8 SEWERAGE AND SEWAGE TREATMENT IMPLICATIONS

8.1 Introduction

8.1.1 Pursuant to the Environmental Impact Assessment Ordinance, an Environmental Impact Assessment (EIA) of the Engineering Feasibility Study for the Comprehensive Development at Yau Tong Bay has been carried out to demonstrate that the nature and extent of environmental impacts arising from the proposed development are not insurmountable.

8.1.1 As part of the Engineering Feasibility Study for the EIA, the impact on Sewerage and Sewage Treatment Implications is one of the issues to be dealt with. The Sewerage Impact Assessment report (SIA) was prepared to assess the sewerage impacts on the existing sewerage infrastructure in East Kowloon, in particular at Yau Tong area, resulting from Yau Tong Bay Development (YTBD) where the proposed reclamation will take place. The location of the proposed development is shown on Figure 8.1.

8.1.2 The SIA has been carried out following the review on Engineering Impact Assessment of the Request for Rezoning Yau Tong Bay Comprehensive Development Area - Further Submission dated August 1998. The conceptual layout plan adopted in the Request for Rezoning Yau Tong Bay Comprehensive Development Area submission has been used as a base layout to develop the four options for Western Coastal Road (WCR) alignment and Industrial / Residential (I/R) interface as described in Section 2. The SIA has been carried out based on the Master Layout Plan (Full Reclamation without I/R interface) hereinafter called “The Conceptual Plan”, being the worst case scenario amongst the four options. The conceptual layout plan for YTBD is shown in Figure 8.2.

8.1.3 The proposed “Refined Schemes” for both Minimised and Full Reclamation are shown in Figures 2.5a and 2.5b respectively, no additional adverse impact is envisaged if the ‘Refined Scheme’ is adopted. The details of the internal sewerage layout for YTBD may need to be revised at the detailed design stage to suit the layout of the ‘Refined Scheme’.

8.1.4 The SIA has investigated and reviewed the adequacy of the existing sewerage and sewage treatment facilities to handle the sewage generated from the proposed development and any additional sewage flows from other planned/committed developments. It also recommends appropriate mitigation measures within the site area and/or at surrounding area where adverse impacts are identified. The SIA has made reference to the findings of EPD’s Review of Central and East Kowloon Sewerage Master Plans, Draft Final Report, April 2001 (Ref./1/).

Project Description

8.1.5 The reclaimed YTBD is located in the southern part of East Kowloon on the waterfront between Kwun Tong and Lei Yue Mun. It is surrounded by Cha Kwo Ling Road and Ko Fai Road. The Eastern Cross Harbour Tunnel is located to the north of the development site and to the south of the development site lies the Yau Tong Industrial Area. The location plan for the proposed development is shown in Figure 8.1.

8.1.6 Yau Tong Bay is proposed to reclaim to form land for Yau Tong Bay Development. The waterfront area is currently occupied by low rise structures such as shipyards, timber yards and sawmills etc. There are also an ice making and cold storage factory operated by Dairy Farm, a salt water pumping station operated by WSD and a site reserved for CED for maintenance depot included as part of the development site.

8.1.7 The proposed development is characterized by an urban development with an integration of a number of high rise residential towers, schools, shopping arcade, retails and Government Institutional and Community Facility (G/IC) activities, office towers, together with pedestrian and vehicular links with the future Yau Tong MTRC station.

8.2 Design Criteria

Design Parameters

8.2.1 The global unit flow factors in Table 2 of the Sewerage Manual (Ref. /3/) are adopted for assessing the current and forecast sewerage flow in Yau Tong area.

8.2.2 Based on Table 2 of Sewerage Manual (Ref. /3/), the following global unit flows assumed for different housing types are presented in Table 8.1.

Table 8.1 Global Unit Flows

Unit Flow Factors (m³/person/d)	Housing Type	Abbreviation
0.025	- Schools (Students)	Sch
0.240	- Village Type Development	V
0.240	- Residential 1	R1 / RA
0.300	- Residential 2	R2
0.370	- Residential 3	R3
0.300	- Government Institutional Community	GIC
0.350*	- Commercial / Retail / Employment in School	Comm
1.000*	- Manufacturing Industry	Ind

Note: * Unit should be m³/employee/day

The unit flow factor of 1.0 m³/employee/day was adopted in the ‘Review of Central and East Kowloon Sewerage Master Plans’.

