Removal of Micropollutants and Closing the Water Cycle Using Hollow-Fibre Nanofiltration Membranes

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1. Introduction

Nowadays, municipal wastewater contains a variety of residues of household chemicals, body
care products, pesticides and pharmaceuticals such as antibiotics and hormone-like
substances. This group of compounds is generally called micropollutants and not all existing
conventional wastewater treatment plants based on biological treatment are capable of
degrading or removing them. As a result, they are commonly discharged into water bodies,
like rivers, and will cause harm to the fauna and flora of these habitats. Moreover, these
receiving water bodies will often be used as the source of water for drinking water production
plants.

Therefore, there is an urgent need to implement new technologies for the treatment of effluents
from conventional wastewater treatment plants. Most technologies applied currently in Europe
are based on adsorption (activated carbon) or advanced oxidation processes (AOP) e.g. O3,
UV/H2O2. However, it must be taken into account that these methods also have drawbacks:
for example, the necessity of saturated activated carbon regeneration or disposal and the
formation of by-products in the case of AOP. In this publication, results obtained on real
effluent from a conventional wastewater treatment plant with hollow-fiber nanofiltration
membranes are presented, including an energy consumption comparison, as well as typical
operation costs (OPEX).

1.1. Hollow-fiber Nanofiltration Membranes

At the end of the 90’s, the first hollow-fiber nanofiltration membranes were firstly developed in
The Netherlands by Arie Zwijnenburg from Stork Friesland [1]. This first-generation hollow
fiber NF membranes were based upon the well-known interfacial polymerization concept
developed by Cadotte and others in the early 70’s. Big disadvantages of this concept are the
restricted chemical resistance of the selective polyamide top layer and the difficulties in the
production process using interfacial polymerization. A new approach to hollow-fiber
membranes is based upon the so-called Layer-by-Layer concept. At the RWTH University of
Aachen [2] and at the University of Twente [3], good results on lab-scale were obtained in the
beginning of the 21st century.

Based upon these research results, NX Filtration (NX) has developed the next generation
hollow-fiber nanofiltration membranes for direct treatment of surface water and biologically
treated wastewater. The base material of the membrane is poly-ether-sulphone (PES), which
has an outstanding mechanical resistance and generates membranes with very small pore
sizes and narrow pore size distribution. The PES base is covered with selective nanolayers
created from water-based electrolytes.

The hollow-fiber configuration improves fouling resistance as the flow is not obstructed by the
spacers used in spiral wound elements. Uniquely to nanofiltration processes, the hollow-fibers
can be backwashed, and NX’s innovative membrane chemistry allows for continuous
operation and cleaning with strong oxidants such as chlorine and hydrogen peroxide at very
high and very low pH. The membranes remove low molecular weight organics, color caused
by natural organic matter (NOM) and remove hardness partially, while allowing a high passage
of monovalent ions.

Two types of nanofiltration membranes are available: dNF80 (800 Dalton MWCO) is typically
used for NOM (color) removal. The denser dNF40 (400 MWCO) is suitable for micropollutant
removal and partial demineralization of water. Both types are available in modules of DN 110
and DN 200 with a membrane area of up to 50 m2 and membrane fiber internal diameter of
0.7 mm. Modules are operated in cross-flow mode with a cross-flow velocity of ≤0.5 m/s and
a maximum transmembrane pressure (TMP) of 6 bar.

1.2. Pilot plant and process

For these trials, a pilot plant has been installed at the municipal wastewater treatment plant
in Glanerbrug, the Netherlands. This WWTP is based on the conventional 3-step treatment
process: primary sedimentation, biological treatment and secondary sedimentation. The final
effluent was used as feed water to the pilot plant without any pretreatment other than a 200
micron strainer to remove large particles and coarse material prior to the membranes.

During continuous operation, filtration cycles of one hour were alternated with flush steps and
backwashes to control TMP rise. Eventually, chemical cleanings were performed to restore
permeability and resume filtration.

2. Results

These membranes have been used for this purposes since the very beginning of NX
Filtration on several municipal effluent streams, and have demonstrated a superior
performance compared to competing technologies.

3. Conclusion

From the results presented, not only on the removal of micropollutant but also in operational
costs, it can be concluded that hollow-fiber nanofiltration membranes can be a realistic
alternative for conventional technologies for polishing of conventional municipal WWTP
effluent commonly used nowadays. The high quality permeate can be further reused as process water in some applications and even as potable water (after further conditioning).
Thus, hollow-fiber nanofiltration membranes demonstrate a great potential to become a state
of the art method for water reuse applications.

4. Acknowledgement

The developments presented were supported financially in part by the EFRO-project
24Water (PROJ-00386) and the Interreg-project MEDUWA Vechte(e) (142118).

5. References

  1. Frank, M. J. W., Frank, M. J. W., Bargeman, G., Zwijnenburg, A., & Wessling, M. (2001).
    Capillary hollow-fiber nanofiltration membranes. Separation and purification technology,
    22(23), 499-506
  2. de Grooth, J. (2015). A tale of two charges: zwitterionic polyelectrolyte multilayer
    membranes. Enschede: Universiteit Twente. https://doi.org/10.3990/1.9789036538015
  3. STOWA, TAPES, Waterschap De Dommel (2015). Costs of removal of micropollutants
    from effluents of municipal wastewater treatment plants.
  4. Miklos et al., Water Research Volume 139, 1 August 2018, Pages 118-131. Evaluation of
    advanced oxidation processes for water and wastewater treatment — A critical review.

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