Each step in the wastewater treatment process is essential to achieve the desired treatment results. However, primary and tertiary treatments are critical to the overall process. During the primary processing, solids are reduced to a large extent. Without this step, subsequent treatment will be less effective. In tertiary treatment, harmful microbiological substances are killed or inactive, so they do not cause disease in those organisms that encounter them.
Each of these processes has several ways in which they can be performed, chemical or non-chemical methods. Each of these wastewater treatment methods has advantages and disadvantages.
coagulation
Wastewater contains varying levels of total dissolved solids (TDS) and total suspended solids (TSS). Course screening and chamber grit will reduce the TSS, but must be followed by a more refined solids removal process.
Popularity
Coagulation has become a popular method for reducing both the TSS and, in some cases, the TDS of wastewater. This process involves the destabilization of charged particles in solution. Because of their equal electrical charges, the particles repel each other and prevent them from settling quickly. To destabilize this electrical charge, an opposite charge must be applied to the solution, allowing colloids and other minerals to aggregate.
There are currently two well-known methods of coagulation:
Chemical coagulation
Chemical coagulation is a well known method for particle coagulation. This process requires the addition of a number of chemical additives to achieve the desired destabilized state. Other additives include polymers, which act to aid in the aggregation of solids.
pros
The main consideration behind the use of chemical coagulation is that it accelerates the time it takes for solids to settle on their own. Consequently, the total retention time of the wastewater treatment process is reduced.
Chemical coagulation can also facilitate the settling of smaller colloidal particles and mineral contaminants. These particles usually may not settle during the sedimentation process and will pass through the subsequent filtration system.
Minuses
At its core, chemical coagulation is an additive process. While this can reduce the amount of solids in the solution, it still requires the addition of chemicals to achieve this. Adding these substances can be quite tricky and requires extensive pot testing. The dosages must be accurate enough to properly handle the inflow optimally. Dosage may require continuous adjustment depending on the changing composition of the wastewater source.
The addition of chemicals also results in a large volume of sludge that will need to be treated and disposed of after further processing. This sludge is also hazardous due to the nature of the added components. The volume and toxicity of sludge can increase disposal costs as they are difficult to dewater.
Electrochemical coagulation
After adjusting the pH, if necessary, this process involves applying a certain amount of power to a number of metallic media. Anodes and cathodes can be either of the same material or different from each other. This material is optimized for the composition of the incoming water. Electrodes release charged ions into solution during oxidation, which leads to destabilization of particles in solution.
pros
Electrocoagulation is a direct process. It has few moving parts, so it can be monitored remotely with reduced supervision and maintenance. The process can also usually be adjusted to accommodate different amounts of particles with little effort if required.
The EC process is also capable of targeting multiple pollutants using a single system, and in some cases, a single treatment pass. The absence of the typical chemical addition results in smaller volumes of sludge, which are usually harmless, easily dewatered and less expensive to handle and dispose of.
Minuses
The EC system may require the addition of acids or bases to adjust the pH, so it is not completely free of additives. In addition, due to the nature of the process, the electrodes are sacrificial and corrode over time, which requires replacement. It can use the CIP process to clean the plate, which will use acid in its cleaning cycle. The nature of the process also requires electrical energy. While it may not take long at a time, in some places around the world, power can be more expensive, which can increase operating costs.
disinfection
In a tertiary wastewater treatment process, wastewater can contain bacteria, viruses, mold, cysts or other pathogens that other treatment processes cannot remove. Before treated water can be discharged into any body of water, microbiological contaminants must be inactivated or destroyed. There are several methods for disinfecting wastewater, but two types are most commonly used: chlorine and ultraviolet light.
Chlorine disinfection
Most are familiar with the use of a chlorine compound to shock swimming pools. Chlorine is toxic to biological organisms and kills them by oxidation. It penetrates the surface of pathogenic microorganisms and, once inside, begins to interact with intracellular enzymes and proteins, making them non-functional. The microorganism will either die or be unable to reproduce.
pros
Chlorine is relatively inexpensive and readily available. In addition, because it is such a powerful oxidizing agent, it can be very effective at rendering a large number of harmful microorganisms inert with suitable reaction times.
Minuses
Chlorine is quite volatile and can lead to disinfection by-products (DBP) that can be harmful to humans, animals and aquatic organisms. It requires careful handling in order to be shipped, stored and used safely. Chlorine disinfection does not affect viruses, Giardia lamblia and cryptosporidium.
UV disinfection
Recently, ultraviolet light disinfection systems have been widely used in many fields due to their non-chemical disinfection capabilities. At certain wavelengths, ultraviolet light can destroy the pathogen’s DNA, breaking down its molecular bonds. Normal cellular function becomes impossible in this state, leaving the microbiological organism, cysts and viruses practically inert.
pros
UV disinfection is a completely physical process, so there are no hazardous chemicals to handle. There are no harmful residual by-products that can form in purified water. It is highly effective against most viruses, bacteria, spores and cysts and requires shorter contact times than other tertiary wastewater treatment methods. In addition, it is compact in size for disinfection.
Minuses
Due to the use of light to decontaminate a solution, high concentrations of total suspended solids (TSS) can render it ineffective. This is not a problem as long as the previous treatment process is effective in removing the TSS. Low doses of UV light may not be effective against some viruses, spores, and cysts, and therefore require longer exposure times or higher exposure intensities. There is also the possibility for photoreactivation of microorganisms, as a result of which the organisms recover after treatment if the ultraviolet dose is not large enough.
