Department Process Engineering

Gravity-Driven Membrane (GDM) technology

Inadequate access to microbiologically safe drinking water continuously threatens the health and well-being of more than a billion people, primarily in developing countries. In many areas worldwide the central water infrastructure is not available at all, or not reliable, leading to unsafe water at the tap. In such cases, decentralized water treatment can be used.

Ultrafiltration is an effective technology to treat water and in principle can be applied on a decentralized scale. Most ultrafiltration membranes have pores which are smaller than the size of bacteria and viruses. Thus, water filtered through these membranes is microbiologically safe.

During dead-end ultrafiltrtion all macro- and microorganisms, particles and colloids accumulate on the membrne surface and a fouling layer is formed. Backflushing or chemical cleaning are usually used during conventional ultrafiltration to remove fouling layer. This prevents the membrane from clogging, which is expected to occur during filtration on a long term. However, backflushing or cleaning results in complex and maintenance-intensive systems, which are difficult to operate on a long term in developing countries.

New releases

The composition and compression of biofilms developed on ultrafiltration membranes determine hydraulic biofilm resistance

This study aimed at identifying how to improve the level of permeate flux stabilisation during gravity-driven membrane filtration without control of biofilm formation. The focus was therefore on understanding (i) how the different fractions of the biofilms (inorganics particles, bacterial cells, EPS matrix) influence its hydraulic resistance and (ii) how the compression of biofilms impacts its hydraulic resistance, i.e., can water head be increased to increase the level of permeate flux stabilisation. Biofilms were developed on ultrafiltration membranes at 88 and 284 cm water heads with dead-end filtration for around 50 days. A larger water head resulted in a smaller biofilm permeability (150 and 50 L m−2 h−1 bar−1 for biofilms grown at 88 cm and 284 cm water head, respectively). Biofilms were mainly composed of EPS (>90% in volume). The comparison of the hydraulic resistances of biofilms to model fouling layers indicated that most of the hydraulic resistance is due to the EPS matrix. The compressibility of the biofilm was also evaluated by subjecting the biofilms to short-term (few minutes) and long-term variations of transmembrane pressures (TMP). A sudden change of TMP resulted in an instantaneous and reversible change of biofilm hydraulic resistance. A long-term change of TMP induced a slow change in the biofilm hydraulic resistance. Our results demonstrate that the response of biofilms to a TMP change has two components: an immediate variation of resistance (due to compression/relaxation) and a long-term response (linked to biofilm adaptation/growth). Our results provide relevant information about the relationship between the operating conditions in terms of TMP, the biofilm structure and composition and the resulting biofilm hydraulic resistance. These findings have practical implications for a broad range of membrane systems.
Derlon, N.; Grütter, A.; Brandenberger, F.; Sutter, A.; Kuhlicke, U.; Neu, T. R.; Morgenroth, E. (2016) The composition and compression of biofilms developed on ultrafiltration membranes determine hydraulic biofilm resistance, Water Research, 102, 63-72, doi:10.1016/j.watres.2016.06.019, Institutional Repository

Biological control of biofilms on membranes by metazoans

Traditionally, chemical and physical methods have been used to control biofouling on membranes by inactivating and removing the biofouling layer. Alternatively, the permeability can be increased using biological methods while accepting the presence of the biofouling layer. We have investigated two different types of metazoans for this purpose, the oligochaete Aelosoma hemprichi and the nematode Plectus aquatilis. The addition of these grazing metazoans in biofilm-controlled membrane systems resulted in a flux increase of 50% in presence of the oligochaetes (Aelosoma hemprichi), and a flux increase of 119–164% in presence of the nematodes (Plectus aquatilis) in comparison to the control system operated without metazoans. The change in flux resulted from (1) a change in the biofilm structure, from a homogeneous, cake-like biofilm to a more heterogeneous, porous structure and (2) a significant reduction in the thickness of the basal layer. Pyrosequencing data showed that due to the addition of the predators, also the community composition of the biofilm in terms of protists and bacteria was strongly affected. The results have implications for a range of membrane processes, including ultrafiltration for potable water production, membrane bioreactors and reverse osmosis.
Klein, T.; Zihlmann, D.; Derlon, N.; Isaacson, C.; Szivak, I.; Weissbrodt, D. G.; Pronk, W. (2016) Biological control of biofilms on membranes by metazoans, Water Research, 88, 20-29, doi:10.1016/j.watres.2015.09.050, Institutional Repository

Impact of aeration shear stress on permeate flux and fouling layer properties in a low pressure membrane bioreactor for the treatment of grey water

