Sensivity Test 5

Sensivity Test

5.8.2.16 A sensitivity test was conducted to investigate the change in water quality impacts as a result of San Wai STW reaching its maximum capacity of 264,000m3/s assuming the adoption of CEPT + disinfection. The calculated annual results are shown in Table 5.24 while the percentage difference with Option 2 are shown in Table 5.25.

Table 5.24 Annual Water Quality Results for Sensitivity Test

Indicator Point	DO (mg/L)	Bottom DO (mg/L)	TIN (mg/L)	UIA (mg/L)	SS (mg/L)	E.coli (count/100mL)	BOD5 (mg/L)
DM1	3.2	3.2	2.33	0.098	39.4	3380	6.08
DM2	3.7	3.7	1.71	0.057	29.6	1343	3.63
DM3	4.5	4.5	1.04	0.018	18.9	196	1.54
DM4	4.7	4.5	0.78	0.007	14.4	111	0.90
DM5	4.9	4.9	0.63	0.004	13.2	42	0.64
NM1	5.6	5.6	0.36	0.002	6.1	4	0.30
NM2	5.5	5.4	0.43	0.002	7.1	72	0.35
NM3	5.4	5.4	0.45	0.003	7.6	6	0.38
NM5	5.1	5.2	0.55	0.004	10.2	23	0.52
NM6	5.1	5.1	0.57	0.003	10.3	5	0.53
NM8	5.3	5.2	0.54	0.003	9.6	2	0.51
C1	5.7	5.7	0.36	0.002	6.0	4	0.29
D1	5.0	4.9	0.63	0.004	12.1	61	0.64
E1	6.2	6.1	0.31	0.002	6.0	6	0.41
E2	5.7	5.7	0.35	0.002	5.8	33	0.28
E3	5.7	5.6	0.35	0.002	5.8	115	0.29
F1	3.5	3.5	2.25	0.095	37.9	5218	5.74
F2	5.3	5.3	0.88	0.010	17.1	3573	1.37
G1	5.1	5.1	0.58	0.004	10.6	30	0.55
H1	5.1	5.0	0.58	0.004	10.7	9	0.54
A1	1.8	1.8	3.25	0.163	53.2	3210	9.92
B1	5.3	5.0	0.58	0.003	10.8	36	0.59
B2	5.2	5.1	0.59	0.004	10.0	207	0.62
D2	5.3	5.3	0.49	0.003	8.6	16	0.42
D3	6.3	5.7	0.38	0.002	8.4	258	0.77
D4	5.3	5.2	0.51	0.003	8.9	2	0.47
L1	5.7	5.3	0.41	0.002	7.7	205	0.46
J1	6.0	5.6	0.40	0.002	7.5	342	0.57
E4	5.7	5.6	0.35	0.002	5.9	49	0.29
E6	5.7	5.7	0.34	0.002	5.8	21	0.28
E7	5.7	5.7	0.35	0.002	5.9	24	0.28
E8	5.6	5.5	0.43	0.002	7.3	152	0.42
E11	5.8	5.5	0.42	0.002	7.7	157	0.52
E13	5.5	5.4	0.44	0.002	7.3	112	0.38
F3	5.1	5.0	0.61	0.004	11.6	11	0.58

