Do sewage treatment plants use chemicals?

 Wastewater originates from households, businesses, industry as well as storm drains and rainwater runoff. In general, wastewater contains approximately 99.9% water, with the remaining 0.1% representing dissolved solids or other suspended material. This material may include excreta, detergents from washing clothes and dishes, food residues, grease, oil, plastics, salt, sand, grit and heavy metals. Some wastewater from industrial or agricultural processes may also contain chemicals that may be harmful to the environment or public health and need to be neutralised or removed from the water before they can be safely reintroduced into the environment. The main objective of the wastewater treatment process is to purify the water and ensure that it is safe for subsequent use once it has been returned to the environment as part of the water cycle.

Wastewater treatment involves several stages involving mechanical (physical), biological, chemical and membrane (filtration) based processes.

This article will review the basics of these stages of the wastewater treatment process and then look more closely at the chemical treatment of wastewater.

1. Mechanical wastewater treatment processes

The initial stage of wastewater treatment involves a mechanical process that removes approximately 20-30% of the solids from the water. Firstly, the wastewater is directed through a screen or drum to filter out relatively coarse impurities such as leaves, textiles, paper or other large materials. A series of screens with finenesses ranging from a few centimetres to a few millimetres are used to screen out the coarse contaminants from the water. The speed at which the waste water flows through the screens at each step is carefully controlled to ensure the effectiveness of the screening process. Any debris collected on the screens is sent to other process steps where the material is dewatered and then incinerated.

The next step is usually to feed the waste water into the sand collection process.

As the water enters the primary treatment tank, the wastewater is further slowed down by widening the tank and the last treatment tank represents the end of the mechanical wastewater treatment process. The removed sludge is sent to the digestion tower where any organic matter is converted into methane gas and can be used to generate electricity to power the process plant.

2. Biological wastewater treatment process

After the mechanical treatment stage, the effluent is transferred to a biologically based process for further purification. Aeration tanks are used to add oxygen to the water and circulate it through the use of propellers. The oxygen stimulates the growth of bacteria and micro-organisms which feed on any organic pollutants in the water and convert them into inorganic substances. This process creates a population of activated sludge that floats freely in the water. From the aeration or recirculation tank the water enters the secondary wastewater treatment tank, where the water is again slowed down for settling. The sludge settles to the bottom of the purified water and can therefore be mechanically removed from the bottom of the tank. A portion of the sludge, known as return sludge, is not removed however, but is sent back to the recirculation tank to ensure that sufficient quantities of bacteria and microorganisms are available to keep the biological treatment process viable. The removed sludge is usually sent to a digester for further treatment and to produce methane gas for power generation.

In many cases, completion of the first two stages of wastewater treatment is sufficient to allow the water to re-enter the river or stream. However, for some industrial and agricultural waste streams, further treatment steps are required. This is where chemical wastewater treatment processes come into play. The remainder of this article will focus primarily on these processes.

3. Chemical wastewater treatment processes

Chemicals are used in wastewater treatment processes in a range of processes to accelerate disinfection. These chemical processes, which initiate chemical reactions, are called chemical unit processes and are used in conjunction with biological and physical cleaning processes to achieve various water standards.

Specialised chemicals such as chlorine, hydrogen peroxide, sodium chlorite and sodium hypochlorite (bleach) are used as reagents for disinfection, disinfection and to assist in the purification of wastewater in treatment facilities.

There are several different chemical unit processes including chemical coagulation, chemical precipitation, chemical and advanced oxidation, ion exchange and chemical neutralisation and stabilisation that can be applied to wastewater from cleaning processes.

Neutralisation

Neutralisation involves the addition of chemicals to adjust the pH of the wastewater. This involves the addition of acids (to lower the pH) or bases (to raise the pH) depending on the initial pH of the influent water.

