Zagreb's sewage requires extra care

Groundwater protection was a particular concern in transporting wastewater near the Croatian capital of Zagreb. Austrian pipeline specialist Hobas found the solution, writes Eduard Hesky of the company

Croatia's capital Zagreb has a population of nearly a million who benefit from high-quality drinking water drawn from wells around the city area. The city council needed to retain the groundwater quality and the river Sava with a new Zagreb wastewater treatment plant.

Austrian pipeline specialist Hobas was asked to supply a 5.9km SewerLine (DN 1,000) to transport all wastewater from New Zagreb in the south (which has 200,000 inhabitants), across the river Sava to the wastewater treatment works in the north-east of the city.

The complete pipeline consists of three major sections. The first was MCP Culinecka, which runs from the treatment plant to the Homeland Bridge. This part was accomplished by open-cut installation with well compacted gravel sized up to 16mm. Hobas T-Pieces manholes (DN 1,000/800) were used and covered with stainless-steel blank flanges.

Mechanical couplings join the manholes to the pipeline and provide flexibility regarding maintenance or repair works. They can be easily disconnected and taken out of their concrete housing any time if necessary.

The second section is the 900m long Homeland Bridge, which carries a road and tramway and holds five parallel pipelines (DN 1,000) inside the construction. Four of the pipelines are for potable water and one is a Hobas CC-GRP sewer main for New Zagreb's wastewater.

The 1km-long Hobas SewerLine was installed on stainless-steel supports with 5.85m clearance. Since the only fixed point lies in the centre of the bridge, thermal dilatations were carried out to both sides with the bridge moving up to 50cm. Kresta compensators were used here to absorb these movements - a demanding task considering the relatively large diameter, the pressure and line movement.

An important consideration for the selection of the right material was the magnetic field created by the tramway rails, which can influence close paralleling lines such as the sewer main. Hobas CC-GRP pipe systems are non-conductive, corrosion resistant, show very little thermal expansion and have a long service lifetime.

Apart from this, the products' low weight put less load on to the bridge and facilitated pipe handling inside the construction. This was one of the reasons that the client chose Hobas CC-GRP over other available materials. Another 4km of CC-GRP BridgeLine (DN 300-400) was installed for the drainage of the Homeland Bridge.

The third and final section of the line leads from New Zagreb to the Homeland Bridge. This 3km-long section runs through a water protection area with Zagreb's springs so that the high-quality pipes needed to be placed into a watertight concrete channel. Concrete supports hold the line every 5.8m where it is fixed with stainless-steel straps.

An 8mm EPDM synthetic rubber layer was applied between support, strap and pipe to allow small longitudinal movement due to temperature variations and to evenly distribute the pipe weight on the supporting areas. The longitudinal stiffness of the CC-GRP pipes minimised the number of necessary supports, which saved time and expenditure.

Easily jointed push-to-fit couplings also contributed to the savings since no welding
is required for Hobas products. All sections of the pipeline were pressure tested up to nine bar.

This was demanding for the manholes because the DN 800 blank flanges fixed on the tees without additional support had to withstand a force of 477kN (equivalent to 48 tonnes).

Pressure tests on the pipeline inside the bridge were also hard to conduct because the bridge temperature kept changing, causing the length of bridge to constantly vary. With a reduction in length the pressure inside the closed pipe increased.

The project was completed last year and the facility was constructed by Zagrebacke Otpadne Vode (Zagreb Wastewaters), but Hobas specialists were present during all stages.

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