Removal and Fouling Influence of Microplastics in Fertilizer Driven Forward Osmosis for Wastewater Reclamation

Abstract

Insufficient removal of microplastics (MPs) and nanoplastics (NPs) may exert negative effects on the environment and human health during wastewater reclamation. The fertilizer-driven forward osmosis (FDFO) is an emerging potential technology to generate high-quality water for irrigation of hydroponic systems. In this study, the removal of MPs/NPs by the FDFO process together with their impact on FDFO membrane fouling was investigated, due to FDFO's low molecular weight cut-off and energy requirement by using fertilizer as draw solution. Plastic particles with two different sizes (100 nm and 1 μm) and extracellular polymers released by real wastewater bacteria were utilized as model compounds for FDFO performance comparison. Results show that FDFO membrane system could generate high-quality irrigation water with only fertilizer, completely removing extracellular polymers, MPs and NPs from wastewater. It was found that the MPs and NPs themselves do not cause a significant membrane fouling. Moreover, it could help to reduce the membrane fouling caused by extracellular substances. That is probably because MPs and NPs helped to form a loose and porous fouling layer. Therefore, the FDFO process could be a long-term stable (low fouling) process for the reclamation of wastewater with high-quality requirements.

Keywords: forward osmosis; membrane fouling; microplastics; nano-scale plastics.

Figure 1

The flow chart of the FO membrane system.

Figure 2

LC-OCD chromatograms of FDFO permeate (diluted draw solution) and initial feed solution with dissolved extracellular polymers released by two wastewater bacteria: Shigella flexneri strain 301 and Escherichia fergusonii ATCC 35469.

Figure 3

FEEM spectra of wastewater with isolated extracellular polymers from wastewater bacteria Shigella flexneri strain 301 and Escherichia fergusonii ATCC 35469 (a), and treated wastewater by the FDFO process (b).

Figure 4

1H solution-state NMR chromatography at 700 MHz of isolated extracellular polymers from Shigella flexneri strain 301 (a), Escherichia fergusonii ATCC 35469 (b) and their corresponding fraction assignments and integration based on the NMR chromatography (c).

Figure 5

The comparison of normalized fluxes as a function of time between blank control and feed solution with only MPs and NPs (a), and between blank control, feed solution with extracellular polymers and feed solution with both extracellular polymers and plastics (b).

Figure 6

The SEM image of FO membranes after experiments treating feed solutions of in Group 1 (a), Group 2 (b) and Group 3 (c). The magnitude of these SEM images is 10,000×.