Biological treatment processes: optimising biodegradation of OMPs in rapid filters

Organic micropollutants (OMPs) in drinking water sources are frequently in the news. To be able to continue supplying safe and reliable drinking water, the drinking water sector wishes, today and into the future, to remove these substances from the water as effectively as possible. We know that some OMPs degrade in certain rapid filters as a result of biodegradation, and that this degradation can be initiated in other filters by seeding the filter with sand from another filter. The transformation has so far been limited in the case of specific OMPs, and has decreased when the OMPs are temporarily not present. In this project we want to investigate what causes this limitation, whether the transformation is metabolic or co-metabolic, and whether, on the basis of these findings, the biological degradation of OMPs can be optimised.

Biodegradation in rapid filters

The great advantage of biological removal processes is that they generate (virtually) no residual streams. Their application is moreover simple and inexpensive. A couple of years ago it appeared that some OMPs could be effectively removed in certain rapid filters, but that this did not apply to all rapid filters. However, the biodegradation in the ineffective filters was improved when they were seeded with sand from the effective filters. But seeding alone is not enough, because the degradation is limited and decreases if the OMPs concerned are not present for a short period. A better understanding of the processes involved might make it possible to improve the effectiveness of the transformation.

Better understanding of biodegradation process

Research into the degradation of pyrazole in a rapid filter has shown that this process can be stimulated through seeding with sand from a rapid filter that is effective in removing this compound. Unfortunately, it also turned out that the maximum removal rate was only 40-45 percent, and that the removal decreased when the pyrazole dosing was stopped for a short period. The present project investigates whether the degradation is a metabolic or co-metabolic process. In the latter case, the degradation could possibly be stimulated by adding certain substances (such as carbon, ammonium or orthophosphate) to the filter.

Optimisation of biodegradation

If more were known about the biodegradation mechanism in a rapid filter, it would also be possible to make use of the knowledge to stimulate the process, for example, by adding specific substances. In this manner, the biodegradation of pyrazole, and perhaps of other organic micropollutants, could be optimised. Furthermore, a study could be done into whether the seeding rapid filters is a suitable method in a full-scale drinking water treatment process, and whether this approach could also lead to a reduction in the environmental impact of the treatment.