Chemical precipitation

Chemical precipitation is the most common method of removing dissolved metals from wastewater solutions containing toxic metals. In order to convert the dissolved metal into solid particle form, a precipitation reagent is added to the mixture. The chemical reaction triggered by the reagent results in the formation of solid particles of the dissolved metal. Filtration can then be used to remove the particles from the mixture. How effective this process is depends on the type of metal present, the concentration of the metal and the type of reagent used. In hydroxide precipitation (a common chemical precipitation process), calcium hydroxide or sodium hydroxide is used as a reagent to produce solid metal hydroxides. However, as many wastewater solutions contain a mixture of metals, it is difficult to produce hydroxides from dissolved metal particles in the wastewater.

Chemical coagulation

This chemical process involves destabilising the wastewater particles, causing them to aggregate in a chemical flocculation process. The fine solid particles dispersed in the wastewater have a negative surface charge (in their normal stable state), which prevents them from forming larger clumps and settling. Chemical coagulation destabilises these particles by introducing a positively charged coagulant, which then reduces the charge of the negative particles. Once the charge is reduced, the particles are free to form larger clusters. Next, an anionic flocculant is introduced into the mixture. Because the flocculant reacts with the positively charged mixture, it either neutralises the particle clusters or builds bridges between them, combining the particles into larger groups. After the formation of larger groups of particles, such as poly aluminium chloride (PAC). Also known as a water purifier or coagulant, it is a water-soluble inorganic polymer between ALCL3 and AL(OH)3.

Chemical oxidation/reduction and advanced oxidation

With the introduction of the oxidant during chemical oxidation, electrons are transferred from the oxidant to the pollutants in the wastewater. The contaminants then undergo structural changes to less destructive compounds. Alkaline chlorination uses chlorine as an oxidant against cyanide. However, alkaline chlorination as a chemical oxidation process can lead to the production of toxic chlorinated compounds and may require additional steps. Advanced oxidation can help remove any organic compounds produced as a by-product of chemical oxidation by processes such as vapour extraction, air vapour extraction or activated carbon adsorption.

Redox reactions are used to treat drinking water. Chlorinated hydrocarbons and pesticides can be effectively removed from wastewater by using ozone and hydrogen peroxide treatment. Advanced oxidation processes are also used to degrade drugs such as antibiotics or cytostatic drugs that may be present in the water. Reduction processes can also be used to convert heavy metal ions into sulphides.

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Ion exchange

When water is too hard, it is difficult to clean with detergents and often leaves a grey residue. (This is why clothes washed in hard water often remain a dull shade.) An ion exchange process, similar to reverse osmosis, can be used to soften water. Calcium and magnesium are common ions that cause water hardness. To soften water, positively charged sodium ions are introduced in the form of dissolved sodium chloride salt or brine. The hard calcium and magnesium ions exchange places with the sodium ions and the free sodium ions are simply released into the water. However, after softening large quantities of water, the softening solution may be filled with excess calcium and magnesium ions, requiring the replenishment of sodium ions into the solution.

Adsorption and chemisorption

Adsorption is the process by which substances collect on the surface of a solid under the influence of van der Waals forces. The process is a physical one - when the same thing happens as a result of chemical bonding, the process is called chemisorption.

In wastewater treatment, activated carbon is often used to bind to soluble elements in water that cannot be removed at an early stage of the treatment process (e.g. mechanically or biologically). Colourants from textile dyeing, drug residues, arsenic and heavy metals are examples of substances that can be effectively removed by this process.

Precipitation

The chemical process of precipitation involves the addition of suitable reagents to the wastewater which transform dissolved substances into less soluble ones. Through this transformation, the material precipitates and reduces the concentration of the material. For example, heavy metals can precipitate as metal hydroxides and anions can precipitate as calcium, iron or aluminium salts.

Flocculation

Flocculation uses flocculants to help remove very fine particles from wastewater that would not normally accumulate into larger agglomerates due to electrical repulsion with the same electrical charge. By adding special chemicals, a larger formulation of particles is created which then settle out during the sedimentation process.

Chemical stabilisation

This chemical wastewater treatment process works in a similar way to chemical oxidation. The sludge is treated with a large amount of a given oxidising agent, for example chlorine. The introduction of the oxidising agent slows down the growth of organisms within the sludge and also helps to deodorise the mixture. The water is then removed from the sludge. Hydrogen peroxide can also be used as an oxidising agent and may be a more cost effective option.