For many different types of waste water, treatment systems have been implemented in the past decades. Waste water treatment is usually performed by biological processes, either aerobic or anaerobic, complemented with physical / chemical post treatment techniques. However, in some sectors of industry, like the tanker cleaning industry, the pharmaceutical industry and the textile industry, waste water treatment is still difficult because of the irregular presence of varying loads of often toxic compounds. To enable the (cheap) biological treatment of such waste water types a pre-treatment process is necessary to protect the biological system against the adverse effects of such sudden loads of toxic compounds.
Suitable pre-treatment processes can be found amongst the Advanced Oxidation Processes (AOPs). These AOPs include a wide variety of oxidation processes, having in common that radical reactions may play an important role. Examples of AOPs are oxidation with ozone and oxidation with hydrogen peroxide and UV illumination.
In this thesis the oxidation with ozone was studied. This process has been used since the early 1900s and ozone oxidation therefore is one of the most developed AOPs.
In the introduction of this thesis it is shown that there is some relation between the octanol-water partitioning coefficient of a compound and its toxicity. As a result of oxidation organic compounds usually become more hydrophilic and oxidation therefore can help in the detoxification of waste waters. The effect of the oxidation of two chlorophenols on biodegradability of the resulting solution was studied here. The results of these experiments clearly showed biodegradability to improve on oxidation.
In spite of the good results that can be obtained with ozone oxidation, the application of ozone in waste water treatment is hindered by the costs of ozone. Especially when ozone oxidation is used for pre-treatment of waste water, care has to be taken to minimise the ozone consumption of the process.
Minimisation of the ozone consumption can be achieved by controlling the selectivity of the process. The selectivity of the oxidation process can be controlled by influencing the reaction mechanism. This mechanism can either be a direct oxidation by the ozone molecule or a radical mechanism, in which hydroxyl radicals are the most reactive reactants. Selecting the predominant reaction mechanism can be done by affecting the rate of ozone dissociation and by the use of compounds acting as radical scavengers.
The selectivity of the oxidation was quantified for different reaction conditions, in which either the direct or the radical reaction mechanism was favoured. This quantification was done by establishing linear structure-reactivity relationships, using the Hammett sigma parameter, but also using the half wave oxidation potential of the organic compounds involved. For this, reaction rates and half-wave oxidation potentials were determined experimentally. As a clear correlation between these parameters and the rate of oxidation was observed, rates of oxidation of related compounds can be calculated using the derived relationships.
The effect of several process parameters, like pH, temperature and UV irradiation, as well as the effect of the reactor design was studied experimentally and by modelling the oxidation process. The conclusions drawn from the experiments performed with simple solutions containing only organic compounds were shown to be valid in solutions that, like real waste waters, contained inorganic salts as well. Modelling of the ozone decomposition and the oxidation kinetics only gave qualitative results though.
By modelling the oxidation reactor it could be shown that a Plug Flow Reactor will be the best system for maximising the oxidation selectivity, but the alternative and easier to construct Cascaded Tank Reactor system yields a comparable efficiency and will generally be more suitably as maximised selectivity will not in all cases the objective of the oxidation process.
|Qualification||Doctor of Philosophy|
|Award date||18 Dec 2002|
|Place of Publication||S.l.|
|Publication status||Published - 2002|
- organic compounds
- waste water treatment