UVPS spells the end for chlorine
Water treatment is undergoing a major change with ever more stringent regulations on drinking water, discharge consent levels, water quality as examples. There are also regulations being driven by the EC to phase out the use of chlorine. This, coupled with increased public awareness, represents a major opportunity for UVPS, which offers a low-energy, non-toxic and sustainable water treatment technology. Professor Peter Robertson explains.Traditional water treatment systems have involved the use of techniques such as coagulation, chlorination or ozonation, which use potentially hazardous or polluting materials. Chlorination presents a particular problem since it will often generate mutagenic or carcinogenic by products when used to treat water contaminated with organic compounds.
Techniques such as granular activated carbon adsorption or air-stripping processes have drawbacks, as the methods are non-destructive. The adsorbing media loaded with the waste must therefore also be treated as a hazardous material.
Incineration of organic waste is not always effective and can disperse large quantities of toxic emissions, such as products of incomplete combustion or heavy metals, into the atmosphere. An effective treatment system is required which can degrade the polluting materials prior to discharge of the effluent.
Advanced oxidation technologies (AOT) are processes that could address these requirements. These are new powerful water treatment technologies characterised by a common feature.
For each process the active reagent is the hydroxyl radical, which is one on the most powerful oxidising reagents available. This reagent is capable of completely mineralising (effectively combusting) most chemical pollutants leaving no harmful by-products.
AOTs are now being considered as complementary or even potential replacements for traditional water treatment technologies such as chlorination. The main processes include:
- Hydrogen Peroxide processes (H2O2/UV)
- Ozone Processes (O3/H2O2, O3/UV)
- Photocatalytic Processes (TiO2/UV)
Since many AOTs operate at ambient temperature they may be regarded as cold-combustion processes. The hydroxyl radical generated by the AOT is far more oxidising than many materials that are commonly used for waste treatment (2.8V), including ozone (2.07V), hydrogen peroxide (1.78V), hypochlorous acid (1.49V) and chlorine (1.36V).
This species is therefore capable of destroying even very stable chemical compounds that other processes have proven to be ineffective in decomposing. The other advantage is that since the process operates at near ambient temperature and atmospheric pressure which potentially simplifies the engineering requirements.
In addition to acting as a single treatment process, these technologies may also be integrated into conventional WTWs as a polishing process to aid compliance with more stringent legislation.
Each of the AOT processes are:
UV/Peroxide (UV/ H2O2):
This is one of the most established AOT processes that has been successfully applied to the destruction of a range of contaminants in effluents including aromatics, phenols, aliphatic and halo aliphatics dye effluents and pesticides.
Although hydrogen peroxide is itself a strong oxidising agent and has been used for water treatment applications in the past, the rates of these processes were relatively slow for effective destruction of large volumes of relatively concentrated effluents, particularly those containing aromatic compounds.
Cleavage of the molecules into hydroxyl radicals significantly increases the oxidising power of this reagent. This is usually achieved using low-pressure mercury vapour lamps or xenon mercury lamps emitting UV light below 240nm.
Eq 1: H2O2 + hv -> 2 HOl
As the process requires UV light of wavelength of less than 300nm, reaction vessels must be constructed from quartz that not only adds to manufacturing cost but also provides manufacturing challenges. In addition, at these low wavelengths there is a range of other compounds that may react with the UV light in competition to the peroxide.
Finally, UV light at these wavelengths is also strongly attenuated by water, only penetrating the reactor vessel to a limited area around the lamp, hence reducing the overall area available for reaction.
The concentration of the peroxide in the reactor vessel is also crucial. It has been found that reaction rates increase with increasing peroxide concentration up to a certain level where the peroxide radicals start to recombine with each other or react with the parent peroxide molecule producing the less reactive HO2 radical.
The process, however, has some advantages. The main reactant (H2O2) is readily available and the capital investment costs for the process are relatively low. Unfortunately, the process is limited to solutions with low light absorption and relatively low concentrations of pollutants.
Ozone is also a strong oxidising agents which is increasingly applied to potable water treatment as an alternative to chlorine disinfection.
