Water Conservation and Demand Management in Mining
Table of Contents
The Water Conservation and Water Demand Management (WC/WDM) Targets for the Mining Sector have been developed as a collaborative project by the Chamber of Mines and the Department of Water and Sanitation with active participation from the mining sector. Most new Water Use Licenses prescribe the implementation of Water Conservation and Water Demand Management strategy and is expected to become a requirement for all WUL’s issued in the future. The development of Guideline and Implementation documents defines a set of Water Use Indicators to measure the mine’s water conservation and demand management in question. Some of the principles outlined in these guidelines have been pulled from international best practices such as the Global Reporting Initiative and the Water Accounting Framework for the Minerals Industry as developed by the Minerals Council of Australia. The diverse resources used to develop these guidelines make them relevant for all mining operations. They should be considered and studied by resources working in the Environmental and Water Management Departments.
There has been a massive shift in the environmental and sustainability awareness in the mining industry, which makes water conservation and demand management in the best interests of the mine, over and above the legislative requirements. Through these guidelines and benchmarks, each mining activity can be evaluated on their water use and create an industry standard based on actual operational results. The guideline prescribes a series of steps to develop a Water Conservation and Demand Management plan. This article aims to provide an overview for operational staff and management to understand the implications.
It is important to note that the WC/WDM is developed from the regulator’s perspective and considers water a national resource to be used sustainably and responsibly. The mining industry is seen as a large consumer of water and can negatively impact the overall surrounding water resource due to discharges. The current guidelines focus on the volumetric consumption and impacts on water resources. However, a mine is required to act responsibly regarding discharge water quality and the quality limits indicated by their relevant Water Use License. Due to the high level of detail in which the water balance is developed, it is recommended that potential water quality improvement interventions also be identified and their feasibility determined.
1 Developing a dynamic water balance
All mines are required to develop accurate and computerized water balances as set out by Best Practice Guidelines G2, or if a Water Use License has additional requirements. The purpose of having a computerized model is to determine the impact of potential WC/WDM interventions or environmental conditions such as drought or stormwater conditions. The guidelines under Regulation 6 prescribe the minimum requirements for the water balance read as follows:
Minerals and Petroleum Resources Development Act shall compile an activity-based dynamic water balance for climatic variations, including all inflows and outflows from the activity, reflecting all surface and groundwater interconnections with the water resource.
A person referenced in regulation 6(1) or a holder of a mining right or production right shall ensure that the water balance:
incorporates accurate values based upon measured volumes for the water abstracted, discharged, beneficiation process water intake, outflow to and return water from waste management facilities, and water abstracted from mine workings;
incorporates accurate values determined from suitable measurement or modelling of rainfall, runoff, seepage, and evaporation;
It is required that the water balance is submitted to the Department of Water and Sanitation as part of the Integrated Water and Waste Management Plans, together with the monitoring data, unless stipulated otherwise in the water use license. The water balance should be kept current by ensuring all measured and modelled data is reflected in the model. Any changes are incorporated and should be updated at a frequency not exceeding a monthly basis. All measuring devices used to develop the water balance shall be easily accessible, properly maintained, calibrated and in good working condition. The water balance should be in electronic format and capable of simulating different operational conditions.
There are two primary reasons for the prescription of a computerized water balance:
The water balance should be a management tool capable of accepting new data inputs and process changes based on the operational requirements of the mine, thereby meeting the dynamic criterion.
It should also be a functional simulation tool that can simulate various intervention projects to evaluate the potential impacts on the mine water balance and, ultimately, the Water Use Efficiency Indicators.
