The four stages of wastewater treatment plants

One of the current challenges at global level is to treat as much wastewater as possible in wastewater treatment plants. According to the United Nations’ 2017 World Water Development Report, entitled Wastewater – The Untapped Resource, wastewater can be a key resource for meeting the planet’s growing demand for freshwater and raw materials.

Wastewater or sewage is water whose quality has been compromised by human intervention, and is classified as urban, domestic or industrial, depending on where it comes from.

Wastewater treatment plants (WWTPs), are in charge of collecting water from a populated area or industrial sector and of removing its pollutants. This process aims to return this resource to the water cycle, either by discharging it into watercourses or reusing it in activities such as agriculture.

What processes take place in wastewater treatment plants?

The water entering WWTPs undergoes a series of physical, chemical and biological processes to remove the pollutants it contains. These processes are usually divided into four stages known as preliminary, primary, secondary and tertiary treatments.

In addition, WWTPs also carry out other processes associated with the by-products obtained in the different treatments. Some of these processes, such as the treatment and management of sludge, are significant because they are complex to manage and are of great interest.

1. Preliminary treatment

Preliminary or pretreatment, is the first stage of wastewater treatment and is used to prepare water for purification during the following phases. Thus, it consists of removing objects that could damage the plant or the equipment that will be used during the purification process.

First, roughing filtration is usually carried out. This process separates out the large and medium-sized solid waste using different thickness screens and sieves. Subsequently, grease and sand particles are removed using desanders and degreasers.

2. Primary treatment

The objective of this stage is to remove part of the suspended solids. To this end, water is retained for one to two hours in decanter centrifuges where gravity helps to separate these particles. Other benefits of this process include flow homogenization and the removal of organic matter linked to the suspended solids.

During this process, chemicals such as coagulants and flocculants can also be added to improve the sedimentation of solids and remove phosphorus. In some cases, bases and acidic agents are used to neutralize the water’s pH.

3. Secondary treatment

This process is designed to remove organic matter from the water, as well as nutrients such as nitrogen and phosphorus.

This secondary treatment, which is mainly biological, generally utilizes bacteria and microorganisms to degrade and eliminate the organic matter and the different nutrients contained in the water. The most widespread treatment is activated sludge, where the water to be treated is left in a tank for several days under varying oxygen conditions (aerobic, anoxic and anaerobic) depending on the required removal requirements. Here, the different types of bacteria that live in the tank or reactor feed on the organic matter and the nutrients contained in the water, removing them from the water and taking them into their organisms.

wastewater treatment plants

A second or secondary settling process is usual after the biological process. Here, the bacteria that have grown in the previous process precipitate to the lower part of the settling tank, generating a mixture of water and solids, which is called biological sludge. This mixture is extracted or flushed out through the lower part of the decanter and the purified water flows out through the upper part without most of the bacteria and solids, giving rise to clarified water.

It is common in wastewater treatment plants for water treatment to end at this point, when the treated water meets the defined discharge requirements and there are no additional water quality requirements for reuse or further use.

4. Tertiary treatment

During tertiary or chemical treatment, the aim is to increase the final quality of the water so that it can be returned to the environment (sea, rivers, lakes and other hydrographic basins) and, in some cases, used for human activity. To achieve this, a series of processes are carried out to eliminate pathogenic agents, such as fecal bacteria.

The techniques used include filtration with sand beds or other materials, and disinfection, either using chlorine (usually sodium hypochlorite) or UV light, to reduce the amount of microscopic living organisms that have been generated in the previous stages.

Sludge treatment, WWTP waste

Certain types of waste are produced as by-products of wastewater treatment. The main one is the generation of large quantities of sludge, which is extracted in the primary and secondary decanting processes.

First, the sludge is thickened to reduce the volume of water to be treated, thus optimizing the existing downstream processes. Subsequently, this mixed sludge is digested by aerobic (with air) or anaerobic processes (in a closed tank called a digester) to stabilize the growth of bacteria and to remove them. This is followed by a process to reduce the amount of water by dewatering, usually with decanter centrifuges. When this sludge complies with dryness, heavy metal, nutrient and pathogen content requirements, it must be managed with the least impact possible on the environment. In Spain, it is commonly reused in agriculture as fertilizer.

Wastewater: a key resource to address current challenges

In compliance with Sustainable Development Goal 6 of the United Nations’ 2030 Agenda, the need to increase the percentage of the world’s population that has access to safe drinking water and sanitation will, in turn, lead to increased wastewater discharges. This implies that its treatment must be more efficient, affordable and sustainable to make better use of this resource.

Some progress is being made in this area at global level. According to UNESCO data, in Latin America, for example, wastewater treatment has almost doubled in the last 20 years. In Spain, Idrica’s GoAigua solution is being used in several WWTPs to efficiently harness end water for reuse in agriculture. This is another example of how technology helps to promote environmental sustainability in the case of wastewater.

However, the percentage of sewage that is discharged without being treated is still high, which is why efforts must continue to be made to reduce this amount. A decisive move will be to generalize the reuse of treated wastewater. In this regard, digital transformation is essential to optimize the management of wastewater treatment plants, sewage networks and storm drainage. GoAigua Wastewater solutions, for example, are designed to improve processes, ensure water quality and reduce the carbon footprint, among other use cases.

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