8.2.3 Global Peaking Factors in Table 3 and Section 4.7.2 of Sewerage Manual (Ref. /3/) are adopted.

8.3 Proposed Yau Tong Bay Development

Development Proposal

8.3.1 The landform for YTBD is by the method of reclamation. The proposed reclamation level for the proposed development is assumed to be approximately +5.5 mPD based on the surrounding existing road levels along Cha Kwo Ling Road and Ko Fai Road.

8.3.2 For the domestic component, a total of 39 nos. high rise residential towers ranging from 44 to 50 storeys are being proposed within the development site. A residential plot ratio of 4.79 on the net site area of 20.30 hectares provides a total domestic Gross Floor Area of 973,000 m² (or 97.3 hectares) in 13,900 units. This represents residential accommodation for 39,337 persons. Whereas for the non-domestic components, a twin-office tower with a gross floor area of 110,000 m², a commercial centre with a gross floor area of 40,000 m² and other G/IC facilities with a gross floor area of 62,430 m² are based on a non-domestic plot ratio of 1.05. Details are presented in Table 8.2 below.

Table 8.2 Development Schedule

Development Parameters
*Domestic Components*
No. of Residential Towers	39
Average Flat Size (m²)	70
Average Household Size (person)	2.83
Height of Residential Towers	32 towers x 44 storey 7 towers x 50 storey
No. of Units	13,900
Residential Population	39,337
Proposed Residential Plot Ratio	4.79
Proposed Domestic GFA (m²)	973,000
*Non-Domestic Components*
Proposed Non-Domestic Plot Ratio	1.05
Retail GFA (m²)	40,000
Office GFA (m²)	110,000
Other G/IC Facilities GFA (m²)	62,430
Proposed Non-Domestic GFA (m²)	212,430

Development Population and Sewage Generation

8.3.3 The residential type within the development will be R2. A unit flow factor of 0.300 m³/d/h is adopted in the assessment. Applying this unit flow rate to the estimated residential population of 39,337 for the proposed development, the sewage flow generation is estimated to be 11,801 m³/d. With the assumption of additional 10% allowance for the two levels of car parking area, the domestic sewage flow becomes 12,981 m³/d, which is equivalent to 0.150 m³/s. Table 8.3 summarizes the total sewage flow generated from residential population.

Table 8.3 Residential Sewage Flow

Type	Population	Unit Flow (m³/d/h)	Sewage Flow (m³/s)	Additional Discharge from Car park (m³/s)	Total Flow (m³/s)
Towers 1 to 32	32 x 985 = 31,520	0.30	0.1094	0.01094	0.1203
Towers 33 to 34	2 x 1,110 = 2,220	0.30	0.0077	0.00077	0.0085
Towers 35 to 39	5 x 1,121 = 5,605	0.30	0.019	0.0019	0.0209
Total:					0.150

8.3.4 Besides the residential population, other activities’ discharges for various G/IC facilities and commercial activities were taken into consideration. There are provisions for five primary schools and three secondary schools in the proposed development. Following the provision standard for schools provided by the Planning Department, primary schools and secondary schools are assumed to have 32.5 people and 45 people per class respectively, and each school has 30 classrooms giving a sum of approximately 975 student places for each primary school and 1,350 student places for each secondary school. It is also assumed that there will be 60 employees for each school. Table 8.4 lists the total sewage flow generated from G/IC and commercial facilities.

Table 8.4 G/IC and Commercial Sewage Flow

Type	Employment Population	Unit Flow (m³/d/h)	Sewage Flow (m³/s)	Additional Discharge from Car park (m³/s)	Total Flow (m³/s)
Primary School	5 x 975 = 4,875	0.025	0.0014	-	0.0014
Secondary School	3 x 1,350=4,050	0.025	0.0012	-	0.0012
School Employment	8 x 60 = 480	0.350	0.0019	-	0.0019
Retail	4,000	0.350	0.0162	0.0016	0.0178
Office	6,500	0.350	0.0263	0.0026	0.0289
G/IC	800	0.300	0.0028	0.0003	0.0031
Club House	20	0.350	0.0002	-	0.0002
Total:					0.055

8.3.5 Combining both the domestic and non-domestic sewage flows, the total sewage flow generated by the proposed development would be 0.205 m³/s (DWF).

