The treated wastewater shall be discharged into the stormwater drain via the catch-pit located adjacent to the tunnel portal . The quantity and composition of the treated wastewater from the premises shall not exceed the following discharge limits specified by the Environmental Protection Department ( EPD ):
● Maximum flow rate is 30 m 3 / day .
● Maximum concentration of suspended solids is 30 mg / L .
● Maximum concentration of Chemical Oxygen Demand is 80 mg / L .
● The range of pH levels is 6 to 9 units .
( b ) General Construction Site Activities On-site construction activities may cause water pollution from the following :
● Uncontrolled discharge of debris and rubbish such as packaging , construction materials , refuse etc ; and
● Spillages of liquid stored on-site , such as oil , diesel , solvents etc .
( c ) Sewage Effluent Domestic sewage would be generated from the workforce during the construction phase . Portable chemical toilets will be provided for the site workers . The Contractor will have the responsibility to ensure that chemical toilets are used and properly maintained and that licensed contractors are employed to collect and dispose of the waste off-site at approved locations . Therefore , no adverse water quality impacts are anticipated .
Mitigation Measures
Construction site runoff and drainage should be prevented or minimised in accordance with the guidelines . The specified mitigation measures and practices include the following :
● Provision of perimeter drains to intercept storm runoff from outside the works area . These shall be constructed in advance of site formation works and earthworks . Earth bunds or sandbag barriers should be provided on-site to direct stormwater to silt removal facilities . The design of the temporary on-site drainage system will be undertaken by the Contractor before the commencement of construction .
● Sand / silt removal facilities such as sand / silt traps and sediment basins should be provided to remove sand / silt particles from runoff to meet the requirements of the Technical Memorandum standard under the Water Pollution Control Ordinance . The design of efficient silt removal facilities should be based on the guidelines . The retention time for silt / sand traps should be five minutes under maximum flow conditions . The detailed design of the sand / silt traps will be undertaken by the Contractor before the commencement of construction .
● Air will be used as the flushing medium of the drilling equipment to avoid groundwater being affected by the flushing medium . In addition , a permanent casing may be provided to the drill hole of the soil nail within the permeable colluvium layer as instructed by the Engineer to minimise the impact on the groundwater table situated at the permeable soil stratum .
● An outlet pipe extending above the slope surface would be installed to facilitate the collection of discharge of air , water and grout from the drill hole inserted with soil nail during grouting .
● All drainage facilities and erosion and sediment control structures should be regularly inspected and maintained to ensure proper and efficient operation at all times , particularly during rainstorms . Deposited silt and grit should be regularly removed at the onset of and after each rainstorm to ensure that these facilities are functioning properly at all times .
● Exposed slope / soil surface should be covered by tarpaulin as soon as possible to reduce the potential of soil erosion . Arrangements should always be in place to ensure that adequate surface protection measures can be safely carried out well before the arrival of a rainstorm .
● Open stockpiles of construction materials or construction wastes on-site of more than 50m 3 should be covered with tarpaulin or similar fabric during rainstorms .
79