LCQ15: Treatment of sewage and rainwater

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 system. The study is scheduled to be completed in 2012.

Ends/Wednesday, February 15, 2012
Issued at HKT 12:52