Study on discharge of microplastics via a waste water plant and potential abatement by using a water bubble curtain

This report describes the research of a consortium to investigate the potential to reduce plastic outflow from a waste water treatment plants (WTTP) with a bubble curtain (The Great Bubble Barrier) The focus lies on plastic in the size ranges of 0.02 mm to 5 mm (microplastic) The ultimate goal of the consortium is to prevent microplastic discharge from effluent towards surface water.
To reach this goal not only an effective removal technique is necessary but also reliable analytical techniques proving the effectiveness. For this, two analytical methods were compared during this research. A pilot set-up was built of a bubble curtain in a WWTP-effluent canal. During the course of six months several samples were taken at multiple points and analysed using two analytical principles: laser direct infrared (LDIR) imaging and optical microscopy. These two methods were evaluated for their comparability for plastic particles analysis in the size range from 0.02 mm to 5 mm.
In the condition where the pilot bubble curtain was set up, it was not possible to conclude that it is capable of reducing the outflow of microplastic particles in the effluent stream of the sewage treatment plant. Changes in particle concentration before and after the curtain were indistinguishable from the variation between samples from the same location. Previous studies however showed that the bubble curtain was capable of blocking buoyant plastic fragments on the surface from 1 mm. The lack of a measurable effect may be due to external influences, including analytical detection limitations. Changing the design of the barrier and the dimensions of the canal may improve the detection of the efficiency towards smaller plastic particles studied here. To evaluate the effect of different properties of the plastic particles we recommend additional research with a focus on (i) how hydrodynamic conditions affect separation (concentration and potential collection of microplastics), considering bubble curtain characteristics, horizontal and vertical water-flow and sampling/collection as well as (ii) the effects of physicochemical characteristics (e.g. size, dimensions, surface characteristics) of the plastic particles on their behaviour in the water column.
In this research we observed a WWTP-outflow of microplastics between 1 – 6 particles/L (optical method) and 40 – 50 particles/L (LDIR method). These results clearly indicate that WWTP-effluent emits as much microplastics as was shown in previous studies. Nevertheless, WWTPs remove a substantial part of the microplastics (90 – 99 %) from the influent water through the sludge line. Comparison of the two analytical strategies shows that these methods have similar results and variation with regard to trends, fluctuations, particle number and fibre number. However, these methods provide different levels of information: LDIR is capable of identifying a range of plastic types and shapes whereas in the optical methods distinction is limited and based on visual assessment only. Additionally, LDIR is able to detect smaller particles explaining why more particle numbers were reported using LDIR. However, the optical method may be more advantageous in samples with a high organic content as this impedes plastic identification using infrared. Both methods showed that especially fibres are a consistent part of the outflow of plastic fragments, also described in other studies. Most frequently identified polymers were polyamide (PA) PET, isoprene, PU/varnish, PP, PE-Cl, PE.
This study highlights the need of analytical strategies in microplastic analysis. Furthermore, it identifies the necessity for standardisation and a deeper understanding of factors of influence, e.g., sampling depth, weather conditions and day-to-day fluctuations.