What is semi-passive water treatment? The key to understanding is the prefix semi. Previously, these types of systems were often referred to as passive water treatment. However, there has been a shift in industry to semi-passive because passive was often being confused with walk away or do nothing. In a few situations, there can be entirely walk away treatment performed, but these are quite rare and unique.  

Walk Away (i.e., Passive Treatment): No operations or maintenance

Passive Care (i.e., Semi-Passive Treatment): Operationally passive, with infrequent periodic long-term maintenance.

Semi-Passive systems have the benefits of having operationally infrequent (periodic) or no: 

• Electricity (or minimal solar/wind)

• Addition of chemicals 

• Substrate replacement

• Day-to-day human intervention 

 However, there are periodic maintenance considerations, such as:

• Pipes

• Liners

• Contaminant loading

• Structural lifespans

• Wildlife

Active treatment systems such as ion exchange or membranes (e.g., reverse osmosis) rely on ongoing human management to maintain and operate the system along with regular replacement of components. Semi-passive systems typically rely on microbes to complete treatment reactions. However, semi-passive systems are not walk away. They do require some periodic, planned maintenance and upkeep that often decreases over time. All types of water treatment systems, including semi-passive ones, should be designed with adaptive management strategies, which will be discussed in further detail in another session.  

Types of biological semi-passive treatment technologies

While there are some semi-passive water treatment technologies that are based on chemical reactions (such as limestone beds and zero valent iron), this session is focussed on biological and biogeochemical technologies. Biogeochemical reactions involve biology, geology, and chemistry all at once.

All these systems exist on a continuum from very active to very passive and there is no one single design type. There are a lot of considerations when planning for semi-passive treatment including host rock geology, water chemistry, etc. More information on each treatment technology will be covered in future videos.

• Constructed Wetlands, also called Constructed Wetland Treatment Systems (CWTS), Treatment Wetlands (TWs), Engineered Wetlands, etc

  • Can be found in both municipal and industrial settings, but designs for municipal wastewater are very different than designs needed for treatment of metals and metalloids

• Bioreactors also known as biochemical reactors (BCRs), such as Gravel Bed Bioreactors (GBBRs), In Ground Reactors, Mulch Reactors, Solid Phase Reactors, Packed Bed Reactors, Trickle Filters, Permeable Reactive Barriers, etc

  • Can be very passive or entirely active 

• In Situ Treatment, including Pit Lakes, Saturated Rock Fills, Tailings Ponds, and flooded underground Mine Pools

  • In Situ means ‘In Place’, so the water is treated in place without being pumped elsewhere 

  • Often done by repurposing existing mine infrastructure

Biogeochemistry is like baking

Essentially all the above semi-passive treatment technologies are variations on bioreactors. For example, in constructed wetlands the plants facilitate the hydrology and provide nutrients where a bioreactor needs nutrients and carbon added, and in situ treatment uses the mine infrastructure as the bioreactor ‘vessel’.   

The foundation of all these semi-passive technologies are coupled biogeochemical reactions (reactions that involve biology, geology, and chemistry all at once). It sounds complicated, but this science is now quite well understood.

Biogeochemical reactions can be compared to baking a cake. The key is in understanding the components (or ingredients) and their purpose for existing in the recipe. Once you know this, it is possible to predict the impact of adding more or less of each individual ingredient. If you triple the baking soda, it will have a big impact on texture, shape, and flavour. However, if you triple the chocolate chips everyone will ask for more! Understanding the reaction components is necessary to understand how changes will affect the outcomes!  

Upkeep of a water treatment system is like upkeep for your house

A good way to understand the differences between active, semi-passive, and passive treatment is using the example of a house. A totally passive approach would be doing nothing. Not cleaning the sink, not patching the roof, and not fixing leaks. Technically you could live there, but over time the condition and ability of the house to serve as a home would deteriorate. If you were going to live semi-passively, this would include fixing the broken items, cleaning up periodically, and having a schedule for major jobs (new paint, shingles, etc.). For this you know what needs to be eventually maintained, but day-to-day you are not working on it. Active treatments can be thought of like an Airbnb. In this situation a lot more day-to-day effort is needed on top of what is required to live semi-passively. Things like cleaning between guests, arranging decorative pillows, and keeping the house Instagram ready are all required regularly. No style is inherently better than the other, but the actions required depend on the choice of house (or treatment system).