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Abstract

Contaminants of emerging concern (CEC) are organic pollutants occurring in wastewater and consequently enter the environment. An alternative to conventional wastewater treatment are algal photobioreactors, which treat wastewater with low energy input while producing nutrient- and energy-rich algal biomass and treated water, which both have the potential to be reused. Literature review yields knowledge gaps about the removal and fate of CEC in algal photobioreactors. Also, further studies are needed to establish risk assessment, future regulation and the safe reuse of algal biomass and reclaimed water in a circular economy. The goal of this thesis was to study CEC, including pharmaceuticals and personal care products, pesticides and bisphenols in laboratory and outdoor algal photobioreactors. For this reason, existing GC-MS and LC-MS/MS based methods were modified to analyse selected CEC in the aqueous phase of algal photobioreactors, and GC-MS/MS methods for the analysis of the aqueous and biomass phase were developed.
The removal and partitioning of 18 bisphenols in laboratory-scale photobioreactors were studied. The overall removal after 13 days was similar in both experiments: 72 ± 2% and 73 ± 5% removed in pH non-adjusted and pH adjusted series, respectively. Total removal ranged from 25 ± 5% to ≈100% and correlated with Chlorophyll a and compound log P. Culture pH also governed the removal of 2,2'-BPF, BPC2, BPAF, BPA and BPC. The significance of this work is in providing insight into the removal and effects of physicochemical parameters and pH on the fate of CEC in algal photobioreactors.
Furthermore, the removal of 28 CEC was compared between a mixed culture of algae and bacteria and a pure algal culture. The removal of bisphenols was higher in the mixed culture (87 ± 5%) than the pure culture (55 ± 7%), while similar removals (85 ± 28% and 79 ± 23%) were obtained for pharmaceuticals. For pesticides, removal (64 ± 21% and 90 ± 11%) was higher in the pure culture. In addition, transformation products of ibuprofen and metoprolol were also detected in both cultures. The importance of this study is in the comparison of 28 CEC removal between a mixed culture and a pure algal culture.
Finally, a pilot-scale high-rate algal pond treating real wastewater containing selected CEC was studied. Removal of BPA was 80%, whereas BPS proved more recalcitrant with an average removal of 32%. Methylparaben, 2,4-dihydroxybenzophenone and oxybenzone reached 92%, 76% and 80% removal, respectively. The most abundant compounds were naproxen and ibuprofen, but their removal efficiency was only 60% and 51%, respectively. This study is novel as it provides information on the removal of CEC estimated under realistic conditions, vital for developing algal photobioreactors for wastewater treatment.
Findings show that algal photobioreactors are a promising way to remove CEC from wastewater. Most CEC removals in algal photobioreactors are comparable to conventional biological wastewater treatment, while algal photobioreactors offer the potential of water and biomass reuse. That said, the work remains ongoing as some CEC remain recalcitrant (e.g. BPS, carbamazepine), and optimisation of operating parameters, along with the development of suitable pre- or post-treatment, is vital. The future of efficient wastewater treatment with algal photobioreactors and regulation depends on the continued study of CEC and their occurrence, removal, fate, and toxicity.