Improved water quality through advanced oxidation and dune infiltration

The Dunea drinking water utility uses surface water to produce drinking water by means of dune filtration.  In the decades ahead, the quality of the river water that Dunea takes in for its drinking water production will be increasingly under pressure, and for longer periods, because of an increase in organic micropollutants (OMPs), such as pharmaceuticals, x-ray contrast agents and industrial compounds. For this reason, in 2018, Dunea launched a large-scale pilot in which a sidestream of the intake waters is submitted to an extra pretreatment stage using an advanced oxidation process (AOP) along with rapid sand filtration.

Dunea’s drinking water treatment in which an advanced oxidation process (AOP) stage is added between sand filtration and dune infiltration.

Approach: extensive sampling and comparison with baseline measurement

This method involves the use of an AOP technique developed in-house, known as ‘GOBAM’ (the Dutch acronym for BergAMbacht Advanced Oxidation), which consists of a combination of ozone/hydrogen peroxide/low-pressure UV. The extra-pretreated water at the Bergambacht site is mixed with untreated rapid filtrate and then undergoes regular purification via dune filtration.

Earlier research and experiments with a small test installation in Bergambacht showed that AOP transforms a range of OMPs in the water.

The focus of the research described here is on the effects of AOP on the natural purification through dune filtration. The chemical AOP combined with the biological dune filtration can work in a complementary and synergetic manner, but must of course be safe for the dunes. The hypotheses of this research are:

  1. AOP and dune filtration are complementary because dune filtration removes the by-products of AOP, and the two processes are synergetic because the easily degradable organic matter (introduced through AOP) stimulates the natural purification in the dunes.
  2. AOP is a safe pretreatment stage and does not present a hazard for dune filtration.

To properly assess the effects of GOBAM, KWR and HWL conducted a comparison between the reference ‘baseline measurement’ (research without GOBAM in 2016 and 2017, described in the KWR report 2018.049) and an ‘effect measurement’ (the present research with GOBAM in 2018-2020), which made use of the same sampling points. The microbiology, the organic and inorganic chemistry, and the toxicology were extensively monitored in the water during the entire treatment process: at the intake, after pretreatment, the infiltration water, in four infiltration ponds and related monitoring wells along the bank, in the water abstracted from the dune, and in the drinking water.

From the pretreatment at Bergambacht, the water is transported via two pipelines to two different dune areas: Meijendel and Berkheide. GOBAM              is connected to the Berkheide pipeline, and since 2018 has treated a sidestream (30-40%) of this water, representing about 20% of Dunea’s total water production. After being mixed with untreated rapid filtrate, the water is infiltrated in the Berkheide dunes.

The assessment aspects of the impact of GOBAM on dune filtration concern the transformation of OMPs and the formation of by-products (including bromate and nitrite), and how these behave during dune filtration. The research also investigated changes in other organic and inorganic compounds and transformation products, the microbiology and the (gen)toxicology during dune filtration and in the final drinking water. In parallel research, the ecology in one infiltration pond was monitored over a long period.

The sampled treatment stages during Dunea’s drinking water treatment in this project, with the advanced oxidation process (AOP) stage added between the sand filtration and the dune infiltration.

Results: GOBAM removes OMPs and has limited synergetic effect on dune infiltration

As expected, the removal of OMPs by GOBAM was very effective: of the 48 investigated OMPs which had been detected at least once above the reporting limits, 31 had removal rates of more than 80%. Another 6 compounds showed a decrease of 50-80%. GOBAM thus makes a net positive contribution to the quality of the water that is to be infiltrated.

The concentrations (ug/L) of groups of organic micropollutants (see legend) in the different treatment stages during Dunea’s drinking water production.

Nevertheless, following dune filtration three compounds were still incidentally, and six compounds were regularly, detected above the reporting limits. The regularly detected compounds are amidotrizoic acid, iopamidol, EDTA, urotropine, acesulfame and primidone. The removal of these compounds during the period of the effect measurement with GOBAM was not (yet) significantly improved compared to the baseline measurement. However, after more long-term monitoring, this in fact became the case for the x-ray contrast agents and urotropine, thereby demonstrating that GOBAM improved the drinking water quality.

Increased concentrations of easily degradable organic matter were detected in the GOBAM effluent. These levels decrease in the transport pipeline from Bergambacht to the dunes, probably due to limited microbial growth and to the mixing with untreated rapid filtrate. The microbial growth has so far not led to an increase in the pipe resistance. Therefore, the easily degradable organic matter formed by AOP decreases during transport and does not end up in the first infiltration ponds, so it cannot stimulate the natural purification. But this might occur if a decision were made to increase the GOBAM sidestream.

Some (mostly undesirable) by-products are formed with AOP. Thus, in the first infiltration ponds slightly increased concentrations of bromate, bromoform and nitrite were detected by the effect measurement compared to the baseline measurement. In one of these ponds (pond 38) minor mutagenic activity occurred. In addition, these first infiltration ponds receive DNA from microorganisms which are present in higher concentrations in the GOBAM effluent. In infiltration ponds located further away, and during the subsequent dune filtration, these effects decrease to the point where they are no longer measurable. This indicates that AOP and dune passage complement each other. With regard to the above-mentioned by-products, there was therefore no difference in the drinking water between the baseline measurement and the effect measurement with the introduction of GOBAM.

Application: combination of GOBAM and dune infiltration over long term

It is expected that the application of GOBAM will lead to a further decrease in the concentrations of x-ray contrast agents, complexing agents and sweeteners in drinking water. An increase in the GOBAM sidestream would immediately result in a reduction of the input of these compounds in the dune, so that the drop in the concentrations in the drinking water would probably be accelerated. However, if the sidestream is increased, it is recommended that the effect on the organic and inorganic chemistry, toxicology, biological stability and activity, and the microbial populations be assessed once again during the treatment.

As a follow-up to this project, a geochemical and microbiological measurement programme and modelling of the bottoms of the infiltration ponds was carried out, in order to better understand and predict the removal mechanisms of priority compounds in the subsurface.