During the programming of the water balance, it is crucial to consider the various classifications of the water sources and sinks as defined by the guidelines. These classifications will be used for the calculation of the Water Use Efficiency Indicators. The classifications are described in the table below, and all values are relevant to a specified period such as per day or annum:
Table 1: Water Source and Sink Classifications
It is important to classify the various water streams in the model with a value identifying each stream towards a specific classification to calculate the WUE indicators automatically. The guideline document also specifies that the WUE indicators are calculated for the overall mine operation under consideration, and also separate calculations for the various operations under the following categories: mining operations, beneficiation plants, and residue disposal sites. By keeping this in mind during the programming of the dynamic model, the streams can be grouped according to the relevant category to allow for ease of calculations.
All modern Water Use Licenses require a mine to measure and monitor listed Water Use Activities under Section 21, which addresses the water abstraction for use, making safe, treating and discharge, disposal of waste containing water, or reusing treated effluent. Due to the water scarcity and reduced availability, mines have realized that it is essential to monitor these flow volumes over and above the requirement of the Water Use License. The volumes that are typically more challenging to measure or from an operational perspective to motivate for the costs of monitoring equipment are the discharge water streams. The importance of recycling and reusing water is emphasized by the WUE indicators addressing the percentage of wastewater generated that is not reused and the percentage of water recycled. If these specific indicators are calculated accurately, there is the potential to immediately identify opportunities to reduce the intake of new water sources, which typically comes at a high cost, which is one reason these indicators were developed. If this motivation is developed, effectively operational staff should motivate for the capital required to install the required measuring and monitoring equipment.
Another verification process for the accuracy of flow measurement is calibration at the required intervals or spot measurements with mobile monitoring equipment. Calibration is also a requirement for the WUL license but is typically seen as a grudge purchase when budgets are allocated. Considering the high costs of potable water or the negative impacts of excessive discharges, it is essential to effectively motivate why calibrations are required and done at the recommended intervals.
Fissure water ingress is typically a problematic flow rate to measure due to the various sources found in underground travelling ways or mining stopes. Some mines estimate these flow rates during shutdown periods when all pumping is stopped, and dam levels are monitored to measure the time required to fill specific dam capacities. This provides an accurate prediction as groundwater ingress is not expected to change significantly over short periods. However, it is recommended that this method of measurement is done at least once a year. The risk remains of mining into new aquifers or groundwater channels underground during mine development which would cause a significant change in the overall water balance. If a new water source is encountered, an attempt should be made to collect the water in a channel and estimate flow until accurate measurement can be done. Suppose the mine already has sufficient groundwater to supply the mining activities. In that case, it is recommended that an attempt be made to seal the ingress through chemical grouting and prevent the water from entering the mine water reticulation system. Excess water ingress into the mine will increase the overall water input, and require additional treatment and pumping costs and negatively impact the WUE Indicators.
From a WC/WDM perspective, all discharges or losses to the environment through seepage or evaporation negatively impact the WUE indicators. The model only allows for one positive form of discharge. That is to a third party user with a defined beneficial use for the water and a memorandum of agreement to prove the validity of the off-take. Potential third-party users could be industrial users with a process water requirement; good quality groundwater could be used for agricultural purposes or livestock watering if there are no harmful elements present in the water. Another possibility is the treatment of impacted water to potable standards and providing that to third-party users at a cost that covers the treatment rates. This requires approval through the Water Act as well as permission from the local water services provider. Considering the current national water shortage, it is in the best interest of all affected parties to increase the availability of potable water sources, which is why such potential projects should be investigated and developed further.
1.2 Calculating Water Use Efficiency Indicators
The figure below indicates an example of a classification of the water streams for the different operational categories.
Figure 1: Water Use Efficiency Indicators Calculation Diagrams adapted from Benchmarks for Water Conservation and Water Demand Management (WC/WDM) in the mining sector
The WUE Indicators are sensitive to the Life of Mine plan, which projects the mining program into the future and the planned tons to be mined, and possible expansions to the mine footprint. The Environmental team responsible for developing the WC/WDM strategy must have an excellent understanding of the proposed Life of Mine to construct an effective, and importantly a realistic WC/WDM strategy.
