Pretreatment processes in water treatment systems are often crucial to maintaining efficiency and longevity of the downstream equipment, such as membrane filtration. Many systems incorporate a coagulation step that will encourage small suspended solids to aggregate and fall out of solution. By eliminating the larger particles in the solution, filtration membranes won’t foul, tear or clog as quickly. In order to cause this treatment process to occur, a coagulant must be added to the solution to neutralize either a positively or negatively charged effluent. Commonly used coagulants include alum, lime, ferric sulfate, and polymers. Unfortunately, there are pitfalls to using this type of chemical coagulation treatment in a water treatment plant.
We will discuss below seven common pitfalls in choosing a chemical coagulation water treatment system for your municipal or industrial water treatment process.
- Sludge volume
One of the most prevalent and obvious disadvantages to any type of coagulation process is the resulting sludge. As the suspended solids cluster together and fall, they gather at the bottom of the tank. In a chemical coagulation water treatment process, the volume of the sludge produced can be significant. It is estimated that the volume of sludge can reach up to 0.5 percent of the volume of the treated water. Therefore, if 1000 gallons (3800 liters) of raw water were treated by chemical coagulation, up to 5 gallons of sludge could be produced as a result.
- Sludge is hazardous and requires costly disposal
Not only is there a high volume of sludge, but often in the case of chemical coagulation, the sludge is hazardous. Non-hazardous sludge can actually be used in land applications. It can be applied to agricultural land or forest land as the case may be. This sludge material can condition the soil to facilitate nutrient growth as well as retain water. It can also act as a pseudo fertilizer, or be combined with an organic fertilizer to replace the more expensive chemical fertilizers.
However, sludge from chemical coagulation water treatment is too harmful to the environment to be used in such a way. This sludge can contain metal hydroxides and the sludge itself is often caustic, with a pH of 10 or higher. Because of how hazardous the sludge is, companies spend considerable amounts of money either treating the sludge, transporting and disposing of it. In some cases, more money is spent disposing of the resulting sludge than actually treating the wastewater itself.
- It is an additive process
The sludge produced in a chemical coagulation water treatment process is partially composed of the chemicals added during treatment. While few water treatment processes are free from requiring additives, chemical coagulation can require large amounts of various chemicals. Depending on the composition of the raw water, large doses of several different chemicals are typically needed. Because of all the addition chemical, the total dissolved solids actually increases. In addition, the additives make up nearly half of the resulting sludge.
- Complex dosage
In order to assure that the chemical coagulation water treatment process performs optimally, the correct dosages of the additive constituents must be considered. The rate and efficiency of the process can vary depending on the pH of the solution. Therefore, acids or bases must be added to ensure that the pH is at a neutral level. After this step, the coagulant must be introduced in the optimal dosage. This particular dosage is required to ensure that the effluent is fully destabilized. If too much chemical is added, the solution will restabilize at the opposite charge of the original mixture. Coagulant aids can also be added to increase the density and toughness of the aggregates. This additional chemical allows the particulate to precipitate faster and remain together during mixing and settling. Therefore, there is three different additives that need to be tested and optimized for a particular particle makeup of the effluent.
- High variance of water composition
After all of the work figuring out how much of which chemicals to add to the effluent to treat it. The chemical dosage may not even work correctly. Since, if the effluent has a varying turbidity or particle composition, results would need to be adjusted accordingly. Even if wastewater comes from the same source, its makeup is not always consistent. As mentioned previously, accuracy is very important when dosing the effluent using chemicals. Therefore, if a treatment system were to draw from one water source and optimize dosage to treat this source. The optimized dosage could be highly variant to treat a different similar water or wastewater source.
- Difficult to process multiple contaminants at once
Wastewater effluent often has many contaminants within it. These contaminants can range from organic compounds to heavy metals to oils, fats and hardness. Certain chemical additives are effective at coagulating certain constituents in the raw water. However, there would be several chemical additives to treat different constituents. In short, one pass is often not enough to process all of the unwanted particles using a chemical coagulation system. In this case, another chemical coagulation step may need to be used. Perhaps another removal method would need to be used altogether.
- High operational costs
The six disadvantages listed above, all contribute to this final disadvantage — operational cost. The volume of the sludge produced makes it more costly to transport. The hazardous nature of the sludge makes it more expensive to dispose of. The amounts of chemicals needed, even if the chemicals themselves aren’t expensive, can add significant cost. A chemical coagulation water treatment system requires complex dosages. Any variances in the source water composition can require extensive amounts of testing. This contributes to additional operational cost.
Finally, all unwanted particles and potential microbiological contamination cannot be removed in one chemical treatment process. Therefore additional step will need to be added. This is especially applicable to water source including microbiological contamination, as chemical coagulation does not affect these issues. These additional steps add to operating cost. None of the above reasons even include the additional cost of cleaning and maintenance in using a chemical coagulation system. Other additional costs include the labor required to provide training, testing, operation, monitoring, or cleaning and maintenance.
An Alternate Coagulation Method….
However, problems like these are avoidable with an alternate non chemical coagulation method known as electrochemical coagulation or electrocoagulation (EC). Genesis Water Technologies, Inc. utilizes this advanced, yet versatile technology in many of our clients water treatment projects. To learn more about EC systems, check out our article about the top 5 advantages of an advanced electrocoagulation system.