An assessment of the environmental impact of wastewater from the current slurry storage shows that it is one of the main sources of pollution.
Research conducted in 2018–2019 (Khayrulina and Maksimovich, 2018) and 2020 proved the long-term intensive effect of highly mineralised effluent from the slurry storage along the slope of the Lyonva River valley. The main pollutant ion content was compared to the background values of the Volim River (upper reach) (Khayrulina, Maksimovich, 2018) located outside the area affected by potash industry waste.
The natural chemical composition of water in the Lyonva River has hydrocarbonate-calcium facies. However, the composition of the river water affected by brine from the slurry storage changed to sodium chloride. The mineralisation is 12.3 g/L, chloride concentrations reach 8.0 g/L, sodium is 5.0 g/L, potassium is 2.2 g/L, and the average pH is 7.29 (Table 2). The elevated content of Cl‒, K+, and Na+ is attributed to the high concentration of water-soluble salts in the wastewater entering the slurry storage (Table 3). Due to the active operation of slurry storage, the concentration of the main pollutants in the Lyonva River currently exceeds background values for all components.
Table 2
Chemical composition of the Lyonva River’s water in the area affected by the slurry storage.
No. on the map | Distance from the slurry storage, n-number of samples | рН | HCO3−, mg/L | SO42−, mg/L | Cl−, mg/L | Ca2+, mg/L | Mg2+, mg/L | Na+, mg/L | K+, mg/L | NН4+, mg/L | Dry residue, mg/L | TDS, mg/L |
1 | Lyonva River (8 km), n = 3 | 7.4 | 193.7 | 62.3 | 3642.3 | 1070.7 | 206.0 | 1264.0 | 1046.0 | < 0.5 | 14 969.0 | 12086.4 |
2 | Lyonva River (1,9 km), n = 3 | 7.1 | 176.3 | 230.3 | 12056.7 | 1004.3 | 202.0 | 3172.3 | 1202.3 | < 0.5 | 16 171.0 | 14136.3 |
3 | Lyonva River (7 km), n = 3 | 7.5 | 158.7 | 152.1 | 8189.0 | 842.7 | 146.0 | 1070.0 | 97.3 | < 0.5 | 7 215.0 | 6054.3 |
4 | Lyonva River (10.8 km), n = 3 | 7.2 | 130.3 | 113.3 | 8311.7 | 1211.7 | 265.0 | 2566.7 | 1873.7 | < 0.5 | 20 087.3 | 18380.3 |
Volim River (background concentrations), mg/L acc. to (Khayrulina, Maksimovich, 2018) | 7 | 77.6 | 11.8 | 11.8 | 26.7 | 7.5 | 77.8 | 2.6 | < 0.5 | | |
The findings show that the physical and mechanical properties of the existing clay barriers at the dam's base and the slurry storage bed are ineffective. Highly mineralised brine containing industrial effluents enters surface waters, causing the chemical composition of water in small rivers to change.
An evaluation of the environmental impact of brine shows that slurry storages are the primary source of pollution. Environmental measures and new technological solutions must be developed in this regard.
Therefore, it is necessary to investigate and develop additional methods of pollution prevention and/or propose new technological solutions. Currently, there are different ways to improve environmental safety. In the Werra District (Germany), T. Pinkse et al. (2019) proposed conditioning of waste brine by MgCl2 and subsequent stowage in mines. In Saskatchewan (Canada), Tallin et al. (2011) analysed problems of brine seepages due to inadequate design, construction practice, and inspection. Belarusian researchers of the Starobinskoye Deposit (Churov et al, 2010) proposed consequent stowage of salt-clay slurry and halite waste relating to storage for minimising the land areas withdrawal and brine drainage. Tallin et al. (2011) analysed problems of brine seepages in Saskatchewan (Canada) due to inadequate design, construction practice and inspection more than different techniques.
The construction of new production facilities on the territory of the Verkhnekamskoe Potash Deposit as well as the accumulated environmental damage require new technologies of salt-clay slurry storage that offer more efficient environmental solutions. Based on the example of the slurry storage construction, a new slurry storage design is proposed, taking into account the need to minimise the negative impact on the groundwater and surface water on the territory of the Verkhnekamskoe Potash Deposit.
