Author of the article: Petar Doroslovacki, computer science, the member of ECOLACUS PR team

Infrastructure facilities are large and expensive. Even though it takes a long time to build them, they are eventually put away from the public eye because they are mostly underground/water, and they are not as attractive as bridges, roads, and large buildings. Therefore, they do not enjoy such a reputation, and their authors are unjustifiably unknown to the public.

The condition of Lake Palic – before the ECOLACUS project implementation

“Lake Palic is the largest in a series of shallow lakes that were formed on the border of the Subotica-Horgos sandy terrain and the loess plateau of Backa. The former healing properties of the water and mud of the alkaline steppe lake encouraged the construction of baths and parks in the 19th century. Untreated or insufficiently treated Subotica wastewaters were flowing into Lake Palic for decades, causing drastic changes in the ecosystem and water in the hypereutrophic lake.

Eutrophication is the response of ecosystems to the influx of nutrients (trophos = food), primarily nitrogen and phosphorus salts. The increase in the content of nutrient salts (nutrients) in water causes excessive growth of certain autotrophic species (cyanobacteria, algae and higher plants). Eutrophication is especially a problem for coastal and inland waters, where there can be a huge increase in phytoplankton (algae), which leads to reduced water transparency, degradation of ecosystems, reduced concentration of dissolved oxygen in the water during the night and fish deaths.

Additionally, some species of cyanobacteria (blue-green algae) secrete toxins that can be dangerous to animals and humans, while some species, by binding atmospheric nitrogen, additionally enrich the water with nutrients.

The unsatisfactory quality of water in Lake Palic, and the living communities associated with it, is evident year after year due to the influence of concentrated and diffuse sources of pollutants, which is an additional source of phosphorus and nitrogen salts. Sources of nutrient salts are mainly the wastewater and agricultural activities. “- is stated in the Plan for improving the ecological condition of Lake Palic and its surroundings (Subotica, July 2014).

Image: Tourist part of the lake_fig 1 (Plan for improving the ECO condition of Lake Palić and its surroundings)

Efficient removal of nitrogen and phosphorus – one of the most important tasks of modern wastewater treatment

The complex biochemical transformation of phosphorus/phosphate during wastewater treatment cannot be easily explained in simple words. Scientists have been researching and trying to explain the mechanism of their metabolism in microorganisms in activated sludge for 70 years now. And there are still unexplained things. – Edita Trungel Mester, the head of the Wastewater Treatment Sector at the Subotica waterworks, is starting to explain.

Excessive load of nitrogen and phosphorus as inorganic nutrients leads to excessive growth and flowering of algae and plant organisms, and the consequences are known: creation of floating foam on the water surface, formation of sticky sludge in water, in baths, production of toxins by cyanobacteria, which can cause skin irritation and respiratory failure, population problems and the death of aquatic animals. One of the most important tasks of modern wastewater treatment is the much more efficient removal of inorganic nutrients, nitrogen and phosphorus – adds Edit Trungel Mester.

Image: Edita_kids (photo of: Mihalj Sobonja)

Phosphorus can be removed from wastewater in solid form only by precipitation. This is possible by incorporating phosphorus into biomass, i.e. bioactive sludge or by phosphate removal with a chemical.

The biological treatment at the Subotica wastewater treatment plant consists of several batch reactors, which are divided into sections: anaerobic, anoxic and aerobic sections.

By changing the anaerobic, anoxic and aerobic conditions in the biological treatment of wastewater, microorganisms grow, which are able to reduce phosphorus in wastewater, and bioactive sludge, i.e. the mass of microorganisms is enriched with phosphorus content. Microorganisms, which are able to reduce phosphorus in the wastewater, are growing through the interchange of the anaerobic, anoxic and aerobic conditions in the biological treatment of wastewater, and the bioactive sludge, i.e. the mass of microorganisms is getting enriched with phosphorus content.

Dosing of ferric chloride is enabled for the chemical precipitation of phosphorus at the overflow of the aerobic section, and the phosphorus is added in case the concentration of phosphorus in the effluent is higher than 1 mg/l.

Total phosphorus concentration was about 9 mg/l at the WWTP inlet during 2021. The average annual concentration of treated water was about 0.5 mg/l. The effect on phosphorus removal is 94%.

There is occasionally an overshoot in case of major breakdowns or in cases of downtime of some parts of the plant. A new polishing station, which is under construction, uses the ferric chloride chemical for phosphate deposit to further remove phosphorus. When the new polishing station is fully operational, it should also reduce phosphorus residues from treated water.

