The water quality standards for countries around the world are built based on different purposes of water use, such as drinking water, domestic water, water for aquatic species (fishes, clams, shells,…), water for sports and recreation, water for agriculture, industry, along with many other purposes.

These standards are mainly based on physiochemical and biochemical indicators for the degree of organic pollution of water (Dissolved Oxygen – DO, Biochemical Oxygen Demand – BOD, Chemical Oxygen Demand – COD) and the degree of nutrient pollution (nitrogen, phosphorus); physical indicators (turbidity, total suspended solids, color,…), indicators for pathogenic bacteria (coliform…) and toxic metal (lead, mercury,…)

The basic standards for clean water must at least guarantee an extremely low level of pathogenic bacteria and toxic metal (less than the maximum allowable level or equal to zero) so as not to affect the health of humans as well as aquatic species and the ecosystem, ensuring human safety when in contact with water sources.

The Clean Water Act defines clean water as water of suitable quality for the conservation and maintenance of aquatic species and wildlife, and for recreational human activities under or above the surface.

In Vietnam, the standard for surface water quality is determined based on 36 different parameters, classified into specific levels:

• A1: for domestic supply (after general treatment), conservation of aquatic plants and animals and other purposes
• A2: for domestic supply with appropriate treatment and other purposes
• B1: for agricultural and irrigational purposes or other equivalent purposes with similar water quality requirements
• B2: for transportation purposes or other equivalent purposes with low water quality requirements

In addition to the above physicochemical and biochemical indicators, water quality is also monitored by the water quality index – WQI.

Source: Teaching materials for Water Conservation for Junior high school