Closing the water cycle

Technology

The plan is to combine ozonation with ceramic membrane filtration for the effluent treatment in Wervershoof. The international literature, as well as our earlier experiments, indicated that this approach can produce a synergy effect, with significantly less membrane fouling as a result. Three possible explanations for this were put forward and this study looked at which one of the three is the most probable in this case. A better understanding of the combined application of ozone and ceramic membrane filtration is needed to optimise the process and/or its effect. The quality requirements were also considered for the use of effluent as water for industry and for infiltration of the water in the dunes, as is the case with WRK water.

Challenge

The ultimate objective of the project was twofold: better water quality, with the associated application possibilities, and less fouling of the ceramic membrane. The improved understanding of these processes, and the associated improvement in sustainability, opens up the prospect of the higher-value application of the treated effluent in practice.

To establish a clearer picture of the process by which ozone with ceramic membrane filtration leads to reduced membrane fouling, we investigated whether ozone leads to the formation of hydroxyl radicals on the membrane surface. Other possible explanations are that ozone brings about changes in the water matrix or in the top layer on the membrane surface.

In addition, the study looked at the extent to which the proposed treatment would allow the WWTP effluent to comply with the strict quality requirements for WRK water and what other uses could be considered for this water. WRK water is infiltrated in the dunes, and one of the difficulties when this happens after ozonation is the possible presence of AOC, which is formed by reaction with ozone. This could result in blockages. The extent to which AOC can be removed by filtration with biologically activated carbon was therefore investigated as well.

Solution

Three explanations were investigated for the synergy effect between ozone and ceramic membrane filtration. It emerged that a change in the water matrix (the composition of the effluent organic matter (EfOM)) was probably the causal factor in this case.

The water quality of the treated effluent is better, and so it can be used, for example, as industrial water (or as a source for that water). Another possible use is to infiltrate the water, like WRK water, in the dunes, eventually transforming it into drinking water. However, treatment with ozone breaks down some of the EfOM into smaller molecules, forming AOC. Passing the treated effluent through a column of biologically activated carbon reduced the AOC content and further removed the remaining organic micropollutants. Given these parameters, infiltration in the dunes should be possible, as with WRK water. However, it emerged that the sodium chloride content of the effluent was relatively high (equal to or even slightly higher than the permitted level for drinking water), and this could complicate this application.