Note: Values are calculated as average of wet and dry season results

Table 5.25 Differences in Water Quality Between Option 2 and Sensitivity Test

Indicator Point	DO	Bottom DO	TIN	UIA	SS	E.coli	BOD5
DM1	0%	0%	0%	0%	0%	0%	0%
DM2	0%	0%	0%	0%	0%	0%	0%
DM3	0%	0%	0%	0%	0%	0%	0%
DM4	0%	0%	0%	1%	0%	0%	0%
DM5	0%	0%	1%	4%	0%	2%	1%
NM1	0%	0%	0%	1%	0%	0%	0%
NM2	0%	0%	0%	2%	0%	0%	0%
NM3	0%	0%	1%	2%	0%	1%	1%
NM5	0%	0%	1%	4%	0%	2%	1%
NM6	0%	0%	1%	3%	0%	1%	1%
NM8	0%	0%	0%	2%	0%	0%	0%
C1	0%	0%	0%	1%	0%	0%	0%
D1	0%	0%	1%	3%	0%	1%	1%
E1	0%	0%	0%	1%	0%	0%	0%
E2	0%	0%	0%	1%	0%	0%	0%
E3	0%	0%	0%	1%	0%	0%	0%
F1	0%	0%	0%	0%	0%	0%	0%
F2	0%	0%	0%	1%	0%	0%	0%
G1	0%	0%	1%	4%	0%	1%	1%
H1	0%	0%	1%	3%	0%	1%	1%
A1	0%	0%	0%	0%	0%	0%	0%
B1	0%	0%	1%	3%	0%	0%	1%
B2	0%	0%	1%	3%	0%	0%	1%
D2	0%	0%	1%	3%	0%	0%	1%
D3	0%	0%	1%	1%	0%	0%	0%
D4	0%	0%	1%	2%	0%	0%	0%
L1	0%	0%	1%	2%	0%	0%	0%
J1	0%	0%	1%	2%	0%	0%	0%
E4	0%	0%	0%	1%	0%	0%	0%
E6	0%	0%	0%	1%	0%	0%	0%
E7	0%	0%	0%	1%	0%	0%	0%
E8	0%	0%	1%	2%	0%	0%	0%
E11	0%	0%	1%	2%	0%	0%	0%
E13	0%	0%	1%	2%	0%	0%	0%
F3	0%	0%	1%	3%	0%	1%	1%

Note: All percentages are calculated as (Annual Sensitivity Test – Annual Option 2)/Annual Option 2 x 100%

Values are presented to the nearest percent

5.8.2.17 As shown in Table 5.25, the water quality impacts due to San Wai STW reaching its capacity were insignificant. Some notable, but small, increases in UIA levels were predicted (1 to 4%) with the highest increase at Station NM5.

Emergency Discharge

5.8.2.18 Depending on the emergency event, treated or untreated sewage would be discharged via Tin Shui Wai Drainage Channel to the Deep Bay waters or directly to North Western waters. It is considered that decay of pollutants in Tin Shui Wai Drainage Channel would be a slow process. The length of the channel is relatively short and there is no major inflow of water to enhance the dilution of pollutants in the channel. It is expected that the pollutant concentrations at the point where the emergency discharge enter the upstream location of Tin Shui Wai Drainage Channel would not be much different from the conditions at the outlet of the channel. Therefore, the pollution loads were introduced at the channel outlet in the model set-up.

5.8.2.19 As discussed in Section 5.6.5.2, a worst scenario was considered to assume that the emergency discharge continued for a period of 12 days (emergency repair and maintenance works of the tunnel). During this period, it is likely that the water quality in the drainage channel would be significantly affected. The degree of impact is related to the quality of the discharged effluent and the duration of emergency discharge. The potential impacts on the drainage channel may include the reduction in light penetration into water for primary production, and damage to aquatic organisms. The increase in nutrient levels would deteriorate the water quality in the channel limiting the activities of aquatic organisms. However, it is important to note that there had not been any emergency discharges occurred for Ha Tsuen Pumping Station and San Wai STW to date. As such, the probability for the occurrence of the emergency discharges is anticipated to be low. Since the frequency for the occurrence of emergency discharge would be low and that the duration of the discharge is relatively short, there would be no long-term impact to the channel. Table 5.26 shows the hourly rate of pollutants discharging into the receiving waters as a result of emergency scenario.

5.8.2.20 Model results for different scenarios are presented in the following paragraphs. Descriptions of the scenarios were shown in Table 5.4.