Development Phasing

8.3.6 It is anticipated that the population in-take year for the proposed development would begin at 2008 in phases up to year 2016. It is premature at this stage to define a detailed development programme, but the sewage generated by the population from the proposed development in phases would be taken into consideration for assessing the sewerage impact. As mentioned in Section 2, the whole development would be divided into four phases, with the first population in-take programmed in 2009. The remaining three phases will commence after 2011.

8.4 Committed Developments And Population Forecasts

General

8.4.1 The proposed YTBD will fall within the boundary of the East Kolwoon Sewerage Basin. In addition to the YTBD, the existing and other planned population and developments within this sewerage basin have been considered in the assessment. The impact of these developments, including YTBD, have also been taken into account in the assessment of the adequacy of the existing sewerage and sewage treatment facilities under the Review of Central and East Kowloon Sewerage Master Plans (RCEKSMP).

8.5 Assessment of Impact and Mitigation Measures

Existing Sewerage System

Provision of Existing Sewerage System in the Vicinity

8.5.1 The existing sewerage system layout collecting sewage from the existing gravity sewers within the vicinity of Yau Tong prior to reaching the Yau Tong Pumping Station (YTPS) at Ko Fai Road is shown in Figure 8.3, which extracts information from Drainage Services Department’s Drainage Record Plans. The 900mm diameter rising main exiting the YTPS which connects to the Kwun Tong Preliminary Treatment Works (KTPTW) is also shown in Figure 8.3. Analysis on the capacity of the aforementioned pumping station and preliminary treatment work will be carried out in this SIA to identify any adverse impact due to the additional sewage generated from the proposed YTBD which will come on-line at year 2008 in phases to year 2016.

8.5.2 The sewerage impact due to this additional sewage generated will be assessed locally and globally.

Impact to Harbour Area Treatment Scheme

Harbour Area Treatment Scheme

8.5.3 The KTPTW forms parts of the Harbour Area Treatment Scheme (HATS), which was previously referred to as the Strategic Sewage Disposal Scheme (SSDS). The additional flow to the KTPTW from the proposed YTBD may, therefore, have an impact to the HATS system. The HATS Stage I tunnel, to which the effluent from KTPTW is discharged, will also be considered in the assessment.

8.5.4 The design capacity of the KTPTW and HATS tunnel is 10.93 m³/s and 7.70 m³/s respectively. Table 8.5 shows the flow, including flow from the YTBD and other planned developments, to the KPTPTW and HATS tunnel.

Table 8.5 Flow to the KTPTW and HATS Tunnel

Flow Type	Design Flow	Year			Ultimate
Flow Type	Design Flow	2006	2011	2016	Ultimate
ADWF (m³/d)	337,000	383,022	419,564	463,240	516,088
ADWF (m³/s)	3.85	4.43	4.86	5.36	5.97
Peak Flow to KTPTW (m³/s)	10.93	9.94	10.82	11.86	13.10
Peak Flow to HATS (m³/s)	7.70	8.87	9.71	10.72	11.95

Notes: 1. Table extracted from ‘RCEKSMP – Draft Final Report’ (Ref. /1/).

2. KTPTW peak flows are based upon the peaking factors included in the DSD Sewerage Manual.

3. Peak flows to HATS are based upon a peaking factor of 2.0 x ADWF, ad adopted for HATS.

Impact to Kwun Tong Sewage Treatment Plant

8.5.5 As shown in Table 8.5, the HATS tunnel will have insufficient capacity to handle the sewage flow by 2006. The HATS tunnel will be under-capacity, even without the proposed YTBD. Nevertheless, as discussed in the RCEKSMP, the forecast SSDS under-capacity beyond 2006 may not actually occur and the HATS system will be under-capacity by 2011 at the latest.

8.5.6 According to the RCEKSMP, the solutions to the potential shortfalls to the HATS tunnel will be based upon the findings of further studies recommended by the International Review Panel (IRP), including the upcoming study ‘Environmental and Engineering Feasibility Assessment Studies’, which will review the HATS system as a whole in 2000.

8.5.7 The capacity of the KTPTW is greater than that of the HATS tunnel. It was designed such that the peak dry weather flows with some allowance for stormwater will pass into the HATS tunnel and finally to the Stonecutter STW. Flows in excess of these levels, that is, the peak wet weather flows, will be allowed to overflow after screening and degritting at the PTWs. This ensures that the most heavily polluted flows will be conveyed to the Stonecutters STW for treatment, while the HATS tunnel will not be oversized for the maximum flow, (Ref. /2/). The PTWs, therefore, should have sufficient capacity to handle all the incoming flow so that any overflow will be screened and degritted prior to discharge for the peak wet weather flow periods.

