Ozone applications Drinking water

This combination has several benefits:

- Removal of organic and inorganic matter
- Removal of micro-pollutants, such as pesticides
- Enhancement of the flocculation/coagulation-decantation process
- Enhanced disinfection and reduction of disinfection byproducts
- Odor and taste elimination

Removal of organic matter and inorganic matter

All water sources contain natural organic matter (NOM). Concentrations (usually measured as dissolved organic carbon, DOC) differ from 0,2 to more than 10 mg L-1 [6]. NOM creates direct problems, such as odor and taste in water, but also indirect problems such as organic disinfection byproduct formation, support of bacterial regrowth in the distribution system, etc. To produce pure drinking water, the removal of NOM is a prior task in modern water treatment.
Ozone, like any other oxidant, seldom achieves a complete mineralisation of NOM. Organic matter is partly oxidized and becoming more easily biodegradable. This results in a higher amount of BDOC (Biodegradable DOC). As a result, ozone improves the removal process of NOM by a subsequent filter, when it is used as a pre-oxidant [33,39,40]. In a research paper of Siddiqiui et al. [40] the effect of ozone in combination with a biological filter is described. The combined treatment resulted in a reduction of DOC of 40-60%. The removal is even greater when ozone is used in combination with a coagulant. This is because ozone can enhance the coagulation process. The combination coagulation–ozone–bio filtration results in a DOC reduction of 64%. When only bio filtration was apllied, the reduction rate was only 13%. The optimal concentration to remove organic matter by ozone was at an ozone dose of: O₃/DOC = 1 mg/mg.
Most inorganic matter can be eliminated by ozone quite fast [15,39]. After ozonation, bio filtration is also required for inorganic matter. Namely, oxidation forms unsoluble compounds that need to be removed during the next water purification step.

Pesticides

Micro- pollutants such as pesticides may occur in surface water, but also increasingly in groundwater. Drinking-water standards for pesticides in the European Union are strict: 0,1 μg l-1 for each compound [38].
Several surveys show that ozone can be very effective for the oxidation of several pesticides. At a water treatment plant in Zevenbergen (Holland) it was proved that three barriers (storage–ozonation–granular active carbon filter (GAC filter)) are effective and safe enough for the removal of pesticides. From 23 tested pesticides, 50 % was degraded sufficiently (80 % degradation). Table 1 shows an overview of pesticides that are easily degraded by ozone. For highly resistant pesticides, a higher dosage of ozone is advised, or ozone combined with hydrogen peroxide [38].

Table 1: degradation of pesticides that are easily degradable by ozonation (%)

Reduction of disinfection byproducts and improved disinfection

Disinfection byproducts (DBP) are mainly formed during the reaction between organic material and a disinfectant. The reaction of chlorine with organic matter can lead to the formation of chlorinated organic DBP’s, such as trihalomethanes (THM). Ozone can also react with organic matter and form DBP’s. These are mainly organic disinfection byproducts, such as aldehydes and ketones, which can be easily degradaded in a bio filter (90-100%). Generally, these organic ozone DBP’s do not form any risk of violation of drinking water standards, when ozone is used a pre-oxidant.
To reduce the amount of DBP’s at a conventional disinfection system (disinfection by chlorine products) it is important that the potential to form DBP’s remains low. This is often expressed as DBP formation potential (DBPFP). The potential to form DBP’s can be reduced by the removal of (most of the) NOM, for example by pre-oxidation with ozone (ozone-filtration). This combination can lower the DBPFP by 70-80%, when chlorine is used as a final disinfectant [40]. This concerns the DBPFP for THM’s, HAA (haloacetic acids) and chloral hydrate.

Ozone is a more effective disinfectant than chlorine, chloramines, and even chlorine dioxide. An ozone dose of 0,4 mg L-1 for 4 minutes is usually effective for pre-treated water (low NOM concentration) [39]. Several studies proved that ozone, unlike chlorine products, can deactivate resistant micro-organisms (see resistant microorganisms page). However, as ozone rapidly decomposes in water, its life-span in aqueous solutions is very short (less than one hour). Therefore ozone is less suitable for residual disinfection and can be used only in particular cases (mainly in short distribution systems). Chlorine and chlorine dioxide often replace ozone as a final disinfectant. For primary disinfection (prior to the bio filtration), ozone is very suitable. This will lead to a more complete disinfection and a lower disinfectant concentration.

Odor and taste elimination

Odor and taste production in drinking water can have several causes. Odor and taste forming compounds can be present in raw water, but they can also be formed during water treatment. These compounds may derive from the decomposition of plant matter, but normally they are a result of the activity of living organisms present in the water [5]. Inorganic compounds such as iron, copper and zinc can also generate some taste. Another possibility is that the chemical oxidation (chlorine treatment) leads to an unpleasant tastes and odors.
Odor and taste forming compounds are often very resistant. This causes elimination to be a very intensive process [33]. For the elimination of taste and odor, several processes can be appropriate, such as oxidation, aeration, granular active carbon (GAC) filtration or sand filtration. Usually, a combination of these techniques is applied.
Ozone can oxidize compounds in a range of 20–90% (dependent on the type of compound) [6]. Ozone is more effective for the oxidation of unsaturated compounds. As was the case for the oxidation of pesticides, ozone combined with hydrogen peroxide (AOP process) is more effective than ozone alone. Geosmin and 2-methylisoborneol (MIB) are examples of resistant odorous compounds, which are often present in the water. These are produced by algae and have a low odor and taste threshold. Nevertheless, ozone is still very affectively removes these compounds, see figure 1.

Figure 1: Removal of MIB and Geosmin (Colorado river water, US)

Generally, the most effective way to remove taste and odor components appears to be a combination of pre-oxidation and filtration [5]. Table 2 shows several combinations of techniques to improve taste. Ozone with sand filtration and GAC filtration is the most efficient combination (82% removal).

Table 2: effect of ozone and subsequent treatment on odor threshold and odor reduction at Saint-Maur pilot treatment (France)