The Pan America Environmental STAX systems are a plug-flow type of pipe flocculator design that provides multiple-chemical reaction contact in a compact, small footprint design without moving parts, wearing components and no need for power. The STAX design uses water turbulence induced by a spiral piping network to mix chemical products into a waste stream.
Within each rection zone the retention time is almost uniform and mixing energy is constant in the pipe cross section as such all particles will be subjected to the same amount of mixing energy and for the same period. This will result in a highly uniform floc with excellent separation characteristics.
The design eliminates the need for mixing tanks, mixers and electrical consumption. The process takes place under highly controlled, well-defined and optimised conditions
With many waste streams chemical pre-treatment is required to effectively remove contaminants in combination with physical separation equipment such as Slant Plate Clarifiers, Dissolved Air Flotation (DAF) and oil water separators.
Chemical processes: Chemical treatment should be considered when your project involves: oil-in-water emulsions, dissolved metals, dissolved solids, suspended solids and colloidal solids.
Coagulation: Virtually all surface water sources contain turbidity. Most of the suspended matter in water is present as finely divided or colloidal particles and these do not settle due to gravitational forces alone. For example in a depth of 1 ft; bacteria would take about 1 year to settle, clay particles would take about 8 hours, sand about 10 seconds.
Colloidal size particles may be formed from soluble compounds (ex. soap. starch, bentonite, carbon) which do not grow into crystals of size large enough to settle or be removed in sedimentation. To assist in the agglomeration of particles, coagulants are used to increase the effective size, which increases settling velocities effective in removing turbidity, color, bacteria, algae and odor producing substances.
Since the properties that prevent the natural agglomeration vary for suspended solids, it is important to review the significant effects.
The majority of particles in surface waters are negatively charged, these like charges cause the particles to repulse, or remain in suspension. Such colloidal suspensions are called stable. Coagulation is an attempt to destabilize the suspensions and allows the particles to come together by neutralizing the particles through the addition of chemicals carrying an electrical charge.
Flocculation: While flocculation is similar to coagulation, it differs in that its purpose is to provide a second, final step of building up particles into a large, heavier mass to speed up the settling or flotation process and more efficiently sweep up all coagulated particles in order to build the larger flock.
pH Adjustment: What is water pH? pH is an indication and quantification of its acid/basic properties. pH is measured on a scale of 0-14. See the reference scale for this range.
pH adjustment is required for several purposes. The most popular purposes are for precipitation of metals, breaking of emulsions, correcting pH for polymer & coagulant use and final pH correction prior to sewer discharge. If the water is basic, an acid, such as sulfuric acid is added to reduce the pH level. If the water is acidic a base such as sodium hydroxide is added to raise the pH level. The particular reagent used to raise or lower the pH is dependent on the nature of the wastewater, abilities, cost and availability of reagents as well as the preference of the end user.
process gets the less reagent is required. The amount of reagent used is dependent on the starting pH level and can be determined by a titration curve.
pH adjustment can be performed on a batch or continuous basis. The chemical metering process can be automated or manually operated as dictated by project or end user.
Emulsion Cracking: Oil-in-water emulsions are very common in groundwater remediation, industrial wastewater, vehicle wash and industrial process waters. Each emulsion is different and each requires jar testing to determine the proper pre-treatment design. Often a two or three stage reaction is successful at breaking most emulsions.
Emulsions Defined: Emulsions are defined as a colloidal suspension of a liquid within another liquid (with droplet sizes typically under 20 micron). A colloidal suspension is a concentration of particles or droplets homogeneously dispersed through the carrier liquid (water). This means the oil droplets are reduced in size to such a degree that the oil’s normal electrical repulsion of the water molecule is overcome due to its minute size.
Oil in water emulsions may contain a variety of oil types and concentrations, as well as various types of solids contaminants. The oil in water emulsion in a stable or unstable state maintains the emulsification through mechanical and/or chemical means.
