Author: Hannah Chiew, P. Eng., Senior Process Engineer, Ensero Solutions
This article is part 2 of a 3-part series on biological water treatment for mine impacted water. It is a reference document for mine planners and engineers to understand when to consider biological treatment options and how to identify important selection considerations for biological treatment systems.
Key Words: Biological Treatment, In Situ Pit Lake, In Situ Underground Mine Pool, In Situ Groundwater Injection, Constructed Wetland Treatment Systems, Biochemical Reactor, Gravel Bed Bioreactor, Passive Treatment, Semi-passive Treatment, Mine-Impacted Water
Biological (Bio) approaches to water treatment are an innovative and often cost-effective part of the technology toolbox for mine operations, closure planning, and site remediation. With application options such as passive or semi-passive, ex situ or in situ, Bio treatment approaches allow mine owners to implement a treatment approach that can be specifically tailored to site needs.
In Part 1 of this series, we introduced Bio systems and which technologies can be applied in situ or ex situ, with increasing degrees of operational effort (passive to active systems). This article discusses in more detail how to determine if a Bio treatment system is right for your site.
Selecting the right treatment technology for your site typically requires a selection process that evaluates available water treatment options. Technology selection should take the site owner’s needs and resources, site-specific water quality objectives, and site constraints into consideration. This collaborative process, as shown below, is between the site owner, engineers, geochemists, and water treatment experts and should evaluate the benefits and risks of all physical, chemical, and Bio treatment approaches considered. Often, because of a lack of familiarity and misconceptions about Bio treatment, Bio approaches are overlooked.
What Will a Bio Treatment System Treat?
The viability of a Bio approach depends first on the site-specific constituents of interest (COI) and water quality objectives. Under certain conditions and COI concentrations, Bio treatment systems remove metals (e.g., antimony, cadmium, cobalt, chromium, copper, iron, nickel, lead, and zinc), metalloids (e.g., selenium), sulphate, nitrogen compounds (cyanide, ammonia, nitrate, nitrite), phosphorous, chemical oxygen demand (COD), and biological oxygen demand (BOD). Bio Treatment systems are not typically used to treat readily soluble monovalent ions such as sodium, potassium, lithium, bromide, chloride, and fluoride.
Bio treatment systems can also handle a wide range on influent flow volumes. However, passive or semi-passive Bio treatment systems are typically applied to low-flow scenarios (< 200 gpm), and high-flow scenarios can be implemented with careful design considerations (1000 gpm or more). There are generally no limitations to the sizing of active Bio treatment systems as they can be designed as modular systems and are only limited by the viability of the mechanical equipment component.
What Site Conditions Support the Use of Bio Treatment Systems?
Bio treatment system site considerations include:
Water quality. The chemistry of the water to be treated largely dictates the design of a Bio treatment system, besides the flowrate. While certain COIs are not amenable to Bio treatment as outlined above, there are limitations in the concentration of the COIs that can be treated. Some parameters at above certain levels can have adverse effects to a Bio treatment system due to toxicity effects. For example, high salinity or some metals above a threshold concentration could be toxic to the bacteria present in the Bio treatment system.
Degree of treatment. There are limitations on the level of treatment that can be achieved by a Bio treatment system, compared to physical/chemical treatment-based systems.
Available land or footprint. An active Bio treatment system will likely require more area than a physical/chemical treatment-based system that achieve the same level of treatment but will occupy comparatively less area compared to a passive Bio treatment system such as Constructed Wetland Treatment Systems (CWTS). System sizing varies according to the basis of design. In-situ Bio treatment technologies (e.g., pit lakes) will also require less area than ex-situ Bio treatment technologies.
Site owner’s needs – see details below.
Specific site considerations that affect the selection of passive/semi-passive Bio treatment systems include:
Location. Passive, semi-passive, and in-situ Bio treatment technologies have low maintenance requirements and as a result, a Bio approach may be the ideal choice for a closed or remote site.
Topography. The topography of the site is critical for the selection of CWTS. For example, areas with steep terrain and/or where the water table is close to the surface would not be suitable for implementation of CWTS.
Climate and hydrology. Areas that have more extreme climate conditions (arid, semi-arid, freezing or frozen surfaces or high freshet flows) may not be suitable for CWTS implementation.
Local material availability. Passive bio treatment systems typically require substantial amount of substrate (e.g., gravel, sand, woodchips, hay, straw) and/or carbon sources, and the cost/availability of the required substrate could determine the feasibility of its implementation.
Site Owner's Needs
The business needs of the site owners are critical in selecting the best water treatment system. Bio system options are sustainable, green, and promote ESG values. They provide an opportunity for improvement if the Site Owner has any of the following challenges.
Local Community Perceptions. Local communities may be more accepting of biological-based treatment systems that are effective yet sustainable.
Cost. The capital and operational costs of bio treatment systems can vary. However, they are often less expensive than an active physical/chemical water treatment approach such as lime precipitation or reverse osmosis. Dr. Vanessa Friesen’s article here contains more information on relative costs of Bio systems.
Timeline. Some Bio treatment options, specifically in-situ treatment options, can treat large volumes of water in a relatively short timeline, whereas CWTS would require a relatively substantially longer period of implementation.
Why does this matter?
Bio treatment approaches allow mine and industrial site owners to implement treatment in more cost-effective and environmentally friendly ways that can be specifically tailored to the site’s needs. If any of the COIs, site conditions, or owners needs outlined in this document describe your project, talk to an engineering consultant about your Bio treatment options.
Missed the first article in this series? You can find it here.