Whether a mine is in the development, operational, or closure phase will determine the extent of the possible interventions and changes on the projected WUE indicators. The dependence of the consumptive water use on the ton of Return Of Mine (ROM) ore means that any mine approaching closure will reduce the WUE indicator throughout the WC/WDM strategy because of the decreased production as the reserves are depleted. This places further emphasis on the role of water use in the final closure strategy and the requirement to measure and manage water use throughout the mining process. For new developing mines, the planned production tons are estimated to ramp up quickly. Still, the proposed impacts on the water sources can be overlooked, such as the potential increase in underground ingress water. This has implications on the pumping and distribution networks regarding their design capacities and directly correlated to the operational costs of continuous dewatering of the mine. Therefore, it is essential that all geohydrological studies and their findings are studied in detail and are still up to date when the proposed strategy is developed.
There may be opportunities to optimize the water use for existing mines already operational through additional flow measurement points in the reticulation system.
The first set of indicators is volumetric based only on the total water use for the mine, consumptive water use, and the volume of wastewater lost from the mine operations. Detailed descriptions and calculations of these variables are indicated in the table below:
Table 2: Volumetric Indicators
These indicators are based on volumetric loading only and do not consider the efficiency of the mine water use through recycling or reuse of water streams. It is important to note that their total Water use can heavily influence mines if the mine is located in a compartment with large amounts of groundwater, which can typically not be prevented from ingress into the mining area. The impact of the consumptive use illustrates the importance of social and labour plans for mines with their surrounding communities. It is seen as a driver for interaction and collaboration with the local community.
Table 3: Water Use Efficiency Indicators
Once these indicators have been calculated through the use of the dynamic water balance, each mine is required to evaluate their scores to the latest national benchmarks, of which the most recent document is the Benchmarks for water conservation and demand management (WC/WDM) in the mining sector (June 2016) report. It is important to note that these benchmarks were developed as part of a detailed analysis of 39 mining operations by consultants and the regulators based on the information available at that time. The benchmarks will be updated as required by the regulator once there have been sufficient submissions from the industry regarding WC/WDM strategies. Due to the abundance of coal, gold, and platinum mining operations, these types of mines have their unique benchmark values. All other types of mining operations have been grouped under the classification of Other. It is foreseen that these will be further expanded over time to identify more realistic benchmark targets specific to those industries.
All mines should strive to meet the benchmark values for the WUE indicators of the top three performing mining operations in that specific category. The WC/WDM strategy should be developed to improve the WUE indicators to meet and exceed the benchmark values.
1.3 External and Internal Variables affecting WC/WDM Targets
There are specific differentiating factors that affect how a mine can achieve the water conservation and demand management targets, and the guidelines identify these according to specific classes. Some of these factors are inherent to the mine resources and area, which cannot be changed, whereas others can actively be impacted through specific actions or management procedures. Understanding these differentiation factors and their definitions is essential in developing realistic WC/WDM targets and highlights why they are unique to each mining operation. The guidelines proposed the following classifications:
Table 4: Classification of variables affecting WC/WDM targets
1.4 Developing a Water Conservation and Demand Management Strategy
Once the baseline for the various WUE indicators has been developed, the mine or an appointed consultant must develop an implementation methodology. This methodology should provide technical guidance on specific intervention projects that will improve the WUE indicators and improve the mine’s overall standing. These interventions should be site-specific, based on the local requirements of the mine, availability of water resources, and the requirements for the mining operations. These interventions should aim to improve the WUE indicators in the shortest possible time. They should be incorporated into the mines Integrated Water and Waste Management Plan (IWWMP), which is also a requirement of the Water Use License. The basic principles that should be followed to identify potential interventions are shown below:
Options to reduce consumptive water use
Options to reuse or recycle water
Options to identify and implement alternative technologies that are less water-intensive
Options to treat poor quality water to meet end-use standards and reduce water purchased or sourced from external parties
As these interventions are developed and evaluated for feasibility, the dynamic water balance can be used to evaluate the efficiency of improving the WUE indicators. It is recommended that the interventions be compiled as a table allowing for a rating regarding implementation cost, effect on WUE indicator, implementation time, and risk of failure or non-action. Part of the technical requirements of this strategy is to develop a high-level capital, and operational cost estimate for the proposed interventions as this directly influences the feasibility of the project. It is essential to consider the projected life of mine for the mine in question as return on investment is critical when new projects are being evaluated. Where further specialist studies or detailed designs are required, it should be included as a first phase of the project for implementation as soon as possible, and as part of the annual updates to the WC/WDM strategy these interventions should be further developed and expanded as the results of the specialist studies become available.
