Syphon cleaning improves network performance for Southern Water
Published: 9 November 2026
Persistent spill events associated with a syphon and upstream sewer network highlighted the need for a detailed investigation and targeted maintenance programme.
Southern Water partnered with us to clean and assess the syphon, clear extensive upstream obstructions, and restore hydraulic capacity across the network. The project combined complex jetting, CCTV surveying, and structural repair to improve performance and strengthen the long-term resilience of this critical wastewater asset.
The challenge
Syphons are critical elements of wastewater infrastructure, but their configuration and access requirements can make inspection, cleaning and maintenance particularly challenging. Accumulated debris can restrict hydraulic capacity, reduce throughput, and contribute to increased pressure across the upstream network.
Initial investigations at this site identified debris accumulation within manholes, restricted access to the syphon, and suspected structural deterioration along sections of the upstream sewer running through woodland.
A detailed assessment found that the incoming 300mm clay sewer was in good condition through the field area. However, significant root ingress and poor structural condition were identified further upstream.
The approximately 185-metre triple-pipe syphon (comprising two 150mm and one 225mm cast iron barrels) was also heavily restricted by debris. This reduced its hydraulic capacity and contributed to ongoing network performance issues and persistent spill events.
Investigation and assessment
Our specialist complex jetting and CCTV teams carried out a detailed assessment of the syphon and upstream sewer network to establish the extent of the restrictions and identify any underlying structural defects.
CCTV surveying provided visibility of asset condition, while misconnection checks and dye testing were undertaken to confirm network connectivity. The investigation identified extensive root growth within the upstream sewer, as well as substantial debris accumulation within the syphon and associated structures.
Access around the syphon hatch boxes presented an additional challenge. A wooden bridge was constructed to provide personnel and equipment with safe, stable access to each hatch box and the surrounding work areas.
The structure remained in place following completion of the project, providing continued access for future inspection and maintenance activities.
Remediation
To restore capacity, our teams deployed high-performance recycler units equipped with large remote reels and hoses.
Significant volumes of debris were removed from the syphon, while extensive root growth was cleared from the upstream network. This included approximately 300 metres of root obstructions within the 500-metre sewer between the combined sewer overflow and the syphon.
Following the cleaning programme, CCTV surveys confirmed the effectiveness of the works and identified a defect within the left-hand syphon barrel. A patch liner was subsequently installed to repair the affected section and improve its long-term structural integrity.
Impact and outcomes delivered
Flow data demonstrated a clear improvement in network performance, with reduced surcharge duration following rainfall, faster hydraulic recovery after storm events, and greater operational stability across the network.
Key outcomes included:
Reduced sustained surcharge behaviour following rainfall
Faster hydraulic recovery after storm events
Fewer spill events through restored flow capacity
Increased storage and network hydraulic performance
Improved visibility of asset condition through post-cleaning CCTV surveys
Enhanced long-term resilience following patch liner installation
Based on the volume of debris identified during the project, our specialists recommended introducing a 24-month inspection and cleaning cycle to help maintain performance, monitor asset condition, and reduce the risk of future blockages.
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