On rainy days, rainwater falls on the soil and vegetation and is absorbed. However, urban roads are covered with non-permeable cement, and rainwater falling on them enters storm drains, via the drain holes you see in streets. As rainwater should be clean, storm drains carry the water directly into rivers or the sea. However, in reality, storm drains not only collect rainwater, but a lot of wastewater including from washing the streets and cars, as well as sewage illegally connected to storm drains. Every day, large amounts of untreated sewage – which includes oil, food waste, chemical cleaners and pesticides – are poured directly into river channels and the sea. These are known as "Non-point- source" pollution, which poses a serious threat to plants and animals in rivers and the sea.
Water pollution can be divided into "point source" and "non-point source". Point source pollution refers to wastewater discharged from a designated location, such as sewage outlets in factories. Pollutants on roads mix with rainwater and flow into the storm drains, eventually being discharged into river channels or the sea. As it is difficult to trace the source of the pollutants, this is categorised as non-point source.
Wastewater from residential, commercial and industrial sources is treated in a public waste water treatment system, and collected via a wastewater pumping station or waste water treatment plant through a wastewater treatment network (or sewer). Based on the "Water Pollution Control Ordinance" that was formulated in 1980, Hong Kong is divided into 10 Water Control Zones (WCZs), and discharge of wastewater is regulated under a licensing system. The "Livestock Waste Control Scheme" promulgated in 1987 and the "Waste Disposal (Livestock Waste) Regulations" that came into effect in 1988 further regulated wastewater from raising livestock such as pigs, chickens and ducks. So point source wastewater is regulated and treated.
However, non-point source water pollution has been ignored by the government and the general public. The streets are fitted with storm drains with the intention of effectively draining rainwater during rainfall, to reduce the risk of flooding. While large items or rubbish are filtered out by a barrier, rainwater is discharged directly into river channels and the sea without any waste treatment, as the storm drains were designed purely for discharging rainwater. Problems will arise when wastewater enters the storm drains.
Storm drains have always received wastewater
The non-point source pollution received in the storm drains includes all kinds of waste, such as dust, dirt and rubbish from vehicles. During rainfall or when streets are washed, these pollutants flow into the storm drains together with rainwater. Meanwhile, the misuse of the storm drains is very common, as many people have a wrong perception of storm drains and see them as "sewer openings" for waste, and often dispose of rubbish and wastewater into the storm drains. Restaurants frequently wash plates and food in back alleys, while car repair shops also wash cars by the roadside. The wastewater from washing the streets also enters storm drains. So while intended to collect clean rainwater, in reality storm drains collect a lot of wastewater containing oil waste, food waste, chemical cleaning agents and pesticides.
Storm drains discharge wastewater directly into river channels, greatly affecting the water quality. This is evident when comparing water quality statistics from the water monitoring stations of Shan Pui River, Yuen Long, and Tung Chung River, Lantau Island. The catchment area of the Shan Pui River water quality monitoring station covers mainly towns and villages, so the water tested comes mostly from storm drains or surface runoff from roads, while the catchment area of Tung Chung River water quality monitoring station covers mostly natural areas, with a vegetated river basin and several villages. As the water tested in the two water quality monitoring stations derives from different sources, the water quality is drastically different.
The suspended solid particles and lipid content in Shan Pui River are significantly higher than in Tung Chung River. Both pollutants are the result of human activity. As for the number of faecal coliforms and E. coli, the measurements for Shan Pui River are 10 times those for Tung Chung River. Both pollutants are closely related to domestic wastewater. In addition, apart from unusually high readings in May, conductivity at the Tung Chung River stations was significantly lower than for Shan Pui River – reflecting lower levels of pollutants.
The water quality of Tung Chung River is good, and there is a high diversity of fish. Plants and insects can be found along the river channel, reflecting a vibrant ecology. In contrast, the water in Shan Pui River has been contaminated and resembles a lifeless "sewer", where the only fish found are those resistant to pollution, all of which are invasive species. The river has become very turbid and smelly due to poor water quality, becoming a nuisance for nearby residents and causing problems to public health and urban appearance. Shan Pui River flows into the sea and has a high possibility of affecting the seawater quality, which would also affect the health of swimmers at nearby beaches.
