Ozonation

Process Type: Chemical

Description:

Ozonation is well established as a treatment technology for drinking waters, or in swimming pools, for which it is used as a disinfectant, to degrade substances of concern, and to enhance the performance of other treatment processes. Although not so widely employed for the treatment of sewage or industrial wastewaters, ozonation has much to offer in specific circumstances.

Ozone is the strongest practical oxidant available for waste water treatment, and is used for:

Oxidation of organic materials, especially recalcitrant organic compounds, to enhance their removal by subsequent treatment especially in biological processes;
Disinfection;
Taste, odour, and colour removal;
As a pre-oxidant stage to enhance removal of turbidity and algae within subsequent treatment processes; and Precipitation of iron and manganese.

Ozone itself (O₃) is an allotrope of oxygen, and is a gas at normal temperatures and pressures. It is relatively unstable, having a half-life of less than 30 minutes in distilled water at 30C (Reynolds, 1982).

Because of this instability, ozone must therefore be generated at the point of use, by passing air or pure oxygen between oppositely charged plates. The gases having been pre-dried to a dew point lower than about -40C. Pure oxygen feed is generally only more cost-effective than air for ozonation systems that are required to generate more than 1 tonne of ozone per day.

For smaller systems (typical leachate applications will require less than 50 kg of ozone per day) then air is generally used.

Once produced, air containing enhanced concentrations of ozone gas is bubbled through the water to be treated in a column, using a bubble diffuser system.

Generally, a batch system of treatment is preferred, with a contact time of between 15 minutes and an hour.

Ozone transfer occurs as fine bubbles containing ozone and air (or oxygen) that rise slowly inside the column, contacting the contaminated water phase. Correct ozone dosage to achieve required oxidation of specific compounds is generally determined using small-scale treatability studies. Pesticides, aromatics, alkanes and alkenes are examples of compounds readily and successfully treatable by ozonation.

Advantages:

Extremely useful to achieve required oxidation of specific persistent organic compounds where other processes may be ineffective.

Disadvantages:

Ozone treatment is generally only appropriate as a polishing step in the treatment of landfill leachates, following extensive biological pre-treatment to remove degradable organic compounds that might otherwise result in excessive consumption of ozone. Removal of suspended solids from water being treated is also essential for efficient treatment. In addition, ozonation is best applied to well nitrified or low ammonia containing effluents, since to some extent ammonia also competes for ozone with the organic compounds being targeted.

Where best used:

Where specific trace organic chemicals still require removal after biological treatment. (Not normally needed for EU Directive and Waste regulations compliant landfills.)

Costs comments:

Capital costs of ozone treatment are relatively high (typically 250K to 350K to dose 150 mg/l into 200 m³ of effluent per day), due to the high cost of equipment for ozone generation. Electricity comprises the majority of operational costs, which can also be high, especially for stronger leachates.

Ozonation should be seen as an expensive polishing option, appropriate only in specific circumstances for leachate treatment, such as complete destruction of less biologically-degradable pesticides in final effluents. Nevertheless, case studies in the UK and overseas have demonstrated that such systems can operate reliably on landfill sites.

Sustainability comments:

Unlike chlorine, the use of ozone for effluent polishing does not result in excessive formation of trihalomethanes. However, as well as directly degrading some organic compounds, ozone can increase the degradability of organic compounds, resulting in increased levels of BOD in effluents.

These can then readily be degraded efficiently, using passive processes such as reed bed polishing.

Particularly during treatment of landfill leachates, ozonation can result in generation of very reactive brominated intermediate compounds (e.g. bromal, =tribromoacetaldehyde).

Experience has demonstrated that although such compounds exhibit significant toxicity, they are readily and completely degraded within an appropriately designed reed bed polishing system.

There is only one example of a full-scale leachate treatment plant in the UK where ozonation has been applied as a polishing stage for leachate treatment. In that instance, ozonation was applied to meet extremely stringent effluent toxicity criteria, before discharge into a very sensitive receiving watercourse. The plant has operated successfully since 1994, particularly for the complete removal of a number of pesticides, such as mecoprop and isoproturon, in biologically pre-treated leachate.

Experience has been that ozonation generally only provides
between 10-15 percent removal of residual hard COD, and that if COD levels in final effluent are a major issue, then alternative polishing processes, such as activated carbon, may be more appropriate.

Where removal of adsorbable organic halogens (AOX) is an issue, ozonation has been shown to be capable of reducing values of AOX from up to 3 mg/l, to below 0.5 mg/l (e.g. see Kaulbach, 1993). Costs of such treatment, where required, must be compared with those of alternative processes, such as activated carbon adsorption.

Although variants of ozonation, involving combined treatment with hydrogen peroxide (H₂O₂), and/or Ultra Violet irradiation, are capable of providing increased oxidation potential by the enhanced generation of hydroxyl radicals, such processes have rarely been applied to treatment of landfill leachates.

The ozone contactor should be designed for efficient adsorption that minimises ozone in the off-gas. Any ozone remaining in the off-gas from the diffusion system must be destroyed before release into the atmosphere. Destruction of excess ozone is accomplished readily using thermal, catalytic, or other processes. (Threshold Limit Value (TLV) for repeated exposure of workers to ozone is 0.21 mg/m 3 in air.)

By decomposing to oxygen as it reacts, ozone provides an environmentally preferable alternative to halogenated oxidants (e.g. chlorine), adsorption (e.g. activated carbon) or even reverse osmosis in some circumstances.

Typical power consumption for generation of ozone from air or oxygen is 16 kWh and 8 kWh respectively, per kg of ozone produced. Process design must take into account the additional costs involved in purchase and safe handling of liquid oxygen, and also the significant costs of pumping liquids and dosing these with the ozone-enhanced air.