Some characteristics of operating slurry storage in one of the mines at the Verkhnekamskoe Potash Deposit was analysed (Glushankova and Rudakova, 2019). The amount of incoming industrial effluent in the form of clay-salt slurry suspension ranges from 4386 to 4490 m3/day. Water-material balance was calculated using data on water consumption, water discharge, atmospheric precipitation and evaporation, and volumes of circulating brine. The facility receives 2878.653 thousand m3 of wastewater per year. Recycled brine accounts for 17.2% of wastewater. Solid wastes are accumulated annually and account for 5.6% of the incoming wastewater. About 23.8% of clarified drainage water enters the water body. The chemical composition of liquid wastes is stratified as follows: the mass contents of KCl and NaCl are 5.7-6.0% and 15.3–20%, respectively; the dry residue content is 21.0–26% (Table 3). Table 3 shows the chemical composition of clarified wastewater and wastewater entering the slurry storage. The data obtained show that the content of suspended solids in clarified water is significantly reduced, and purification efficiency is greater than 99.9%. In the clarified circulating brine, the content of mineral impurities decreases: the content of chloride ions decreases by 46%, and the content of dry residue decreases by 45.6%. Decomposition of organic amines used as flocculants can explain the increase in the content of ammonium ions.
Table 3
Chemical composition characteristics of wastewater and recycled brine
Component | Measurement unit | Content | Change in concentration (±%) |
Wastewater | Clarified water (recycled brine) |
Suspended matter | g/L | 60.1 | 0.009 | -99.98 |
Dry residue (TDS) | g/L | 351.9 | 191.5 | -45.6 |
Cl− | mg/L | 192.8 | 104.7 | -46.0 |
NH4+ | mg/L | 2153 | 484 | + 55.0 |
K+ | mg/L | 59.8 | 28 | -53.2 |
Ca2+ | mg/L | 2060 | 2287 | + 11.0 |
Mg2+ | mg/L | 684 | 511 | -25.3 |
Na+ | mg/L | 79300 | 48500 | -50.5 |
SO42− | mg/L | 2690 | 2403 | -10.7 |
The reduction of clarified water mineralisation is connected with the processes of salt crystallisation under the influence of climatic factors, temperature, and density stratification along the vertical extent of the slurry storage. The qualitative and quantitative characteristics of highly mineralised wastewater clarification and the proposed technology of wastewater treatment in a slurry storage have been studied. A settling pond is a periodic earthwork of 1.5–2.5 m depth, usually consisting of two sections connected in series. Slurry storage is designed to separate fine suspended impurities during the retention period of at least 2–5 days. The effectiveness of water clarification and the size of the structures depend on the rate of sedimentation (settling) of suspended solids. Large particles settle in horizontal settling ponds, and sandy particles (0.2 mm and larger) settle in sand traps. Particles smaller than 0.2 mm can be deposited in continuous slurry storage.
The granulometric composition of the solid phase of clay-salt slurry entering the slurry storage was analysed. The results showed that the main fraction of the slurry consists of particles smaller than 0.045 mm (70–80%). The slurry entering the slurry storage is a cohesive, hard-to-separate colloidal system. Such calculations usually use empirical dependences of sedimentation rate on particle size. Table 4 shows the average sedimentation rate of soil particles in water as a function of particle size.
Table 4
Soil particle sedimentation velocity
Particle diameter D, mm | Average sedimentation velocity W, mm/s |
Max | Min |
2 | 1 | 94 |
0.5 | 0.25 | 45 |
0.25 | 0.1 | 10 |
0.1 | 0.05 | 1.2 |
0.05 | 0.005 | 0.19 |
0.005 | 0 | 0.009 |
When selecting gravity structures, the sedimentation rate of impurities is determined by theoretical and experimental methods. The capacity of the structures is calculated based on the obtained data and the required degree of purification. It should be noted that when clay-salt slurry is disposed of, suspended solids settle in concentrated sodium chloride solutions (16–21 wt.%) with a high density (1120–1160 kg/m3 or 1.12–1.16٠10− 3 g/mm3). At 20°C, the dynamic viscosity of the solutions ranges from 13.14٠10 − 4 to 15.49٠10− 4 Pa٠s and at 10°C from 34.91٠10 − 4 to 49.05 ٠10− 4 Pa٠s. Table 5 shows the calculated settling time for particles at different temperatures, solution concentrations, and layer heights.
