FLOC SYSTEMS INFO

Contaminants in wastewater often cause the water to appear turbid or colored. These impurities include suspended and colloidal materials and soluble substances. Because the density of many of these particles is only slightly greater than water, agglomeration or aggregation of particles into a larger floc is necessary in order for them to be removed by sedimentation and filtration. The clarified water then can be either discharged or reused and the settled floc can be collected, dewatered and disposed of as solid waste.

Flocculant and coagulant separation technology play an important part in treating almost every kind of industrial waste stream. Just like all other major divisions of wastewater treatment technology, they have their advantages and limitations. The key is to very clearly determine the best technology for the given situation.

Definitions

Flocculant

Long chained polymers of tremendous molecular weight are gently mixed to increase contact through collisions. The result is a faster settling action within a design specific vessel. Typically adequate mixing and settling periods must be provided in order for the coagulation, flocculation, and sedimentation to take place.

Coagulation

The process that causes a reduction of repulsion force between particles or colloids (Zeta Potential) that is present of the charges on the particles. A chemical that when added to water, suspended solids mixture negates the electrical charge (Zeta Potential) in the water which repels particles and keeps them in suspension. When the coagulant is introduced, the zeta potential in the water is altered thus allowing the individual contaminant particle to agglomerate. Coagulants can be a liquid or a solid and can be added in batch or continuous type reactors.

Encapsulants

Sometimes the material that is flocced out of water contains contaminants that are toxic. In that situation an additional material may be added to the process, called an encapsulant. The purpose of the encapsulant is to bind and surround the toxic substances in the flocced material in a matrix. In addition, the encapsulants add the necessary mass to the coagulated particles enabling them to settle quicker. Encapsulants that are designed to be non-leachable will not allow the contaminant to re-dissolve once it is captured. RGF’s EC-2000 and QF series flocs incorporate these encapsulants allowing the settled sludge cakes to pass rigorous leachability tests.

Leachability

The relative measure of how much of a given solid contaminant will dissolve when put under conducive conditions. Heavy metals have certain leachability limits for which a solid must pass to ensure the metals won’t leach out of the solid at some point in the future.

Precipitation

All metals, when introduced to water, will dissolve to a certain extent and form ions. In the case of heavy metals, these ions are very toxic. One way to remove them from water is to cause them to turn back into a solid by adjusting the pH of the water. Two things then occur, they become less toxic, and most importantly they are now in a particle form which can be filtered out by flocculation or ultrafiltration. A typical flocculation system designed to remove heavy metals from water would have a precipitation step subsequent to the injection of the flocculant.

Zeta Potential

The small electrical imbalance created in water by the interaction of water molecules and the small particles and colloids suspended in water. The usually negative charge keeps the particles apart and prevents them from settling. When a coagulant is added to the water, the zeta potential is reduced to zero, allowing the suspended particles to coagulate.

Specific Gravity

The ratio of the density of a liquid relative to that of pure water. Pure water has a density of 1.0g/ml and therefore, by definition, a specific gravity of 1.00. Substances heavier than pure water have specific gravities of greater than one and conversely, substances lighter than water have specific gravities less than one. For example, sea water has approximately 32,000 ppm Total Dissolved Solids thus is more dense than pure water. Its density is 1.032 divided by 1.0000 or 1.032. On the other hand, oil is lighter than water and has a density of .8g/ml.

Adsorption

The process that occurs when one substance is attracted and adheres to the surface of another. An example of adsorption is when oil particles are drawn out of water by polypropylene fibers and affix to the surface of the fiber. The word "adhere" is a good way to remember the mechanism of adsorption.

TCLP

Solids must pass a series of leachability test in order for them to be disposed of as non hazardous waste. The Toxic Chemical Leachability Profile Tests are designed to provide a standardized means of testing the leachability characteristics of solids. They look for common heavy metals and other organic toxins which are classified as hazardous by regulators.

Bentonite and other Organic Clays

These are the coagulation and encapsulating agents normally included in dry flocculants to aid in the floccing and settling process. In addition to creating the coagulant effect in the wastewater, they adsorb many contaminants and bind them to the floc matrix. In addition the floc particles are weighted by the clay particles which allows them to settle much more quickly.

Sludge Cake

The solid mass resulting from the addition of the floc to the wastewater. Contained in the cake are all the contaminants that were drawn out of the water as it was clarified.

Heavy Metals

Heavy metals are the most serious parameter of inorganic material in wastewater because of their toxic effects on life forms. They are most toxic when in their dissolved state. Some common heavy metals are Zinc, Chromium, Nickel, Lead, Mercury, and Cadmium.

Colloid

A homogeneous mixture of compounds in water which remain suspended and contribute to the turbidity and color. They are not individually visible to the human eye. Colloids may also contain toxic substances in need of removal. Coagulation/Flocculation is one of the best technologies for removing colloidal suspended solids from water.

Filtration Aids

Coagulants can be added to a waste stream to increase the filtration efficiency of standard filtration media by causing colloidal suspensions to agglomerate. These larger agglomerated particles are then big enough to be caught in the applicable filter media.