Configuration and monitoring significance of automatic water quality monitoring station

- Aug 14, 2020-

The automatic water quality monitoring station consists of a station room, instrument analysis unit, water intake unit, water distribution unit, control system, data acquisition and processing transmission system, lightning protection equipment, etc. The monitoring indicators are mainly water temperature, pH, dissolved oxygen, conductivity, turbidity, Ammonia nitrogen, heavy metals, chemical oxygen demand and permanganate index, etc. By connecting with the relevant data monitoring platform of the ecological environment department, automatic water quality monitoring can be realized.

According to the instructions issued by the platform, the remote setting, calibration, cleaning, emergency monitoring and other monitoring operations of the water station are realized, so as to achieve real-time control of the water quality of the section water body, early warning and forecasting.

  Surface water quality automatic monitoring station instrument configuration and operation mode

   The monitoring items of the automatic water quality monitoring station include water temperature, pH, dissolved oxygen (DO), conductivity, turbidity, permanganate index, total organic carbon (TOC), ammonia nitrogen, and lake water quality. The monitoring items of the automatic monitoring station also include total nitrogen and total phosphorus. In the future, some sites will be selected for pilot work on volatile organic compounds (VOCs), biological toxicity and chlorophyll a.

   The monitoring frequency of automatic water quality monitoring stations generally adopts sampling and analysis every 4 hours. Each monitoring item can get 6 monitoring results every day, and the monitoring frequency can be increased according to management needs. The monitoring data is transmitted to the trusteeship stations of the automatic water quality stations, the provincial monitoring center stations and the China Environmental Monitoring Center through the public and external network VPN.

The monitoring frequency of each water station is once every 4 hours, and the monitoring is started on the hour of 0:00, 4:00, 8:00, 12:00, 16:00, 20:00, and 24:00, and the released data is the latest Monitoring value.

   The monitoring items issued by each water station are pH, dissolved oxygen (DO), total organic carbon (TOC) or permanganate index (CODMn) and ammonia nitrogen (NH3-N). Implement the corresponding standards in the "Surface Water Environmental Quality Standards" (GB3838-2002), and give the corresponding water quality categories for the results of each monitoring project. There is currently no evaluation standard for total organic carbon (TOC).

In order to make the expression of water quality easier to understand, the water quality status is divided into excellent (I, II water quality), good (III water quality), light pollution (IV water quality), moderate pollution (V water quality) and Severe pollution (poor grade V water quality).

   Meaning of main monitoring indicators

   pH: An indicator that characterizes the acidity and alkalinity of the water body. When the pH value is 7, it is neutral, less than 7 is acidic, and greater than 7 is alkaline. The pH value of natural surface water is generally between 6-9. When algae grow in the water body, the pH value of the surface layer will increase due to the absorption of carbon dioxide by photosynthesis.

   Dissolved oxygen (DO): represents molecular oxygen dissolved in water. The dissolved oxygen index in water is one of the important indicators reflecting the quality of water bodies. Surface water that contains organic matter pollution will consume dissolved oxygen in the water when the organic pollutants decompose under the action of bacteria, making the water body black and smelly, and causing fish , Shrimp and other aquatic organisms die. In natural water with good fluidity (good exchange with air), the saturated concentration of dissolved oxygen is related to temperature and air pressure. At zero degrees, the saturated oxygen content in water can be 14.6 mg/L, and at 25°C it is 8.25 mg/L. When algae grow in water bodies, oxygen is generated due to photosynthesis, which will cause the surface dissolved oxygen to rise abnormally and exceed the saturation value.

   Permanganate Index (CODMn): The amount consumed when processing surface water samples with potassium permanganate as the oxidant, expressed in mg/L of oxygen. Under these conditions, both reducing inorganic substances (ferrous salts, sulfides, etc.) and organic pollutants in the water can consume potassium permanganate, which is often used as a comprehensive indicator of the degree of surface water pollution by organic pollutants. The potassium permanganate method, also known as chemical oxygen demand, is different from the chemical oxygen demand (COD) of the potassium dichromate method, which is often used for wastewater discharge monitoring.

   Total Organic Carbon (TOC): Another comprehensive indicator representing the content of organic matter in water. Using the organic matter in the combustion water sample, by measuring the carbon dioxide (CO2) content generated, the total organic carbon content is expressed by the amount of C element. For water samples with the same chemical composition, there is a certain correlation between total organic carbon and permanganate index.

Ammonia nitrogen (NH3-N): Ammonia nitrogen exists in water in the form of dissolved molecular ammonia (also known as free ammonia, NH3) and ammonium salt (NH4+). The ratio of the two depends on the pH and temperature of the water to contain The amount of N element represents the content of ammonia nitrogen. The sources of ammonia nitrogen in water are mainly domestic sewage and certain industrial wastewater (such as coking and synthetic ammonia industries) and surface runoff (mainly making fertilizer used in farmland enter rivers, lakes and reservoirs through surface runoff).