In the absence of light ozone it has been demonstrated that ozone is capable of decomposing organic compounds, although the rates are relatively slow. Ozone is usually generated in situ by applying an electric discharge in a stream of oxygen or air.
The effluent being treated must then be saturated by the ozone and there are potential mass transfer problems with this process. As a result, it is necessary to strongly agitate the solution using for example line mixers. The solution containing the ozone is then irradiated with UV light below 254nm and the ozone splits into a reactive oxygen species and an oxygen molecule (eq 2). The reactive oxygen species subsequently reacts with water forming hydrogen peroxide, which then in turn react with the UV light forming the reactive hydroxyl radicals (eq 3,4).
Eq 2: O3 + hv -> O2 + O1
Eq 3: O1 + H2O -> H2O2
Eq 4: H2O2 + hv -> 2 HOl
This may appear to be a rather convoluted way of generating hydrogen peroxide directly in water, however ozone absorbs UV light more effectively at 254m than does H2O2, so this technology may be applied to solutions containing UV absorbing contaminants. UV/Ozone has been applied to the treatment of potable water, removal of dyes for effluents and destruction or aromatic and aliphatic hydrocarbons.
Photocatalytic Process (TiO2/UV):
This involves the use of a non-toxic semiconductor catalyst such as titanium dioxide (TiO2). When semiconductors are illuminated with light of an appropriate wavelength they become powerful oxidants which will convert most organic materials to carbon dioxide and water.
This process has been effective in destroying a range of organic materials typical of those generated by the industries mentioned above. The use of titanium dioxide as a photocatalyst for the destruction of polluting materials has now been demonstrated to be an effective process. The overall process may be described as follows. A reaction vessel containing the waste material and the semiconductor is illuminated and the waste is converted to a non-toxic form either by an oxidation or reduction process.
An example of typical process would be the destruction of halo-aromatic compounds, resulting in the production of carbon dioxide, water and the halide ion. The overall process for the photo-oxidation of waste materials (P), sensitised by TiO2 is shown below:
P + O2 _ _ _ _ CO2 + H2O
+ Mineral acids
At the surface of the TiO2 particle surface adsorbed water (hydroxide ions OH-) are oxidised to hydroxyl radicals by losing an electron to the hole in the valence band. These radicals subsequently oxidise the polluting material, while at the conductance band an electron is donated to oxygen thereby generating the superoxide radical anion.
Photocatalytic systems are relatively simple to implement and the process should have a number of advantages over traditional technologies including:
- Small foot print
- Requires low energy sources / power
- No waste (except CO2 and H2O)
- No consumables, e.g. filters
In the AWTS, the light energy is provided by low-energy black ultra-violet fluorescent tubes (UV-A). These have a number of advantages including having better water column penetration due to less attenuation than with UVB or UVC light, they have a longer lifetime and there are savings on running costs.
The form of the catalyst used within the AWTS is a specially produced, pelletised variety. This greatly enhances ease of separation from the processed water and does away with the need for any filtration on the outlet. The photocatalytic unit is a compact, modular twin-tank design that will fit into standard, half-size offshore containers.
In addition, it uses standardised spare-parts, reducing marginal costs. The unit also has a flexible configuration that may allow for larger flow rates or more persistent pollutants.
The photocatalytic system has been tested and proved successful in destroying a range of compounds and bacterial species plus their toxins. Many of the compounds treated are highly soluble and are thus difficult, if not impossible to remove by traditional techniques.
It should be stressed that the AWTS is complementary to existing water treatment methods as it is a water polishing technology. Used as an addition to traditional methods, the AWTS is capable of exceeding current legislative requirements and of future-proofing the Oil & Gas industry against the OSPAR and zero-emission legislation.
As a polishing system and in order to operate at maximum efficiency, the AWTS should be provided with a pre-treated effluent stream to reduce the levels of free hydrocarbons, sediment and grit to a minimum.
Prof Peter Robertson is part of the research team at Robert Gordon University's Centre for Research in Energy and the Environment. T: 01224 262000.