A reader wrote to me about their wastewater treatment system and asked about flocculation/coagulation and electrocoagulation.
Frank AltmayerThey wrote, “My company is considering upgrading our wastewater treatment system, which currently treats a wide variety of metals, including some that are complexed, but relatively low in concentration. We periodically experience problems with “pinfloc” and wish to pursue a more reliable flocculation/coagulation system. I have heard a lot about electrocoagulation as an alternative to chemical additions. What has been your experience with this technology?”
Let’s begin with a discussion of coagulation and flocculation:
Coagulation
When water-based solutions containing soluble heavy metals are adjusted in pH to a level where the chemistry favors the formation of a metal hydroxide, these metal hydroxides usually form a so-called floc, which can be a voluminous solid particle or a very small particle. In either case, the density of these particles is very close to that of the water matrix. Because metal finishing wastewater often contains metals such as iron and aluminum that naturally form well-settling flocs, coagulation and flocculation of some wastewaters may be very easy.
However, when the waste stream is very dilute or when the wastewater contains traces of emulsified oils or complexing agents from cleaners or other processing solutions, the flocs produced remain small and tend to repel one another. Such particles tend to float and/or take a long time to settle in the clarifier because these small flocculant particles strongly repel one another due to electrostatic charges on their surfaces.
One way to cause these “pinfloc” particles to agglomerate into larger, heavier, denser particles is to add positively charged ionic species to the wastewater to reduce their electrostatic charge.
The most commonly used coagulants are ferric iron salts, such as ferric chloride or sulfate, and aluminum salts, such as aluminum chloride and aluminum sulfate, commonly called “alum.” Since iron may be regulated in some locations and aluminum tends to be unregulated, the most commonly used coagulant in the metal finishing industry is alum. Each of these salts provides the wastewater to be treated with trivalent metal ions, which not only eliminate electrostatic forces but also form large flocculant particles that physically “sweep” the wastewater as they settle. Ferric iron coagulants can greatly increase the amount of sludge to be handled during final solids removal.
Most often, salts such as ferrous sulfate, ferric chloride, or aluminum sulfate are used at concentrations of 100 to 300 mg/L as coagulant additions to the waste stream. Coagulants are typically added to the pH adjust system ahead of the flocculation step (assuming one is employed). As the metal hydroxide solids are produced, the anion (sulfate or chloride) becomes free. As a result, the pH of the treated wastewater tends to decrease, while the total dissolved solids content increases. Either the initial pH should be high enough to allow this pH depression, or additional alkali must be provided.
Flocculation
Flocculation is further agglomeration of insoluble particulates in an aqueous medium, caused by the addition of an organic “flocculant,” a.k.a., “polymer” or “polyelectrolyte.” These organics are water-soluble long-chain hydrocarbons that have chemical “branches” attached with either a negative (anionic polymer), positive (cationic polymer), or neutral charge (nonionic polymer). The charged flocculant attracts oppositely charged particles.
Most metal finishing wastewaters form metal hydroxides, which respond well to anionic polymers. However, in some cases, cationic polymers, combinations of anionic and cationic polymers, or nonionic products have produced better results. Therefore, extensive jar testing is often necessary to determine which polymer is best suited for the wastewater to be treated in any given case.
Flocculants are added to a special reaction tank equipped with a low-speed, low-shear agitator (adjustable between 25 and 100 rpm). The addition rate of polyelectrolyte is typically based on the manufacturer’s recommendation. It is difficult to control properly. Theoretically, the polymer feed rate should be based on the concentration of metal hydroxides produced in the pH adjust system. Since this varies and is nearly impossible to determine, the most common method of adding polymer is by adding a fixed rate, regardless of the concentration of metal hydroxides produced. This often results in floating (when the polymer addition rate is too high) or pinfloc (when the polymer addition rate is too low). A knowledgeable wastewater treatment operator will periodically use jar tests to verify that the polymer addition rate is within a workable range.
Factors that affect good flocculation: The higher the metal concentration in the effluent, the larger the floc formation and weight gain by the flocculant particles. This has some practical limitations, however, because very high metal concentrations produce sludge that is difficult to clarify due to its high solids content.
