2                        Project Description

2.1                 Introduction

The TPSTW Stage V project comprises the expansion and modification of the existing Stage I/II works and Stage IV works of the TPSTW (there is no Stage III works). uUpon completion of the Project  the capacity of the STW would be increased from the present daily average flow of 88,000 m3/day to 130,000 m3/day.   

2.1.1           The key elements of the expansion and modification works include the following:

(a)       Additional inlet and preliminary treatment works;

(b)      Additional primary clarifiers;

(c)       Additional secondary treatment facilities (new bioreactors and final clarifiers);

(d)      Modification of existing bioreactors;

(e)       Reconstruction of existing final clarifiers;

(a)        Additional inlet and preliminary treatment works;

(a)        Additional primary clarifiers;

(f)        Disinfection facilities;

(g)       Additional sludge treatment facilities; and

(h)   Ancillary works to existing treatment facilities.

2.2                 Project Location and Site History

2.2.1           The TPSTW is located within the Tai Po Industrial Estate.  It currently comprises four Stages: I, II , IVA and IVB works.  Stage I was completed in 1979 and has provided secondary treatment since then.  In 1982, Drainage Services Department (DSD) carried out the Stage II extension, which consisted of the extension to the Stage I treatment facilities and the construction of sludge treatment facilities.  Stage IVA and IVB works were two other extensions to the TPSTW at a site adjacent to Stage I/II works at 1986 and 1995 respectively, with an increased capacity of 54,000 m3/day.  Stage I/II and Stage IV are designed to operate independently although they are interconnected in order to provide flexibility of operation. The Stage V works will be constructed within the existing TPSTW site boundary. It would entail upgrading of existing treatment facilities and provision of additional facilities to increase treatment capacity.  A location plan of the Project and preliminary general layout arrangement are enclosed in Figure 2.1 and Figure 2.2 to this report.


2.3                 Project Design

2.3.1           The detailed  design of the Project is still in progress. The major design features of the TPSTW Stage V that are currently available  are summarized in the following sections.


2.4                 Basis of Project Design

2.4.1           The design of the Project is based on the population forecast and estimated flows and loads in the Tai Po New Town area.


·             Population

With reference to Population information is based on the Projections of Population Distribution 2003 – 2012 prepared by Planning Department., tThe 2012 projected population of the catchment of TPSTW is 282,945.


·             Flows

According to the projected population, the design flow is estimated to be 100,000 m3/d. With a reserve capacity of 30% for future development, the  TPSTW  Stage V project is designed to cope with 130,000 m3/d and the Project will be implemented in 2 phases.


        The design capacity capacities of Phase 1 and Phase 2 works are 100,000 m3/d and 130,000 m3/d respectively.


·              Loads

 The design load concentrations to be adopted by DSD are summarized as flows:



Load (tonnes/d)

Concentration (mg/l)














2.4.2           The design E.coli concentration in the influent is 2 ´107 counts/100 ml .


2.5                 Treated Effluent and Dry Solid Standards

2.5.1           The current and the new effluent standards upon commissioning of the project Project are summarized as follows:




Current Standards

New Standards

95 percentile


95 percentile


Annual Average

Monthly Geometric Mean

BOD5 (mg/l)







TSS (mg/l)







Total N (mg/l)







NH3-N (mg/l)







E. coli (counts/










2.5.2           In addition, the dewatered sludge shall should have a solid content not less than 30% for disposal at landfill.


2.6                 Scope of Works

2.6.1           The general layout of the works involved in this Project is shown in Figure 2.2

              The key elements of the Phase 1 works (design capacity up to 100,000 m3/d) include:

(a)                      Additional inlet and preliminary treatment works;

(b)                  Modification of existing secondary treatment process units (retrofit existing bioreactors and adding 2 new final clarifiers);

(c)                      Reconstruction of 6 existing final clarifiers;

(a)                     Additional inlet and preliminary treatment works;

(d)                     Additional sludge treatment facilities; and

(e)                      Ancillary works to existing treatment facilities.


The key elements of the Phase 2 works (design capacity up to 130,000 m3/d) include:

(a)                Construction of additional secondary treatment process units (1 primary clarifier, 3 bioreactors and 2 final clarifiers);

(b)               Reconstruction of 4 existing final clarifiers;

(c)                Disinfection facilities;

(d)               Additional sludge treatment facilities; and

(e)                Ancillary works to existing treatment facilities.


2.6.2    Disinfection will be achieved by ultra-violet irradiation.  It is noted that consideration may be given to the use of chlorination for the Shatin STW, in which case there may be scope to include the Tai Po STW effluent as well.  This scenario, should it transpires, is to be considered as part of any necessary studies for the chlorination.


2.7                 Works Programme

2.7.1           The construction of Phase 1 works is scheduled to commence in 2005 for completion in 2009 whilst the Phase 2 works, being subject to actual flow build-up, is scheduled to commission before 2016.  

2.8                 Other Related Projects

2.8.1           Since the Project is located within the boundary of the existing TPSTW, it is anticipated that there would be no direct interfaces between this project and any other project.  NonethelessHowever, the findings of the following studies and projects and other on-going studies/ projects in the area were have been taken into consideration where necessaryappropriate during the Assignment:


(a)                North District and Tolo Harbour Sewerage Master Plans (SMP);


(b)               Review of North District and Tolo Harbour Sewerage Master Plans;


(c)                Performance Verification of Discharge into Victoria Harbour from Tolo Harbour Effluent Export scheme, and


(a)               Harbour Area Treatment Scheme.




2.9                 Alternative Option Consideration


2.9.1           By virtue of its upgrading natures, no alternative site is applicable tothis Project.  However, alternativelayouts (Figure 2.3) was considered for the air quality assessment.  As thework sites are limited during construction and the work areas are alllocated within the TPSTW, no adverse dust impact would be expected from theBase Option and Alternative Scheme.  During operation phase, the potentialodour sources under Stage V of TPSTW of Base Option are basically similartoAlternative Scheme, except the installation of the filtrate treatment SBRat the south of StageI/II Primary Sedimentation Tanks in the Base Option.  Since the odouremission from the SBR is of a very small amount, the potential odour impactforboth schemes would be similar.  Only one noise sensitive receiver wasidentified, andthe noise impact for the two schemes is similiar.  No difference isexpected for the water quality impacts due to the difference of layout. The layout for further assessment was selected as Figure 2.2.

2.9.2           For water quality assessment, different bypass discharge locations (atShatin pumping station and at Tai Po pumping station) were alsoconsidered and the results were compared (see Section 4).