Raw Sewage to Deep Bay (Scenario iii)

5.8.2.21 Figures 5.19, 5.20, 5.29 and 5.30 show the increases in E. coli at selected indicator points in Deep Bay during the emergency discharge period in the dry and wet seasons. Both the results for the normal operation scenario (Option 2) and raw sewage discharge scenario are presented in the figures.

5.8.2.22 Tsim Bei Tsui SSSI is an ecological sensitive area and is located nearest to the outlet of Tin Shui Wai Drainage Channel. The E. coli levels in this location increased dramatically during the emergency discharge period. The highest E. coli levels were almost 80x104 count/100mL in the dry season and 40x104 count/100mL in the wet season. The influence on water quality in terms of the increases in E. coli at DM1, DM2, DM3 and Pak Nai SSSI, which are located further away from the channel outlet, reduced with increasing distance.

5.8.2.23 Figures 5.23, 5.24, 5.33 and 5.34 show the TIN results and Figures 5.25, 5.26 5.35 and 5.36 show the UIA results for the wet and dry seasons. The highest TIN and UIA were recorded at Tsim Bei Tsui SSSI during wet and dry seasons. The predicted levels are 5.50 mg/L (TIN) and 0.46 mg/L (UIA) for wet season and 5.75 mg/L (TIN) and 0.20 mg/L (UIA) for dry season. Meanwhile, at DM1, the predicted highest TIN and UIA were 4.4 mg/L and 0.35 mg/L respectively in the wet season and 5.2 mg/L and 0.19 mg/L in dry season. For Tsim Bei Tsui SSSI and DM1, the dry season results were generally lower than those predicted for the wet season for UIA while the predicted levels are similar for TIN. The results recorded at DM2, DM3 and Pak Nai SSSI showed that the influence from the emergency discharge became less significant.

Table 5.26 Rate of Pollutant Discharging into Receiving Waters during Emergency Scenario

Emergency Scenario¹	Receiving Water Body	Discharge Option	The Worst Emergency Discharge Duration	Flow m³/hr	BOD g/hr	SS g/hr	Org-N g/hr	NH3-N g/hr	E.coli no./hr	Copper g/hr	TP g/hr	Ortho-P g/hr	Silicate g/hr	TON g/hr
(i) San Wai STW not functioning and no treatment of the incoming sewage	North Western Waters	Raw Sewage	12 days (See Note 2 below)	11,831	1,952,957	1,843,074	169,650	240,235	3.27E+15	325	55,891	36,280	106,483	26,029
(ii) NWNT Tunnel/Outfall not in operation	Deep Bay Waters	Preferred Option (i.e. Treatment Option 2)			896,103	599,221	97,310	217,756	1.80E+12	90	27,213	19,209	106,483	26,029
(iii) San Wai STW and NWNT Tunnel/Outfall not in operation together or breakdown of Ha Tsuen Pumping Station	Deep Bay Waters	Raw Sewage			1,952,957	1,843,074	169,650	240,235	3.27E+15	325	55,891	36,280	106,483	26,029

Note: 1 Please refer to Table 5.4 for emergency discharge scenario

2 Please refer to para. 5.6.5.2 and para. 5.8.2.25 for raw sewage discharge to Deep Bay

5.8.2.24 With reference to the WQOs for Deep Bay (TIN < 0.7 mg/L; NH3-N < 0.021 mg/L; E. coli 610 count/100mL), the predicted E. coli, TIN and UIA levels at Tsim Bei Tsui SSSI during the emergency discharge period were rather high. Elevations of E. coli, TIN and UIA levels were observed immediately after the occurrence of emergency discharge (July for wet season and February for dry season). The levels of these parameters, however, reduced rapidly after the termination of emergency discharge. About 5 to 8 days after the termination of emergency discharge, the TIN and UIA levels dropped to the levels almost the same as the baseline conditions. Meanwhile, the E. coli levels decayed more rapidly and returned to the baseline levels shortly after the termination of emergency discharge.