Impact to Kwun Tong Sewage Treatment Plant

8.5.8 As shown in Table 8.5, the flow to the KTPTW is 10.82 m³/s and 11.86 m³/s for 2011 and 2016 respectively. In the RCEKSMP, it is assumed that the population in-take of the YTBD will take place between 2006 – 2011. This implies that the under-capacity of the KTPTW in 2016 is not due to the YTBD and hence, the YTBD should have not adverse impact on the PTW. As the current development programme of YTBD starts from 2008 and ends by 2016, some of the spare capacity of the KTPTW allowed for the YTBD in the RCEKSMP may be taken up by other planned developments which will have population in-take after 2011. The KTPTW may not be able to handle the sewage flow generated from YTBD after 2011, depending upon the programme of other planned developments within the sewerage catchment. In the event that the KTPTW is overloaded after 2011, a mitigation/contingency measure in the form of an on-site retention tank is proposed for the subsequent phases of the development after 2011. Nevertheless, upgrading of the KTPTW or other alternatives such as sewage flow diversions will be required to ensure the PTW would be able to handle the total sewage flow within the catchment by 2016. Based upon the IRP recommendations on the HATS, further studies, including the ‘Environmental and Engineering Feasibility Assessment Studies’, will be required to ascertain the scope and programme of further upgrading work of HATS.

8.5.9 As part of the HATS, the need and the implementation of the upgrading works of the KTPTW are uncertain. Hence, a contingency measure such as temporary on-site retention tank would have to be provided to mitigate the potential shortfall in the KTPTW after 2011. The retention tank is a holding facility which stores the excessive sewage temporarily and discharging it during off-peak hours to eliminate the peak flow effect on KTPTW. The retention tank will be demolished once the upgrading of KTPTW is completed.

Mitigation / Contingency Measures

8.5.10 An on-site retention tank will be provided as a contingency/mitigation measure to attenuate the sewage generated by the population intake in YTBD after 2011. The requirement for the tank will be dependent on the actual sewage build up at KTPTW in 2011 and beyond.

8.5.11 By interpolating the sewage flow build up to the KTPTW between 2011 and 2016, the PTW will become overloaded before 2012. As only Phase 1 of the development will take place before 2011, the sizing of the retention tank will be based upon the sewage flow generated from the remaining three phases. The retention tank should only intercept the additional flow from the remaining phases of YTBD after 2011 in the event that there is no spare capacity at KTPTW. Phase 1 includes 11 residential towers and 2 schools (1 primary and 1 secondary) and will account for an ADWF of 0.039 m³/s. The remaining phases will account for an ADWF of 0.166 m³/s and will need to be attenuated prior to discharge into the existing sewerage system.

8.5.12 A 24 hour sewage diurnal curve under average daily conditions extracted from the ‘RCEKSMP – Working Paper No.2 Hydraulic Modelling (revised)’ has been used to determine the daily operation of the retention tank including the retention and discharging period. The profile of the diurnal graph is included in Appendix 8A. To consider the flow to the KTPTW, it is preferable to adopt the diurnal profile for the Ho Man Tin area, as it takes into account various types of development within the area whereas the other curves represent only one particular type of development.

8.5.13 For sizing the retention tank, a peaking factor as recommended in Table 3 and Figure 3 of the Sewerage Manual is adopted.

8.5.14 Based upon the diurnal curve for Ho Man Tin area, there are two peak flow periods, with one in the morning around 9:00 a.m. and the second one at night time around 12:00 a.m.. The additional sewage flow from YTBD during these two peaks periods will be stored in the retention tank and then be discharged into the existing sewerage system after each peak period. The stored peak flow in the morning will be discharged after the morning peak, whereas the night peak flow stored in the tank will be discharged at mid-night and early morning. Since the morning peak flow is significantly higher than the night peak, the morning peak will be used for determining the storage period required.