High concentrations of wetting agents, cleaner compounds, silicates, and chelates tend to suspend the flocculant materials and produce milky suspensions that don’t clarify well.
Precipitation with calcium hydroxide (lime) in the pH-adjustment system, either as an alternative to sodium hydroxide or as an additional neutralizing agent along with sodium hydroxide, while creating a larger volume of sludge, will tend to provide a faster-settling precipitate.
During neutralization, it is necessary to stir the effluent to ensure good mixing and to promote rapid chemical reaction between the neutralizing chemicals and the contents of the waste stream. After five minutes, or sometimes less, the flocculant particles will start to appear. Excessive, violent stirring at this point tends to break down the small floc into colloidal material that will be nearly impossible to settle. After neutralization is complete, allow 5 to 10 minutes for flocculation to occur. A typical flocculation chamber uses slow agitation to allow flocculant particles to contact one another and be preconditioned for the next step.
Electrocoagulation
Electrocoagulation (EC) may be an alternative to using metal salts as coagulants or polyelectrolytes in some metal precipitation systems. Consider the following advantages and disadvantages:
Advantages:
- Simple to operate.
- Solids produced have low water content and are more easily settled or filtered.
- Flocs are much larger, contain less bound water, and separate from water faster.
- Lower TDS, since no chemicals are added.
- Effective on colloidal wastewater.
- No “chemical” cost, storage, or handling.
Disadvantages
- The metal anode needs to be replaced regularly.
- The cost of electricity needs to be accounted for.
- Films produced on the cathode may require frequent cleaning.
- It may not work well at higher dissolved metal concentrations found in some metal finishing operations.
- Gases produced may affect settling in the clarifier.
EC manufacturers have successfully lowered the concentrations of COD, BOD, TOC, TSS, TDS, FOG, MTBEX, heavy metals, organic and inorganic colloids, inks and dyes, chlorinated PCBs, cyanides, suspended particles, chemical and mechanical polishing waste, synthetic detergent effluents, and more.
One EC installation (a job shop performing aerospace plating and anodizing) that I have had some experience with failed to work well on raw waste, but worked very well at reducing heavy metals in the effluent from the clarifier after “normal” treatment of the wastewater.
How it Works
An electrocoagulation system consists of an electrolytic cell employing anode (s) and cathode(s). The electrodes may be made of several metals, including titanium, graphite, platinized titanium, and others, but the most common ones are iron (steel) and aluminum. The figure shows a schematic of an EC system using a tube-within-a-tube construction. Other designs use parallel plates that alternate anodes and cathodes.
When connected to an external DC power source, several reactions between the anode and the waste and between the cathode and the waste occur simultaneously:
Flocculation: The anode corrodes under the power supply potential, producing dissolved ions that act as coagulants.
Deemulsification: The oxygen and hydrogen ions produced at the anode and cathode, respectively, react with oil molecules, creating water-insoluble residuals that precipitate from the water. Organic/metallic dye residuals in wastewater from anodizing operations may be broken down into solid residues. The oxidative conditions at the anode may also break down traces of cyanide.
In a typical EC process, treated wastewater separates into a floating layer, a sludge layer, and clear water. The three phases are then separated using gravity and or filtration. To avoid flotation in a clarifier, caused by gases generated by the electrodes, deaeration may be required.
Control of EC systems
Control of the amperage, flow rate, and pH of the waste to be coagulated is critical to the successful use of this technology. Still, it is typically not difficult to do once the most effective operating parameters have been identified. Routine inspection and cleaning of the cathodes may be required. Temperature and operating pressure need not be carefully controlled. The EC system works best when the characteristics of the raw wastewater are invariant.
Frank Altmayer is a Master Surface Finisher, an AESF Fellow, and the technical education director of the AESF Foundation and NASF. He owned Scientific Control Laboratories from 1986 to 2007 and has over 50 years of experience in the metal finishing industry. He received the AESF Past Presidents Award, the NAMF Award of Special Recognition, the AESF Leadership Award, the AESF Fellowship Award, the Chicago Branch AESF Geldzahler Service Award, and the NASF Award of Special Recognition.
