Enhanced micropollutant removal and nutrient recovery in municipal wastewater treatment

Abstract

Depleting natural resources such as fresh water and phosphate minerals creates pressure to pursue towards resource recovery in municipal wastewater treatment. In addition, the aquatic environment is under stress due to eutrophication as well as increasing presence of harmful micropollutants, such as pharmaceuticals, hormones and pesticides. Traditional municipal wastewater treatment is not designed for micropollutant removal and treatment plants also struggle with tightening discharge limits. Advanced technologies are required to tackle these challenges.

This thesis examines advanced wastewater treatment technologies for enhanced micropollutant removal and nutrient recovery by focusing on technologies that minimize the use of chemicals. Nanofiltration, reverse osmosis, electrodialysis and pulsed corona discharge oxidation studied gave promising results concerning micropollutant removal. For instance, >90% removal of target pollutants diclofenac, carbamazepine and furosemide could be obtained. Oxidation degraded all target pollutants below 0.1 μg/L with only 0.2 kWh/m3 oxidation energy when MBR permeate was treated. Oxidation was identified as the most promising technology when only pollutant degradation from MBR permeate is required. However, nanofiltration could be suitable in applications where enhanced micropollutant removal as well as COD, DOC and phosphorus is required.

Size exclusion, hydrophilicity and electrostatic interactions were the main micropollutant removal mechanisms in nanofiltration, whereas size exclusion was the key factor in reverse osmosis. The molecular weight, the amount of double bonds within the molecule and OH degradation constant were the main parameters for pollutant degradation in oxidations according to regression analyses. Molecule hydrophilicity and electrostatic interactions had the strongest influence for pollutant transport in electrodialysis. Overall, the micropollutant removal in the studied technologies appeared to be a combination of several pollutant properties, water matrix and process specific conditions.

Two-stage nanofiltration process coupled with phosphorus precipitation was able to minimize the amount of membrane concentrate by reaching a final VRF value of 300 with minor membrane fouling whilst calcium phosphate was spontaneously precipitated with a 52% recovery rate. Pilot scale electrodialysis produced a highly concentrated fertiliser product rich in NH4 +-N (7.1 g/L) and K+ (2.5 g/L) by concentrating nutrient ions from centrate wastewater with a low energy consumption of 4.9 kWh/kg NH4 +-N. The studied processes could help to guarantee a safer aquatic environment for future generations.