Reducing microplastic pollution from urban wastewater : a cost-effectiveness analysis

Microplastic pollution is a globally increasing issue in both terrestrial and aquatic environments. One of its pathways to ecosystems is urban wastewater, as the treated effluent has been observed to contain remarkable quantities of plastic particles. In addition, as the treatment process separates the nutrients and solids into a sludge, the particles have been found in the sludge as well. The aim of this thesis is to investigate the costs of removing microplastic particles from the wastewater, and to assess the total microplastic pollution mitigation potential of selected wastewater treatment and sludge management methods. To examine this, I conducted a cost-effectiveness analysis, in which I constituted five different technology scenarios and calculated their incremental cost-effectiveness ratios. The ratios were then compared against a business-as-usual baseline scenario and each other to define the lowest cost of removing microlitter. The cost-effectiveness analysis was conducted for four different wastewater treatment plant sizes. The technology scenarios constituted of combinations of three wastewater treatment and two sludge management methods. The wastewater treatment methods were conventional activated sludge, rapid sand filtering and membrane bioreactor, and the sludge management methods were anaerobic treatment followed by land application as a fertilizer, and sludge incineration. The cost data on the selected wastewater treatment and sludge management methods were obtained from various documented sources, whereas two studies on the microlitter content in wastewater (Talvitie et al., 2017a & 2017b) comprised the effect data. To achieve an overview of the magnitude and the mitigation capacity of the issue, I compared the costs and the quantity of released microlitter of each technology scenario to the baseline scenario. In addition, to assess the impact of changing economic attributes and wastewater quality on the cost-effectiveness ratios, I performed a univariate sensitivity analysis. The results of the analyses prove that removing microplastics from wastewater is both feasible and cost-effective. The membrane bioreactor combined with sludge incineration resulted the most economical scenario in all circumstances and in each wastewater treatment capacity.