DOC and Trace Organic removal via ozonation & underground passage - expected benefit and limitations
The project OXIRED was initiated to assess the potential of a combination of natural systems such as bank filtration (BF) and artificial recharge (AR) and oxidation processes in order to improve the degradability of DOC and the removal of trace organics during water treatment. In this literature study, treatment schemes, which combine subsurface passage with oxidation processes, were evaluated with regard to the potential removal of DOC and trace organics, by theoretical considerations and case study analyses. The objectives were i) to estimate the degradation of bulk organic matter and trace organics in such combined systems, ii) to assess the potential for toxic by-products and iii) to describe different possible schemes combining natural systems (BF & AR) and oxidation processes. Available data generally shows good removal of the substances identified as persistent during BF & AR by oxidation processes. Carbamazepine, for example, is poorly degradable during bank filtration, but ozonation leads to a transformation of more than 97%. If ozonation alone does not suffice, advanced oxidation processes may enhance the transformation. E.g. literature gives a values of < 50% removal of Iopamidol by ozonation. However, transformation increases up to 88% using advanced oxidation processes, such as O3/H2O2 and O3/UV. Investigations on the formation of possible toxic by-products have shown the general possibilities to control the formation of bromate by decreasing the pH, avoiding free dissolved ozone in the reactor and/or by adding H2O2. Only a low risk of exposure of the potentially forming nitrosamines in drinking water after artificial recharge could be identified. Especially the cancerogenic metabolite NDMA is degraded during subsurface passage. Three reference treatment schemes were identified: (A): surface water is treated via oxidation before infiltration into artificial recharge ponds.(B): a river bank filtration with short retention times (<5 days) is used as a pretreatment step before the successive oxidation and artificial recharge (AR). (C1/C2): oxidation is applied subsequent to subsurface passage after bank filtration and artificial recharge. Due to the possible formation of toxic by-products and the increased assimilable DOC in scheme C (Examples for C1 Mülheim Styrum-East and Le Pecq Croissy & C2 Prairie Waters Project and the Bi´eau Process) a post-treatment including disinfection after oxidation is necessary. Additional post-treatment in schemes A (implemented at Mülheim Dohne) and B depends on the redox conditions and the travel times during the subsurface passage. However, although there is a lack of practical data, the enhancement of BDOC via oxidation prior to the underground passage seems theoretically more promising than the reverse configuration. It is therefore recommended that any further experimental program in OXIRED should focus on the schemes A and B and include a cost-benefit analysis of the additional first BF step.