Water-friendly culture has been mentioned in the Policy Address for the past two years. Water-friendly activities mean that the public can get access to water bodies such as rivers for recreation. A prerequisite is that the water is of a safe standard. And this standard may be different for human and aquatic life. There was a case in which a copper-silver ion system, used for disinfecting drinking water for people, was adopted in an aquaculture practice that resulted in massive deaths of the shrimps and crabs reared. There is a set of water quality objectives for rivers in Hong Kong. These objectives may safeguard human use during water activities but do not guarantee ecological health. Is it time we considered organisms other than human beings?
There are three sets of water quality objectives in Hong Kong – for beaches, seawater and rivers, for monitoring sewage discharges and protecting different water bodies. The simplest of the three is the one for beaches, which monitors only the level of Escherichia coli (E. coli) – which should be below a geometric mean E. coli level of 180 per 100mL to ensure the safety of swimmers.
The objectives for marine water are more complicated. There are more than 20 physical and chemical parameters, including odour, colour, temperature, salinity, pH, dissolved oxygen, inorganic nitrogen, and E. coli Dissolved oxygen, total inorganic nitrogen, unionised ammonia and E. coli are used to assess achievement of the objectives.
The numerous parameters may not sufficiently protect marine health. Studies overseas have indicated that human pathogens may infect dolphins via sewage discharges, and currently pathogens are not included in the water quality indicators. As early as the 1990s, local environmental groups argued that sterilisation must be added to the regular sewage treatment procedures for Siu Ho Wan Sewage Treatment Works, as treated water is discharged into the habitat of Chinese White Dolphin (Sousa chinensis). Only after eight years of operation did the government instal an ultraviolet sterilisation system in the plant.
Slack regulation compared to international standards
There are even more parameters for river water quality monitoring. Nearly 50 physical, chemical and biological parameters – including organics, nutrients, metals and E. coli – are on the list. The Environmental Protection Department also set up Water Quality Objective compliance and Water Quality Index (WQI) gradings for inland waterways in Hong Kong. The former is based on pH, suspended solids, suspended solids, 5-day biochemical oxygen demand and chemical oxygen demand while the latter is based on dissolved oxygen, 5-day biochemical oxygen demand and ammonia-nitrogen level, to assess organic pollution and general health of the waters; there are five gradings from Excellent to Very Bad.
And similarly, even if the water quality indicators are achieved, the river may still not be a suitable habitat for aquatic life. Studies elsewhere show that the criterion for minimum dissolved oxygen for freshwater life is 5mg/L, yet the water quality requirement in Hong Kong is 4mg/L. A low oxygen level in water means a harsh environment for fishes. Hence, in Hong Kong rivers, only hardy foreign species such as Tilapia (Tilapia spp.) and Golden Apple Snail (Pomacea canaliculata) can thrive with the low dissolved oxygen levels. Local fishes such as Beijiang Thicj-lipped Barb (Acrossocheilus beijiangensis) and Freshwater Minnow (Zacco platypus) will on the other hand face great difficulty in surviving. Recent studies by a local university found that the reproductive capacity of Japanese Rice Fish (Oryzias latipes) will be lowered in a low-oxygen environment, and the situation will passed on for two generations.
The failure of sewage treatment
Large amounts of wastewater are generated in our daily life. Domestic sewage originates from kitchen use; and cleaning ourselves, industrial, commercial and agricultural activities all generate sewage. Wastewater is collected and treated in the sewage treatment works.
At present, some 280 cubic metres of wastewater are collected daily from all sources, equivalent to 1,480 standard swimming pools. After treatment, the sewage is discharged into the sea via pipes. You may be surprised that in some sewage treatment works, there is just primary treatment, meaning only solid waste and grit are removed before the sewage is discharged into the sea. And in most plants, there is only secondary treatment.
Secondary treatment means that wastewater is treated first for screening and de-gritting, then sedimentation to remove total suspended solids and finally utilising microorganisms to consume organic pollutants in the water. The objective is to reach a low total suspended solids and low biochemical oxygen demand in the discharged wastewater.
The water quality indicator of suspended solids is based on "not exceeding a certain percentage of the original level" or "not posing a threat of adverse impacts to aquatic life". There is yet to be a clear definition of what counts as "adverse impacts". As for biochemical oxygen demand, standards in European countries are below 30 mg/L while that of local rivers in Hong Kong are at 30 mg/L.
Pollutants in wastewater from domestic, industrial, commercial and agricultural activities vary. Even though technically they can be treated, in reality the massive amount of sewage generated daily makes proper treatment difficult. Treated sewage is still wastewater, only with the amounts of pollutants reduced to "acceptable levels".
It should be noted that the present sewage treatment in Hong Kong does not target non-natural substances including heavy metals, organic chloride and pharmaceutical drugs such as antibiotics and oestrogens for contraception. Though in seemingly minute amounts, these pollutants can pose serious threat to aquatic life, in some cases changing the sexes of marine organisms or killing them directly. There are no water quality indicators for these substances. In addition, in treating wastewater, chlorine products are used for sterilisation, and upon chemical reaction with ammonia present in the sewage, these create toxic substances such as chloramine and trihabomethanes, directly harming aquatic life.
There have been studies on toxicological impacts on aquatic life in which the animals are put in wastewater after secondary and tertiary treatment. Tertiary treatment is a high level sewage treatment procedure, with aeration tanks and filtration used to remove organic pollutants, total suspended solids and nutrients to extremely low levels. Yet, studies results show a poor correlation between the chemical parameters of conventional water quality tests and toxicology in animals. Low concentrations of chemicals do not necessarily mean low toxicity in aquatic life. Plus, ozone is used for sterilisation in tertiary sewage treatment, which increases water toxicity that harms or kills aquatic life.
Water quality beyond "human use"
All the above shows that human and aquatic life may have different demands form water quality. At present, water quality indicators in Hong Kong are based on human use, meaning only human health – and no other aquatic life, is taken into consideration in setting the standards. Therefore, even all the standards are fulfilled, the water bodies may still not be suitable for the survival of aquatic life.
In many European countries and the United States, whole effluent toxicity testing has been introduced. A variety of species in the aquatic food chain have been chosen for monitoring their survival, growth and reproduction in the waters. In California, more proactive Backflow Prevention is adopted to avoid polluting the more ecologically rich water bodies, even by water that has meets standards for water quality indicators.
In Hong Kong, the government has in recent years attempted to restore urban waterways that had been turned into concrete channels, such as Kai Tak River in Wong Tai Sin and Shan Pui River in Yuen Long. To restore the rivers, there must be sufficient water sources. The government thus intends to introduce treated sewage into the rivers. This may be workable if the rivers serve human recreational use, but if we also consider aquatic life that includes vegetation on the banks, and fishes, frogs and dragonflies, we may find the proposal is not practical at all.
Hong Kong rivers are generally not large and are rather concentrated. The river systems are more easily affected by introduced water than the marine system. The government should take the on-going implementation of Biodiversity Strategy and Action Plan as an opportunity and formulate Ecological Water Quality Objectives with toxicology testing. This will certainly provide better protection for rivers (such as Tung Chung River) and marine waters (such as Hoi Ha Wan) of high ecological values, as well as waters of high commercial value for fisheries.
Rivers are important ecological systems. We should protect them not only for recreational uses but also as the natural habitats of aquatic life.
Text ｜ Dr. Cheng Luk-k & Henry Lui