Wastewater is generated by residences, companies, industries, storm drains, and rainwater runoff. In general, wastewater is 99.9 % water by weight, with the remaining 0.1 % made up of dissolved solids or other suspended elements. Excreta, detergents from washing clothes and dishes, food scraps, grease, oils, plastics, salts, sand, grit, and heavy metals are examples of this substance. Some wastewaters from industrial or agricultural activities may also contain chemicals that are dangerous to the environment or public health and must be neutralized or eliminated before safely returning to the environment. The primary goal of wastewater treatment techniques is to purify the water and ensure that it is suitable for use after being returned to the environment as part of the water cycle.

Wastewater treatment consists of several steps comprising mechanical (physical)-based, biological-based, chemical-based, and membrane (filter) processes.

This article will go through the fundamentals of these stages of wastewater treatment procedures, followed by a more in-depth look into chemical-based wastewater treatment.

Processes for Mechanical Wastewater Treatment

The first step of wastewater treatment comprises mechanical techniques that remove 20-30% of the particles in the water. First, the wastewater is routed through a screen or a sieve drum, which filters away relatively coarse contaminants such as leaves, textiles, paper, or other significant objects. Next, a succession of screens with varying degrees of fineness from several centimeters down to a few millimeters is utilized to screen out coarse pollutants in the water. Ensure the efficiency of the screening process, the speed with which the wastewater passes through the screens at each phase is managed carefully. Any debris gathered on the screens is transported to the following process stage, dewatered, and subsequently incinerated.

The effluent is then frequently sent through a sand collector operation. A specific sedimentation tank is employed to remove coarse particles such as stones, particles of glass, sand, or any uncaptured organic material. The pace of the wastewater is reduced in this stage to allow the sediments to settle out of the water and gather at the bottom of the tank. Several sorts of sand collectors may be employed, such as those using aeration to transport fats and oils to the water's surface or centrifugal force to separate water from other components.

The wastewater speed is further reduced by extending the basin as it reaches the first treatment tank. The slower velocity allows finer particles to settle out and form sludge at the tank's bottom. This last treatment tank signifies the completion of the mechanical wastewater treatment procedure. Sludge is removed and delivered to a digestion tower, where any organic material is turned into methane gas, which may be utilized to generate electricity to operate the processing facility.

Processes for Biological Wastewater Treatment

Following the mechanical treatment stage, the effluent is sent to a biological-based process for further purification. Aeration tanks are used to provide oxygen to the water and circulate it through the use of propellers. The oxygen promotes bacteria and microbes growth, which consume organic pollutants in the water and convert them to inorganic compounds. This technique generates activated sludge flocks, which float freely in the water. Next, the water is moved from the aeration or circulation tanks to a secondary wastewater treatment tank, where the velocity of the water is reduced again, enabling sedimentation to occur. The sludge settles to the bottom of the filtered water and may thus be removed mechanically from the tank's bottom. However, a percentage of the sludge, known as return sludge, is not collected and is put back into the circulation tank to ensure a sufficient number of bacteria and microorganisms are available to maintain the biological treatment process functional. The sludge collected is usually transported to the digester for additional processing and methane gas production, which is then utilized to generate electricity.

In many circumstances, finishing the first two stages of wastewater treatment is enough to allow the water to be reintroduced into a river or stream. However, further processing procedures are required for various industrial and agricultural waste streams. Chemical wastewater treatment technologies come into play here. The remainder of this paper will mainly concentrate on these procedures.

Chemical Wastewater Treatment Methods

Chemicals are utilized in a variety of wastewater treatment methods. Chemical unit procedures are chemical processes that cause chemical reactions and are used in conjunction with biological and physical cleaning procedures to achieve specific water standards.

Chlorine, hydrogen peroxide, sodium chlorite, and sodium hypochlorite (bleach) are specialized chemicals that disinfect, sterilize, and aid in the purification of wastewater at treatment facilities.

Chemical coagulation, chemical precipitation, chemical oxidation, and advanced oxidation, ion exchange, and chemical neutralization and stabilization are all unique chemical unit processes that may be used to wastewater during cleaning.

Neutralization

The use of chemicals to modify the pH of wastewater is known as neutralization. Depending on the initial pH of the influent, acids (to reduce pH) or alkalis (to raise pH) are added.

