WATER MANAGEMENT
> By Monique Simair
FIVE THINGS
you need to know about modern ‘passive’ bioreactors
B
iochemical reactors (BCRs) have a longstanding history in the environmental field; however, they have only been recently employed as a tool in water treatment of the mining sector. As this field of technology continues to grow, bioreactors are gaining interest in the world of “passive” water treatment and the technology is modernizing at a rapid rate. These passive bioreactors are built into the ground and designed with the intention to require little operator intervention. There are more than a dozen names and acronyms in use to describe different bioreactors depending on the style or brand of system. The generic term “BCR” will be used here for simplicity, this includes types such as mixed-media reactors and gravel bed bioreactors (GBBRs). Here are five things that you should know about modern in-ground BCRs.
1
IT’S NOT JUST PASSIVE OR ACTIVE
There are many approaches to designing in-ground BCRs. Designs must be matched not only to what is being treated, but also to site-specific objectives. Although a “walk away” method is sought after in water treatment, it is usually more reasonable to set a goal of operating as passively as possible. With BCRs, design measures can be put in place to adjust operations as needed, hence the modern language term “semi-passive” (as opposed to completely passive). Flexible operating modes can be adjusted depending on site specific variability, such as seasonality, flows, chemistries, and so on. These factors require modern in-ground BCRs to be supplemented with nutrients and other reagents as needed. Systems are also plumbed with the ability to flow water in different directions, making it possible to clean the system when necessary.
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2
SOMETIMES TEMPERATURE MATTERS, SOMETIMES IT DOESN’T
Some constituents are directly transformed by the microbes in the BCR (such as cyanide, thiocyanate, ammonia, biological oxygen demand, nitrite, nitrate, and sulphate). For this reason, the treatment of these constituents is temperature dependent. This means that the BCR must be sized accordingly. Although cold temperature bioreactors for these constituents may need to be sized larger, there are several examples of BCRs operating with water barely above freezing. In contrast, many metals and metalloids are treated indirectly in BCRs by the bacteria using coupled biogeochemical processes. This means the microbes create compounds in the BCR, which in turn treat the water. Under the right conditions, microbes can create stockpiles of these treatment compounds in the BCR (which we can measure and track). This ‘stockpiling’ of materials makes it possible to design and operate BCRs effectively in cold temperatures when treating metals and metalloids, with change in temperature having little effect.
3
THE “BLACK BOX” IS OPEN AND UNDERSTOOD
One of the greatest misconceptions that remains is that BCRs are a “black box.” BCRs are more complex than conventional physical and chemical water treatment due to their biological component. However, they are not purely biological. BCRs function through coupled reactions including physical, chemical, microbiological, and biogeochemical reactions. These must all be considered when selecting appropriate substrates and reagents for a system. The use of reagents should be purposeful and targeted. For example,
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