Biogeochemical water treatment, including semi-passive treatment (e.g., constructed wetlands, bioreactors, and in situ treatments) can be used to treat different constituents of concern in the mining, oil, and gas industries. You can learn more about what semi-passive treatment is in the previous video and article titled What is Semi-Passive Water Treatment? The technologies we are focussing on here use biogeochemical processes to treat water. Just like active treatment, they can have different effectiveness and capabilities depending on their design.  

Biogeochemical treatment relies on transformations, which fundamentally changes the properties of a constituent or compound. In contrast, a transfer changes the place/location of the contaminant without changing the properties and can be paired with biogeochemical reactions. You can learn more about these processes here: How are Contaminants Treated?

What cannot be treated biogeochemically (semi-passively)?

Sometimes it is helpful to first rule out what cannot be treated.  In general, elements commonly found as salts, such as sodium, potassium, chloride, and possibly fluoride (Na, K, Cl, and F), are not considered passively treatable as individual elements. However, compounds containing these elements can be treated, such as fluorocarbons and chlorinated compounds. For our fellow science and engineering nerds out there - a good rule of thumb is that a compound or element requires redox sensitivity to be treated biogeochemically. In some semi-passive treatment systems, such as treatment wetlands, concentrations of salts can sometimes increase due to evaporation, so this needs to be kept in mind.

What can be treated biogeochemically?

Moderately treatable semi-passively: 

• Sulphate 

  • Can be reduced to sulphide, but the amount of sulphide that can then be removed from the water is limited stoichiometrically by concentrations of metals such as iron

  • These approaches are more commonly semi-active as significant amounts of electron donors (e.g., carbon) and additions of metals such as iron are often required to achieve treatment balance for sulphate.  

• Acidity

  • Reducing/anerobic systems produce some alkalinity, but in a limited amount.

  • Systems can be built with lime incorporated for alkalinity, or it can be added periodically and operated semi-passively. However, the lifespan of the lime needs to be considered for replacement along with maintenance for armouring and clogging.  

• Herbicides & Pesticides

  • Not usually a factor at mines, oil, and gas operations, but may sometimes be encountered, especially during closure and reclamation activities if attempting to control invasive plants.

  • Some solvents and reagents (e.g., xylene) are also classified as herbicides.

    Need to be considered in design, but can also be treated for (e.g., may use a bioreactor instead of a treatment wetland).

The concept for the figure above came from Gusek 2009 (link below, as well as a newer version - note, we have no affiliation with these authors or articles, but found them to be interesting and informative!). Here we have built upon that concept and created a new table focused on showcasing the biogeochemical treatability of common constituents of potential concern found in waters associated with mining, oil, and gas operations. Elements are categorized by treatment mechanism (for more information see our post How are Contaminants Treated?). 

While a great deal of contaminants are treatable, semi-passive systems are not a “magic bullet” for these contaminants. Semi-passive systems must be designed properly for the unique site-specific considerations in order to ensure functionality. A system’s treatment capacity is dependent on many parameters, including design aspects such as:

• Inflow Concentration (Ci)

• Desired Outflow Concentration (Cf)

• Flow Rate (Q)

• Area (A) 

• Comprehensive Water Chemistry

• Site-specific considerations (seasonality, periodicity of flow, etc)

Moreover, considering the big picture and incorporating site-specific opportunities and considerations into design is critical. For example, you could have a readily treatable contaminant, but lack an ideal pH for that process. Other considerations include temperature, infrastructure, and timeline. Some reactions will occur more slowly at low temperatures, while others do not. Additionally, space and timeline considerations need to be accounted for.

While there are a lot of factors to consider in determining what can be treated at a given site, these site-specific considerations are taken into account in the mScout free web app water treatment technology selection tool. You can use it or read more about it here.

References

Gusek, J. (2009). A periodic table of passive treatment for mining influenced water, 26th Annual Meetings of the American Society of Mining and Reclamation and 11th Billings Land Reclamation Symposium, Billings, MT. doi: 1. 10.21000/JASMR09010550.

Gusek, J. J. (2013): A Periodic Table of Passive Treatment for Mining Influenced Water - Revisited. – In: Brown, A.; Figueroa, L. & Wolkersdorfer, Ch.: Reliable Mine Water Technology (Vol I). – p. 575 – 581; Denver, Colorado, USA (Publication Printers).