The concentration of total nitrogen at the inlet of WWTP was about 62 mg/l in 2021, and the average annual concentration in the treated water was about 6 mg/l. The purification effect on nitrogen is 91% – adds Edita Trungel Mester at the end of her broader systematic presentation.

Any activity for the rehabilitation or revitalization of the lake is meaningless if the sources of ecosystem degradation are not removed in parallel and at the same time.

Before embarking upon any activity of rehabilitation or revitalization of the lake, it is necessary to remove the most significant sources of burden and degradation of ecosystems and reduce the inflow of nutrients and pollutants to a minimum.

It turned out that, although the negative impact of the previous main lake polluter (Wastewater Treatment Plant of the city of Subotica) has been significantly reduced, the problem has not been resolved to an acceptable extent. Even though the city wastewater treatment plant operates according to the designed parameters (phosphorus, nitrogen, HPK, BOD, suspended matter), it is still the significant source of nutrients (phosphorus and nitrogen compounds) due to the condition of Lake Palic – – adds Petar Pizurica, the Executive Director of the Production Department of Subotica’s “Vodovod i kanalizacija”.

The significance of the Biodiversity and Water Protection Lake Palic and Lake Ludas project, which is being implemented with a donation of KfW Bank, comes out from the approach to make efforts and rehabilitate other sources that pollute the lake. Any activity aiming at the rehabilitation or revitalization of the lake is meaningless unless all significant sources of environmental burden and degradation of the ecosystem are removed at the same time. – adds Petar Pizurica.

Image: WWTP_lagoons (Dinovizija)


Image: Sluice gate_OK_1 (Dinovizija)

Petar Pizurica says: When it comes to the combination of responsibilities and competencies of PUC Vodovod i kanalizacija within the implementation of this project, but also within the investment and regular maintenance, which ultimately influence the reduction of the “entry” of inorganic substances in Lake Palic (and Ludas), I would like to remind what those activities are –

▪ Introduction of new technological processes on the waterline of the city Wastewater Treatment Plant – a device for additional phosphorus removal before the treated water exits into the lagoon system (up to Sector I)

▪ Installation of a new sand trap with a sand washer that removes sand from the wastewater. This way the organic matters return to the treatment process, the result of which is an efficient reduction of the total nitrogen in the water.

▪ Provision of the spare hydro mechanical and measuring and regulating equipment that provides the maintenance of the installed equipment and its continuous functional condition.

▪ Introduction of frequency conversion at the sludge recirculation pumps, which provides the number of required microorganisms at the optimal level and more stable quality of treated wastewater

▪ Installation of a mixer in the old aeration tank to optimize the flow of treated wastewater, which was not implemented due to lack of funds.

▪ The location for disposal of digested sludge has been changed from the WWTP location in Subotica and it is transported in a more rational and economical way (new vehicle from the KfW donation funds) to the Bikovo Regional Landfill where its life cycle continues through composting and phytoremediation processes. The total amount of sludge is 10,000 tons per year, while the designed capacity for the Regional Landfill is half as small. This means that additional drying (dewatering) of this sludge should be started. There is the modern solar method, for which it is necessary to provide an additional 2.5 million Euros.

In addition to the above, it should be noted that the completion of the construction of the sewerage network around Lake Palic and in the residential area of this settlement and its connection with the city network and drainage and treatment at the city plant. The most important thing is that this created conditions for the connection of households, but the priority is the connection of major polluters (weekend settlement, Zoo, the Large Park, the West bank of Sector IV and the surrounding farms near the lake), but also other lake basin facilities (public companies, hotels…).

If I add to all this the setting up the buffer zones to reduce the influx of nutrients (phosphorus and nitrogen) and harmful substances deriving from the agricultural activities that flow into the lake, in combination with the predator fish catching, as well as tourist friendly measures such as bike paths, bird watching towers, and rest areas around the lake, which are under the jurisdiction of PE “Palic – Ludas” – then I can conclude that we have done a lot for the biodiversity and water protection of Subotica lakes – concludes his presentation Petar Pizurica.

Image: Biodiversity (from the PP presentation of Vinko Tamas)

Information sources – interlocutors:

  • Edit Trungel Master, B.Sc. in Chemistry, Head of the Wastewater Treatment Sector and
  • Petar Pizurica, B.Sc. in Mechanical Engineering, Executive Director of the Production Department of PUC “Vodovod i kanalizacija” Subotica