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.

Subsurface passage as utilized during river bank filtration and artificial groundwater recharge has shown to be an effective barrier for multiple substances present in surface waters during drinking water production. Additionally it is widely used as polishing step after wastewater treatment. However, there are limitations concerning the removal of DOC and specific trace organics. The project ”OXIRED“ aims at assessing possibilities to overcome these limitations by combining subsurface passage with pre-oxidation by ozone. In the first phase of the project, laboratory-scale column experiments were conducted in order to quantify removal for different settings under varying conditions. In a previous study different combinations of advanced oxidation and subsurface passage were evaluated concerning their potential removal efficiency and practical implementation on the basis of existing, published experiences and theoretical considerations. Two different scenarios were identified as promising for experiments in laboratory-scale columns with surface water and sewage treatment plant effluent: (A) surface water - oxidation - groundwater recharge and (B) surface water - short bankfiltration - oxidation - groundwater recharge. The investigations were designed to lead to recommendations for the implementation of a combined system of subsurface passage and advanced oxidation in pilot scale experiments that will be carried out in the second phase of the project. Prior to column experiments, batch tests following the RCT-concept by Elovitz and von Gunten (1999) were carried out to characterize the reaction of ozone with the investigated water qualities [1]. Additional batch ozonation tests with subsequent analysis of biodegradable dissolved organic carbon (BDOC) were conducted in order to determine optimal ozone doses for DOC removal in column experiments. For laboratory-scale experiments a set of 8 soil columns (length: 1 m; diameter: 0.3 m) was operated at TUB to evaluate the effects of pre-ozonation of different source waters (secondary effluent, surface water, bank filtrate). Ozonation was conducted with gaseous ozone in a 13-L stirred tank reactor. Specific ozone doses of 0.7 mg O3/mg DOC0 and 0.9 mg O3/mg DOC0 were investigated. Trace organic compounds to be targeted were identified in a prior literature study on existing data on subsurface removal. Results from laboratory-scale soil column experiments led to recommend specific ozone doses (z) of 0.7 mg O3/mg DOC0 for the following technical- and pilot-scale applications. Removal of surface water DOC in the soil columns was increased from 22% without ozonation to 40% (z = 0.7) and 45% (z = 0.9) with preozonation and the DOC in the column effluent reached the level of tap water in Berlin within less than one week of retention time. At bank filtration and artificial recharge sites in Berlin similar removal rates were only observed within 3 - 6 months of retention [2]. The transformation of many trace compounds was efficient with specific ozone doses of 0.6-0.7 mg O3/mg DOC0. Realistic surface water concentrations of carbamazepine,sulfamethoxazole, diclofenac and bentazone were reduced below the limits of quantification (LOQ). The pesticides diuron and linuron were reduced close to LOQ. The substances MTBE, ETBE and atrazine were only partly transformed during ozonation. For efficient transformation of these substances, higher ozone doses or an optimisation of the oxidation process, for example as advanced oxidation process (AOP), should be considered. Operating a preceding bank filtration (scenario B) will enhance the transformation efficiency of MTBE and ETBE. With similar ozone consumption the transformation of MTBE and ETBE was increased by 27-31% and 28-33% of the original removal, respectively. Other investigated compounds were efficiently transformed during ozonation of surface water independently of the preceding bank filtration step. For the removal of bulk organic carbon only little improvement was observed for scenario B. Overall DOC removal increased from 45% with direct ozonation of surface water to up to 50% with a preceding soil column. Despite the presence of relevant bromide concentrations (~ 100 µg/L) formation of the oxidation by-product bromate was not observed (< 5 µg/L). However, this could also be a result of analytical problems, as later spiking tests showed. Formation of brominated organic compounds was also not observed. Adsorbable organic bromide (AOBr) even decreased by 50 - 60% for secondary effluent and 80 - 90% for surface water. The reduction of AOBr concentrations was accompanied by an increase of inorganic bromide by up to 40 µg/L during ozonation of surface water. In the two conducted in vitro genotoxicity tests (Ames test, micronucleus assay) no genotoxicity caused by ozonation of water samples was observed. Testing for cytotoxicity (glucose consumption rate, ROS generation) showed positive results in several samples. However, a systematic attribution of toxic effects to ozonation or subsequent soil passage was not possible. Reasons for cytotoxic effects were not evaluated within the scope of this project but it is assumed that they were caused by unknown cofactors. These results show that the objectives of enhanced removal of trace organics and DOC by combining ozonation and subsurface passage are well met. Further investigations need to confirm this for the pilot scale, especially taking into account the formation, retention and toxicity of oxidation by-products.

The combination of advanced oxidation (e.g. ozonation) and subsurface passage could overcome known limitations of MAR techniques with respect to dissolved organic carbon (DOC) and trace organics removal. The objective of the OXIRED project is to assess possibilities and limitations as well as practicability and technical feasibility of different combinations of advanced oxidation and subsurface passage with respect to this topic. As part of the first project phase, existing data on subsurface removal of organic trace substances was evaluated in order to identify substances that should be targeted in laboratory and technical scale experiments. This report summarizes the outcomes of this evaluation.