8.5.15 Since the retention tank is designed for mitigating the overloading of the KTPTW due to the additional flow from YTBD, only the flow from the development in excess of the peak base flow to the PTW should be considered. As a result, the storage period required for the retention tank will be the period in which the flow from YTBD will exceed the peak base flow to the KTPTW. According to the diurnal curve for Ho Man Tin area in Appendix 8A, the storage period should be 1 hour. Based on a conservative assumption, a retention period of 1.5 hours for peaking flow from YTBD (i.e. peaking factor x ADWF of 0.166 m³/s) will be used for estimating the required volume of the retention tank. It is proposed to provide an underground retention tank with an effective storage volume of approximately 2,712 m³. Calculations for the storage volume are provided in Appendix 8B. The proposed dimensions of the site area required for the tank are 22 m x 53m. The retention tank will attenuate the peak discharge during the rising phase of the sewage diurnal curve by locally storing excess sewage and later discharging it into the sewerage system during the receding phase of the sewage diurnal curve.

8.5.16 The flow can be controlled by the submersible pumps with intermittent timer built in at the control panel of the control room. The size of the pump is determined based on the incoming flow during the critical retention and discharging period. It is proposed to provide 3 nos. of pumps, 2 duty and 1 standby, to facilitate the discharging and storage operation. In order to discharge the stored sewage plus the continuous sewage during the non-peak hours, the type of pumps to be selected must each be able to handle a flow of 0.15 m³/s for 7.5 m head as a minimum requirement.

8.5.17 It is proposed to locate the underground retention tank within the 30 m set back strip along Ko Fai Road. A possible location of the retention tank is shown on Figure 8.6. The invert level of the inlet chamber is proposed to set at a level in order to accommodate the inlet pipe, while the base level of the retention tank is set at an approximate level of +0.5 mPD in order to cater for the required storage volume. Figures 8.4 and 8.5 show the typical plan and cross section of the proposed sewage retention tank respectively.

8.5.18 It is proposed that the design of the retention tank will adopt submersible pumps with only wet well for storage of excess sewage to minimize the tank’s plan dimension. Since this proposed retention tank is only a contingency measure, it is therefore not recommended to design a temporary storage tank with wet and dry well arrangement.

8.5.19 However, the necessity and exact size of the retention tank should be further reviewed during the detailed design stage when the need and programme for KTPTW upgrading and the development schedule of the proposed YTBD and other developments are confirmed. If a retention tank is necessary as a mitigation measure, the retention period should be reviewed in conjunction with the available PTW flow patterns in the detailed design stage. Excessive sewage discharge to the PTW via the Yau Tong Pumping Station during the PTW diurnal peak flow periods should be avoided. The peak pumped sewage discharge rates from the retention tank of YTBD should also be reviewed to ensure that the capacity of the Yau Tong Pumping Station would not be exceeded.

Maintenance Requirement, Environmental Impacts and General Risk Consideration

8.5.20 For maintenance purposes, an access cat ladder will be provided to allow maintenance crews to gain access to the bottom of the tank directly. A basket screen well will be provided to remove any debris that may lead to failure of the pumps. An access cover will also be provided directly above the basket screen well to allow a channel for debris removal operation.

8.5.21 In order to control the sludge built up and sedimentation on the sump floor, submersible mixers would be provided to perform an aeration action to re-suspend settled solids.

8.5.22 It is recommended that the storage tank should be regularly inspected and debris removal carried out to minimize the risks associated with the tank to an acceptable level.

8.5.23 Further details of the maintenance requirement will be developed in the detailed design stage.

8.5.24 The construction and operation of the retention tank will be funded, implemented, managed and maintained by Main Wealth Development Limited. This will also include landscaping works.

8.5.25 To ensure there will be no adverse odour impact on the surrounding are due to the operation of the retention tank, an activated carbon filter will be installed as recommended in the section of odour impact from sewage retention tank (Section 4.11). It is recommended that the carbon filter should be changed monthly for keeping the required deodouring efficiency. With the carbon filter installed as a mitigation measure, the residual odour impact upon the odour sensitive receivers is expected to be to an acceptable level.

8.5.26 To control the septicity, one option is to add chemicals to remove the sulphide in the wastewater either by oxidation or precipitation. Chemical such as hydrogen peroxide and nitrate may serve the function. Alternatively, by improving the oxygen balance to maintain the DO levels greater than 1.0 mg/L, it can prevent the sulphide buildup and maintain aerobic condition. This can be done either by oxygen injection or air injection. However, the type of mitigation measure will be selected during design stage.

8.5.27 As the pumps inside the retention tank will be underground, it is envisaged that noise impact due to the operation of the tank will be insignificant.

8.5.28 As the retention tank will be located to the north of Ko Fai Road, opposite to the existing G/IC facilities, the potential visual impacts to the surroundings should be minor. Within the 30 m set back strip along Ko Fai Road, where the retention tank will be located, landscaping will be provided and this will ensure there is no adverse visual impact.

