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.
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.
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).
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.
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.
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.