled by one another.

Zeta Potential refers to the electrostatic potential generated by the accumulation of ions at the surface of the colloidal particle. Particle charge can be controlled by modifying the suspending liquid. Modifications include changing the liquid's pH or changing the ionic species in solution.

In the treatment plant, to assist in the removal of colloidal particles from suspension, chemical coagulation and flocculation are required. Chemicals are mixed with wastewater to promote the aggregation of the suspended solids into particles large enough to settle or be

Sources

"Coagulatin-Flocculation". Lenntech

"About Coagulation and Flocculation". Water Specialists Technologies

"Properties of Colloids". Clean Manufacturing

"Zeta-Potential". Zeta Meter Inc.

Assignment

Complete the questions for Assignment 9 . When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and directly submit it into the database for a grade.

Lab

Read the Turbidity and Jar Test labs and do any corresponding assignments associated with the virtual labs listed.

Quiz

Answer the questions in Quiz 9 . When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and directly submit it into the database for a grade.