Introduction to Coagulation and Flocculation of Wastewater

Term Project by Sheree DiTerlizzi, Fall 1994

Abstract

Coagulation and flocculation of microorganisms is of practical importance in wastewater treatment because flocculated organisms are relatively easy to collect from the various streams in a wastewater treatment plant. Chemical coagulation of biologically treated waste waters is most often the initial step in water renovation systems. Coagulants used today generally consist of alum, lime or a synthetic polyelectrolyte. Separation of the floc is accomplished by flotation or sedimentation.

A process developed by the Microfloc Corporation involves the addition of alum and a polyelectrolyte. The coagulated mixture is directly applied to a mixed media separation bed. The separation bed is made up of several materials of different specific gravity and particle size, resulting in a graded filter media from coarse to fine. The addition of coagulants and removal of the floc on mixed media separation beds eliminates the need for clarification tanks.

Poor settling produces "bulking" of sewage sludge, caused by proliferation of filamentous bacteria or fungi. Bulking will also be exhibited by diffuse bacterial flocs. Heukelkian and Weisberg showed that high loading conditions produced a grayish floc with a high bound water content and poor settling characteristics. At low loading, the floc was brown and dense with a low bound water. The conclusion drawn from their investigations was that rapid growth rates in the presence of high substrate concentration yielded a microbial structure containing considerable bound water and a resulting low specific gravity. Long aeration periods in the absence of a food source will convert the floc to the dense form. Floc characteristics as related to growth rate will be influenced both by the availability of the substrate and the mode of introduction of the waste to the system.

Introduction

Coagulation of wastewater may be accomplished with any of the common water coagulants including lime, iron and aluminum salts, and synthetic polymers. The choice is based on suitability for a particular waste, availability and cost of the coagulant, and sludge treatment and disposal considerations. For example, iron is sometimes available at no cost as a waste product in the form of pickling liquor, and its presence in sludge presents no particular problems for anaerobic digestion. Lime generally provides good clarification, a rapidly settling sludge, and permits the use of a simple method of recovery that also insures destruction of most sewage solids in the resulting sludge.

An important aspect of wastewater purification and clarification is the removal of suspended colloidal material and as much of the solutes as possible. To remove colloidal material, a floc forming chemical is needed. Most floc forming chemicals are tied up with peripheral chemicals which cause the resulting effluent to have a greater total dissolved solids (TDS) content. If this water is being reused, the high TDS levels contribute to a high final water usage cost.

Perhaps the floc chemical could be put into the contaminated water in its ionic form, without adding the peripheral chemicals. This should be ideal, because the colloidal contaminants and soluble contaminants that would react with the floc chemical could be removed. The results would be a clarified water with a reduced TDS. Considerable research has been undertaken to evaluate methods by which these objectives could be obtained. One possible approach is treatment through electrochemistry.

Simply put, electrochemistry is defined as the use of direct current to cause sacrificial electrode ions to move into an electrolyte and remove undesirable contaminants either by chemical reaction and precipitation, or by causing colloidal materials to coalesce and then be removed by electrolytic flotation. The electrochemical system has proven to be able to cope with a variety of wastewaters. These waters include paper pulp mill waste, metal plating, tanneries, canning factories, steel mill effluent, slaughter houses, chromate, lead and mercury-laden effluents as well as domestic sewage. These waters may be reduced to clear, clean, odorless and reusable water that may even be of better quality than the water from which it originated.

Chemical Applications

Colloid Stability

The stability of solids and suspensions can be interpreted by the same laws that apply to colloidal solutions. The aggregation of colloids is known as coagulation or flocculation. In the past, these two terms were used interchangeably, but now there is a trend to distinguish between aggregation due to simple ions (coagulation) and aggregation due to polymers (flocculation).

Except under idealized lab conditions, colloid stability is a very complex issue. Several forces can operate between the colloids; some attractive, others repulsive. These forces may react in different ways upon variations in the conditions (pH, T, salt concentration, etc.) surrounding them. One must also consider the rate at which particle surface properties can alter relative to the rate at which two particles approach one another. Both static and dynamic properties are of significance.

The most frequently occurring forces between colloids are: van der Waalls forces, electrostatic forces, and forces due to adsorbed macromolecules. Adsorbed polymers or lower weight materials can only operate if they are present, but that is frequently the case in natural waters. In addition, specific forces may act in special cases. For instance, magnetic colloid particles may attract each other magnetically or chemical bonds may be found between two colloids.

