Metro Tunnelling Dewatering

Project Overview

A 12 km metro line extension beneath a major European capital, involving twin-bore tunnels with diameters of 6.2 m excavated by earth pressure balance TBMs. The project generated 450-600 m³/day of slurry water from TBM cutterhead lubrication, segment grouting, and groundwater inflow. Discharge to sewer was prohibited by the city's water authority, requiring full on-site treatment and recycle.

The Challenge

Tunnelling slurry presented a unique treatment challenge distinct from conventional wastewater:

• High suspended solids: 8,000-25,000 mg/L TSS from bentonite, rock flour, and grout residues
• High pH: Grout bleed water reaching pH 12.5, requiring neutralisation before advanced biological treatment
• Variable chemistry: Transition from clay to sand to chalk geology changed slurry characteristics weekly
• Space constraints: No room for permanent civil structures; treatment plant needed to move 3 times during project
• Zero discharge requirement: All water to be recycled as TBM makeup or dust suppression

The Solution

Reynolds & Bauhm proposed a fully containerised, relocatable Treatment Process comprising four 40-foot high-cube shipping containers:

Container 1: Screening & Grit Removal

• Coarse screen: 6 mm step screen for rock fragments and debris
• Desanding cyclone: Hydrocyclone separator removing sand >75 µm at 95% efficiency

Container 2: Equalisation & pH Correction

• 40 m³ balance tank: With submersible mixer and pH probe array
• Automated acid dosing: HCl dosing to maintain pH 7.0-8.5 for downstream processes

Container 3: DAF & Lamella Clarification

• QuickDAF unit: 50 m³/h capacity with integrated flocculation and flotation
• Lamella clarifier: Polishing stage reducing TSS to < 50 mg/L
• Chemical dosing: PAC and anionic polymer for colloidal clay removal

Container 4: Filtration & Disinfection

• Self-cleaning filter: 100 µm automatic backwash filter protecting TBM pumps
• UV disinfection: Medium-pressure UV for bacterial control (Legionella prevention)

Results

Project Impact

Compliance & Environmental

The project maintained full compliance with the city's groundwater protection ordinance and construction site discharge regulations. Third-party water quality auditing confirmed zero uncontrolled releases to the urban sewer network or surrounding groundwater aquifer. The project received the client's annual sustainability award for water stewardship.

Key Learning

Containerised treatment is not merely a space-saving expedient — it fundamentally changes project logistics. By eliminating the need for civil construction at each worksite, the treatment plant moves with the TBM advance, reducing both capital cost and environmental permitting burden. The QuickDAF container format has since been adopted as standard for all tunnelling projects in the client's portfolio.