Wastewater Treatment Methods: Advantages and Disadvantages of Coagulation and Disinfection

Each stage in the wastewater treatment process plays a crucial role in achieving the desired treatment outcome. However, primary and tertiary treatments are especially critical. Primary treatment is responsible for substantially reducing solids, without which subsequent treatment stages would be less effective. Tertiary treatment is responsible for neutralizing harmful microbiological matter, ensuring that it does not pose a risk to living organisms that come into contact with it.

The two wastewater treatment methods are coagulation and disinfection, which can be achieved through various chemical or non-chemical techniques. Each method has its own advantages and disadvantages.

Coagulation

Wastewater influents consist of different amounts of total suspended solids (TSS) and total dissolved solids (TDS). While coarse screening and grit chambers can decrease the TSS, a more precise process is required for the removal of solids. Sedimentation and filtration have been historically used for this purpose, but they may not be effective in eliminating the smallest particles.

Coagulation has gained popularity as a method of decreasing both TSS and, in some cases, TDS in wastewater. This process involves destabilizing the charged particles in the solution. Due to their similar electrical charges, the particles repel each other and resist settling quickly. To destabilize this electrical charge, an opposite charge must be introduced to the solution, allowing the colloids and other minerals to aggregate.

There are currently two commonly used methods for coagulation treatment:

Chemical Coagulation

Chemical coagulation is a widely used method for particle destabilization, which involves adding various chemical additives to achieve the desired state. Commonly used additives include alum, ferric chloride, ferric sulfate, ferrous sulfate, and lime, which help neutralize the charged particles. Polymers can also be added as supplements to aid in the aggregation of solids.

Advantages

Chemical coagulation is primarily used to accelerate the settling time of solids, thereby reducing the overall detention time of the wastewater treatment process.

Chemical coagulation can also facilitate the settling of smaller colloidal particles and mineral impurities that would not settle during sedimentation alone and may bypass the subsequent filtration stage.

Disadvantages

At its core, chemical coagulation is an additive process that relies on the addition of chemicals to reduce the amounts of solids in a solution. However, achieving the desired results can be complex and require extensive jar testing. The dosages of chemicals need to be precise to properly process the influent. In addition, the dosages may require continuous adjustment based on the varying composition of the wastewater source.

Adding chemicals in the coagulation process can lead to the generation of a significant amount of sludge that requires treatment and proper disposal after treatment. The nature of the constituents being added makes the sludge hazardous. As a result, the volume and toxicity of the sludge can increase the overall cost of disposal, as it is difficult to dewater.

Electrochemical Coagulation

In recent times, electrochemical coagulation has emerged as an advanced technique in wastewater treatment. This process involves supplying precise power to a set of metallic media after adjusting the pH, if required. The anodes and cathodes used in this process can either be of the same material or different from each other, depending on the composition of the influent water. Materials such as aluminum and iron are commonly used in this process. During oxidation, the electrodes release charged ions into the solution, leading to the destabilization of the particles in the solution.

Advantages

The electrocoagulation process is simple and has minimal components, making it easier to monitor and maintain remotely with less oversight. Additionally, the process can be easily adapted to handle varying levels of particles with minimal adjustments.

The electrocoagulation (EC) process has the advantage of targeting multiple contaminants with a single system and often with a single treatment pass. Since there is no typical chemical addition, it produces smaller volumes of sludge, which are typically non-hazardous, easily dewatered, and less expensive to process and dispose of. Additionally, the EC process has few moving parts, making it easy to remotely monitor with reduced oversight and maintenance. The process can also be adjusted easily to accommodate differing amounts of particles if required.

Disadvantages

The EC system is not entirely additive-free as it may require the addition of acids or bases for pH adjustment. In addition, the electrodes used in the process are sacrificial and will corrode over time, necessitating replacement. While the system does not require a significant amount of power, in certain parts of the world where electricity is more expensive, operating costs may be higher. The cleaning of plates in the system may also involve the use of acids through a CIP process.

Disinfection

The tertiary treatment of wastewater aims to remove any remaining pathogens, such as bacteria, viruses, mold, cysts, or other microorganisms, that may have survived previous treatment processes. Disinfection is required to render the treated water safe for discharge into water bodies. Chlorine and ultraviolet (UV) light are the two most widely used disinfection methods for wastewater treatment.

Chlorine Disinfection

Chlorine is commonly known for its use as a shock treatment in swimming pools. This compound is toxic to biological organisms and operates through oxidation to kill them. Once it penetrates the surface of pathogens, it interacts with intracellular enzymes and proteins, rendering them non-functional. The micro-organisms either die or are unable to reproduce as a result.

Advantages

Chlorine is readily available and relatively inexpensive. Due to its strong oxidizing properties, it can effectively neutralize large quantities of harmful microorganisms given sufficient reaction time.

Disadvantages

Chlorine, although effective, has some limitations in wastewater treatment. Its volatility can lead to the formation of disinfection by-products that may be harmful to humans, animals, and aquatic life. Safe handling is required during transportation, storage, and use. Additionally, certain pathogens such as viruses, Giardia lamblia, and cryptosporidium remain unaffected by chlorine disinfection treatment.

UV Disinfection

Ultraviolet light disinfection (UV disinfection) systems have become increasingly popular in various applications due to their non-chemical disinfection capabilities. UV light, at certain wavelengths, can break molecular bonds in a pathogen’s DNA, disrupting its normal cellular function and rendering microorganisms, cysts, and viruses inert.

Advantages

UV disinfection is a physical process that does not involve any hazardous chemicals to handle, and there are no harmful residual byproducts generated in the treated water. It is highly effective against most viruses, bacteria, spores, and cysts, and requires a shorter contact time compared to other tertiary wastewater treatment methods. Moreover, it has a compact footprint for its disinfection capability.

Disadvantages

The effectiveness of UV disinfection can be reduced by high concentrations of total suspended solids (TSS) in the treated water. Therefore, it is important to ensure that the preceding treatment process is effective at removing TSS. Additionally, low doses of UV light may not be sufficient to deactivate certain viruses, spores, and cysts, requiring longer exposure times or higher intensity exposure. There is also the risk of photo reactivation, where microorganisms can repair themselves if the UV dose is not strong enough.