Yeah, they really do. In this article I’ll present two of Budapest’s wastewater treatment facilities, the North Pest and South Pest plants. The first one I can introduce to you (and illustrate with self-made pictures) because a few years ago I wrote my thesis about it, and the second one because in 2019 I had the opportunity to attend their public open day on the occasion of World Water Day. By the way, open days are organized each year, it’s worth a visit, even with children, since they explain the plant’s operation in plain language. Written by Noémi Szabó.

BSW, operator of the wastewater treatment plants

The main activity of Budapest Sewage Works Pte Ltd. (BSW) is the drainage of sewage and rainwater, but it also deals with many other things and contributes to the introduction of various environmentally friendly developments and technologies in Hungary. Imagine that not only the subway, but also some of the wastewater from North Buda passes under the Danube. The North Pest and Sout Pest plants, with the help of the central facility, are able to treat 100% of the wastewater of Budapest.

What else does BSW do?

  1. Apart from Budapest, it also operates the sewerage network of Budaörs, including the continuous monitoring of the sewers’ technical condition.
  2. It maintains, cleans and repairs the heavily stressed sewer network.
  3. It operates pumps to bring wastewater from all parts of the city to the plants.
  4. Both plants receive and treat organic waste from different companies.
  5. It operates most of the public toilets in the capital.
  6. It performs flood and inland water protection tasks.
  7. Performs maintenance work on small watercourses of the capital, that is, on various streams and ditches with a combined length of 170 km.

The total length of the sewerage network is 6,399 km in Budapest and 163 km in Budaörs. The northern site can treat 200,000 m3 and the southern site 80,000 m3 of wastewater per day.

Facts and details

Together, the two sites are capable of biological treatment of nearly half of the wastewater from the capital, which means that water free of phosphorus and nitrogen will be returned to the Danube.

“In the aerated sand and fat collector, non-organic matter (sand, small gravel) settles and is removed from the bottom of the basin by means of a scraper. The fat and oil-like floating materials are collected in a sump and discharged into the digesters.”

But what does biological treatment mean?

In short, it means that bacteria purify our wastewater.

Primary clarifier: Fine fractions of non-organic and settling matter remaining in the wastewater are separated here. The accrued sludge is pumped to the sludge treatment facilities.”

But how?

After the grit collection unit, water first passes through automatic screens to filter out coarse pollutants like wet wipes. Then, raw sewage passes through a sand and grease trap, where sand settles and grease accumulates on the surface so both can be removed with scrapers.

Chemical phosphorus removal is carried out prior to biological treatment by the addition of ferric chloride.

The next step is the primary clarifier, followed by biological treatment, but before that, phosphorus removal starts with the addition of ferric chloride.

And now bacteria come into play!


In convenient wastewater treatment, nitrogen removal is based on nitrification and denitrification. Autotrophic nitrification, the first step of the process, takes place in an aerob environment, and molecular oxygen serves as an electron acceptor. Ammonium is oxidized to nitrate, which has a high oxygen demand, requiring intensive aeration. Nitrification and denitrification are carried out in different environments and by different types of microorganisms, so these processes must be separated in space or time.
The second step in the nitrogen removal process is heterotrophic denitrification under anoxic conditions, which occurs rapidly with numerous different electron donors (methanol, ethanol, glucose). In denitrification, nitrite (NO2) and nitrate (NO3) are the starting materials and nitrogen (N2) is the end product.

Nitrification and denitrification are carried out in the anoxic (no or very low dissolved oxygen content) and oxic (high oxygen content) zones. Nitrification takes place in several consecutive steps, with the genus Nitrobacter converting ammonium into nitrite and the genus Nitrosomonas converting nitrite into nitrate. All this takes place in the aerated basin with periodic or continuous aeration from below. The purpose of the recycling circuit is to return the nitrate to the anoxic zone where the final product is nitrogen. Treated water and activated sludge gravitationally enters (i.e. drains into) the secondary clarifier, where scrapers remove and pumps recirculate the excess sludge to the outflow of the primary clarifier, inoculating incoming wastewater.

Treated water enters the chlorinating channel through perforated pipes or spill edges then through the end channel into the Danube.

At the South Pest site, the journey of wastewater doesn’t end, in order to protect the Ráckeve-Soroksár-Danube branch, water passes through a purpose-built fixed filtration system where the remaining nutrients are removed from the wastewater.

Spill edges

The plants are almost self-sufficient, as they produce biogas from the sewage sludge, and generate electricity and heat from the gas.

How does all this work?

It’s simple. Pumps transport the mixed sludge settled in the primary clarifier to the thickener unit, where sludge is thickened to 6%. This is followed by a dewatering process. Sludge is then digested by microorganisms at 37°C to produce biogas. After desulphurization, the gas is stored in one of the two storage tanks, and it’s burned in gas engines to produce electricity and heat (excess gas is flared if necessary).


The South Pest Wastewater Treatment Plant uses living machines to deliver water of higher quality to the Danube than EU standards.

But what is a living machine?

“Essentially, the Living Machines technology consists of an ecosystem of about 2,000 to 3,000 species installed over aerated reactors commonly used in activated sludge processes. Depending on the composition of wastewater and treatment demand, an anaerobic pre-clarifier, an anoxic zone and, if required, an aerated biofilter as post-treatment are added to the process sequence. In addition to bacteria, treatment also involves zooplankton, various plants, and even mussels and snails. Plants are installed on a grid at the top of the aerated tanks, and through with their roots extending 1-1.5 m deep into the wastewater, they provide a large surface and excellent habitat for the multitude of organisms that take part in the treatment. In the climate of Hungary, the technology is placed in greenhouses.”

Thanks to a Hungarian development, the Organica Living Machines system, the treatment plant has increased its efficiency with live vegetation and an engineered root system.

In this case, following mechanical treatment (grit collection and separation basin, sand and fat collector and primary clarifier), microorganisms in activated sludge basins covered with greenhouses decompose the dissolved organic matter content of the wastewater. The root system of waterborne plants provides a perfect habitat for the microorganisms.

The root system of plants reaches into the wastewater

Bacteria and plants

Just think about it: all the wastewater produced during your everyday life (doing laundry, bathing, washing the dishes, using the toilet) can be treated by bacteria and plants. These awesome and interesting technologies allow us to return treated water into the Danube instead of sewage!

The South Pest plant – just like a park, source:

BSW and society

The company pays particular attention to supporting environmental initiatives, such as raising awareness to environmental issues. Not only do they hold a public open day each year on World Water Day (the open day in 2020 is cancelled due to the pandemic), but they also support educational institutions with open days combined with environmental education seminars. On these occasions, children are taught to take responsibility for their environment and to protect one of our most important natural treasures, Earth’s freshwater supply.

Some additional pictures of the structure and processes of the plants

Progress flow (South Pest)
Structure of the South Pest plant (1-3. mechanical treatment, 4-6. biological treatment, 7-14. sludge and waste treatment, 15. treatment of technological waste, 16-20. other structures)

Source of featured image:
Translation by Ádám Hittaller

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