Introduction

Within the rehabilitation concept of Lake Palic a plant for waste water treatment was built in the 1970s as a secondary phase for treating all waste water generated on the territory of the city of Subotica.

The secondary treatment of waste waters covers a complex of facilities and equipment for the treatment of waste waters generated by households, the industry and previously treated industrial waste waters, applying mechanical and biological methods. These methods are adjusted to the reduction of mostly organic matters from waste waters. The system is composed of two units: the water line and the sludge line.

The Waste Water Treatment Plant (WWTP)

The Water Line

The system (water line) has been designed to consist two parts: mechanical and biological treatment by biologically active sludge and additional treatment in the lagoon system.

Through the main collector (about 1,7 km long open channel), all waste waters from the households, institutions, crafts and industry, and also stormwater collected in the sewerage system, are channelled to the WWTP. The first facility is a manually cleaned coarse screen.

Behind it is the main pump station for the primary lifting of waste water to other treatment facilities. There are three units of screw pumps for lifting up to 3,2 m with an individual capacity of 200 l/sec., 400 l/sec. and 300/600 l/sec. and installed motor power of 11 kW, 22 kW and 30/42 kW. After pumping there is a fine screen with mechanical cleaning (2 units). Wastes from the screen are transported by a belt conveyor to a container, and are disposed on the solid waste landfill of the city of Subotica. The next facilities are the sand traps (2 units). The tangential sand trap has a total volume of about 20 m3 and is used as a sand trap only. The volume of two-chamber aerated sand trap-grease separator is about 260 m³ and beyond sand separation it separates grease as well. The separated products (grease) are transported to the city landfill, while the sand and other sedimentary matters from the aerated sand trap are disposed by giant pumps to a natural depression nearby the Plant. Even besides this two-stage sand separation, sand sedimentation on various points of the whole line is apparent.

The next facility is the primary sedimentation tank, which is a “radial sedimentation tank with horizontal streaming". The tank’s diameter is 28 m, its average depth is 2,5 m and volume is 1423 m³. After the primary treatment the water enters the aeration (or biological) tanks for the biologically active sludge process. Aeration tanks are facilities with two compartments, each 19.6 m in width, 39.6 m in length and 3.0 m of water depth (their total volume is 4656 m³)

Until 2003, the aeration system was surface aeration (by aeration turbines) while in mid-2003 a deep aeration system was installed, whereas air is blown in from the bottom of the tank by 4 blowing units (the capacity of each motor is 55 kW) with an effective oxygen input of 3.5 kgO₂/ kWč.

The next facilities are the secondary sedimentation tanks (a total of three). These tank are ``radial tanks with horizontal streaming``. The tank diameter is 28 m (there are such tanks) and 50 m (1 tank) respectively. Tank volume are 2 x 1500 m³ and 1 x 4600 m³. The recirculation of active sludge (from the secondary sedimentation tank to the aeration tank) is made by screw pumps of 2x100 l/sec. capacity and 2,2 m of lifting height.

The Lagoon System

There are three serially connected lagoons attached to the WWTP. Their area is approximately 50.000 – 80 000 m²/each depth is about 2,0 meters and volume is 114000 –196 000 m³/each. Reliable data on the lagoon status after 35 years of operation are not available. There is an apparent fact about the significant decrease in the size of the lagoon system if compared to its original status, and the aeration equipment for the aeration lagoon stopped operating.

The Sludge Line

Excess sludge is collected from the tank of the sludge recirculating pump station. Pumping units (submersible wet-well sludge pumps) with 2x33 l/sec. capacity and 15 meters of lifting height. Excess sludge treatment facilities are not operating. The sludge is transported from the installation to both fixed and mobile cassettes through pipelines. Unstabilised liquid sludge is then deposited into earthen cassettes (located outside the Plant) or in the sludge storage lake within the Plant. The sludge storage lake was the first facility for sludge storage having an area of 15 ha. According to the designs, a disposal of 300000 m³ thickened and pressed sludge has been foreseen. As indicated above, sludge stabilisation and pressing are not performed, hence the raw sludge occupies more space, therefore additional storage capacities (sludge cassettes) were built.

Supporting Facilities

With regard to the general feature of the city’s sewage system, wastewaters are often diluted by stormwater. The evacuation of these waters is made by a special unit through a coarse screen (2,2 m opening width , water height 2.0 m and rod with of 40 mm) and pump station with submersible pump units ( 2 pieces each of 450 l/sec. capacity, with 4,5 m lifting height). Through this system the water is directed to lagoon no. 3. As designed, these waters may be directed to the Plant, lagoon 3 or the surface storage for stormwater, yet the system as a whole has never been accomplished, namely excess diluted wastewater to be channelled to surface storage is directed to other routes if needed. The designed volume of the surface storage is 139600 m³, but sludge sedimentation caused an apparent and significant reduction of its capacity.

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