Once the most effective and feasible interventions have been identified and agreed upon by the mine operations, they are combined into a five year WC/WDM strategy with implementation dates that the mine has committed to achieving. This strategy should demonstrate that the mine and the consultants have considered all the viable water conservation and demand management interventions possible. Based on their specific mining conditions and environment, the optimal Water Use Efficiency targets.
The entire WC/WDM strategy will have to be submitted via the Standardized Water Accounting Framework (SWAF), an online database currently under development by the Department of Water and Sanitation as of the end of 2020. This online database will serve as an automatic tracking system for the progress with the submitted plan and the implementation of the various proposed interventions. It is also envisaged that the online system will update the WUE indicators for all mines which have submitted their WC/WDM strategies and data for the national benchmarks to be updated at a specified time. All mines will be required to update the WC/WDM strategy every five years to ensure it stays relevant and considers operational changes to calculate the WUE indicators accurately. It is crucial to study the current benchmarks and implementation documents for Water Conservation and Water Demand Management. The requirements for submissions of WC/WDM strategies on the SWAF online system will be stringent once implemented.
1.5 Potential Water Conservation and Water Demand Management Measures
The total mining footprint is inherently determined by the location of the ore reserves that are identified through exploration projects. The surrounding geology and aquifers determine the abundance or scarcity of water, and the mine is forced to develop the mining infrastructure around this.
1.5.1 Underground and Surface Mining Management Measures
To improve WUE indicators, the priority is to avoid ingress of excessive groundwater and management of water across the shaft and mining areas. This can be achieved through the following interventions:
Adjust underground mining and development plans to avoid water-bearing strata or aquifers, and where it cannot be avoided, developed engineered solutions such as bulkheads to prevent contamination of the ingress water and separate distribution system to ensure water remains pure until it can be discharged.
Improve / Optimize underground ventilation systems to reduce the requirement for underground cooling, a large consumer and source of contamination for process water due to the cyclic increase in salt load.
Backfill and seal off old underground mine workings that have been mined out to reduce potential ingress points. Preventing contact with fresh groundwater containing dissolved oxygen will also reduce the kinetics for the dissolution of sulfidic compounds that can generate acid and cause additional pollution.
Install online flow meters throughout the distribution network for both process and potable water to monitor leaks and wastage. Online metering and database monitoring allow for identifying trends or sudden spikes, which would typically indicate a leak or change in operation that should be addressed. This also allows for an operational usage audit of various sections of the mining footprint to identify poor practices and set specific targets to improve consumptive water use.
A quality audit of the water uses can determine whether the water source for specific activities is fit for use. There are usually opportunities to reuse service water or reduce additional water intake if specific quality requirements are met. This might be achieved by installing a basic filtration system to allow for process water to be reused for gland service instead of additional potable water being purchased.
Potable water purchased from municipalities or local water boards is one of the more expensive operational costs. It increases the overall water use of the mine, which detrimentally impacts the WUE indicators. Available water sources should be analyzed regarding the quality and potential for treatment to potable standards. This will improve the water reuse indicator and reduce overall mine water use by reducing the water pumped to the mine.
