Simulation of the interception of plastic fluxes in rivers

Abstract

One of the preferred features of plastic is its high
durability. This merit is its main disadvantage in the natural environment.
When poorly disposed of, plastic waste can enter rivers from surface waters and
subsequently end up in the ocean. Here, plastic spreads out over the entire
ocean and ultimately ends in ocean gyres. Plastic material in the ocean causes
environmental damage. Marine animals can get entangled in plastic debris,
plastic accumulation occurs when consuming seafood and the material spreads
toxins in the seawater. It has also caused economic damage: marine plastic
accumulation led to €5 to 18 billion to key economic sectors for 87 coastal
countries in 2020 (Deloitte, 2021). Attempts have been made to capture buoyant
plastics from the ocean with interception devices, with mixed success.

The placement of an interception device in a river is
thought to be a better approach to the problem. A standard interception design
is that of a floating boom. Plastic debris is transported by the river flow and
these objects guide buoyant plastics into a collection tray. Currently, these
interception designs are still in their infancy. A difficulty in the
optimisation of these interception devices lies in the limited knowledge of the
trajectories of plastic waste. Computational fluid dynamics (CFD) modelling can
help to create a better understanding of these aspects. This tool can provide a
rapid assessment of different flow conditions with different types of plastic
debris modelled. Further knowledge into this field can ultimately help into
better retention of plastics in rivers.

Plastic modelling for buoyant particles has been performed
in the marine environment (Van Utenhove, 2019) and in the river (Van Welsenes, 2019)
with a Lagrangian study. However, both works did not perform a validation
study. The combination of a CFD model and the results of physical model tests
can be valuable in removing uncertainties related to plastic trajectories and
the modelling of these.  One physical
model test performed on plastics is the release of plastic particles in a flume
(Zaat, 2020) Small films were released in a 2DV physical model, representing
plastic bags. It was found that these particles followed an inverted Rouse profile
for spherical particles times a shape factor.

The study by Zaat (2020) was replicated in the CFD modelling
environment of OpenFOAM in this thesis. Subsequently, the retention of plastic
particles was investigated. Three hydrodynamic conditions were investigated,
with uniform inlet flow velocities of 0.10, 0.55 and 0.90 m/s. The $k-\epsilon$
turbulence model was used. An Euler-Euler approach was applied with a discrete
and continuous phase. This modelling approach is computationally less expensive
compared to a Euler-Lagrangian framework and can be easily established for
different cases. The drag models Gibilaro, Wen-Yu and Syamlal-O'Brien were
tested to investigate which concentration profile best fits an inverted Rouse
profile for rising particles. The interception device was modelled as a square
obstruction of 10\% of the water depth and the model was assumed as a
rigid-lid. The retention of particles by the system was explored for neutrally
buoyant particles, light material and high-density
polyethylene (HDPE) and polypropylene (PP) films.

This study found relative differences in near-surface flow
and the analytical approach of 3 to 4%. The Gibilaro model was best applicable
and model coefficients for the virtual mass, lift and turbulent dispersion of
0.5, 1.6 and 1.0 were found. The concentration profiles followed the inverted
Rouse profile and that for films closely for the medium and high flow cases,
but not for the low flow case. A lower retention of plastics was observed for
increasing flow velocities. In the extreme case of low flow and light particles
a build-up of particles near-surface can be observed and runs against the
streamwise direction.

Several limitations were present in this study. Only two
parameters of the plastic particles were adjustable, which were the particle
diameter and density. In reality, plastic debris has different shapes will
experience drag differently, especially the friction drag. The rigid-lid model
is valid only if the velocity head is smaller than that of 10% of the water
depth, which is not the case for the high flow velocity case.

The results show that on the basis of a rise velocity and a
density a representative diameter can be determined from Stokes’ law. The
Euler-Euler modelling approach provides an accurate assessment and gives a
proper representation of the particles for the Rouse distributions. This is
done without handling the particles individually and thus results can be found
more quickly. Both for HDPE and PP inverted Rouse profiles were found that
followed the theoretical profile with an underestimation of 10\%-20\%
near-surface. The retention was influenced mainly by the size of the wake. It
is evident that the interception system can work for buoyant particles. The
object can be placed during low flow conditions for the highest efficiency. For
high discharge in rivers, more particles will be in suspension and thus further
away from the water surface, which makes this design less preferred during high
flow conditions.

Recommendations for further research are to perform more
physical model tests, implement a non-spherical drag model applicable for
plastic material and to apply a three-phase model. This removes uncertainties
related to the model and to the current knowledge in particle dynamics. The
addition of physical model tests would benefit the current knowledge the
greatest. It is only possible to expand the complexity of the CFD model with a
larger availability of these physical model tests.