8.5.29 The general risks associated with the storage tank have been considered. A standby pump has been provided as a contingency measure in case one of the duty pumps fails. In the event that the pumps are malfunctioned due to the cut off of power source, the built up sewage in the tank will be released via the overflow pipe connecting to the proposed downstream sewerage system which conveys to the existing YTPS by gravity and pumped to the existing KTPTW.

8.5.30 Should the YTPS be unable to convey sewage flow from the existing sewer and YTBD, due to pump failure, the excess sewage would then be released at the emergency outfall at the southern end of Ko Fai Road, which follows the current practice.

8.5.31 The risk associated with failure of the tank would be limited to the period after the peak hours at 9:00 a.m. and 12:00a.m. of the day and when the tank would operate and attenuate flows. The discharging period between 10:00 a.m. to 11:00 p.m. and 1:00 a.m. to 8:00 a.m. would be considered to be the off-peak hours, as for normal flow without the tank, the proposed sewerage system is designed to cater for the entire development and that there is spare capacity for the existing YTPS. Hence any over flow of sewage due to tank failure should be able to be handled by the proposed sewerage system, YTPS and KTPTW. Dual power supply should be provided to ensure reliable operation of the tank and to minimize overflow. If the retention tank is not manned 24 hours, telemetry system should be provided to other 24-hour facilities to ensure the operation of the retention tank will be monitored continuously and any malfunctioning of the retention tank will be readily detected and rectified.

Impact to Yau Tong Pumping Station

Yau Tong Pumping Station (YTPS)

8.5.32 At present, the sewage generated from the Yau Tong area is conduited by gravity to YTPS which it is located at the southern end of Ko Fai Road. The collected sewage is then pumped to KTPTW via a 900 mm dia. rising main. The pumping station has three sewage pumps that operate in a 2 duty and 1 standby mode with each pump having a design capacity of 0.55 m³/s. Therefore, the ultimate design capacity for YTPS is 1.1 m³/s.

8.5.33 According to the RCEKSMP, the YTPS, together with the rising main, will have adequate capacity to handle the additional sewage flow from the YTBD. Hence, the YTPS should have no adverse impact on the YTPS and upgrading works of the pumping station will be not required.

Impact to the Existing Rising Main

8.5.34 Based upon the findings in the RCEKSMP, the rising main downstream of the YTPS would be able to handle the sewage flow from the proposed YTPS. Currently, there is only one rising main conveying the sewage flow from YTPS to KTPTW. However, the RCEKSMP recommended the provision of an additional rising main to provide standby capacity.

8.5.35 The RCEKSMP investigated the opportunity of constructing a second rising main and the possible alignment, which was on the western side of the YTBD and parallel to the proposed West Coast Road (WCR). Since part of the second rising main will fall within the YTBD, a sewerage reserve of approximately 7 m would be required. However, this alignment may impose a constraint to the development within the YTBD. The alignment would also have to take account of existing underground utilities. Taking into account the conceptual plan of the development, a possible routing of the second rising main within the development extending from the southern entrance to the northern entrance may be considered. Further investigations, including checking of the potential conflicts with the stormwater and sewerage system, would be required to confirm the routing. The RCEKSMP stated that the construction of the second rising main might be carried out under either the WCR project or the YTBD. Agreement between Main Wealth Development Limited and the Government would be required should a sewerage reserve for the rising main be required through the development.

Impact to Existing Sewers Connecting to Yau Tong Pumping Station

General

8.5.36 As a baseline condition, an assessment was carried out based on the estimated sewage flow from other developments/redevelopments neighboring within the sewerage catchment of the local sewers, to which the sewage flow from the YTBD is discharged. An assessment with the inclusion of the proposed YTBD have also carried out to identify the impact on the existing local sewers due to the proposed developments.

8.5.37 For the assessment of the existing local sewer, appropriate peaking factors in accordance with the Sewerage Manual Part 1 Table 3 (Ref. /3/) were used. These peaking factors have taken into account of the total cumulated population from other contributed planning areas within the sewage catchment basin.

