U. von Gunten

Combination of Ozone with Activated Carbon as an Alternative to Conventional Advanced Oxidation Processes

The objective of this study was to compare the efficiency of O3/granular activated carbon (GAC) to enhance ozone transformation into ·OH radicals, with the common advanced oxidation processes (O3/OH−, O3/H2O2). The results obtained with model systems under the given experimental conditions showed that the system O3/OH− (pH 9) and O3/H2O2 (pH 7, [H2O2] = 1·10−5 M) are more efficient than O3/GAC (pH 7, [GAC] = 0.5 g/L) to enhance ozone transformation into ·OH radicals. However, in Lake Zurich water the O3/GAC process has a similar efficiency as O3/H2O2 for ozone transformation into ·OH radicals. The results also show that the presence of GAC during Lake Zurich water ozonation leads to (i) removal of hydrophilic and hydrophobic micropollutants, (ii) reduction of the concentration of CO3 2−/HCO3 −, and (iii) decrease of the concentration of dissolved organic carbon (DOC) present in the system.

Ozonation of iodide-containing waters: Selective oxidation of iodide to iodate with simultaneous minimization of bromate and I-THMs

The presence of iodinated disinfection by-products (I-DBPs) in drinking water poses a potential health concern since it has been shown that I-DBPs are generally more genotoxic and cytotoxic than their chlorinated and brominated analogs. I-DBPs are formed during oxidation/disinfection of iodide-containing waters by reaction of the transient hypoiodous acid (HOI) with natural organic matter (NOM). In this study, we demonstrate that ozone pre-treatment selectively oxidizes iodide to iodate and avoids the formation of I-DBPs. Iodate is non-toxic and is therefore a desired sink of iodine in drinking water. Complete conversion of iodide to iodate while minimizing the bromate formation to below the guideline value of 10 μg L⁻¹ was achieved for a wide range of ozone doses in five raw waters with DOC and bromide concentrations of 1.1-20 mg L⁻¹ and 170-940 μg L⁻¹, respectively. Lowering the pH effectively further reduced bromate formation but had no impact on the extent of iodate and bromoform formation (the main trihalomethane (THM) formed during ozonation). Experiments carried out with pre-chlorinated/post-clarified samples already containing I-DBPs, showed that ozonation effectively oxidized I-THMs. Therefore, in iodide-containing waters, in which I-DBPs can be produced upon chlorination or especially chloramination, a pre-ozonation step to oxidize iodide to iodate is an efficient process to mitigate I-DBP formation.

Oxidation of manganese(II) during chlorination: role of bromide.

The oxidation of dissolved manganese(II) (Mn(II)) during chlorination is a relatively slow process which may lead to residual Mn(II) in treated drinking waters. Chemical Mn(II) oxidation is autocatalytic and consists of a homogeneous and a heterogeneous process; the oxidation of Mn(II) is mainly driven by the latter process. This study demonstrates that Mn(II) oxidation during chlorination is enhanced in bromide-containing waters by the formation of reactive bromine species (e.g., HOBr, BrCl, Br2O) from the oxidation of bromide by chlorine. During oxidation of Mn(II) by chlorine in bromide-containing waters, bromide is recycled and acts as a catalyst. For a chlorine dose of 1 mg/L and a bromide level as low as 10 μg/L, the oxidation of Mn(II) by reactive bromine species becomes the main pathway. It was demonstrated that the kinetics of the reaction are dominated by the adsorbed Mn(OH)2 species for both chlorine and bromine at circumneutral pH. Reactive bromine species such as Br2O and BrCl significantly influence the rate of manganese oxidation and may even outweigh the reactivity of HOBr. Reaction orders in [HOBr]tot were found to be 1.33 (±0.15) at pH 7.8 and increased to 1.97 (±0.17) at pH 8.2 consistent with an important contribution of Br2O which is second order in [HOBr]tot. These findings highlight the need to take bromide, and the subsequent reactive bromine species formed upon chlorination, into account to assess Mn(II) removal during water treatment with chlorine.

Ozonation as Pre-Treatment Step for the Biological Batch Degradation of Industrial Wastewater Containing 3-Methyl-Pyridine.

Abstract A heteroaromatic substance, 3-methylpyridine, as model pollutant, was degraded by ozonation bubble-column. This was the first step of a sequencing batch biofilm reactor process (SBBR). The modeling of this batch ozonation was developed in different steps. The model included hydraulics of the reactor, chemical kinetics and effects of gas-liquid transfer processes. The crucial and most sensitive part of the model was the observed fast consumption of ozone during gas-liquid transfer reactions. Without including these processes into the model, it was impossible to explain the experimental observations. The ozone consuming processes in the gas-liquid film are not totally understood at present.

Modelling Full-Scale Advanced Micropollutant Oxidation

Advanced oxidation has proven to be an efficient process to increase micropollutant removal compared to conventional ozonation and to lower considerably the load to subsequent activated carbon treatment. Laboratory tests to determine the most important reaction rate constants involved in conventional and advanced oxidation and the OH radical/ozone ratio in the specific water to be treated can provide the basic parameters to simulate direct ozone and OH radical reactions. The combination of tracer based hydraulic reactor and chemical reaction modelling made it possible to simulate full-scale reactor performance. Model extrapolations used to optimise the peroxide dosing point for maximum micropollutant removal showed that disinfection for Cryptosporidium may severely be impaired.

An American in Zurich: Jerry Schnoor as an Ambassador for U.S. Environmental Science and Engineering.

Reference EPFL-ARTICLE-221712doi:10.1021/acs.est.5b06233View record in Web of Science Record created on 2016-10-18, modified on 2016-11-02