A sludge separation plant for a project using a water shield It is sized based on the excavation diameter, the advance rate, the volume of excavated material, the circulation slurry flow rate, the soil particle size distribution, the fines content, the target slurry density, the pumping distance, the required treatment capacity, and environmental limits for disposal, reuse, or waste management. Its function is to separate solids from the excavation fluid to maintain a stable slurry circuit, control face pressure, and enable continuous and safe excavation.
In projects of pipe ramming, microtunneling y crossings over rivers or waterways, the hydro-shield for water-logged terrain It uses a slurry circuit to stabilize the cutting face, transport the excavated material, and control hydraulic pressure. Therefore, the screening plant must be designed as part of the excavation system, not as a standalone auxiliary piece of equipment.
What the separation plant must achieve
The plant must meet four main objectives:
| Objective | What this means on-site |
|---|---|
| Separating solids from sludge | Remove sand, gravel, silt, and fine particles from the excavation area |
| Maintain fluid properties | Monitor density, viscosity, solids content, and flowability |
| Maintain production | Handle the required flow rate without causing the tunnel boring machine to stop |
| Waste and Water Management | Comply with environmental requirements, as well as those related to collection, transportation, disposal, or reuse |
If the plant does not have sufficient capacity, the sludge may become saturated with solids, lose its transport capacity, increase in density, cause blockages, reduce performance, or compromise front control.
Basic Design Parameters
To determine the plant's capacity, the following factors are analyzed:
1. Diameter and Cross-Section of the Excavation
The diameter of the tunnel boring machine determines the volume excavated per meter advanced. The larger the diameter, the greater the volume of material produced and the greater the need for separation.
2. Expected feed rate
The feed rate determines the flow rate of solids entering the circuit. It is not enough to calculate the average output; one must also consider peak feed rates, changes in terrain, and periods of high output.
3. Soil particle size distribution
The type of soil determines the separation stages:
- Gravel and coarse sand: screening and desanding.
- Fine sand: hydrocyclones or fine separation.
- Sludge and clay: settling, flocculation, centrifugation, or specific treatment.
- Mixed-media systems: a combination of screens, cyclones, and clarification equipment.
4. Fine Particle Content
Fine particles are problematic because they can remain in suspension, increase density and viscosity, hinder recirculation, and raise operating costs.
5. Slurry Flow Rate
The plant must treat the flow circulating between the tunnel boring machine and the surface. This flow depends on the diameter, distance, head loss, pumping capacity, face pressure, and volume of material transported.
6. Pumping Distance and Head
The length of the circuit, the depth of the vertical wells, Head and pressure drops affect pumps, piping, pressure, flow, and operational continuity.
7. Environmental Requirements
The management of excavated sludge must take into account storage sites, dewatering, transportation, waste characterization, disposal, water reuse, available land, and permits.
Typical stages of a separation plant
| Stage | Function | Material to be removed |
|---|---|---|
| Primary screening | To retain gravel, small pebbles, or coarse material | Coarse solids |
| Desanding | Separating sand using hydrocyclones or other equipment | Medium and fine sand |
| Unclassified | Reduce sludge and fine particles | Suspended solids |
| Decanting or clarification | Separating solids by sedimentation or chemical treatment | Silt and fine particles |
| Flocculation | Aggregate fine particles to facilitate separation | Clays and colloidal fines |
| Centrifugation or filter press | Dehydrating sludge and reducing waste volume | Concentrated sludge |
| Fluid Adjustment | Adjust the density, viscosity, or properties of the reused slurry | Recirculation fluid |
Not all projects require all of these stages. The configuration depends on the terrain, the hydroshield system, the available space, and the slurry management requirements.
How Does the Soil Affect the Plant?
Sand and Gravel
They typically require screens, desanders, and hydrocyclones with sufficient capacity to separate coarse and medium-sized particles. The main risk is overloading the plant or causing wear on pumps, pipes, and cyclones.
Limos
Silt can remain in suspension and increase turbidity. It may require settling, flocculation, or fine separation to keep the sludge within operating parameters.
Clays
Clays are more complex because they can disperse, increase viscosity, and make separation more difficult. In these cases, chemical treatment, flocculation, centrifugation, or filter press processing may be necessary.
Mixed-use properties
In soil consisting of alternating layers of gravel, sand, silt, clay, or crushed rock, the machine must be flexible and able to accommodate variations in particle size, density, and flow rate without stopping the tunnel boring machine.
Rock or abrasive materials
Crushed rock and abrasive materials can increase wear on pumps, pipes, screens, and hydrocyclones. Wear resistance and scheduled maintenance should be reviewed.
Relationship Between the Plant, the Hydroshield, and Front Control
The separation plant directly affects the performance of the hydroscreen. If the recirculated slurry does not maintain adequate density, viscosity, and solids content, it can affect the pressure at the cutting face, the material transport capacity, and the stability of the excavation.
For this reason, the following are monitored during construction:
- Supply and return flow rates.
- Slurry density.
- Viscosity.
- Solids content.
- Particle size distribution of separated solids.
- System pressure.
- Pressure drops.
- Performance of screens and hydrocyclones.
- Excavated volume versus processed volume.
- Water consumption, bentonite, or additives.
- Volume of waste generated.
- Quality of clarified water, if applicable.
- Incidents involving blockages, wear, or saturation.
Risks of an Undersized Plant
An improperly sized plant can cause:
- Sludge saturation with solids.
- Loss of transport capacity.
- Increase in density or viscosity.
- Circuit blockages.
- Tunnel-boring machine stops.
- Lower feed rate.
- Accelerated wear and tear on pumps and pipes.
- Difficulty maintaining forward pressure.
- Excessive consumption of water or additives.
- Excess liquid waste.
- Problems with collection, transportation, or disposal.
- Cost overruns and delays.
At submarine emissaries, sea water catchments or crossings under watercourses; these risks may also affect environmental permits, turbidity control, and process water management.
Information Required for Design or RFQ
To design a slurry separation plant for a water shield, the following information should be provided: excavation diameter, driving length, well depth, expected advance rate, volume excavated per meter, circulation flow rate, pumping distance, head, working pressure, geotechnical data, particle size distribution, percentage of fines, plasticity, abrasiveness, water table, permeability, slurry type, target density and viscosity, available space at the job site, water supply, power supply, storage system, reuse criteria, disposal limits, need for flocculation, centrifugation, or filter press, waste management, environmental permits, and quality documentation requirements.
Request a Technical inspection of a sludge separation plant for a hydro-shield before finalizing the excavation method, the construction plan, or the RFQ documentation.
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