By Dr. J.H. Wakefield, consulting analytic chemist and a practicing chemical engineer
Principles and operational parameters to help today’s wastewater plant operators figure out where AOP should and shouldn’t be used
One of the current buzzwords in the waste treatment arena is AOP (advanced oxidation processes). It may have several different meanings depending on who is doing the talking, but it usually refers to enhanced methods of oxidizing wastestreams or components thereof. It should be used primarily in either the collection system or in the outgoing effluent in the final treatment phase of the wastewater treatment process. It may be used in the waste treatment plant as well, but it — like fire — is a faithful servant but a terrible master. Using enhanced oxidation processes within most treatment plants are ill-advised undertakings. So, in this introduction, I will confine my remarks to the collection system, the final “polishing” system, and a careful examination of the true nature of what AOP is.
The most commonly used applications involve odor suppression, degradation of recalcitrant components of the wastestream, and control of microorganisms, usually pathogenic ones. How, when, and where these are affected will comprise the remainder of this introduction.
There are four basic applications where AOP usage is of great benefit:
Let us concentrate our efforts with respect to ozone, especially with aerobic mixers, to three application areas: grease traps, selected lift stations, and downline treatment areas such as polishing ponds and effluvia of various sorts.
Ozone has usually been applied as an oxidizing agent and as a sterilant. The latter results from its oxidizing characteristics exclusively. We may utilize it to do both, though a primary use is for it to break down recalcitrant molecules encountered in FOG (fats, oils, and grease) deposits as well as to attack the unusually occurring compounds encountered in various wastestreams that are difficult to treat in other manners. Examples of these may include various hydrocarbons — heterocyclics, polyaromatic hydrocarbons, and other aromatic hydrocarbons that are difficult for bacteria to degrade and/or are toxic for specific microorganisms used in the degradation processes or for some other reason present aggravation in this manner.
In the cases where ozone treatment is effective, the resulting compounds formed are carbon dioxide, water, and various other oxides.
Another use found for ozone is to place an adsorbed charge on microparticulates engendered in aerobic mixers. This enables us to prevent them from settling out in the lines, to stabilize the suspensions formed so that they are pumpable, and to enable plant treatment operators to avoid “shear” problems in their clarifiers.
Another related application for aerobic mixers is the delivery of hybrid ozone to fluid streams containing metallic ions so that they precipitate or become complexed, making them more insoluble and easily removed.
The sizing of the delivered ozone depends on the actual problems encountered, the horsepower of the aerobic mixer’s blower, the number of ozone tubes necessary to deliver whatever level of ozone we feel is adequate for the actual problems encountered, and taking into account any downline considerations such as impacting on microbial populations that are not targeted.
Grease traps require the highest ozone levels to effectuate grease management, to control odors (particularly H2S), and to stabilize the suspension formed. The effect on the microbial community here is a relatively minor concern, as any buildup in microbial numbers would adversely affect any force main in the collection chain. Therefore, the sterilization effect of the ozone is a good thing in grease traps. Because of the relatively immediate reaction of the ozone we are using, dwell time and downline residuals can be safely ignored.
A more complex decision situation occurs where aerobic mixers are placed in lift stations. Here, we must consider several additional factors, mainly the distance from the waste treatment plant and, of course, the size of the lift station and where the situs of delivery is — that is, is it a force main, a secondary station, or a gravity-feed to the waste treatment plant? The reason that this must all be carefully considered is that we want to treat the wastestream and not adversely impact the treatment system by wantonly killing off essential microorganisms in the process.
At this point, the ozone generation becomes of interest as different ozones have different reaction characteristics, half-lives, and residual effects. We are discussing the use of a hybrid ozone that delivers an almost instantaneous reaction from both ozone itself and free-radical hydroxyl ions formed in this process of ozone generation. Residual moieties may be encountered, which are normally peroxide activeradicals. We have to be careful to limit these other moieties as they may be transported through the collection piping to affect the microorganisms in the waste treatment plant. Naturally, both the location and velocity of the wastestreams are taken into account. The sterilant effects of hybrid ozone come to the forefront in application to downstream effluvia, polishing ponds, and the use of various aerators to apply in most of these locales.
Let us examine the chemical nature of ozone so that we may better understand its effects.
