Why I Worry About Advanced Oxidation Water Treatment

Advanced oxidation processes are becoming more prevalent in wastewater, recycled water, and drinking water treatment. There is no doubt that these processes are very effective at treating a wide range of otherwise difficult to treat for chemicals from whatever source you start with. But what happens to the chemicals we are treating for when we use advanced oxidation? And could we be creating a bigger problem than we started with?

Advanced oxidation (AO) refers to treatment to remove chemicals by oxidation through reactions with hydroxyl radicals.  Most commonly, this is achieved by the addition of either ozone (O3) or hydrogen peroxide (H2O2) and then exposure to UV light.  The process is very effective; if you have a given chemical in your source and then treat it by an AO process and re-test it, the chemical will be found at a greatly reduced level or even be completely gone.  But where did it go?  This is not an adsorptive process like ion exchange or treatment with GAC; the chemical is not being physically removed from the water. The Law of Conservation of Mass, as well as common sense, dictates that it cannot simply disappear. And AO treatment does not break chemicals down all the way to their individual atomic constituents. So what’s really happening?

The AO process simply changes the chemical into something else.  Usually, a chemical is broken down into smaller chemicals, although that is not always the case.  Sometimes its form is simply modified.  So what you end up with after advanced oxidation is not contaminant free water.  You have simply traded one contaminant for one or more others.  That is the point at which I start to worry about the AO process.  To oversimplify, the AO process takes one contaminant that we may or may not understand the toxicity of, and modifies it into one or more different contaminants that we probably know even less about.

Some research has been done on this issue, but not nearly enough.  One group of researchers show how the cancer drug cyclophosphamide (1), when treated by AO, has as its main reaction product 4-ketocyclophosphamide.  You can see from the chemical structures in Figure 1 that the reaction product is not much changed from the parent compound.

 If you analyzed your water after treatment, it would appear the cyclophosphamide had gone, which it has, but only to be replaced by a very similar compound.  Is that good? Is the water after treatment more protective of the environment and of public health?  I don’t think we have any idea, which is exactly the point.

Another excellent paper that came out in 2007 in The Journal of the International Ozone Association (2) reviews the knowledge of a wide range of compounds and how they react in the AO process. In the paper, the authors state “In some cases, disappearance of parent pharmaceutical compounds does not indicate successful treatment because the degraded products may be as biologically active as the parent compounds.”  The degraded products may be as biologically active as the parent compounds.  Or they may not.  Or we may have absolutely no idea if they are or not, so we may have no idea whether what we are considering treatment isn’t itself a source of contamination. 

In the same paper, one of the compounds reviewed is carbamazepine, a widely used anticonvulsant that “has been found ubiquitously in the aquatic environment.”  The reaction products of carbamazepine after AO are several, and have names far too long for me to type out here. But the authors recognized that these reaction products were “polycyclic heteroaromatics known to be toxic to aquatic organisms.”  Are they more or less toxic than the carbemazapine itself? Do they have synergistic effects that cause them to be more toxic working together than separately? Again, we just don’t know.

Just this year, a paper was published describing a new tool that is available to try and answer the question of how compounds break down when subjected to AO. In the ACS Journal Environmental Science & Technology, Xin Guo, et al (3) gives the basis for a model that can be used to “predict the degradation mechanisms and fates of intermediates and byproducts produced during aqueous-phase advanced oxidation processes for various organic compounds.”  That’s sounds like a great tool that those who implement these processes should look into to help predict what’s actually happening during treatment.

AO is a promising technique that has proven applications in many water treatment scenarios, whether you’re dealing with wastewater, drinking water, or water recycling.  But a great deal more study needs to be done to make sure we aren’t creating bigger problems than we currently have.  I call upon all of those involved in the issue, manufacturers, end users, and industry associations like AWWA, WRF, and WEF, to support the investment needed for research into these questions.

1)                  Hernandez C, Fernandez LA, Bataller M, Lopez A, Veliz E, Ledea O, Alvarez C, Besada V, Cyclophosphamide degradation by ozoneunder advanced oxidation process conditions, IOA 17^th World Ozone Congress, Strasbourg, 2005, VI.3.2-1-11

2)                  Ikehata, K.; Naghashkar, N.J.; Ei-Din, M.G. Degradation of aqueous pharmaceuticals by ozonation and advanced oxidationprocesses: A review. Ozone Sci. Eng. 2006, 28, 353–414.

3)                  Computer-Based First-Principles Kinetic Modelingof Degradation Pathways and Byproduct Fates in Aqueous-Phase Advanced OxidationProcesses, Xin Guo, Daisuke Minakata, Junfeng Niu, and John Crittenden; Environmental Science & Technology 2014 48 (10), 5718-5725