Proper stormwater management and infrastructure ensure that the maximum percentage of rainwater is contained and available for effective reuse. If the trenches and drain systems are insufficient, this could result in contamination of the water source and potential impacts on the surrounding environment. Ensuring there is sufficient storage capacity in the stormwater and pollution control dams allow for the maximum buffer capacity for the reuse of rainwater, especially in areas constrained with groundwater.
An evaluation of the open water storage dams and reservoirs on the mine can identify the extent of water losses due to evaporation and whether there is potential for interventions to reduce the surface area available for evaporation.
1.5.2 Minerals Processing Management Measures
Depending on the type of mineral being mined and processed, the various metallurgical processes can vary greatly. However, there is always a portion of the operations dependent on water availability to ensure it can work effectively. Through implementing specific control measures or best practices can result in improved water use efficiencies, such as the examples below:
Where possible, the dry conveyance of ore is recommended to replace the use of slurry pipelines. This requires significantly more capital and maintenance and is limited to the maximum distances feasible. However, it dramatically reduces water use requirements and the potential impacts in spillage.
Ore and waste rock stockpiles provide a large mass and surface area for contamination when rainwater is not effectively diverted. Stormwater management principles should be followed strictly to prevent contaminated runoff from entering the surrounding environment and minimizing seepage from these facilities. It is also essential to manage the levels and capacity of all storage dams or reservoirs to ensure there is sufficient capacity in the case of high rainfall events to prevent spillages. In some cases, the build-up of silt in these dams reduces the overall capacity and no longer meets the design specifications to ensure GN 1147 compliance is met regarding 1 in 50-year storm events.
Where leaching processes are used, as in gold processing, the Specific Gravity (SG) of the underground ore slurry feeding the leaching tanks are controlled through the primary thickeners. There are typically online monitoring and control measures capable of optimizing this SG. However, any additional water users such as for hose down, flushing, or make-up are collected and pumped into the final residue tanks in all the subsequent process steps. This results in significant dilution of the solid content of the final residue, which results in significant losses of process water to the tailings storage facility, where the bulk of the water is lost due to evaporation or seepage. Increasing the average residue slurry density from 1.3 to 1.35 will reduce the volume of water pumped to the tailings storage facility by 15.4%, which is an immediate reduction in water lost due to evaporation and seepage. This would require additional water recovery or management processes to be installed through monitoring or mechanical equipment installed in the residue management area of the plant.
1.5.3 Residue Disposal Management Measures
Residue disposal facilities or Tailings Storage Facilities (TSF) is the final step in the ore beneficiation process once the valuable minerals have been extracted from the ore. The responsible and sustainable disposal of the waste products will minimize the potential impact on the receiving environment and mitigate impacts on the water resources.
Historic waste rock dumps contain inherent leaching or stormwater runoff contamination risks. The reclamation or rehabilitation of these footprints can reduce the potential impacts and reduce the dust suppression requirements.
Guidelines recommend waste disposal into old opencast pit workings to reduce the surface footprint of waste after taking due consideration and having evaluated the potential impacts on water quality. This will effectively reduce the available contamination area for stormwater and assist with the free drainage of pits once adequately rehabilitated.
Seepage from Tailings Storage Facilities is usually managed by installing liners or drainage systems. However, there is always a risk of increased seepage due to external factors or imperfections in the designs. Seepage from tailings facilities poses a quality contamination risk to the surrounding environment and the loss of return water from the dams. Hydrogeological studies can identify the quantitative losses due to seepage. Based on the findings, it may be viable to install curtain drains or boreholes to collect the seepage water for reuse and pollution prevention.
New requirements under the National Environmental Management Act – GNR 1147 require that concurrent rehabilitation be implemented at all mining operations and included in the mining operational plan. By actively implementing rehabilitation methods such as sloping and vegetation cover, the water losses related to the facility can be reduced by decreasing the dust suppression requirements and reduction of seepage losses.