Baseline Condition

8.5.38 The baseline condition is established based upon the population and employment projections for 2011 and 2016 most likely scenarios in the RCEMSMP, (Ref. /1/). The population and employment projections in the RCEKSMP are presented in terms of TPU. As YTBD has been included in the RCEKSMP, the population within the development was taken out to establish the baseline condition. As shown in Figure 8.6, the discharge point for the sewage flow from YTBD to the existing local sewer is proposed to be at the existing manhole Ex. 8. As a result, only the section of local sewer from manhole Ex. 8 to the gravity sewer discharging to the downstream YTPS have been considered in the assessment. Refer to Figure 8.3 for location of the local sewers.

8.5.39 One discharge point is proposed for YTBD such that the sewage flow from the development can be easily attenuated by passing through the proposed retention tank.

8.5.40 The concerned existing sewers have been checked with the design flows derived from the contributed catchment area from Yau Tong area. The hydraulic performance on the existing local sewer for year 2011 and 2016 without YTBD was assessed using the SPIDA program and the results are presented in Appendix 8C.1.

8.5.41 The results in Appendix 8C.1 indicate that the section of sewer from manhole Ex. 8 to the inlet of YTPS will have adequate capacity to handle the flow in both 2011 and 2016 excluded flows from the YTBD.

Sewage Flow with the Inclusion of Yau Tong Bay Development

8.5.42 The discharge point Qa is shown in Figure 8.6.

8.5.43 The impact of the sewage flow from the YTBD to the existing sewerage system immediately downstream of the discharge point Qa is assessed by adding the flow to the baseline scenario. The impact is assessed using the SPIDA program for both 2011 and 2016 most likely scenarios. The results of the assessment are included in Appendix 8C.2.

8.5.44 Based upon the assessment result, the existing local sewer from manhole Ex.8 to the inlet to YTPS will be able to handle the additional flow from YTBD in both 2011 and 2016 scenarios. Therefore, no upgrading works of the local sewers will be required and there will be no adverse impact on the existing local sewers due to the development.

8.6 Conclusion

8.6.1 In accordance with the findings in the RCEKSMP, the HATS tunnel will have insufficient capacity to handle the sewage flow by 2006, even without the proposed YTBD. Nevertheless, the forecast SSDS under-capacity beyond 2006 may not actually occur and the HATS system will be under-capacity by 2011 at the latest. The solutions to the potential shortfalls to the HATS tunnel will be based upon the findings of further studies, including the upcoming ‘Environmental and Engineering Feasibility Assessment Studies’, recommended by the International Review Panel (IRP).

8.6.2 According to the RCEKSMP, the KTPTW should be able to handle the flow from YTBD provided the development comes on-line by 2011. However, under the YTBD programme, the development will come on-line in phases between 2008 and 2016. There may be insufficient capacity available for YTBD at KTPTW after 2011. As the need and implementation of the upgrading works of the KTPTW, which will be based upon further investigations as recommended by the IRP, are uncertain, a mitigation/contingency measure in a form of an on-site retention tank is proposed. The necessity or the exact size of the retention tank will need to be reviewed during the detailed design stage when the need and programme for the KTPTW upgrading from findings of further studies, including the ‘Environmental and Engineering Feasibility Assessment Studies’ and the development schedule of the YTBD and other developments within the catchment are confirmed. If a retention tank is necessary as a mitigation measure, the retention period should be reviewed in conjunction with the available PTW flow patterns in the detailed design stage. Excessive sewage discharge to the PTW via the Yau Tong Pumping Station during the PTW diurnal peak flow periods should be avoided. The peak pumped sewage discharge rates from the retention tank of YTBD should also be reviewed to ensure that the capacity of the Yau Tong Pumping Station would not be exceeded.

8.6.3 The YTPS and associated rising main exiting from the pumping station to KTPTW will have sufficient reserve capacity to cope with the additional flow from the proposed YTBD. If an additional rising main is provided by Government for standby capacity as recommended in the RCEKSMP, a sewerage reserve of approximately 7 m would be required through the development site. Further investigations would be required to confirm the routing. Agreement between Main Wealth Development Limited and the Government would be required should a sewerage reserve for the rising main be required through the development.

8.6.4 The assessment has shown that the existing public sewer downstream from the proposed development’s sewage discharge point would be able to accommodate the additional sewage generated by YTBD and therefore no upgrading works will be required.

8.7 References

1. Review of Central and East Kolwoon Sewerage Master Plans – Draft Final Report, Hyder Consulting Limited, April 2001.

2. Review of Central and East Kolwoon Sewerage Master Plans – Interim Report, Hyder Consulting Limited, December 1999.

2. Sewerage Manual Part 1, Drainage Services Department, May 1995.