Microplastic discharge from a wastewater treatment plant: long term monitoring to compare two analytical techniques, LDIR and optical microscopy while also assessing the removal efficiency of a bubble curtain

In this study we compare two parallel analytical methods while also testing a microplastics mitigation method. We assess the effectiveness of a bubble curtain to reduce microplastics in a wastewater treatment plant (WWTP)-effluent canal during the course of six months (>70 samples) using two analytical techniques: laser direct infrared (LDIR) and optical microscopy (OM) covering a size range of 0.02 to 5 mm. Comparison of the two analytical strategies shows similar trends, fluctuations, and correlating particle and fibre numbers. However, absolute values of particles differ, and the strategies provide different levels of information: LDIR is capable of identifying the plastic type as well as shape, while OM cannot determine the plastic type. Furthermore LIDR has a lower size limit (10-20 μm) than OM (50 μm). While information obtained by OM in general is far less detailed it is more affordable. This research also shows that the bubble curtain pilot does not have a measurable effect on the particle concentration. Possible effects of the curtain are hidden in the temporal variations. This research also reveals that individual samples show a large variation in particle numbers, illustrating that single measurements might give a poor representation of environmental particle number.