5

5.8.2.9 The difference between Option 4 and Option 3 is that the former includes nitrogen removal in the treatment processes. Figures 5.11 and 5.12 present graphically the absolute differences in UIA and TIN between Option 4 and Option 3. The absolute differences for UIA and TIN were minimal and the areas of increase were small.

5.8.2.10 The predicted annual water quality results for Option 4 are summarised in Table 5.22. The percentage differences in UIA and TIN levels of Option 4 relative to Option 3 are shown in Table 5.23. With the nitrogen removal, the results for Option 4 induced lower UIA and TIN levels at the nearest indicator points DM5, NM3, NM5, Chinese White Dolphin feeding ground in the Urmston Road Channel (G1) and marine park at Sha Chau/Lung Kwu Chau (H1) when compared to the results for Option 3. The differences at these sensitive receivers were small (TIN: 0.01 mg/L; UIA: 0.0004mg/L). The time series plots for TIN and UIA during wet and dry seasons at the selected indicator points are shown in Figures 5.15a to 5.18b. Although some notable reductions in UIA were observed by adopting nitrogen removal (Option 4), it is considered that the predicted TIN and UIA levels were similar between the two options. No abrupt changes in TIN and UIA were observed from the time series plots.

Table 5.20 Wet Season Water Quality Results for Option 4

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	2.3	2.2	2.15	0.125	41.1	1506	5.38
DM2	2.6	2.6	1.66	0.072	31.8	560	2.90
DM3	3.4	3.4	1.11	0.021	20.9	90	1.00
DM4	3.6	3.3	0.90	0.007	16.6	63	0.58
DM5	3.9	3.8	0.78	0.003	17.5	27	0.47
NM1	4.9	4.9	0.52	0.002	7.3	3	0.24
NM2	4.6	4.6	0.61	0.002	8.7	43	0.27
NM3	4.6	4.6	0.62	0.002	9.3	3	0.28
NM5	4.2	4.2	0.72	0.003	12.8	15	0.37
NM6	4.1	4.0	0.75	0.003	12.7	4	0.37
NM8	4.2	4.1	0.73	0.003	11.5	2	0.33
C1	5.0	5.0	0.51	0.002	7.1	2	0.24
D1	3.9	3.9	0.79	0.004	15.2	47	0.45
E1	5.1	5.0	0.48	0.002	7.0	32	0.26
E2	5.0	5.0	0.50	0.002	6.8	23	0.24
E3	5.0	4.9	0.50	0.002	6.9	92	0.24
F1	2.3	2.3	2.16	0.128	40.9	2708	5.28
F2	4.5	4.4	0.87	0.008	17.7	2824	0.69
G1	4.1	4.1	0.74	0.003	13.3	21	0.39
H1	4.1	4.0	0.75	0.003	13.2	6	0.38
A1	0.6	0.5	3.22	0.222	60.9	2325	11.31
B1	4.4	4.1	0.75	0.003	12.8	25	0.38
B2	4.2	4.1	0.77	0.004	11.8	101	0.48
D2	4.4	4.5	0.66	0.003	10.6	8	0.30
D3	5.4	4.7	0.57	0.002	9.2	147	0.54
D4	4.3	4.3	0.69	0.003	10.7	1	0.31
L1	5.0	4.6	0.60	0.002	8.6	249	0.34
J1	5.1	4.6	0.59	0.002	8.8	196	0.43
E4	5.0	4.9	0.51	0.002	6.9	31	0.24
E6	5.0	5.0	0.49	0.002	6.8	15	0.24
E7	5.0	5.0	0.50	0.002	7.0	20	0.24
E8	4.8	4.6	0.62	0.002	8.5	138	0.31
E11	5.0	4.7	0.62	0.002	8.3	149	0.33
E13	4.6	4.6	0.63	0.002	8.9	71	0.30
F3	4.0	3.9	0.77	0.003	14.6	7	0.41

