What influences mine effluent toxicity?

Clients and investors are increasingly giving consideration to companies’ environmental performance and social licence of a project, and these factors may determine their future investment decisions. Over the years, this has prompted many companies to establish strict environmental criteria aimed at helping them manage and reduce environmental risks. Toxicity analyses are essential for assessing and controlling the quality of discharged water and, more importantly, for detecting effluent toxicity.

However, once the test results are in, determining the source of the toxicity can be a complex process. This article discusses some of the issues that should be considered when testing effluents to reduce the risk of toxic emissions. It also outlines a methodology for considering all potential sources of toxicity.

Defining toxicity

Under the Canadian Environmental Protection Act, a substance is toxic if it is entering or may enter the environment in a quantity or concentration or under conditions that have or may have an immediate or long-term harmful effect on the environment or its biological diversity; constitute or may constitute a danger to the environment on which life depends; or constitute or may constitute a danger in Canada to human life or health.

Tests are generally conducted to determine effluent toxicity. Two types of tests are used: chronic toxicity tests and acute toxicity tests on contaminant-sensitive organisms. Chronic toxicity tests are used to estimate an effluent’s long-term effects by observing a test species and its ability to reproduce, grow and exhibit normal behaviour. Meanwhile, acute toxicity tests are used to estimate the short-term effects of effluent, including the mortality and mobility of a test species. In Canada, current regulations generally define acute toxicity test parameters, allowing for a rapid assessment of any immediate risk to aquatic life and quick action to reduce impacts.

There are several advantages to conducting toxicity tests. First, they determine whether an effluent is toxic. The toxicity may be caused by a single contaminant or a combination of contaminants and factors. Toxicity tests are conducted on selected control species, which are those that are sensitive to harmful substances and usually present in ecosystems. These tests provide a general idea of how the effluent would affect the receiving environment. While this analytical method has its advantages, it isn’t useful for determining which substance(s) may be causing the toxicity. The source of the toxicity may be unique, multiple or complex. Therefore, it must be established using a methodical investigation so that appropriate, standards-compliant corrective action can be taken.

Performing a thorough investigation

To evaluate the impact of mining effluents on receiving environments, several criteria are assessed before it is discharged. Some examples of assessed factors include heavy metals, hydrocarbons and suspended solids.

But effluent composition can sometimes be complex. Even when contaminant concentrations are reduced to fall below regulatory threshold values, chemical interactions between compounds can affect an effluent’s toxicity.

Having a solid understanding of the processes used in the mine and very good effluent characterization can help you determine the interactions between the different products being used and investigate the problem more effectively. In fact, a study on the identification and reduction of toxicity and toxic products^[1] could serve as a basis for establishing and sequencing the various steps of an effective investigation.

Gaining a thorough understanding of the effluent’s physico-chemical characteristics is helpful for predicting certain interactions and finding a solution to the problem.

Using effluents’ physico-chemical characteristics to your advantage

Parameters such as pH, conductivity, redox potential, dissolved oxygen, hardness and alkalinity can help characterize the effluent and identify the source(s) of toxicity.

For example, alkalinity and hardness are non-standardized parameters that influence acid buffering and water’s chemical stability respectively. Changes to these parameters can significantly impact the bioavailability of metals in effluent. An increase in freshwater hardness—mainly calcium and magnesium ions—can inhibit the uptake of metals by the cell membranes of some organisms, due to competitive interactions. Adjusting the alkalinity by adding carbonates can alter the chemical speciation of some metals. In turn, this can result in the formation of insoluble compounds and reduce the concentrations of toxic chemical species in the effluent. If effluent presents low alkalinity or hardness, it would be wise to analyze how increasing these parameters might impact the water quality.

Thiosalts are another factor that can affect mine effluent toxicity. Even though thiosalts aren’t listed as toxic substances, the consequences of their oxidation include the consumption of dissolved oxygen and the release of protons (H+) into the receiving environment’s water. This causes delayed acidification of the environment and affects aquatic organisms. Therefore, the critical threshold of thiosalts that can be released depends on the aquatic environment’s capacity. If the environment has a good dissolved oxygen levels and good buffering capacity, thiosalts can be discharged with little impact. However, in sensitive environments, thiosalt concentrations should be at the lowest levels. This is why having a thorough understanding of the receiving environment is important and can help you understand the situation and determine the best treatment method.

Modifying effluent treatment systems

Proper characterization of the effluent and receiving environment, as explained above, provides crucial information for identifying the various steps involved in defining the proper water treatment system to treat the effluent and/or acting at the source to remediate toxicity. To illustrate the need for understanding an effluent’s physico-chemical characteristics, let’s consider how effluent treatment systems can be adjusted to address ammonia. Total ammonia consists of ionized (NH4+) and un-ionized (NH3) forms. Both are toxic, but un-ionized ammonia is the more toxic form. Even when the total ammonia concentration falls within water quality guidelines for the protection of aquatic life,^[2] higher levels of the un-ionized form may increase the toxicity risk upon discharge.

Since the concentration of each ammonia species varies with pH, temperature and ionic strength, changes such as pH adjustment and operations within specific pH range can reduce the amount of un-ionized ammonia that gets released and, in doing so, reduce the risk of effluent toxicity.

Another case for modifying effluent treatment systems involves adjusting a biological treatment plant to remove sulphur or nitrogen compounds. The presence of certain heavy metals—even at concentrations below regulatory threshold levels—can inhibit bacterial activity, reduce the treatment capacity and continue to render the effluent toxic. Prior to the biological treatment phase, adding a pre-treatment step would carry out metal precipitation and removal below the levels permitted by regulation. Doing so can greatly improve the performance of biological treatment systems.

Conclusion

As we’ve seen in this article, good effluent characterization and a thorough understanding of the receiving environment are needed to establish an effective effluent treatment system. This information makes it possible to identify which substances and parameters are problematic and could be the cause of the effluent toxicity. A detailed analysis is needed, such as studies to identify toxicity and toxic substances and strategies for reducing their toxicity. This will help you determine the issue at hand, the most effective water treatment technologies (or the work required at the source) and, ultimately, the project cost.

Considering that discharging toxic effluents into the natural environment can adversely affect public opinion, it’s undoubtedly important to have a thorough understanding of water quality and the various factors that can influence it. Not only will this knowledge lead to optimal engineering solutions that meet the various environmental criteria defined by both regulatory bodies and the organization, it will also help the project’s social licencing.

Using a holistic approach that involves actively seeking client input, BBA can determine potential sources of contaminants linked to effluent toxicity and ensure compliance with regulatory requirements by implementing effective effluent treatment solutions. In addition, BBA’s multidisciplinary teams are familiar with mining processes and issues and can use this knowledge to design the best possible water treatment solutions for your operational capacity and budget.

If you have questions on this subject, please contact our water treatment experts.

References

[1] Guide d’évaluation et de réduction des toxiques, Quebec Ministry of the Environment and Wildlife, February 1996

[2] CCME, Canadian Water Quality Guidelines for the Protection of Aquatic Life, 2000, 2009 and 2010