Questions about ozone
Questions about ozonation
In Switzerland, Art. 6 of the WPO stipulates the prohibition of pollution. It is therefore forbidden to introduce substances into a body of water that may pollute the water. This applies irrespective of possible limit values under food law (e.g. the Ordinance of the Federal Department of Home Affairs on Drinking Water and Water in Publicly Accessible Baths and Shower Facilities). For bromate, for example, the drinking water limit value of 10 µg/L applies, as bromate has a potentially carcinogenic effect. Bromate is formed from bromide during ozonation and is not degradable in the environment under aerobic conditions. How much bromate is formed in the ozonation depends mainly on two factors: the bromide concentration and the ozone dose. There will always be some bromate that is formed by oxidation with ozone (if bromide is present in the wastewater), but bromate formation at “typical” ozone doses in municipal wastewater treatment (0.6 – 0.8 g ozone/g DOC) is only significant at bromide concentrations of greater than 100 µg/L. The influent concentrations for wastewater of a mainly municipal catchment are usually well below 100 µg/L bromide. Only in the case of special dischargers (such as waste incineration plants with wet flue gas scrubbing, chemical industry, landfills, hazardous waste incineration plants, etc.) may the bromide concentration be significantly higher.
Within the framework of proper and appropriate planning as defined in Art. 63 of the WPO, it is therefore essential to ensure that only WWTPs where bromate formation is minimal are expanded with ozonation. For example, targeted minimization of bromide discharge in the catchment area through measures at the source has proven to be a very effective measure for reducing undesirable bromate formation. However, it should also be mentioned here that treatment with activated carbon should be considered in unclear cases. Where measures at the source or a treatment with activated carbon are not possible, combining ozone and activated carbon where ozonation is operated at significantly lower (and, in terms of bromate formation, unproblematic) ozone doses also represents an alternative.
At most WWTPs in Switzerland an ozonation system can certainly be built with a clear conscience. These are WWTPs with a largely municipal catchment area, where treatment with ozone has many positive effects on the quality of the treated wastewater (reduction of micropollutants and ecotoxicological effects, hygienization, decolourization). The construction of an ozonation system may also be possible for WWTPs with a certain proportion of industrial dischargers. In principle, however, the boundary conditions must be clarified carefully for each individual case. For this purpose, the VSA recommendation by Wunderlin et al. 2017/2021 should be applied in advance as part of the preliminary study. Among other things, it considers the formation of known problematic transformation products such as bromate and nitrosamines. In addition, ecotoxicological investigations are recommended. In contrast to individual chemical measurements, they capture the cumulative effects of possible, unknown problematic transformation products. It is also important to keep an eye on the further development of the catchment area. Future industries there must be considered. Finally, an overall assessment of the wastewater’s treatability with ozone is possible. In unclear cases, alternatives to ozonation are recommended.
The VSA recommendation by Wunderlin et al. 2017/2021 contains the indication that future changes in biological wastewater treatment should already be included in the preliminary assessments. The following considerations indicate that the informative value of the assessments with non-nitrified wastewater is limited:
- It is not yet clear among experts how strongly ammonium and nitrite influence ozone chemistry (ozone and OH radical stability). Bromate formation is underestimated because ammonium reacts with an intermediate product during the reaction of bromide to bromate and thus less bromate is formed.
- In the case of non-nitrified wastewater, the significance of the bioassays (e.g. fish egg test, daphnia) is limited due to the increased ammonium concentration. If ammonium and thus, depending on the pH value, ammonia are present, this increases the toxicity and masks any other effects.
Therefore, it is recommended to carry out the assessments with wastewater that has been nitrified in the laboratory.
A number of safety requirements must be taken into account in ozonation. Ozone itself is an irritant gas with a relatively low effect threshold, i.e. ozone leakage into the ambient air must be prevented (by sealing the reactor and drains and discharging exhaust air via a residual ozone destroyer). Ozone sensors are used to monitor the atmosphere. In addition, the materials and pipes used must be ozone-resistant. Safety aspects for handling ozone have been published in a factsheet (in German, French and Italian).
In addition, it must be considered that ozone is often produced from pure oxygen. There are also safety regulations for oxygen, mainly because of the risk of explosion/ignition. Safety aspects for handling oxygen have been published in a factsheet (in German, French and Italian).
Questions about post-treatment after ozonation
Yes, biological post-treatment is mandatory for ozonation. Treatment with ozone can produce labile, toxic reaction products. The main task of the biologically active post-treatment is to abate these substances and their toxic effects.
The aim of a biological post-treatment is the degradation of reactive intermediate products. Sand filtration is suitable for this purpose. It has been used in previous experiments and large-scale implementations in Switzerland. In Germany, a plant with a treatment pond is operated at the WWTP Bad Sassendorf. A fluidized bed system, as installed at the WWTP in Duisburg Vierlinden, Germany, is possible in principle, but further investigations are recommended. A filter with GAC can also be used as a post-treatment stage. Since the GAC also removes micropollutants, the ozonation can be reduced accordingly. The use of filters as a post-treatment process is described in the report “Process Overview for Biological Post-treatment in Ozonation” (in German and French). Criteria for the selection of the post-treatment stage and experiences with different processes are also presented.
An important dimensioning parameter for sand filters is the maximum filtration velocity (at Qmax), which is usually around 15 m/h. It must be taken into account that one filter cell is always taken out of operation for rinsing, and that the backwash water produced loads the filter additionally. Depending on the number of filter cells and the backloading, this results in a maximum filtration velocity of about 12.5 m/h in relation to the total filter area. The contact time at maximum flow is about 5 minutes (about 15 minutes at dry weather flow) (see also the report “Process Overview for Biological Post-Treatment in Ozonation”). Further information on the dimensioning and design of filter systems can be found in the specialist literature.
In principle, a fluidized bed system is conceivable as a possible post-treatment process. However, further investigations are recommended.
The post-treatment has the task of eliminating the labile, toxic reaction products that may be formed during ozonation. However, no periodic monitoring is stipulated in this respect. The report “Process Overview for Biological Post-Treatment in Ozonation” assesses whether the processes are suitable as post-treatment.