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Where are the sources of wastewater?

The wastewater flows into the wastewater treatment plant (WWTP) from several sources. Households and municipal wastewater, along with the agriculture and industry as the biggest polluters, constitute a large share of wastewater inflow. Typical examples include food industry, textile industry, chemical and production plants, heating plants, and mines.

This might surprise you, but storms are also one of the sources of wastewater. The rain washes away the impurities from roads and land which then end up in sewers or water bodies, contaminating the water and making the treatment necessary.

What is the content of wastewater at the time of entering the WWTP?

For starters, you should know that only 1% of wastewater is contaminated and the rest is just water. However, in order to reuse the water, it is crucial to remove the one percent of harmful substances.

Pathogens are frequently contained in wastewater and pose a threat to public health. The same applies to toxic compounds that can cause your healthy cells to become carcinogenic.

Wastewater also contains phosphorus and nitrogen, substances that can disturb the balance of other organisms. Released phosphorus results in massive reproduction of algae in freshwater bodies. Nitrogen kills fishes and has detrimental impact on biotopes.

Wastewater has very diverse content. Apart from the said nitrogen, phosphorus and pathogens, it also contains heavy metals, insoluble materials, and biodegradable organics.

The share of particular substances in wastewater depends on its source, i.e. whether it’s municipal wastewater, household wastewater, or industrial water.

Why is water treatment in cities vital?

Since wastewater contains harmful toxins and microorganisms, the purpose of the treatment is to remove them.

Wastewater must not have any negative impact on environment or public health in order to be suitable for reuse.

What is the ideal temperature for wastewater treatment?

When it comes to wastewater treatment, both extremes have adverse effects. In other words, neither too high nor too low temperature is suitable. The ideal temperature lies in the middle at about 20 to 35 degrees Celsius.

Should the temperature drop or rise after all, the treatment is no longer effective, especially at the secondary stage, when bacteria are added to the water to kill the harmful organics.

High temperature could inactivate the bacteria. Low temperature will not harm the bacteria but could decelerate their activity, and thus the entire treatment process.

WWTPs have special systems in place to achieve the ideal temperature.

How long does the treatment take?

In large treatment plants, which handle thousands of hectolitres of water per day, the process takes several hours.

First, solid pollution, such as rocks, sand, glass or even textile, must be captured. The next step is to separate grease and soap scum, which float to the top, from water.

Following procedures ensure the removal of organics by injecting bacteria.

Lastly, chlorine or other disinfectant is added to the water to kill any pathogens and viruses. The water then gets released back to the ecosystem into the river.

North Jersey’s sewage treatment plant on the banks of the Hackensack River employs a similar process. The entire treatment process from the first moment to the release into the river takes 6 hours.

Treating drinking water is slightly different and takes about 5 hours.

Actually, in case of aeration of the wastewater the entire process takes significantly longer as this step takes one or two days.

What is the basis of primary, secondary, and tertiary treatment?

As mentioned before, wastewater treatment is a multi-stage process that includes primary, secondary, and tertiary treatment.

The primary treatment follows the removal of solid materials, such as rocks, textile, plastic, or wood debris. The water passes through primary treatment tanks. The sludge sinks to the bottom, while lighter solids, such as grease and soap scum, float to the top.

Adding chemicals to remove phosphorus is also a part of the primary stage.

The purpose of secondary treatment is to remove biodegradable substances by filtration and aeration that can take up to 30 hours. By the end of the second stage, the water is 90% clean.

Tertiary treatment is a vital step because the effluent is treated in order to achieve drinking water quality and get released into water flows. The harmful pathogens are killed during this stage.

The unremovable microorganisms are made sterile and safe by means of UV light.

What is the role of treated wastewater?

This might be surprising, but Singaporeans drink the treated wastewater. Due to the high population, the country aims to use up to 55% of wastewater as drinking water.

Industry and agriculture are also big consumers of wastewater. Carlsberg, a Danish brewery, operates its own wastewater treatment plant and aims to reduce the “clean” water use by up to 50%. Instead, Carlsberg will use recycled water to brew their beer.

In other parts of the world, the treated wastewater functions as an addition to concrete mixtures or for cooling of industrial towers.

In the U.S., more than 56% of recycled water is used for irrigation of agricultural land.

All these solutions reflect the baseline problem, i.e. we have little water, while the demand continually grows.

What is nitrification and denitrification?

These are biological processes used to remove nitrogen from wastewater. The ammonia is transformed into nitrite followed by the oxidation to nitrate.

Nitrification is followed by denitrification, where nitrate is reduced and gaseous nitrogen is produced.

The importance of these processes is evident: they transform the harmful nitrogen into substances with no negative environmental impact (e.g. on water, land).

What is the purpose of treating wastewater?

It is true that the majority of harmful substances and waste is removed throughout the secondary stage. However, in order to treat drinkable water and release it into water flows, it must be perfectly clean.

This means the water must be disinfected. Disinfection kills harmful viruses, bacteria, mould, and fungi.

Typical sources of disinfection are chlorine, UV light, or ozone. If you are interested in wastewater disinfection, read our article where we described the process in detail and find out more.