Ozone is an allotrope of oxygen; that is to say, it is oxygen that manifests in a different atomic form. Normally, oxygen may exist in one of three allotropic forms — in the first, there is a single oxygen atom (O); in the second, there are two oxygen atoms (O2); and in the third, there are three oxygen atoms (O3). The first is known as monomolecular oxygen, the second is commonly encountered and called diatomic oxygen and more commonly referred to as molecular oxygen, and the third is called triatomic oxygen, more commonly referred to as ozone.
Chemically, oxygen is the second most active nonmetal. Metals are atoms that lose electrons and engender regions around the atom that are positively charged (+); conversely, nonmetals are atoms that gain electrons and engender regions around the atom that are negatively charged (-). All atoms in their native state consist of a neutral region caused by the balance of + and - charges, and these, in turn, are the result of the gain (or loss) of electrons. As an atomic nucleus has a + charge, the loss of an electron results in a net positive charge, and the gain of an electron conversely results in a net negative charge.
As these elemental atoms become larger, the tendency to become more active metals increases as the positive pull of the nucleus becomes weaker as a consequence of the outermost electrons being further removed from the nucleus. In nonmetals, this is just the opposite, as nonmetals are more active as the pull of the + nucleus affects the outermost electrons that are nearer to it, and nearer, in this case, relates to the decreased atomic size of the atoms.
As mentioned earlier, ozone is primarily used as an oxidant, and most of the other applications can be traced to this. To measure the activity of various oxidants, we can look at the oxidation-reduction potential of common oxidants comparatively.
The following are measured at 25°C (77.0°F) and are reported in volts:
Similar comparisons hold for various commonly used sterilants measured against ozone.
Hydroxyl radicals are generated by two means: (1) by breaking down hydrogen peroxide (H2O2) by irradiating the water with 254 nm UV or (2) combining ozone and water in the presence of 254 nm UV.
The AOP that we are discussing is employed in aerobic mixers by the second method of generating hydroxyl radicals in the presence of 254 nm UV. This system is compact and produces a concentrated and more effective oxidation process when applied using aerobic mixers from specific wastewater technology vendors.
When ozone reacts with BOD (biochemical oxygen demand) compounds, COD (chemical oxygen demand) compounds, FOG compounds, and combined halogens, it reduces (actually oxidizes) contaminants to nonproblematic compounds, even to carbon dioxide and water. Inert materials may be saline, silicon dioxide, or various other degraded compounds that are usually insoluble.
Ozone attacks many heavy metals in solution, e.g., iron, manganese, zinc, copper, and others. This advanced oxidation processing results in the breakdown of many pharmaceuticals as well as the killing of coliform bacteria and many viruses. Ozone may also oxidize hydrogen sulfide (H2S) to (1) sulfur dioxide, SO2, or (2) sulfite, SO3=, or even (3) sulfate, SO4=
This is the major reason AOP is so effective for air quality and odor control.
AOP (ozone combined with hydroxyl radicals) is 100 to 200 times more effective when properly injected directly into the water column where the hydrogen sulfide is produced. This hybrid ozone carries a half-life of up to 15 minutes. This half-life is variant depending on the wastestream treated; therefore, it is not to be confused with the “half-life” used as a designation for many radioisotopes, which are constant.
Usually ozone is applied as a fogging agent; this does not reach the source of the hydrogen sulfide. This method of fogging ozone is ineffective at destroying the source of the problem and often creates a more severe corrosion problem. The fogging approach reacts only with H2S gas as it is released from the water column. To effectively eliminate H2S, one must properly inject the ozone directly into the water column where the H2S is originating.
The most efficient and, therefore, effective means of injecting ozone or concentrated oxygen into a fluid column requires a unique combination of coarse and fine bubble diffusion, released in a confined space or vessel under minimal pressure (<2.5 psi). Air under pressure generates heat, and heat can reduce oxygen transfer by as much as 80 percent. This may be achieved by means of a low-pressure, high-volume regenerative air blower.
Ozone And Advanced Oxidation Combined
Hydroxyl radicals are an even more potent and powerful oxidizer than ozone alone. The combination of ozone and hydroxyl radicals provides one of the most powerful oxidation products, which substantially reduces organic loading as well as microorganisms in wastewater.
This hybrid ozone generation method allows one to:
AOP combined with an aerobic mixer delivery technology can now be used on varying scales to address a wide variety of difficult or previously impossible cleanup problems applying to wastewater treatment and environmental issues.
Dr. J.H. Wakefield has been a consulting analytic chemist and a practicing chemical engineer (environmental/materials) for more than 30 years. He thanks DO2E for providing the AOP system and the aerobic mixers he discusses here.