5.8.2.25 Although a very worst-case emergency discharge duration of 12 days was assumed in the assessment for scenario (iii), the estimated duration of emergency discharge under scenario (iii), if happened, is expected to be much shorter. Shut down of the STW and the NWNT tunnel together is even extremely remote. Standby pumps and back-up power in the form of dual power supply will be installed for the expanded Ha Tsuen Pumping Station, the chance of pumping station breakdown is also very remote. However, if pumping station breakdown due to power supply failure ever happened, it is expected that the power supply and hence the operation of the pumping station should be able to recover in hours as normally experienced in Hong Kong.

Emergency Discharge to Deep Bay - Option 2 (Scenario ii)

5.8.2.26 The raw sewage after the CEPT or CEPT with disinfection at San Wai STW reduces the pollution levels. Figures 5.21, 5.22, 5.31 and 5.32 present the changes in E. coli levels at selected indicator points during the emergency discharge period for Option 2 discharge scenario. The results for the scenario under normal operation are also included for comparison.

5.8.2.27 The Option 2 emergency discharge case did not show any significant deviations from the normal operation conditions. Both the dry and wet season results indicated that the E. coli levels would be similar to the normal operation conditions. Dilution and decay of E. coli in the seawater during the transport processes may result in small increases in E. coli at the monitoring points.

Raw Sewage Discharge to Urmston Road (Scenario i)

5.8.2.28 Figures 5.27, 5.28, 5.37 and 5.38 present the predicted E. coli, TIN and UIA levels at NM5 and the Chinese White Dolphin feeding ground in the Urmston Road Channel (G1) for the emergency discharge of raw sewage. Results for Option 2 were also included for comparison. The TIN and UIA levels for the emergency discharge case and Option 2 were similar without significant differences. However, the E. coli levels at these sensitive receivers were remarkably higher. High peaks recorded in the wet season were about 13.5x103 count/100mL at Urmston Road Channel and 9.6x103 count/100mL at NM5. Meanwhile, for dry season, the E.Coli levels were 6x103 count/100mL at Urmston Road Channel and 10x103 count/100mL at NM5.

5.8.2.29 In summary, the model results showed that the increases in E. coli, TIN and UIA would cause a short-term deterioration of the water quality conditions in the inner part of Deep Bay during the emergency discharge period. The impacts are apparently related to the duration of emergency discharge period. The water quality conditions would quickly recover after the termination of emergency discharge for both wet and dry seasons. The potential impacts to the water quality sensitive receivers located further away from the discharge location would be less significant. Meanwhile, as incorporated in the models, the tidal current speed at Urmston Road were high and that the pollutants in that region would easily be dispersed by the fast moving tidal currents leading to a better flushing effect. Therefore, the emergency discharge at Urmston Road would cause lower water quality impacts when compared to the discharge through Tin Shui Wai Drainage Channel. Meanwhile, within Tin Shui Wai Drainage Channel, the water quality impacts would be short-term. Furthermore, since the ecological values of Tin Shui Wai Drainage Channels are low, the water quality and ecological impacts would be limited.

5.8.2.30 Although as discussed previously, the probability of occurrence of the emergency discharges is low, the following precaution measures are recommended to further minimise the occurrence of the emergency discharges:
· Regular maintenance and checking of plant equipment to prevent equipment failure;
· Use standby pump in case of pump failure or maintenance required and;
· Use dual power supply to keep the pump in operation in case of main power failure.

5.8.2.31 A contingency plan should be developed at the detailed design stage to deal with the emergency discharges that may occur during the operational stage of the project. It is recommended to include the following items in the contingency plan:

· Locations of the sensitive receivers in the vicinity of the emergency discharges at Deep Bay and Urmston Road;
· A list of relevant government bodies to be informed and to provide assistance in the event of emergency discharges. Information on key contact persons and telephone numbers should be included;
· Reporting procedures required in the event of emergency discharges; and
· Procedures listing the most effective means in rectifying the breakdown of San Wai STW, Urmston Road Tunnel or Ha Tsuen Pumping Station in order to minimise the discharge duration.