MBR AOP Desalination Corrosion

Trussell Technologies has the expertise to evaluate innovative oxidation technologies for water and advanced wastewater treatment, including UV/Photolysis and Advanced Oxidation Processes, such as UV/Hydrogen Peroxide, Ozone, and Ozone/Hydrogen Peroxide.
Processes
UV/Photolysis and advanced oxidation processes (AOPs) are complex technologies affected by many factors. Trussell Technologies has the expertise to provide guidance to utilities in their application.

UV/Photolysis
UV/photolysis is a process in which compounds absorb photons and the energy released drives oxidation processes induced by light. The photolysis rate of a compound can be estimated based on the compound’s light absorption rate and quantum yield. Some organic compounds, such as NDMA, can be reduced by photolysis alone. The reduction of many other compounds is aided by the addition of hydrogen peroxide to generate hydroxyl radicals in an advanced oxidation process, as discussed below.

UV/Hydrogen Peroxide
In the UV/Hydrogen Peroxide (UV/H2O2) process, the photolysis of hydrogen peroxide generates hydroxyl radicals, which oxidize target compounds. Many compounds are not amenable to UV/Photolysis but can be readily degraded using advanced oxidation processes, including such compounds as 1,4-Dioxane and MtBE. Advanced oxidation processes (AOPs) are an attractive technology because the reaction rate of target compounds with hydroxyl radicals is often several orders of magnitude higher than with any conventional oxidant.

Ozone
Ozone is classified as an AOP because the ozonation process generates hydroxyl radicals and target compounds are oxidized both by the direct reaction with ozone and by reaction with hydroxyl radicals. The reaction with hydroxyl radicals is much more important because the rate constant for the reaction of the target compound with hydroxyl radicals is typically several orders of magnitude higher than the apparent rate constant for the reaction of the target compound with ozone. For some recalcitrant compounds, it is necessary to add hydrogen peroxide to increase the production of hydroxyl radicals, as discussed below.

Ozone/Hydrogen Peroxide
In the Ozone/Hydrogen Peroxide (O3/H2O2) process, ozone reacts with hydrogen peroxide to generate hydroxyl radicals. The hydroxyl radicals in turn oxidize target organics. The O3/H2O2 process has been applied to full-scale facilities for the oxidation of taste and odor (T&O) compounds MIB and Geosmin during seasonal T&O episodes.

Factors Affecting Advanced Oxidation Processes
There are numerous factors that affect advanced oxidation processes. These factors will be broken down into factors that affect all AOPs, factors that affect the UV/H2O2 AOP, and factors that affect the O3/H2O2 AOP. Trussell Technologies has the knowledge to help a utility optimize an AOP considering the factors discussed below and more:

Process Modeling and Evaluation

Trussell Technologies has the expertise to guide utilities in the application of Advanced Oxidation Processes. Dr. Rhodes Trussell, Dr. David Hokanson, Ms. Aieta, and Dr. Shane Trussell recently completed a 10% design for a UV/H2O2 full-scale process application in California. At the preliminary design stage, both the Ozone/ H2O2 process and the UV/H2O2 processes were considered and it was determined that the UV/H2O2 process was most applicable. The evaluation considered all of the factors affecting AOPs shown on this web page. The analysis applied the AdOx™ model discussed below.

Dr. David Hokanson is a Co-Inventor of Advanced Oxidation Process Simulation Software (AdOx™), which can dynamically simulate parent compound destruction considering all identified photochemical and chemical reactions for the UV/Photolysis and UV/H2O2 processes for both low pressure UV (LPUV) and medium pressure UV (MPUV) lamps.

With AdOx™, Trussell Technologies can readily:

  • Optimize energy efficiency by comparing electrical efficiency per log order (EE/O) of compound destruction
  • Consider LPUV and MPUV lamps for an application
  • Optimize hydrogen peroxide dosage
  • Consider background water quality including alkalinity and presence of NOM
  • Consider non-ideal mixing in the photochemical reactor

Factors that Affect All AOPs
1) Presence of carbonate species
Bicarbonate and carbonate ions present in the background water matrix will scavenge hydroxyl radicals and reduce the reaction rate with organics.

