Every stage in a wastewater treatment process is important to achieve the desired treatment results. However, primary and tertiary treatment is critical to the overall process. During the initial treatment, solids are largely reduced. Without this step, subsequent treatment would be less effective. In tertiary treatment, harmful microbiological material is killed or inactivated so that the affected organs do not become ill.
These wastewater treatment methods are coagulation or disinfection. Each of these processes can be carried out in a number of ways, either by chemical or non-chemical techniques. Each of these wastewater treatment methods has its own advantages and disadvantages.
Wastewater feeds contain different amounts of total dissolved solids (TDS) and total suspended solids (TSS). Section sieving and granular chambers reduce the TSS but must be followed by a more refined solids removal process. Sedimentation and filtration are methods that have been used in the past, but these methods cannot remove many of the smallest particles.
Coagulation has become a popular method to reduce both the TSS and, in some cases, the TDS of wastewater. This process involves destabilizing the charged particles in the solution. Because of their similar electrical charge, the particles repel each other and prevent them from settling quickly. To destabilize this electrical charge, an opposite charge must be placed on the solution to allow the colloids and other minerals to accumulate.
There are currently two known methods of coagulation:
Chemical coagulation is a well-known method of particle coagulation. This process requires the addition of a variety of chemical additives to achieve the desired destabilized state. Alum, ferric chloride, iron sulfate, iron sulfate, and lime are some of the additives used to neutralize the charged particles. Other additives include polymers that aid in the aggregation of solids.
The main consideration behind using chemical coagulation is that it speeds up the time it would take for the solids to settle on their own. Therefore, the total residence time of the wastewater treatment in Pakistan process is shortened.
Chemical coagulation can also aid in the settling of finer colloidal particles and mineral contaminants. These particles may not settle during a sedimentation process and are passed through a subsequent filtration system.
Chemical coagulation is essentially an additive process. While it can reduce the amount of solids in a solution, it does require the addition of chemicals to achieve this. Adding these substances can be complex and require extensive glass research. The dosages have to be fairly accurate in order to optimally process the influence. The dosage may require continuous adjustment due to the different composition of the wastewater source.
The addition of chemicals also creates a large volume of sludge that must be treated and disposed of after treatment. This sludge is also dangerous due to the nature of the ingredients added. The volume and toxicity of the sludge can increase disposal costs because it cannot be easily dewatered.
Electrochemical coagulation has recently found its way into wastewater treatment in an optimized form. In this process, a number of metallic media are supplied with specific energy after a required pH adjustment. The anodes and cathodes can either be the same material or differ from one another. This material is optimized depending on the incoming water mix. Aluminum and iron are two such materials that can be used in this process. The electrodes release charged ions into the solution during the oxidation, which leads to the destabilization of the particles in the solution.
Electrocoagulation is a straightforward process. It has only a few moving parts and can therefore be remotely monitored with less supervision and maintenance. The method can also typically be adapted in order to take up different amounts of particles, if necessary, with little effort.
The EC method is also able to detect multiple contaminants with a single system and in certain cases with a single treatment run. The lack of a typical chemical addition results in smaller amounts of sludge that are normally harmless, easy to dewater, and cheaper to process and dispose of.
An EC system may require the addition of acids or bases to adjust the pH so that it is not completely free of additives. Because of the nature of the process, the electrodes are sacrificial and will corrode over time, requiring replacement. A CIP process that uses acid in the cleaning cycle can be used to clean the plates. The nature of the process also requires electrical energy. In some places around the world, you might not need much at once, but electricity can be more expensive, which can add to operating costs.
In tertiary wastewater treatment, the wastewater can contain bacteria, viruses, molds, cysts or other pathogens that other treatment methods cannot remove. Before the treated water can be discharged into a body of water, the microbiological impurities must be inactivated or killed. Various disinfection methods are available for wastewater treatment. The two most commonly used are chlorine and ultraviolet light.
Most are familiar with the use of a chlorine compound for shock treatment of swimming pools. Chlorine is toxic to biological organisms and kills them through oxidation. It penetrates the surface of pathogens and begins to interact with intracellular enzymes and proteins inside, rendering them inoperable. The microorganism dies or no longer reproduces.
Chlorine is relatively cheap and readily available. In addition, because it is such a powerful oxidizing agent, it can be very effective in rendering large amounts of harmful microorganisms inert with the appropriate reaction time.
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 to be safely shipped, stored, and used. Viruses, Giardia lamblia and Cryptosporidium are not affected by the chlorine disinfection treatment.
Ultraviolet light disinfection systems have recently become widely used in many applications because of their non-chemical disinfection capabilities. At certain wavelengths, UV light can disrupt the DNA of a pathogen by breaking its molecular bonds. Normal cell function becomes impossible in this state and the microbiological organism, the cysts and the viruses remain practically inert.
UV disinfection is a purely physical process so that no dangerous chemicals are produced. There are no harmful residual by-products that could be generated in the treated water. It is highly effective against most viruses, bacteria, spores and cysts and requires a shorter contact time than other tertiary wastewater treatment methods. In addition, it has a compact footprint for its disinfectability.
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 if the previous treatment procedure effectively removes the TSS. Low doses of UV light may be ineffective on some viruses, spores, and cysts, requiring longer contact times or higher intensity exposure. Photoreactivation can also occur in the microorganisms, in which the organisms repair themselves after the treatment if the UV dose is not strong enough.