What is a two-level sewer system?
A two-level sewer system is a specific sewer arrangement in which two conduits are laid one above the other in the same trench. The lower conduit conveys sanitary sewage, while the upper conduit carries stormwater. This configuration reduces the amount of underground infrastructure and lowers construction costs. It was used historically in some Polish cities, including Bydgoszcz, Grudziądz, Malbork, Toruń and Brodnica.
Why is a two-level sewer system difficult to modernize?
Despite its functionality, a two-level sewer system presents significant challenges during renovation or when installing new connections:
- Limited access to conduits — their vertical arrangement restricts inspection and repair options.
- Complex trenchless technologies — refurbishing both levels requires precise methods and advanced equipment.
- Risk of mutual interference — damage to one conduit can affect the other, potentially disrupting the entire network.
- No alternative flow routes — temporary bypasses are required during works, complicating operations.
- Depth of works — these networks are often located at depths of 4–6 metres, increasing execution difficulty.
New sewer connection in Bydgoszcz — executed by MIDO
MIDO recently executed a new sewer connection to an existing shaft within a two-level sewer system in Bydgoszcz. The work was part of a development on Gen. Józef Drewnicki Street.
Key aspects of the execution:
- Trenchless installation — the entire process was completed without open-cut excavation, avoiding disruption to road traffic and minimizing impact on the surroundings.
- Speed and efficiency — the operation was completed within a single day, reducing the duration of disturbances.
- Cost reduction and limited site disturbance — the absence of open excavations eliminated extensive earthworks and reduced dewatering costs.
- Materials used:
- One 15 m section of PP225 — 25 modular pipe units were used.
- The new connection was installed at approximately 5 m depth, about 1 m below the groundwater table, without the need to dewater the shaft — a key technical aspect of this execution.
Although two-level sewer systems were once applied as an innovative infrastructure solution, they now create multiple challenges for modernization and expansion. Modern trenchless technologies, such as those applied by MIDO Wiertnica Grawitacyjna, enable rapid, precise and cost-effective connections of new structures to existing networks. The Bydgoszcz execution showed that, even under demanding conditions, accurate sewer works can be performed with minimal disturbance to the environment, supporting the applicability of these methods in contemporary water and sewer engineering.
