Harbour Area Treatment Scheme

Hong Kong, Hong Kong

Year

2017

Duration

15 years

Cost

HKD 25.8 billion (£2.3bn)

Location

Hong Kong

Project achievements

Solved the problem

Substantially improved water quality.

Used engineering skill

Used an E. coli sensor to optimise disinfection.

Environment benefitted

Used an E. coli sensor to optimise disinfection.

Improve water quality for revival of a natural asset

The Harbour Area Treatment Scheme (HATS) is the largest environmental infrastructure project in Hong Kong’s history, implemented to restore the water quality of Victoria Harbour following decades of fast development.

With a population of 7.2 million, Hong Kong is one of the most densely populated cities. Over 60% of people live around the Harbour, putting severe stress on its water quality.

The project comprises:

A 44km network of deep underground sewage tunnels at over 100m average depth (168m at the deepest)
16 preliminary sewage treatment works
The Stonecutters Island Sewage Treatment Works (SCISTW) providing chemically-enhanced primary treatment and disinfection

The HATS is a compact and energy-efficient system. With a footprint of only 10 hectares, the SCISTW is designed to handle a sewage flow of 2.45 million m³ per day (around 1000 standard swimming pools’ volume) and serve a population of 5.7 million.

Since the commissioning of HATS, the water quality of the harbour has been greatly improved. A number of beaches, which have been closed due to poor water quality, were re-opened for public enjoyment. Hong Kong’s annual cross-harbour swimming race was re-convened after 40 years’ suspension, reviving an important memory of the Hong Kong community.

Did you know …

Hong Kong’s largest ever environmental infrastructure project
World’s deepest sewage tunnel (168m below ground) and Asia’s longest very deep sewage tunnel (total length of 44km)
World’s largest chemically enhanced primary treatment plant (serving 5.7 million people) with a treatment capacity of 2.45 million cubic metres per day

Project achievements and benefits

Land resource in Hong Kong is scarce. With only ten hectares footprint of SCISTW, the HATS system can serve 5.7 million of the Hong Kong’s population in over 10 districts (around 0.04m² land per m³ treated sewage).

The HATS has greatly improved the water quality of the Victoria Harbour, which is an important natural asset of Hong Kong. The sewage treatment of HATS has removed about 70% (400 tonnes) of Biochemical Oxygen Demand and 80% (350 tonnes) of Suspended Solids daily.

With the efficacy of HATS and high assimilative capacity of the Victoria Harbour, the water quality of the harbour has been substantially improved. Prescribed water quality objectives, including dissolved oxygen and ammonia nitrogen, have been fully complied with, and E.coli level has been substantially reduced by 93%.

Project elements

Construction Challenges of the deep sewage tunnel

One of the major challenges is effectively controlling the ingress of underground water especially the section underneath Victoria Harbour. High-pressure grouting was applied to seal up the rock joints before excavation, which greatly reduced the groundwater inflow.

To overcome the complicated ground condition, a section of the tunnel adopted the slurry type tunnel boring machine. This was supplemented with specialist artificial ground freezing for break-in and break-out, which is a first in Hong Kong.

Energy efficient design

The deep sewage tunnel is designed with the shortest distance and an inverted syphon to reduce the energy consumption for pumping. Around 40% of the relevant operational cost would be saved. To avoid deposition of the inverted syphon, the tunnel is designed to maintain a self-cleansing velocity at all flows. The Sewage Tunnels (Statutory Easements) Ordinance was also enacted in 1993 to provide for the creation of easements and other rights over land in connection with the sewage tunnels.

Effective use of land

Double-tray sedimentation tank is used in SCISTW, which greatly reduce the use of the scarce land resource in Hong Kong. Notwithstanding the limited site coverage of 10 hectares, the daily treatment capacity of SCISTW is 2.45 million cubic metre (around 0.04m² land per m³ treated sewage).

“Diesel-electric” cargo vessels

Two vessels with alternative maritime power (AMP) and daily carrying capacity up to 1,000ton are designed and constructed to transport sewage sludge to incineration to minimise the impact on road traffic and the public. Compared with the ordinary diesel vessels, it generated 130,000 kg less carbon dioxide, which is equivalent to the carbon dioxide absorption of about 6,000 trees per year.

Innovative technology

An innovative online E.coli sensor, ColiMinder, is installed in SCISTW, which is the first application in Asia. The sensor gives real-time indication of the E.coli. level for optimising the disinfection system with a view to effectively utilising the energy and resource input for disinfection.