Table 5.21 Dry Season Water Quality Results for Option 4

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	4.1	4.1	2.50	0.073	37.8	7629	6.82
DM2	4.8	4.8	1.75	0.041	27.7	3231	4.38
DM3	5.6	5.6	0.96	0.015	17.1	431	2.09
DM4	5.9	5.7	0.64	0.007	12.2	196	1.21
DM5	6.1	6.1	0.45	0.004	8.9	57	0.77
NM1	6.4	6.4	0.20	0.002	5.0	6	0.35
NM2	6.3	6.3	0.23	0.002	5.6	122	0.42
NM3	6.3	6.2	0.27	0.003	6.0	9	0.46
NM5	6.2	6.1	0.37	0.003	7.6	31	0.63
NM6	6.3	6.2	0.37	0.003	7.9	7	0.67
NM8	6.4	6.4	0.33	0.003	7.7	2	0.68
C1	6.4	6.4	0.20	0.002	4.9	6	0.34
D1	6.1	6.0	0.46	0.004	9.1	78	0.80
E1	7.4	7.3	0.14	0.002	5.0	1	0.56
E2	6.4	6.3	0.19	0.002	4.8	49	0.32
E3	6.4	6.3	0.19	0.002	4.8	143	0.33
F1	4.6	4.6	2.34	0.063	35.1	10090	6.23
F2	6.2	6.2	0.88	0.012	16.6	4522	2.04
G1	6.1	6.1	0.39	0.003	7.9	37	0.66
H1	6.2	6.1	0.39	0.003	8.1	12	0.68
A1	3.1	3.1	3.28	0.104	45.6	4461	8.56
B1	6.2	6.0	0.39	0.003	8.8	53	0.77
B2	6.2	6.1	0.39	0.003	8.2	422	0.73
D2	6.2	6.2	0.30	0.003	6.5	31	0.52
D3	7.3	6.8	0.17	0.002	7.5	452	0.99
D4	6.3	6.3	0.32	0.003	7.0	2	0.61
L1	6.4	6.2	0.21	0.002	6.7	170	0.58
J1	7.0	6.6	0.20	0.002	6.2	597	0.71
E4	6.4	6.3	0.19	0.002	4.8	78	0.34
E6	6.4	6.3	0.19	0.002	4.8	29	0.32
E7	6.4	6.3	0.19	0.002	4.8	28	0.32
E8	6.5	6.4	0.22	0.002	6.0	167	0.52
E11	6.7	6.3	0.20	0.002	7.0	166	0.70
E13	6.4	6.3	0.24	0.002	5.7	176	0.45
F3	6.2	6.1	0.42	0.003	8.5	16	0.72

Table 5.22 Annual Water Quality Results for Option 4

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	0.0988	39.5	3390	6.10
DM2	3.7	3.7	1.71	0.0569	29.7	1345	3.64
DM3	4.5	4.5	1.03	0.0179	19.0	197	1.54
DM4	4.7	4.5	0.77	0.0070	14.4	111	0.89
DM5	5.0	4.9	0.62	0.0037	13.2	40	0.62
NM1	5.7	5.6	0.36	0.0020	6.1	4	0.30
NM2	5.5	5.4	0.42	0.0022	7.1	72	0.34
NM3	5.4	5.4	0.45	0.0025	7.6	6	0.37
NM5	5.2	5.2	0.54	0.0033	10.2	22	0.50
NM6	5.2	5.1	0.56	0.0029	10.3	5	0.52
NM8	5.3	5.2	0.53	0.0026	9.6	2	0.51
C1	5.7	5.7	0.35	0.0019	6.0	4	0.29
D1	5.0	5.0	0.62	0.0040	12.1	60	0.62
E1	6.2	6.1	0.31	0.0016	6.0	6	0.41
E2	5.7	5.7	0.34	0.0019	5.8	33	0.28
E3	5.7	5.6	0.35	0.0019	5.8	115	0.29
F1	3.5	3.5	2.25	0.0957	38.0	5227	5.76
F2	5.3	5.3	0.87	0.0100	17.2	3574	1.36
G1	5.1	5.1	0.56	0.0035	10.6	28	0.53
H1	5.1	5.1	0.57	0.0032	10.6	9	0.53
A1	1.9	1.8	3.25	0.1630	53.3	3221	9.94
B1	5.3	5.1	0.57	0.003	10.8	36	0.58
B2	5.2	5.1	0.58	0.004	10.0	206	0.61
D2	5.3	5.3	0.48	0.003	8.6	16	0.41
D3	6.3	5.8	0.37	0.002	8.4	257	0.76
D4	5.3	5.3	0.50	0.003	8.9	2	0.46
L1	5.7	5.4	0.41	0.002	7.6	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	5.7	5.5	0.42	0.002	7.3	152	0.41
E11	5.8	5.5	0.41	0.002	7.7	157	0.52
E13	5.5	5.4	0.43	0.002	7.3	112	0.38
F3	5.1	5.0	0.60	0.003	11.5	11	0.57