Table 5
Particle settling time and sedimentation velocity at various temperatures and brine concentrations
T °C | Solution concentration, wt.% | Sedimentation of particles, mm/s | Settling time at a layer height H, hrs |
5 | 10 | 20 |
20 | 16 | 0.048 | 29.0 | 56.0 | 112.0 |
-10 | 16 | 0.018 | 77.1 | 154.2 | 308.4 |
20 | 21 | 0.047 | 28.0 | 54.5 | 110.0 |
-10 | 21 | 0.013 | 106.9 | 213.8 | 427.6 |
According to the assessment, slurry storage capacity should provide at least 10 days of contact time for wastewater in the structure to ensure effective impurity sedimentation. It should be noted that coagulation processes in the electrolyte environment can accelerate the settling of small suspended solids. The settling of suspended solids in slurry storages at the Verkhnekamskoe Potash Deposit takes more than 30 days, ensuring 99.99% water purification efficiency.
Thus, slurry storages can be categorised as settling ponds or wastewater clarifiers depending on their mode of operation. Due to their large size, the structures can be used for a long time both as slurry storages and clarification ponds (Fig. 4). The following major processes can occur in slurry storage, allowing it to be considered as a structure for wastewater treatment:
1. Dilution of wastewater by atmospheric precipitation;
2. Clarification of water as a result of prolonged sedimentation;
3. Dissolution of solid-phase salts and their crystallisation at the bottom of clay-salt suspension storage;
4. Density stratification of mineralised water along the depth of the clay-salt slurry storage. In the bottom layers, saturated salt solutions can form. Low winter temperatures cause the solutions to become more viscous, creating a layer of ice. As the size of slurry storages depends on their capacity (60-250 ha), parts of the structures are covered with a layer of ice and snow.
A layer of slightly saline water is formed when ice on surface layers of slurry storage melts during the spring pre-flooding period. At this time, clarified and demineralised water can be discharged into a water body, taking into account the hydrological and hydrochemical regimes of the receiving watercourse. The volume of discharged water should be calculated so that the content of mineral impurities in the reservoir water does not exceed the required standard values.
5. Decomposition of organic impurities by halotolerant and halophilic bacteria;
6. Disinfection of wastewater. Processes of water disinfection occur in the slurry storage as a result of the osmotic pressure effect caused by different concentrations of salts in highly mineralised effluent and in the cytoplasm of bacterial cells. High osmotic pressure causes dehydration of microbial cells and their death. Disinfection of pathogenic microorganisms is 98-99% effective.
Thus, a complex of treatment facilities enables the treatment of industrial wastewater in accordance with the requirements for brine reuse. The recommendations for the design of a slurry storage that will be used as a settling pond have been developed based on the results obtained. The following recommended construction and design improvements can increase the effectiveness of slurry storage (Fig. 3):
- Construction of a contour dam around the perimeter of the slurry storage to prevent brine from spreading beyond its boundaries;
- Installation of an impervious blanket of polymer geomembranes along the bed of the slurry storage, as well as along the upper slope of the contour dams to prevent brine filtration and its penetration into groundwater;
- Installation of a drainage system under the filtration screen to divert some of the groundwater displaced through the bottom of the slurry storage and brine settling tank, as well as to remove drained water from the soil pores at the base of the pond under the weight of incoming wastewater;
- Installation of an interception ditch for drainage of surface water from the adjacent areas behind the enclosing dam of the slurry storage;
- Installation of the slurry pipeline end outlet 1.5-2.0 m below the surface of the slurry storage to reduce mixing of slurry and clarified water and minimise flow turbulence.
To improve operation of the facility and increase its durability it is recommended to take clarified wastewater for brine reuse from the middle of the facility and controlled discharge of desalinated and clarified wastewater into water bodies during floods. The volume of wastewater should be calculated using existing methods based on the analysis of wastewater samples taken from the top to the bottom of the slurry storage.