Emulsions Are Created In Two Ways
Mechanical Emulsions: In mechanical emulsions a common method of creating the emulsion is by violent mixing or shearing of the oil droplet in the waste stream with a high shear transfer pump, vigorous mixer or other device that might disperse the oil droplets into minute droplets. Given enough time, the mechanical emulsion may break without any treatment. But with most processes and manufacturing time frames this time may be too long for practical use.
Chemical Emulsions: Are created when a surface-active chemical or chemicals are used, such as alkaline cleaners containing surfactants, soaps and detergents having ionic or non-ionic characteristics. These chemicals interfere with the natural coalescing of oil droplets and generally creates a permanently stabilized emulsion with little chance of breaking by itself.
Emulsion cracking can be facilitated by the use of a coagulant/flocculent or pH changing chemical(acid/caustic). The chemical used depends on the particular emulsion. These chemicals change droplet electrical charges by the effect of their own charge, usually a cationic (positive) charge. This charge manipulation allows the oil droplets to become free and lets them coalesce (gather together) into larger droplets. The oils don’t always break out of emulsion into a free distinct layer. Sometimes they may be combined with solids or create a light mass that is in suspension, sinks or (usually) floats. The form and appearance of chemical emulsion cracking results can vary quite a bit from emulsion to emulsion. This variation helps to determine the type of equipment and processes required to treat the water.
Coagulation / Flocculation: The destruction of the emulsifying properties of the surface-active agent or neutralization of the charged droplet can be effected with the use of polymer products. A polymer or combination of polymers destabilize the electrical bond between oil and water allowing oils to free themselves, creating droplets and a distinct layer or flocculated mass. Flocculation refers to the successful collisions that occur when the destabilized particles (oil) are agglomerated via a bridging effect due to the flocculating polymer. The polymer acts like a broom, effectively stretching out in the water and bringing many small micro-floc particles together into a larger visible mass. Depending on the nature of the flocculated mass in may sink, float or stay in suspension. Numerous bench tests should be performed to determine the proper chemical types, concentrations and combinations in order to achieve optimum treatment.
Metals Precipitation: Most common metals, when dissolved in a waste stream can be precipitated from their ionic state via an increase in pH level to an optimum point of insolubility, usually in the neutral through low, strongly basic pH range. Typically, a two or three stage reaction system is required to best pre-treat metals for settling prior to a Slant Plate Clarifier or DAF system. A hydroxide precipitation system using a coagulant, sodium hydroxide and a cationic flocculant can be one of the best processes for most of the common metals such as: zinc (Zn), nickel (Ni), iron (Fe), copper (Cu), aluminum (Ag), chromium 3 (Cr3), lead (Pb) and cadmium (Cd).
A common problem with chromium is that it can be frequently found in the hexavalent (Cr6) form in industrial process waters (plating, rinse etc). In order to remove the Cr6 it must first be reduced to the trivalent form (Cr3) via a reduction process using sodium metabisulfite at a pH of 2-3. The closer to pH 2 the faster the reaction. Once the reduction is complete hydroxide precipitation/flocculation can be performed followed by settling.
Chelating agents (i.e. EDTA, DTA) can interfere with the hydroxide precipitation of metals by holding the metallic ion in solution. The chelants must first be neutralized prior to precipitation, jar testing with analytical testing will best identify the proper chemistry.
Performance: STAX Series
Metal Reduction Precipitation Form (mg/L)
– .005 Ferric hydroxide
– .50 Sulfate
– .05 Hydroxide
– .02-.07 Hydroxide
– .001-.01 Alum. co precip
– .12 Hydroxide
– .05 Sulfide
– .10 Hydroxide
Following the precipitation/flocculation stages if the particulate settle then a Slant Plate Clarifier is required. The Slant Plate Clarifier is the device of choice and will provide the following typical settling performance:
SPC discharge (mg/L)
If the reacted solids or emulsions float or flotation is determined to be the proper separation device the DAF system will provide the following performance
DAF discharge (mg/L)