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

E.Coli values are calculated as geometric mean

Table 5.23 Differences in TIN and UIA Levels Between Option 3 and Option 4

Indicator Point	TIN	UIA
DM1	0%	0%
DM2	0%	0%
DM3	-1%	-1%
DM4	-1%	-3%
DM5	-1%	-9%
NM1	-1%	-2%
NM2	-1%	-4%
NM3	-1%	-6%
NM5	-2%	-10%
NM6	-1%	-7%
NM8	-1%	-4%
C1	-1%	-2%
D1	-1%	-7%
E1	-1%	-1%
E2	0%	-1%
E3	-1%	-1%
F1	0%	0%
F2	-1%	-1%
G1	-2%	-10%
H1	-1%	-8%
A1	0%	0%
B1	-1%	-7%
B2	-1%	-7%
D2	-1%	-7%
D3	-1%	-3%
D4	-1%	-5%
L1	-1%	-4%
J1	-1%	-3%
E4	-1%	-1%
E6	0%	-1%
E7	-1%	-1%
E8	-1%	-4%
E11	-1%	-4%
E13	-1%	-5%
F3	-1%	-7%

Note: All percentages are calculated as (Option 4 – Option 3)/Option 3 x 100%

Values are presented to the nearest percent

5.8.2.11 In summary, it is predicted that all four options during normal operation would significantly improve the water quality in inner Deep Bay when compared to baseline scenario. In the region, significant reductions were predicted for mean depth-averaged TIN, UIA, SS, E.Coli and BOD5 while there were increases in bottom and depth-averaged DO. However, Option 1 would have effect on the E. coli levels in the receiving water and the predicted E. coli levels at the indicator points near to the discharge location at Urmston Road were considerably higher than baseline and the other 3 options with disinfection.

5.8.2.12 Based on the model results, significant reductions in E. coli levels were predicted for Option 2 (CEPT + disinfection) when compared with both baseline and Option 1. The predicted E.Coli levels at the sensitive receivers near the Urmston Road Outfall are well below the relevant WQOs. Except for the differences in E. coli levels, the model results between Option 2 and Option 1 were basically the same. Option 2 is preferable to Option 1 as disinfection of effluent would minimise the impact to the aquatic environment in terms of spreading of faecal coliform.
5.8.2.13 The secondary treatment for Option 3 has a higher removal efficiency of BOD5 when compared to the CEPT method for Option 2. However, as discussed in Section 5.8.2.8, the differences in BOD5 levels at the indicator points nearest to the discharge location between the two options were small with absolute differences of less than 0.03 mg/L. Furthermore, as discussed in Section 5.8.2.7, the absolute BOD5 differences between Option 2 and Option 3 were only observed in a very localised area. Therefore, it is predicted that the adoption of secondary treatment in terms of water quality impacts may not have a significant advantage over Option 2.

5.8.2.14 The comparison between Option 3 and Option 4 indicated that there would be slight reduction in UIA and TIN levels in the receiving water body for adoption of Option 4 for sewage treatment. Based on the results recorded at the indicator points nearest to the discharge location, the improvement of water quality as a result of reduction in UIA and TIN levels was rather small.
5.8.2.15 From a water quality perspective, the higher level of treatment should improve the water quality in the vicinity of Urmston Road Outfall and thus, the preferred option would be Option 4, followed by Option 3, 2 and 1. However, as discussed above, in comparison with the four treatment options, the predicted water quality from Option 2 in North Western and Deep Bay regions would not significantly be lower than Option 3 and Option 4. Indeed, in the view of small differences in TIN and UIA levels between Option 2/3 and Option 4 in the vicinity of the Outfall, treatments without nitrification would not contribute to the non-compliance to relevant WQOs in North Western and Deep Bay WCZs. Moreover, as shown in Table 5.14, the high DO levels predicted in Option 2 and the small absolute differences in DO levels between Option 2 and 3 in Table 5.19 indicates that biological treatment is not necessary to meet the relevant WQOs. It is predicted that no adverse water quality impacts were observed based on the model predictions for Option 2. In addition, as disinfection is included in Option 2, it has an advantage over Option 1 to significantly reduce the E. coli levels in the receiving water body. Disinfection is important to satisfy the precautionary requirements for the protection of dolphins. Provision of disinfection also helps to protect bathing water quality at recreational areas in the vicinity.