Water, in a liquid or solid form and mainly as seawater, covers over 70% of the globe's surface area. Water is also the most common solvent in the world, capable of dissolving many substances. As water flows, it dissolves different sorts of substances along its way. Rainwater falling onto a mountainside or into a reservoir has a shorter distance to travel and generally accumulates fewer substances. However, after reaching the sea, as the run-off distance is longer, more substances are collected by water as it is carried along. Since ancient times, due to the metabolic and chemical action of numerous marine organisms, seawater has harboured high chemical levels and accumulated elements. Scientists have discovered that seawater contains more than 90 substances and elements. Oxygen and hydrogen have the highest levels, as well as chlorine, sodium, sulphur, magnesium, calcium and potassium, etc.

Various treatment processes including filtration and disinfection are thus required for turning naturally collected raw water into potable water for human consumption, which involves the removal of a large number of visible and invisible substances. Drinking water standards are set to limit these substances below the permissible health levels. The Hong Kong Drinking Water Standards (HKDWS) have been developed with reference to World Health Organization recommendations and international practices. Furthermore, a set of drinking water standards suitable for local context has been established after being reviewed by water experts. The drinking water standards promulgated in April 2021 include 57 chemical parameters, 2 radiological parameters and 1 microbiological parameter. Each of these has an upper limit. The Government of the Hong Kong Special Administrative Region (HKSAR Government) also includes a series of aesthetic guidelines to ensure that the appearance, taste and odour of water are acceptable (see "Hong Kong's Drinking Water Standards" in Chapter 5 for details).

The previous chapter discusses Hong Kong's efforts and achievements over the last century in exploiting and harvesting its water resources. These efforts resulted in the creation of a reservoir system that has become a collection of iconic and elegant architectural structures, and the introduction of Dongjiang water has put an end to Hong Kong’s century-long water supply issues. The raw water treatment or filtration presented in this chapter shows the growing demand for enhancing water quality, following the stabilisation of the water source supply. While water purification and filtration are concepts that have been developed by different societies since ancient times, the regulation of water quality is a more recent practice. The City of London in the United Kingdom was the first to enact legislation in 1852¹ and has become a precedent for many places to follow. In Hong Kong, the first Albany Filter Beds at Mid-Levels were completed in 1890 in conjunction with the construction of the city’s second reservoir system, the Tai Tam Reservoirs.