Retrofit existing drainage systems
In countries with scarce water resources, such as Singapore, there are heavy penalties for improper use of storm water drains, as rainwater collected from these drains is filtered, disinfected and re-used even as drinking water. The maximum penalty is SG$10,000 (HK$60,000). In Hong Kong, according to the "Water Pollution Control Ordinance", discharge of wastewater into storm drains is an offence. First-time offenders face a maximum penalty of HK$200,000 and 6 months imprisonment, which is more severe than in Singapore. Yet, apart from illegal connection of wastewater to storm drains, the authorities do not enforce the law against roadside discharge of wastewater into storm drains. Furthermore, under normal circumstances, it is not feasible to connect to the sewer system for work such as cleaning in restaurants, car washing and street cleaning.
To solve the problem of wastewater affecting river and sea water quality and ecology, the most effective way is to connect storm drains to wastewater treatment plants. To drain effectively, storm drains empty directly to rivers and the sea. For such large scale changes to storm drains, consideration must be given to the capacity of wastewater treatment, which is based on the existing sewage system receiving volume, which may not be adequate for handling additional wastewater from storm drains. For this reason, improvement to the existing storm drain system poses a great challenge.
Based on past experience, water that is polluted from non-point source pollution in storm drains and discharged into river channels beside residential areas, the wastewater becomes a nuisance for residents, and the government needs to take action. Shing Mun River, for example, has been affected by non-point pollution, with poor water quality. Shing Mun River has undergone water quality improvement works for over a decade, costing billions of dollars, involving various government departments. Unfortunately, the Shing Mun River's water quality has never reached acceptable standards that would make it okay for people to be in contact with it. This is because the problem of non-point source pollution was never resolved. Therefore, improving the existing storm drainage system is extremely difficult without dealing with the source of the pollution problem. To spend more resources would be just waste of effort.
Innovative rainwater collection system
To thoroughly resolve the non-point pollution from storm drains, the best solution is to focus on the issue and solve it in the planning stage in new development areas. The government will soon begin the "Tung Chung New Town Extension Project", which involves tens of hectares of space, with an estimated population increase of more than 20,000 people in Tung Chung West. This project will be huge, involving public and private housing developments with traffic amenities. If these follow previous developments, storm drains connections will be close to the river. Hence, the storm drains of the Tung Chung New development area will be connected to the Tung Chung River. Tung Chung River is one of the few large rivers that is largely preserved in its natural form and is ecologically rich, making it exceptionally important to protect the water quality of Tung Chung River from non-point source pollution from storm drains.
With strong demands from environmental groups to protect the Tung Chung River, there will be an innovative rainwater collection system set up in the Tung Chung New Town Extension, which will collect rainwater from storm drains in the New Town and villages. There will be periodic checks, and if the pollution exceeds a certain standard, the water will be transferred to a wastewater treatment plant. If the pollution levels have not exceeded the standard, they will flow through a newly built wetland filter before discharge into Tung Chung River. Wetland filters are modelled on the pollutant filtration and degradation functions of natural soil and vegetation.
There are successful examples of wetland filters abroad. However, as the designs of the current Tung Chung New Town Extension are still concepts, there are still no details. In addition, wetland filters cannot degrade some modern chemical pollutants, such as pollutants resulting from residual drugs. Therefore, environmental groups still have doubts regarding how effective wetland filters will be in practice.
Yet, this is a crucial step for the government in facing storm drain pollution. Environmental groups also hope that finding a solution for storm drain pollution of the New Town Development can successfully protect the water quality within the natural environment, in turn setting an example for other new development areas. Developed areas should also gradually improve existing storm drains, and not cling to the notion that storm drains only contain clean rainwater, or depend on enforcement to solve problems. The problem of water pollution in urban river channels and the harbour has troubled people for decades. We must not allow non-point pollution in storm drains to endlessly pollute rivers and the sea.