Coal mine water[1–3] is the underground water that accompanies the coal mining process. Generally, the coal-bearing strata are located below underground aquifers, and 97% of coal mines in China are underground mines. Therefore, a large amount of mine water must be discharged to ensure that coal mining is safe. Natural and man-made factors influence the production of mine water. Natural factors include geological conditions and surface water bodies. Human factors include water leakage from exploration boreholes and water accumulation in goaves. According to statistics, approximately 4.2 billion m3 of mine water resources are discharged during coal mining in China; thus, treating mine water resources is imperative[4–6].
At present, China classifies mine water into five categories[7–10]: clean mine water, mine water containing suspended solids (SS), mine water with high salinity, acid mine water, and special polluted mine water. Among them, mine water with SS accounts for 60% of the five types of mine water. This type of mine water has the following water quality characteristics: neutral, mostly gray-black color, and high amount of suspended matter and turbidity. According to the actual measurement statistics of the SS content in mine water, the mines with SS content of less than 300 mg/L account for approximately 80%, while the mines with SS content of more than 500 mg/L account for less than 12%. Suspended matter in water refers to the solid matter trapped in a 0.45 um filter membrane from water and is dried to a constant weight at 103°C–105°C. Turbidity in water is the measurement of suspended and colloidal particles in water with size greater than 0.15–0.2 µm. Both suspended matter and turbidity are indicators that reflect the coarsely dispersed particles in the water, and a certain connection exists between the two.
SS content and turbidity are the most important water quality indicators for mine water containing suspended solids, and their real-time monitoring is required before and after treatment of mine water. However, SS content is determined according to the national standard GB 11901-89, which is a gravimetric method[11–13]; this method is simple but cannot be used for real-time monitoring. Turbidity is measured using the national standard GB 13200-91, which is an optical method[14]. Compared with the method for measuring suspended matter, that of turbidity is simple, convenient, and involves real-time monitoring. Furthermore, the real-time monitoring of SS and turbidity may be realized by establishing the relationship between mine water turbidity and SS content, thereby allowing the SS content to be calculated based on turbidity. However, no existing relevant literature has discussed the relationship between wastewater turbidity and SS in coal mine water[15–20]. Therefore, this area of research is important.
The accurate conversion relationship between SS and turbidity in coal mine water was determined by collecting six different coal mine waters, and one sample was selected as the original water sample to configure mine water with different SS content gradients. The actual SS and turbidity in water were measured using national standards. According to the actual measured turbidity and SS values, the relationship between the two was studied, and the relationship formula between turbidity and SS under different gradients was established. Finally, the five other actual detection values of coal mine water and the accuracy of correction model formula were verified.
SS in mine water refer to solid particles, such as coal dust, rock dust, and clay, produced by a series of physical, chemical, and biochemical reactions that occur when rock layers and buried coal come into contact with groundwater during coal mining. The SS in mine water are characterized by small particle size, low density, poor sedimentation effect, and poor sensory properties, and their content is generally tens to hundreds of milligrams per liter. Suspended matter is mainly obtained in moving water but can be separated in still water as light ones float while heavy ones sink. Mine water containing suspended solids is generally black, but its total hardness and salinity are not high.
Turbidity refers to the degree of obstruction of the solution to the passage of light, including the scattering of light by suspended matter and the absorption of light by solute molecules. The composition of mine water is simple and its organic solute content is low. The main factors affecting its turbidity are not only related to the content of suspended matter in the water but also to the size, shape, and refractive index of the suspended matter. When the concentration of suspended solids in the liquid increases or decreases, the turbidity value also increases or decreases. However, the amount of scattered light, chroma, and particle size, shape, and composition are all related to the reflection index. When the concentration of the suspended matter is the same but the material composition is different, scattering the same amount of light is impossible. Therefore, turbidity is related to suspended matter, but this relationship cannot usually be quantified.
The method for measuring SS is the national standard GB 11901-89, which is a gravimetric method. SS are dried after the mine water sample is filtered, weighed under laboratory conditions, and weighed after cooling. SS content can be determined by comparing the result of the subtracted mass of the two weighings with the sample volume. The turbidity measurement method is the national standard GB13200-91, which is the optical method. A portable turbidity meter can be used for sampling and determination in the laboratory or in the field. When determining turbidity, the sample should be shaken vigorously. Suspended matter content and turbidity are both important water quality indicators of mine water, but their measurement methods have certain differences. According to national standards, the SS content must be determined in a laboratory, which requires a matching drying device and cooling device and takes a long time. When the sample is sent from the mine to the laboratory for measurement, too much time is consumed, the cost is increased, and obtaining continuous dynamic changes in the content of SS in mine water is impossible. Turbidity is measured in much simpler and faster method. A sample can be measured directly on the spot, and the method can realize continuous dynamic online monitoring of mine water. Thus, exploring the relationship between SS content and turbidity in mine water is of considerable significance for simplifying the determination of SS content and realizing the online continuous dynamic monitoring of SS content.