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Outline of Japan-Korea Tunnel Plan-Environmental Assessment (Environmental Impact Assessment)-

Wastewater treatment in mountain tunnel construction

Surrounding environment measures / wastewater treatment

In mountain tunnel construction, in the spring water from the face and construction water generated by tunnel excavation, drilling, grouting, fine-grained soil and concrete placement by grout transportation, cement by chemical injection, injection material, or various types Oil leaking from underground work machinery is often mixed in and discharged as turbid water.The amount and quality of this muddy water will differ depending on the scale, extension, excavation method, geology, amount of underground spring water, etc. of the tunnel.Since tunnel construction requires a relatively long construction period, if turbid water is discharged directly into rivers, lakes, agricultural waterways, etc., water pollution will occur, causing damage to the living environment and natural environment.
 

In general, SS (suspended solids), pH (hydrogen ion concentration) and oil content are problems in environmental protection in turbid water of tunnel construction.Depending on the construction, the regulated value of treated water may ask for biochemical oxygen demand (BOD), chemical oxygen demand (COD), chromium content, etc.In addition, spring water containing heavy metals may be a problem.When excavating a place where industrial wastewater has flowed into the ground or a ground containing heavy metals, it is necessary to separately consider how to deal with it.
 

When planning wastewater treatment, it is necessary to set the expected amount of wastewater and understand the wastewater standard.

① Expected amount of drainage

Raw water for turbid water in tunnel construction is divided into underground spring water and construction water.Of these, the amount of spring water in the mine is often significantly different from the forecast, and it can be said that it is difficult to accurately predict the amount of raw water at the time of planning.For this reason, a method is also adopted in which a minimum amount of turbid water treatment equipment is installed at the start of tunnel excavation, and equipment is added in accordance with the increase in the amount of spring water as the excavation progresses.In addition, when the amount of brave water is large, if there is no problem with the components of the spring water itself, the fresh water and turbid water are separated, and the fresh water that does not contain fine-grained soil or cement is directly discharged by another system, a so-called turbid separation method. Is often adopted.

② Drainage standard

Before the start of construction, the source of polluted water and its influence, the degree, related laws and regulations, regulation values, etc. must be investigated, and appropriate purification treatment must be carried out to drain the water.In addition, it is necessary to measure and report the amount of discharged water, water quality, etc. in accordance with the regulations.

③ Wastewater treatment

In general, the points to keep in mind and countermeasures for the treatment of SS, pH, and oil, which are problems as water pollution sources, will be described.

 

[SS (Suspended solids)]
The particle size composition of soil particles contained in the drainage during tunnel construction differs depending on the type and properties of the rock, the sliding method, the drainage method, etc., but the proportion of fine particles such as silt and clay is often high. ..Since the sedimentation velocity of these particles is very low and it takes a long time to settle, physical and chemical treatments are applied to aggregate the particles and flock (= large particles generated by agglomeration action, fluffy suspended matter). The method of forming an aggregate of) and accelerating the sedimentation is taken.The flocculants used for floc formation include inorganic flocculants and polymer flocculants.PAC (polyaluminum chloride) is mainly used as an inorganic flocculant for turbid water treatment of tunnels.Polymer condensing agents are sold by many manufacturers under their own brand names, and although there are many types of products, most of them are anionic.These inorganic flocculants and polymer flocculants are often used in combination with the expectation of a synergistic effect.

 

[PH (Hydrogen ion concentration: Potential of hydrogen)]
The pH of spring water generated by tunnel construction is generally about 6.5 to 7.5, but the pH is often 9 to 13 due to the mixing of sprayed concrete and chemical injection agents.In addition, depending on the rock quality and soil quality of the ground, the spring water itself may be strongly acidic or alkaline.When draining to public water areas, the pH must be adjusted to the range of the drainage standard value. The pH adjustment may be performed before or after the sedimentation separation, and the former has been widely adopted recently because the aggregation treatment can be performed in the optimum pH range, but the latter is a neutralization reaction in turbid water. A large amount of substances that inhibit the pH is mixed in, and the amount of the neutralizing agent used is slightly larger than that of post-neutralization.Carbon dioxide is generally used for alkaline wastewater, but dilute sulfuric acid may be used temporarily when the pH value is extremely high.For acidic wastewater, slaked lime or caustic soda is used.

 

[Oil (normal hexane extract)]
If oil and fat leaking from tunnel excavation machines (rock drills, rock excavators, locomotives, rails, shear carriers), etc. are mixed into the spring water, the concentration may reach 10 to 15 ppm.Oil content is a problem mainly due to the generation of offensive odors and the odor of seafood, which may cause odor even at low concentrations, so it is necessary to consider removing it as much as possible.When oil is mixed in the raw water, a polyethylene-based cloth-like adsorbent or the like is used in the raw water tank.
 
(Reference material: "From survey / design to construction of mountain tunnel construction method", Japanese Geotechnical Society)

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