2) Presence of natural organic matter
Natural organic matter (NOM) present in the background water matrix will scavenge hydroxyl radicals and reduce the reaction rate with organics.

3) pH
The pH dictates the level at which certain ions important to AOPs will be present, including carbonate ion, bicarbonate ion, and the anion of hydrogen peroxide (HO2-). The pH affects the charge on target organics if they are weak acids or bases and in some cases the ionic form has a rate constant one or two orders of magnitude higher than the molecular form.

4) Presence of reduced metal ions
Reduced metal ions, such as Fe(II) and Mn(II), present in the background water matrix will scavenge hydroxyl radicals and reduce the
reaction rate with organics.

5) Reactivity of the target compound with hydroxyl radicals.
The general reaction for destruction of a target compound in an AOP is:

The second order hydroxyl radical rate constant is an indication of how the AOP
reactions will proceed. AOP reactions tend to be quite rapid with second order
hydroxyl radical rate constants on the order of 108 to 1010 L/mole•s. The higher the
second order hydroxyl radical rate constant, the more amenable the compound is to
reduction by an AOP. A sampling of second order hydroxyl radical rate constants is
provided in the table.

Although the second order rate constants of bicarbonate ion, carbonate ion, and
NOM are typically lower than the target compounds, their concentrations are often
several orders of magnitude higher than the target compounds, increasing the
importance of the presence of these species in the background water matrix.

6) Assimilable Organic Carbon
Reactions between hydroxyl radicals and natural organic matter present in the background water matrix will produce assimilable organic carbon (AOC). By definition, AOC is biodegradable and EPA encourages biological filtration to remove AOC when ozonation is applied. The UV/H2O2 process has typically been applied to waters with low TOC so the biological filtration step has not been consistently applied. AOC should be considered in the application of an AOP.

Factors that Affect the UV/H2O2 AOP
7) Photolysis of Hydrogen Peroxide
Photolysis is a process in which compounds absorb photons and the energy released drives oxidation processes induced by light. The photolysis rate of a compound can be estimated based on its light absorption rate and quantum yield. The extinction coefficient represents the phenomenon that as wavelength decreases, more photons are absorbed.

It is the photolysis of hydrogen peroxide that generates the hydroxyl radicals that drive the UV/H2O2 AOP according to the reaction:

8) Absorption of UV Light by NOM
The natural organic matter (NOM) present in the water can also absorb UV light before it is able to form hydroxyl radicals. This reduces the amount of UV light available in support of the desired reactions in an AOP involving UV light. The extinction coefficient for NOM, which is a measure of the amount of light absorption by NOM, varies widely and is site specific. Certain other constituents (e.g., iron) in the background water matrix can also absorb UV light and reduce the amount of UV light available for the desired reactions in an AOP.

9) UV Lamp Technology
There are two types of lamps commonly applied in the water industry for destruction of target compounds:
a. Low Pressure UV (LPUV) Lamps
b. Medium Pressure UV (MPUV) Lamps

Application of AOPs for Emerging Contaminants

In drinking water, advanced wastewater, and water reuse applications, there are emerging contaminants challenging the industry. Emerging contaminants include persistent organic pollutants (POPs), endocrine disruptors, and pharmaceutically active agents, some of which are recalcitrant and cannot be reduced by traditional processes. For such recalcitrant compounds, AOPs represent a promising technology for target compound reduction.

To evaluate the reduction of emerging contaminants in an AOP, it is necessary to determine the hydroxyl radical rate constants for each emerging contaminant. For processes involving UV light, it is necessary to determine quantum yield and extinction coefficients to evaluate reduction by direct photolysis.

