How to read the water quality indicators of tailwater from sewage treatment plants?
In today’s era of increasing environmental awareness, the water quality of tailwater from sewage treatment plants has become a hot topic of concern. The tailwater of sewage treatment plants, that is, the water that is finally discharged after a series of treatment processes, has a water quality that not only affects the stability and health of the surrounding ecological environment, but also has a profound impact on the sustainable use of water resources.
I. Main indicators of tailwater quality of sewage treatment plants
(I) Physical indicators
Temperature: The temperature of tailwater affects the rate of chemical reactions in water and the living environment of aquatic organisms. Generally speaking, a suitable temperature range helps maintain the ecological balance in water. For example, too high or too low temperatures may cause changes in the dissolved oxygen content in water, affecting the breathing and reproduction of aquatic organisms such as fish.
Color and transparency: The tailwater after normal treatment should have a relatively clear appearance without obvious abnormal color. If the tailwater is turbid or colored, it may indicate that it still contains a lot of suspended impurities or incompletely degraded substances, such as colored complexes formed by the reaction of certain organic dyes or metal ions with substances. Tail water with high transparency usually indicates that the suspended solids content is low and the water quality is good.
(II) Chemical indicators
pH value: pH value reflects the acidity and alkalinity of tail water. Generally speaking, the pH value of tail water from sewage treatment plants should be close to neutral, between 6 and 9. Excessively acidic or alkaline tail water discharged into natural water bodies may interfere with the acid-base balance of aquatic organisms, resulting in the inhibition of enzyme activity in the organisms, affecting their normal physiological functions, and may even affect the solubility of metal ions in the water body, causing further changes in water quality.
Dissolved oxygen (DO): Dissolved oxygen is a key indicator for the survival of aquatic organisms. Sufficient dissolved oxygen in tail water can maintain the respiration of aquatic organisms and promote the self-purification ability of water bodies. It is generally required that the dissolved oxygen content in tail water should not be lower than a certain standard. For example, when discharged into surface water, the dissolved oxygen content should be able to meet the survival needs of aquatic organisms, usually 2-5mg/L or higher, depending on the nature and function of the receiving water body.
Chemical oxygen demand (COD): COD measures the amount of oxygen required for organic matter in tailwater to be oxidized by chemical oxidants, and it reflects the content of organic matter in tailwater. A lower COD value indicates that the tailwater contains less organic matter and the treatment effect is better. Different regions and receiving water bodies have strict regulations on tailwater COD discharge standards. For example, in some sensitive waters, the COD concentration may be as low as 30mg/L or even lower to prevent water quality deterioration caused by eutrophication and organic pollution.
Biological oxygen demand (BOD): BOD refers to the amount of oxygen consumed by microorganisms in water to decompose organic matter under aerobic conditions. The BOD value is closely related to the concentration of biodegradable organic matter in water. Generally, the BOD of tailwater from sewage treatment plants should be controlled at a low level, and the general standard requires it to be around 10-20mg/L, which helps to reduce the biological oxygen demand for receiving water bodies after tailwater discharge and prevent the occurrence of water hypoxia.
Ammonia nitrogen (NH₃-N): Ammonia nitrogen is one of the main forms of nitrogen in tailwater. High concentrations of ammonia nitrogen can lead to eutrophication of water bodies, promote the proliferation of algae, consume dissolved oxygen in water, cause water quality deterioration, and affect the survival of aquatic organisms. Therefore, the ammonia nitrogen content of tailwater must be strictly controlled, and the general discharge standard is about 5mg/L or lower, especially for those receiving water bodies with higher water quality requirements, such as those near drinking water source protection areas. The discharge requirements will be more stringent.
Total phosphorus (TP): Phosphorus is one of the key elements of eutrophication of water bodies. The total phosphorus in tailwater mainly comes from domestic sewage and industrial wastewater. Controlling the total phosphorus content in tailwater is of great significance to preventing algae outbreaks in water bodies. The common tailwater total phosphorus discharge standard is about 0.5mg/L. The total phosphorus concentration in tailwater can be effectively reduced by chemical precipitation, biological phosphorus removal and other methods.
Heavy metal content: Even if the content of heavy metals such as mercury (Hg), cadmium (Cd), lead (Pb), and chromium (Cr) in tailwater is extremely low, it may be enriched in aquatic organisms and transmitted through the food chain, causing serious harm to human health. For example, mercury can be converted into methylmercury in water, which is highly biologically toxic and can damage the human nervous system and kidney function. Therefore, the tailwater discharge standard has extremely strict restrictions on the content of heavy metals, usually requiring mercury content to be less than 0.001mg/L, cadmium to be less than 0.01mg/L, and lead to be less than 0.1mg/L.