Danièle Mousel

Energy demand for elimination of organic micropollutants in municipal wastewater treatment plants

Organic micropollutants (OMP), e.g. pharmaceuticals and household/industrial chemicals, are not fully eliminated in state-of-the-art municipal wastewater treatment plants and can potentially harm the aquatic environment. Therefore, several pilot and large-scale investigations on the elimination of organic micropollutants have taken place in recent years. Based on the present findings, the most efficient treatment steps to eliminate organic micropollutants have proven to be ozonation, adsorption on powdered activated carbon (PAC), or filtration through granular activated carbon (GAC). Yet a further treatment step implies an increase in energy demand of the wastewater treatment plant, which has to be considered along with OMP elimination. To this aim, data on energy demand of ten large-scale municipal wastewater treatment plants (WWTP) with processes for OMP elimination was collected and analyzed. Moreover, calculations on energy demand beyond the WWTP for production and transport of ancillary materials were performed to assess the cumulative energy demand of the processes. An assessment of the greenhouse gas emissions of the processes was achieved, which shall facilitate future life cycle analyses. The results show that energy demand of ozonation at the wastewater treatment plant is dependent upon the ozone dosage and is significantly higher than energy demand of PAC addition or GAC filtration (2 to 4 times higher without consideration of delivery heads). Despite uncertainties regarding the energy demand for production of activated carbon, it could be shown that the cumulative energy demand of adsorption steps is significantly higher than the energy demand at the WWTP. Using reactivated GAC can lead to energy and greenhouse gas emissions savings compared to using fresh GAC/PAC. Moreover, energy demand is always plant-specific and depends on different factors (delivery heads, existing filtration or post-treatment etc.). Since processes for elimination of organic micropollutants are still in a developing phase, future optimization steps shall minimize their energy demand.

Ozonation of nursing home wastewater pretreated in a membrane bioreactor

Nursing home (NH) wastewater was pretreated in an ultrafiltration membrane bioreactor (MBR) and subsequently ozonated in a pilot plant in order to evaluate the elimination of pharmaceutically active compounds (PhACs). Dosing of the pre-treated wastewater with 5 mg ozone (O3) L-1 led to the elimination of >50% for nearly all investigated PhACs in the ozonation plant, whereas dosing 10 mg O3 L-1 increased elimination to >80%. A total hydraulic retention time of 12.8 min proved sufficient for PhAC elimination. Specific ozone consumption and influent dissolved organic carbon (DOC) (8.2-9.5 mg L-1) were in similar ranges for all three performed trials. Combining the MBR with subsequent ozonation at a dosage of 5 mg O3 L-1 achieved elimination of >90% and effluent concentrations below 250 ng L-1 for nearly all the investigated PhACs. Influent concentrations of the MBR were comparable to those found in municipal wastewater. Thus, the recommended dosage for PhAC elimination of 5 mg O3 L-1 (i.e. a specific consumption of 0.6 g O3*(g DOC)-1) is in the same range as for municipal wastewater. However, due to a smaller plant size, the specific costs for treating NH wastewater would significantly exceed those of treating municipal wastewater.