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LCQ15: Treatment of sewage and rainwater

LCQ15: Treatment of sewage and rainwater

Hong Kong (HKSAR) – Following is a question by the Hon Frederick Fung and
a written reply by the Secretary for Development, Mrs Carrie Lam, in the
Legislative Council today (February 15):


At present, the Drainage Services Department is responsible for the
sewage and stormwater treatment and drainage systems in Hong Kong. The
stormwater drainage system is mainly used for flood prevention and for
coping with floods caused by heavy rainstorms, and stormwater is
basically untreated before it is discharged into the sea directly. In
this connection, will the Government inform this Council:

(a) of the number and contents of the complaints received by the
authorities in the past three years about the hygiene problems brought by
the outfalls and gullies of the stormwater collection system, as well as
the solutions provided by the authorities; whether the authorities have
conducted any study or used any new technology to improve the relevant
hygienic conditions;

(b) given that it is possible that the sewage produced in our daily lives
may be discharged into the stormwater collection system directly (e.g.

the sewage produced from street cleaning, especially during the dry
seasons when there is not enough rainwater to dilute the sewage before it
is discharged into the sea directly), thereby polluting the coastal water
in the vicinity of outfalls and giving out foul odour, whether the
authorities have, in the past, conducted water quality tests on the
sewage discharged through the stormwater collection system or the coastal
water in the vicinity of the outfalls at different times in each year; if
they have, of the test results in the past three years (including the
impacts of seasonal factors, etc.); if not, the reasons for that;

(c) whether the authorities have examined or considered applying the
technologies (including connecting all or some of the stormwater drains
to the sewage drainage system and putting in place a switching system
that allows the connection of stormwater drains to the sewage drainage
system) used in the stormwater collection systems in other places, so as
to reduce water pollution caused by the direct discharge of stormwater;
if not, of the reasons for that; and

(d) whether the authorities have, in the past, conducted any study on
stormwater collection systems with a view to utilising stormwater
resources in a more effective way to reduce the consumption of potable
water, e.g. building large-scale or regional stormwater harvesting
systems and secondary water supply systems (i.e. systems other than the
existing plumbing systems for fresh water supply at the taps) for the
purpose of utilising stormwater for various non-potable uses (e.g.

toilet flushing, landscape irrigation and cooling air-conditioning
systems, etc.); if they have, of the findings?



In Hong Kong, the sewerage system and the stormwater collection system
operate independently to enable the separate treatment of sewage and
rainwater. In general, stormwater is directly discharged into the sea. To
prevent polluted water from entering the stormwater collection system due
to various reasons thus polluting the environment, we have implemented a
handful of measures to reduce the discharge of polluted water into the
stormwater collection system, such as tackling the problem at source by
rectifying misconnections to stormwater drains; installing dry weather
flow interceptors at suitable locations; and regulating improper
discharge of polluted water at roadside.

Moreover, we would carry out regular cleansing work to remove sludge at
the stormwater collection system so as to reduce nuisance to the public
arising from the odour generated by the accumulated sludge.

My reply to the four parts of the question is as follows:

(a) From 2009 to 2011, the Highways Department and the Drainage Services
Department received a total of 563 complaints about foul odour from
drains and gullies of the stormwater collection system.However, we have
no further breakdown of the complaints into different problems, such as
hygiene problem. On receipt of a complaint about foul odour, the
departments concerned would immediately arrange contractor to carry out
cleansing work. Generally, the main causes of foul odour are
misconnection of sewers to the stormwater collection system and improper
discharge of polluted water into stormwater drains.

Apart from rectifying the misconnections to resolve the problem at
source, enforcement departments concerned would also conduct regular
inspections to deal with any illegal discharge of polluted water into
roadside drains on the spot.Appropriate enforcement actions would also be
considered when breaches of relevant legislations (such as the Water
Pollution Control Ordinance) are found. On the technical front, the
Highways Department would install gully traps at roadside gullies at
black spots of foul odour to reduce odour releasing from the drains. In
addition, a local university is conducting a study on reducing foul odour
arising from sludge in box culverts.

The study is expected to complete in 2013.

(b) The Environmental Protection Department (EPD) has set up a total of
94 monitoring stations in Hong Kong waters, inner bays, typhoon shelters
and anchorages to monitor marine water quality on a regular basis. The
selection of sites for these stations and the monitoring methodologies
are set in accordance with internationally recognised scientific
practices of the relevant disciplines, including oceanography and
statistics. As the collected water quality data are mainly used for
studying the long-term trend of variations in marine water quality, the
monitoring stations are generally located offshore instead of near-shore
areas to avoid recording widely divergent data arising from abrupt
pollutant sources near the shore.

Otherwise, the data may lead to over- or under-estimation of the water
quality of the receiving waters concerned. As such, the Marine Water
Quality Monitoring Programme of the EPD would not conduct water quality
tests at near-shore areas, particularly at stormwater outfalls or their
nearby waters where there may be pollutant discharges.

Apart from EPD’s regular water quality monitoring as mentioned above, the
Civil Engineering and Development Department (CEDD) has also conducted
baseline water quality monitoring at the waterways adjacent to Kai Tak
Development (including To Kwa Wan Typhoon Shelter) since December 2009,
in connection with the planning and design of the Kai Tak Development.
The CEDD collects water samples every three months to analyse a number of
physical, chemical and microbiological parameters, including dissolved
oxygen and coliform count.

The monitoring results are available for public viewing on the website of
Kai Tak Development (

Regarding the monitoring of inflow of polluted water into the stormwater
collection system, we consider that regular inspections and immediate
tackling of pollution at source are more effective in preventing the
polluted water from entering stormwater drains.

(c) As far as practicable, we would incorporate sewage collection
installations, such as dry weather flow interceptors, into stormwater
collection systems. These installations can intercept and divert polluted
water flow to sewerage system for treatment during dry seasons.

Hong Kong is located in the subtropical region with high annual rainfall.
Connecting all or some of the stormwater drains directly or via a
switching system to the sewerage system would lead to huge volume of
stormwater entering the sewerage system.Coping with the large amount of
additional stormwater flow would require many-fold enlargement in the
size of the existing sewer pipes and substantial expansion in the
capacity of the sewage treatment works.

As a matter of fact, most of our urban underground spaces are already
congested with various kinds of pipes and ducts. It is not practical to
lay larger sewers in these areas. Besides, it is not cost-effective to
enlarge the sewers and expand the capacity of sewage treatment works to
deal with the additional rainwater inflow.

(d) We have been undertaking studies on the more effective use of
rainwater resources and reduction of potable water for non-potable uses.

At present, we have selected some parks and public housing development
projects for trial schemes on harvesting rainwater via rainwater
harvesting systems for toilet flushing and irrigation purposes. Findings
from these schemes will serve as references for setting the future
standards of rainwater harvesting system. Besides, the Water Supplies
Department has commissioned a consultancy study on the development of
design guidelines and water quality standards for rainwater harvesting

The study is scheduled to be completed in 2012.

Source: HKSAR Government

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