project

Preventing the flow of microplastics via effluent into surface water

Microplastics are small (< 5mm), synthetic particles that can have a big impact if they flow into large surface water bodies. Larger pieces of plastic waste cause harm to river and marine life, where for example animals mistake them for food or become tangled up in them.  The longer the plastic remains in the water, the more sunlight degrades it into ever smaller particles that can more easily be taken up in the food chain. These small plastic particles bind to organic micropollutants in the surface water and have already been detected in the food chain. Bubble Barriers applied to WWTP effluent could offer a solution to prevent the flow of microplastics into surface water.

Technology: Bubble Barrier

The traditional wastewater treatment plants (WWTPs) remove nitrate, phosphate and suspended particles from wastewater. Larger pieces of plastic and other materials are filtered out from the water at the WWTP. Literature data show that the plants also remove a large portion of the microplastics via the sludge line. At this time, knowledge about the presence of microplastics in WWTP effluent is still very limited. Effective solutions for the capture of microplastics in the WWTP effluent are still hardly available, and no single solution enables the retention and removal of microplastics over the whole width and depth of wastewater.

The application of the Great Bubble Barrier is a proven, effective technology for the capture and removal of macroplastics from flowing rivers and canals. The Bubble Barrier’s tube contains small holes through which air is pumped out, creating a wall of bubbles. Thanks to the river’s natural current and the diagonal positioning of the Bubble Barrier on the river bed, the plastic waste is guided to the river bank. There, it can then be removed from the water, without interfering with the vessels or the fish. A similar set-up at the WWTP in Wervershoof is testing whether the barrier also has an effect on microplastics. This experiment should reveal where exactly the boundary lies with regard to plastic particle size.

Research status

Starting in June 2019, at different times and positions before and after the Bubble Barrier, water samples were collected, according to the (adjusted) measurement protocol developed in the TRAMP project to determine the number of plastic particles per litre. Besides establishing the number, type and size-distribution of microplastics in the treated wastewater (effluent), the research also focuses on the operation of the Bubble Barrier regarding microplastics measuring 0.02 mm – 0.5 mm, and on improving and standardising the measurement protocol for microplastics. Based on these initial results, it can be concluded that microplastics have indeed been found in the effluent of the Wervershoof WWTP.

The initial results have also led to an adjustment in the method used to reprocess the samples. Because of the presence of a fatty substance in the effluent, an additional reprocessing step was required. KWR has now begun to analyse the samples using Laser Direct Infrared (LDIR) chemical imaging, which identifies microplastics extremely fast using a sensitive laser. Het Waterlaboratorium (HWL) is conducting parallel microscopic analysis of the same samples, which is easier in terms of the reprocessing but more labour-intensive for the detection. The final sampling took place in late November 2019, and the very lengthy sample reprocessing and analyses are currently underway. The definitive results will be reported in March 2020.

The partners organised an on-site press day on 13 September 2019, which attracted a great deal of interest from the local and national media, including the NOS Journaal news programme.

Read more about the TRAMP project here.

Challenge

TThe Great Bubble Barrier (TGBB), water utility PWN, PWN Technologies (PWNT), Hollands Noorderkwartier Water Authority (HHNK), and water research institute KWR jointly research the degree to which microplastics are present in a WWTP’s effluent, and how effectively a Bubble Barrier can prevent microplastics from being discharged with the effluent. All the partners united in the consortium assume their societal responsibility with this experiment.

Solution

This research project involves three cornerstones:

  1. Determining the physical factors involved in the use of a Bubble Barrier to retain microplastics for the protection of surface water.
  2. Further developing the existing measurement method for microplastics to encompass plastic particles of 10 µm and larger, so that their presence can be better quantified. This will produce a dataset on the presence and character of microplastics in WWTP effluent.
  3. Determining the effectiveness of a Bubble Barrier in intercepting microplastics.