PROJECT Modernization of the primary and secondary clarifi ers
CUSTOMER Wodociągi Słupsk Sp. z o.o.
OBJECT WWTP Slupsk, Poland


The wastewater treatment plant is located in the northern part of the Słupsk city, on the right bank of the Słupia River. It is a mechanical-biological treatment plant, adapted for deep removal of biogenic compounds in a threephase, modified Bardenpho process, supplemented with a predenitrification chamber. The capability of the treatment plant corresponds to 200,000 population equivalent, and the average daily fl ow of about 19 647 m3/day.

In November, Esmil Prodeko-Ełk Sp. z o.o. has been selected as a General Contractor and signed an agreement with Słupsk Waterworks for the task: “Delivery of a dewatering centrifuge and scrapers to the municipal sewage treatment plant in Słupsk at ul. Sportowa 73 – task no. 2 – Delivery of the scrapers along with the performance of necessary construction, assembly and installation works.

As part of the contract, the following facilities were modernized:

  • Preliminary settling tanks with a diameter of D = 34m – 2 sets
  • Secondary clarifiers with a diameter of D = 40m – 2 sets

The modernization included dismantling of the old scrapers, delivery, assembly and kick-start of the new ones and also commissioning and warranty tests.

Initial Stage

The preliminary settling tanks were built in the first stage of construction of the wastewater treatment plant in 1986 and equipped with radial scrapers according to the solutions used in the 1970s. As a result of the modernization of the sewage treatment plant in 2006-2009, one of the primary settling tanks has been adapted to the retention and overflow function. The technical condition of the scrapers at the initial stage of the project was very poor and qualified them for replacement. In case of secondary clarifiers,
the situation was similar, with the diff erence that the facilities were built in 1978 and modernized in 2001.

Project Goals

The main goal of the signed modernization project was to improve the efficiency of sludge removal from both primary and secondary clarifiers.

In order to solve the problems and ensure stable and proper conducting of the technological process in both the sewage and sludge part, it was necessary to replace the scrapers in two primary settling tanks with models ensuring greater efficiency and reliability. Ultimately, this meant to improve the balance of the initial and excessive sludge, and thus increase the biogas production and reduce the operating costs of the treatment plant. Also the scrapers had to be replaced on the two existing secondary clarifiers. Part of the modernization was to supply the equipment with so-called side scraper, similar to ones mounted on two clarifiers that were built in 2008-2009.

Project Implementation

The modernization project of the preliminary and secondary clarifiers has been carried out between 15th of October 2018 and 28th of March 2019 and consisted of:

  • disassembly of the old (existing) scrapers and placing them in the containers provided by the INVESTOR,
  • preparation and delivery of the new scrapers made entirely from AISI 304 (1.4301) steel, sized to existing facilities,
  • installation of the new scrapers in an alternating system, i.e. first preliminary, then secondary, then again preliminary and finally secondary clarifier
  • technological start-up of each of the settlers and putting them into operation.

Results and Conclusions

As a result of modernization of the preliminary settling tanks, by installing new scrapers, problems related to ensuring stable and correct conducting of the technological process in both the sewage and sludge part were solved. The new scrapers are characterized by greater efficiency and operational reliability, which significantly contributed to the improvement of the initial and excessive sludge balance, and thus to increased biogas production and reduction of the operating costs of the treatment plant.

However, as a result of replacing scrapers in the existing secondary clarifiers with the devices equipped with so-called side-scraper, the removal of activated sludge excess to the surface of the settling tank in relation to normal operating parameters has been eliminated. This, in turn, resulted in a decrease of the total suspended solids discharged with sewage,and with it the load of carbon, nitrogen and phosphorus in the treated sewage to the Słupia river.

By replacing scrapers with ones equipped with the catching scrapers, the duration of active sludge retention within the secondary clarifiers has been shortened, which has prevented the formation of anaerobic conditions and secondary biological dephosphorization, and as a consequence – the process of secondary release of the orthophosphates into the wastewater from the activated sludge accumulated at the bottom.