There are two main high-level ways to conceptualize how contaminants are treated: transfers and transformations. Understanding these two general mechanisms can help you with the outcomes of any form of water treatment and better evaluate the risks and benefits of particular treatment options.

Transfers move constituents from one place to another, but their form remains the same. In contrast, transformations involve a fundamental change in the properties of the constituents itself.

Transformations

Changes form

• Reduction

• Oxidation

• Ionization

• Photolysis

• Hydrolysis

• Mineralization

• Biodegradation

• Assimilation

Transfers

Changes place

• Membrane Separation

• Ion Exchange

• Precipitation

• Sorption

• Volitization

• Settling

• Filtration

• Dilution

• Dispersion

All transfers include a movement of constituents from one location to another. However, the elements stay the same. For example, the same compound is present in the water and in the resin during ion exchange, it just moved out of the water and into the resin.

Transformations are often chemical or biological reactions that change the fundamental structure or composition of a constituent. For example, we will consider how road salt builds up on cars in the winter and how calcium builds up on taps. The process of salt building up is a transfer, it goes from the water on the road to your car. In this situation the salt does not change form. This is why the salt can easily be washed off (i.e., it was transferred to the car, and can be transferred back to water). However, the calcium buildup is an example of mineralization. Transformations such as mineralization affect the stability of the newly formed compound and cannot be readily reversed without applying additional reactions or processes. Calcium buildup cannot be readily removed with more water, and instead require another reaction (such as soaking in vinegar).

In the same way in which we can target transfers in active water treatment (i.e., creating a settling pond or designing a membrane) we can target transformations in a semi-passive system. However, a combination of both processes is usually utilized as this improves robustness and predictability. For example, selenium might be absorbed onto organic compounds in a wetland and then reduced by microbes.

One important drawback to using only transfers is that you will need a plan for what happens to the concentrated constituents after treatment. Unless you can pair it with a transformation you will need to manage the disposal. Constituents haven’t changed form, only changed their location, so a disposal plan is required to remove the risk of recontamination. Not all constituents behave the same way or can be treated with the same transformations. You need to know the properties of the constituent and conditions necessary to undergo the transformation you are targeting.

References:

Haakensen (Simair) et al., (2015). Key Aspects for Successful Design and Implementation of Passive Water Treatment Systems. Journal of Environmental Solutions for Oil, Gas, and Mining, 1(1), 59–81. doi: 10.3992/1573-2377-374x-1.1.59