Trussell Tech can provide guidance and design experiments for determining rate constants, quantum yields, and extinction coefficients for emerging contaminants including POPs, endocrine disruptors, and pharmaceutically active agents.

For UV/H2O2 processes, once the hydroxyl radical rate constants, quantum yields and extinction coefficients are determined, Trussell Tech can run its UV/H2O2 model, AdOx™, to determine the potential for reduction of the compound in a specific application.

 

LPUV lamps may be either low intensity or high intensity lamps. MPUV lamps applied in the water industry are high intensity. LPUV lamps emit UV light only at a wavelength of 254 nm. MPUV lamps emit energy over the 200 through 400 nm range but only the 200 to 300 nm is important in the UV/H2O2 process because hydrogen peroxide only absorbs UV light at wavelengths less than 300 nm.

There are various advantages and disadvantages to each type of lamp and Trussell Tech can help evaluate which type of lamp is appropriate for a given application. For example, LPUV lamps generate UV light more efficiently than MPUV lamps. MPUV lamps can operate at a higher power input so fewer lamps may be needed, but the power requirements are greater for each lamp.

10) Electrical Efficiency per Unit Order of Compound Reduction
Photolysis reactions require a large amount of energy so it is important to optimize efficiency on the basis of energy required per amount of compound removed. One measure is electrical efficiency per log order of compound reduction (EE/O). AOP models utilized by Trussell Tech allow for comparison of competing UV technologies in terms of minimizing the EE/O based on varying various factors affecting the process (e.g., type of lamp, hydrogen peroxide dosage, level of pretreatment):

where EE/O = electrical efficiency per log order reduction, kWh/kgal
P = lamp power output, kW
Q = water flow rate, kgal/hr
Ci = initial concentration, mg/L
Cf = target concentration, mg/L

11) Hydrogen Peroxide Dosage
Relatively high hydrogen peroxide dosages compared to the O3/ H2O2 process are needed for the UV/H2O2 process to generate
sufficient quantities of hydroxyl radicals because hydrogen peroxide does a poor job absorbing UV light, especially compared to NOM and iron if they are present. Higher H2O2 dosages will produce significant amounts of residual H2O2 that must be removed from the water. Trussell Tech can provide guidance in dealing with the H2O2 residual. The chemical costs of H2O2 need to be balanced
against the energy costs of the UV lamps when evaluating an appropriate UV/H2O2 process (LPUV or MPUV) for a given application.

Factors that Affect the O3/H2O2 AOP
12) Ozone Dosage
The overall reaction for the formation of hydroxyl radicals in the O3/ H2O2 process is:

Because ozone is more reactive with NOM and inorganics than hydrogen peroxide, the amount of ozone applied must be greater than that estimated from stoichiometry. Care must be exercised in choosing the appropriate O3 dosage because too much ozone will result in the scavenging of hydroxyl radicals.

13) Hydrogen Peroxide Dosage
As mentioned above, excess hydrogen peroxide can scavenge hydroxyl radicals so it is important to avoid excess dosages. H2O2 dosages required for the O3/H2O2 are less than those required for the UV/H2O2 but care should be taken to optimize H2O2 dosage at the lowest appropriate level to minimize the amount of H2O2 residual.

14) Bromate Formation
Processes involving ozonation can produce bromate when bromide ion is present in the raw water. Trussell Tech can provide guidance on appropriate strategies to control bromate formation when the O3/H2O2 process is applied.

Reference
Some information presented on this web page is based on:
Crittenden, J., Trussell, R., Hand, D., Howe, K. and Tchobanoglous, G., 2005, Water Treatment: Principles and Design, MWH, 2nd
Edition, John Wiley & Sons, N.Y., 1948 p.

Links to Key Advanced Oxidation Technology Manufacturers:
Trojan Technologies, Inc. http://www.trojanuv.com/en/homeframe.htm
Calgon Carbon Corporation http://www.calgoncarbon.com/uv/index.html
Applied Process Technologies, Inc. http://www.aptwater.com/