Precipitation of Chemicals

Chemical precipitation is the most often used technique for eliminating dissolved metals from harmful metal-containing wastewater solutions. A precipitation reagent is added to the mixture to transform the dissolved metals into solid particles. The reagent induces a chemical reaction that causes the dissolved metals to solidify. The particles in the mixture can then be removed via filtration. The efficiency of the process is determined by the kind of metal present, the concentration of the metal, and the type of reagent utilized. For example, calcium or sodium hydroxide is employed as the reagent in hydroxide precipitation, a popular chemical precipitation technique, to form solid metal hydroxides. However, because many wastewater solutions contain mixed metals, it might be challenging to produce hydroxides from dissolved metal particles in the wastewater.

Coagulation of Chemicals

This chemical process destabilizes wastewater particles, causing them to agglomerate during chemical flocculation. Fine solid particles scattered in wastewater have negative electric surface charges (in their usual stable condition), preventing them from aggregating and settling. Chemical coagulation destabilizes these particles by injecting positively charged coagulants, lower the charge of the negatively charged particles. When the charge is lowered, the particles form bigger groups more readily. The combination is then treated with an anionic flocculant. Because the flocculant reacts with the positively charged mixture, it either neutralizes the particle groups or forms bridges between them, allowing the particles to be bonded into bigger groups. Sedimentation can be used to remove particles from a mixture after bigger particle groups have formed.

Advanced Oxidation and Chemical Oxidation/Reduction

Electrons migrate from the oxidant to the contaminants in wastewater when an oxidizing agent is introduced during chemical oxidation. The contaminants are then structurally modified, resulting in less toxic molecules. For example, chlorine is used as an oxidant against cyanide in alkaline chlorination. However, alkaline chlorination, being a chemical oxidation process, can result in the formation of hazardous chlorinated chemicals, necessitating subsequent processes. Through methods such as steam stripping, air stripping, or activated carbon adsorption, advanced oxidation can assist remove any organic molecules formed as a result of chemical oxidation.

For the treatment of potable water, redox processes are utilized. The use of ozone and hydrogen peroxide treatments can successfully remove chlorinated hydrocarbons and pesticides from wastewater. Advanced oxidation mechanisms are also utilized to degrade drug compounds present in water, such as antibiotics or cytostatic medicines. Reduction methods can also be utilized to convert heavy metal ions into sulfides.

Ion Exchange

When water is too hard to clean, it is difficult to use and frequently leaves a grey residue. (This is why clothing that has been washed in hard water frequently keeps a dingy tint.) for the softened water, an ion-exchange method similar to reverse osmosis might be utilized. Water hardness is caused by ions such as calcium and magnesium. Positively charged sodium ions are added into the water in dissolved sodium chloride salt or brine to soften it. The hard calcium and magnesium ions trade places with sodium ions, resulting in the release of free sodium ions into the water. However, after softening a considerable volume of water, the softening solution may get clogged with excess calcium and magnesium ions, necessitating recharging with sodium ions.

Adsorption and Chemisorption

Adsorption is the process by which chemicals aggregate on the surfaces of solid bodies due to the van der Waal forces. that is a physical process; when the same thing happens as a result of a chemical connection, it is referred to as chemisorption.

Activated carbons are frequently employed in wastewater treatment to bind to soluble substances in the water that could not be removed in previous treatment process stages, such as mechanical or biological. Colorants used in textile dyeing, pharmaceutical residues, arsenic, and heavy metals are just a few contaminants that may be efficiently eliminated with this method.

Precipitation

Precipitation is a chemical process that includes appropriate agents to wastewater that can convert dissolved chemicals to those not readily soluble: the substance sediments and the concentration of the substance decrease due to this transition. Thus, heavy metals can form metal hydroxides, while anions can form calcium, iron, or aluminum salts.

Flocculation

Flocculation employs flocculants to assist in removing extremely tiny particles from wastewater that would otherwise aggregate as more oversized agglomerates due to their electrical repulsion from having the same charge. More effective particle formulations will result from the addition of specialty chemicals, which will then settle out in a sedimentation process.

Stabilization by Chemicals

This chemical wastewater treatment technology functions similarly to chemical oxidation. First, sludge is treated with a high concentration of an oxidant, such as chlorine. The addition of the oxidant inhibits the pace of biological growth inside the sludge and also aids in the deodorization of the mixture. Water is then extracted from the sludge. As an oxidant, hydrogen peroxide can also be employed and maybe a more cost-effective option.

Summary

To obtain satisfactory solutions to water purification and remediation challenges, wastewater processing, or recycling and reuse, represents a fusion of several different technologies, including specialized chemicals, mechanical filtration products, purification systems, specially designed equipment, and wastewater treatment services.

The purpose of this article was to give an overview of wastewater treatment procedures, with a focus on the chemical wastewater treatment process.