A calculation was conducted to demonstrate that the treated effluent discharge from the submarine outfall would not induce a water quality impact, in terms of TIN level, worse than the conditions without the implementation of the proposed project.
In order to determine the TIN concentration in the Picnic Bay contributed from the discharges of untreated or partially treated sewage, the following assumptions were adopted:
(a) Picnic Bay has a volume of about 7.5 million cubic metres and takes 5 days for pollutants discharged to the head of the bay to escape into the East Lamma Channel according to Outlying Island SMP Final Report
(b) A total of around 24 kg/d TIN loading from the Sok Kwu Wan area would be collected and be transferred to the proposed STW for treatment
Maximum TIN loading from the untreated or partially treated sewage accumulated within the bay = 24 kg/d x 5
= 120 kg
TIN concentration from the untreated or partially treated sewage within the bay
= 120 x 1000 / 7500000 mg/L
= 0.016 mg/L …………………………………………………….(1)
TIN Level Contributed from the Submarine Outfall Discharge
Based on the modelling results, the TIN levels contributed from the submarine outfall discharge during various conditions are shown in Table 1 below.
Table 1 TIN Levels Contributed from the Submarine Outfall Discharge During Various Conditions
|
Parameters |
TIN Concentration, mg/L |
|||||
|
Wet Season Without Stratification |
Wet Season With Stratification |
Dry Season |
||||
|
0.1 m/s |
0.5 m/s |
0.1 m/s |
0.5 m/s |
0.05 m/s |
0.5 m/s |
|
|
Background [1] |
0.082 |
0.082 |
0.082 |
0.082 |
0.229 |
0.229 |
Predicted Overall TIN Levels1 |
||||||
|
At Gazetted FCZ [2] |
0.090 |
0.084 |
0.107 |
0.087 |
0.242 |
0.231 |
|
At Secondary Contact Recreation Subzone at Mo Tat Wan [3] |
0.089 |
0.084 |
0.102 |
0.086 |
0.241 |
0.231 |
|
TIN Levels Contributed from the Submarine Outfall Discharge Only1 |
||||||
|
At Gazetted FCZ [2] – [1] |
0.008 |
0.002 |
0.025 |
0.005 |
0.013 |
0.002 |
|
At Secondary Contact Recreation Subzone at Mo Tat Wan [3] –[1] |
0.007 |
0.002 |
0.020 |
0.004 |
0.012 |
0.002 |
Note:
1 values have included background levels
2 values have excluded background levels
Annual Average TIN Levels
To calculate the annual average of TIN levels, the following assumptions were made:
· Wet season: from April to October (five months without stratification and two months with stratification)
· Dry season: from November to March (five months).
Based on the hydrographic survey, the background TIN levels were 0.082 mg/l for wet season (both unstratified and stratified conditions) and 0.229 for dry season.
Annual average background TIN level = (0.082 x 7 + 0.229 x 5) / 12 mg/l
= 0.143 mg/L ……………………………………(2)
TIN Level with the Exclusion of TIN Level Contributed from the Discharges of Untreated or Partially Treated Sewage
Based the results in Table 1, (1) and (2), annual average TIN levels with the exclusion of TIN contributed from the discharges of untreated or partially treated sewage have been calculated and are shown in Table 2.
Table 2 Comparison of Predicted Annual Average TIN Levels with Annual Average Background TIN Level at Sensitive Receivers
|
Sensitive Receivers |
Background TIN Level in Annual Average, mg/l |
Background TIN Level Excluded Untreated Discharges in Annual Average1, mg/l |
TIN Contribution from Submarine Outfall Discharge in Annual Average, mg/l |
Predicted Annual Average Concentration of TIN2, mg/L |
||
|
Low Current Speed |
High Current Speed |
Low Current Speed |
High Current Speed |
|||
|
Gazetted FCZ |
0.143 |
0.127 |
0.013 |
0.003 |
0.140 |
0.130 |
|
Secondary Contact Recreation Subzone at Mo Tat Wan |
0.011 |
0.002 |
0.138 |
0.129 |
||
Note:
1 Value equal to (2) minus (1)
2 Values have been included the background levels but excluded the TIN contribution from the discharges of untreated or partially treated sewage.
As indicated in Table 2, the annual average TIN levels predicted at the sensitive receivers for both low and high current speeds would not be higher than the annual average background level. Hence, it was concluded that the submarine outfall discharge would not induce water quality impact, in terms of TIN level worse than the conditions without the implementation of the Project.