Some of these forces, such as van der Waals and electrostatic repulsion, have a long range. This implies that they can operate over several tens of nanometers. It is on this principle that the fundamental picture of colloid stability is based; solutions stabilized by electric repulsion between particles can be destabilized by electrolytes. Chemical bonds are short-range, and therefore can only come into operation if there are no other forces keeping approaching particles apart.

The flocculation of particles in a liquid depends on collisions between particles, caused by their relative motion. This relative motion may be caused by Brownian movement, by fluid movement giving rise to velocity gradients, or by particle motion due to an external force (e.g. gravity).

Flocculation Rates

The rate of flocculation is determined by the collision frequency induced by the relative motion. Because this is caused by Brownian movement, it is called perikinetic flocculation. That which is caused by velocity gradients is called orthokinetic flocculation. If there is no surface repulsion between the particles, then every collision leads to aggregation and the process is called rapid flocculation. If a significant repulsion exists, then only a fraction of the collisions results in aggregation. This is called slow flocculation.

A special case of orthokinetic flocculation is provided by the floc blanket clarifier. In addition to the fluidized bed giving rise to velocity gradients, the fluidized particles are participating in the process of agglomeration. If particles are settling at different velocities, then the faster settling particles may collide with slower settling particles, leading to aggregation. The aggregates will then settle faster due to their increased mass, and possibly experience further collisions and aggregations.

Floc Breakup

Compared with present understanding of floc formation through perikinetic and orthokinetic mechanisms, understanding of floc breakup in agitated systems is much more qualitative and speculative, despite research efforts. Proper characterization of floc disruption is an important problem since it is well documented that breakup can appreciably affect the performance of solid-liquid separation processes downstream. Floc breakup in dilute agitated suspensions is governed by the interaction of individual flocs with fluid forces. Depending on its constituent materials, a floc can be viewed roughly as an aggregate of primary micro particles that are bound together to form a matrix possessing a substantial fraction of fluid within its framework. The size and compactness of the matrix, size and shape of the microparticles, and number and strength of the bonds at microparticle contacts can all be expected to contribute to floc structure and the ability to withstand disruption by fluid forces.

Flocculation by Inorganic Salts

When inorganic salts dissolve in water, dissociation into constituent ions occurs and the ions may take part in various reactions with water or with other solutes present. The nature of the resulting aqueous species largely determines the effect of the added salt on colloid stability. Among possible types of aqueous species are the following:

Colloidal particles are usually charged and this charge is frequently responsible for their stability. Basically, added salts can affect stability in two ways: through their effect on the extent of the diffuse layer around the particles and by their "specific"effect on the electric potential controlling colloid stability.

Effects of Polymers on Colloid Stability

Many polymers adsorb readily on colloidal particles and can profoundly affect the interaction between particles. Colloid stability can be either increased of decreased by adsorbed polymers, both of which will be discussed in this section.

Polymer adsorption can increase colloid stability by increasing the electrical repulsion between particles, (decreasing the van der Waals attraction) or by introducing a "steric" component of repulsion. The stabilizing effect of adsorbed materials is often called "protection". Strictly, none of the effects mentioned depend on the polymeric nature of the adsorbed material and all can be achieved by relatively small molecules, such as surfactants. Polymers do tend to give fairly thick adsorbed layers, and therefore produce the most marked effects.

Adsorbed layers can decrease colloid stability for a number of reasons, and this effect is called "sensitization". A special case of sensitization occurs with certain high molecular weight polymers, which can form "bridges" between particles and hence promote the formation of aggregates.

Case Studies

Several case studies were reviewed for different aspects of coagulation-flocculation systems. One case study researched the design and construction of a lab continuous flow coagulation-flocculation model system. It was determined by this study that for the design of mixing and flocculation tanks, the combination of detention time and energy input are of vital importance in flocculation efficiency. A second case study that was done in conjunction with the first discussed the turbidity removal efficiency of the continuous flow model, and also investigated the ranges of feasible trade-offs between coagulant dose, energy input, and detention time. It was concluded that desired flocculation efficiencies can be achieved by combinations of chemical doses, energy input, and detention time. The combinations indicated the feasible trade-offs between these parameters.

Conclusion

The coagulation-flocculation process in wastewater treatment has been shown to be a complex one. Many hours of testing and research have gone into the perfecting of this process over the past years, and it will continue to be the subject of many future studies. Understanding the many chemical processes behind these processes is only the beginning to understanding them fully. Desired efficiencies must always come with trade-offs for other desired effluent qualities. Perhaps a solution will arise in the future that will combine all of the desired features without the need for any trade-offs.