Two different aeration regimes were studied in a low pressure gravity driven membrane bioreactor without any flushing or (back-) washing. In one reactor, the aeration was positioned below the membrane module, thus exposing the membranes to aeration shear stress. A second reactor was operated at low shear stress by placing the aerator in a different compartment. Flux stabilization at 2.0 L/(m2 h) occurred in the reactor with low shear stress while no flux stabilization was observed in the reactor with aeration shear stress, resulting in a flux of 0.5 L/(m2 h) after 120 days. The thickness of the bio-fouling layer in the reactor with aeration shear was smaller (129 vs. 344 µm), which implies that shear stress resulted in a thinner, denser and less permeable bio-fouling layer. The results can be explained by differences in (1) the morphology of the bio-fouling layer and (2) the EPS contents (proteins and polysaccharides) in the bio-fouling layer. The low-shear system provides a suitable solution for decentralized grey water treatment, or other conditions where maintenance and energy consumption should be minimized. Furthermore, the results can contribute to decrease the energy consumption in MBR systems.
Ding, A.; Liang, H.; Li, G.; Derlon, N.; Szivak, I.; Morgenroth, E.; Pronk, W. (2016) Impact of aeration shear stress on permeate flux and fouling layer properties in a low pressure membrane bioreactor for the treatment of grey water, Journal of Membrane Science, 510, 382-390, doi:10.1016/j.memsci.2016.03.025, Institutional Repository

Contact

Dr. Nicolas Derlon Tel. +41 58 765 5378 Send Mail

Research

Formation of biofilms on membrane surfaces is usually considered to be detrimental as filtration performances are decreased. However, controlling the formation of biofilms remains challenging and requires a significant energy- and chemical- demand. Our research aims at developing a new paradigm for operating membrane systems that consists in taking advantage of the presence of biofilms on membrane surfaces. The formation of biofilms on membrane surfaces indeed helps to stabilise the permeate flux over several month. The feed water composition determines the physical and biochemical structure of the biofilms, and ultimately its hydraulic resistance. The biofilm also helps to increase the permeate quality. The retention of biodegradable compounds or viruses is higher in the case of a "biofilm+membrane" composite system than for systems based on membrane only.

Hydraulic resistance of biofilms

Several factors influence the hydraulic resistance of biofilms formed during GDM filtration and in turn the permeate flux: the feed water composition in terms of (i) substrate concentration and (ii) microbial diversity and (iii) the operating conditions in terms of external forces acting on the biofilm structure (e.g. TransMembrane Pressure). These factors determine the physical and biochemical structure of the biofilms and ultimately its hydraulic resistance, i.e., the quantity of water that is filtered. Stable permeate flux of 5 to 20 L m-2 h-1 are usually observed during GDM ultrafiltration of surface water.

An increasing substrate concentration in the feed water results in a higher biofilm accumulation and in a lower flux. However, our results indicate that flux stabilisation always occurs, whatever the type of feed water: contaminated drinking water, surface water, used wash-water, etc. 

Dr. Nicolas Derlon Tel. +41 58 765 5378 Send Mail
Prof. Dr. Eberhard Morgenroth Tel. +41 58 765 5539 Send Mail

Effect of predation

Predation by metazoa and protozoa improves the performance of membranes used for the filtration of contaminated water. Membrane filtration allows to securely remove most pathogens from contaminated water but efficient application of membranes is hampered by the development of biofilms on the surface of the membrane reducing the water flux. Strategies available to reduce biofilms on membranes are mostly energy and chemical intensive and lead to increasing operating costs. In our work we are developing an innovative approach that relies on biological mechanisms influencing the permeability of the biofilm to maintain water flux rather than chemical cleaning or energy intensive cross-flow.

Membranes are operated at low pressures and in a dead-end mode. Without mechanical stress a porous and heterogeneous biofilm structure develops that is susceptible to predation by higher organisms. In presence of predators (upper panel in Figure), an open and heterogeneous biofilm structure developed with only partially coverage of the membrane surface (85%). In absence of predation (where higher organisms are chemically inhibited, lower panel in Figure) a flat and compact structure covering the whole membrane surface developed. We were able to demonstrate that metazoa (rotatoria, nematoda, and oligochaeta) were the main group of higher organisms responsible for the development of open biofilm structures.

Dr. Nicolas Derlon Tel. +41 58 765 5378 Send Mail

Towards lower footprint and capital costs of GDM systems

New GDM platform to test up to 30 modules

GDM is a relevant and adequate approach to produce safe drinking water due to low operation requirements and marginal need for external energy inputs. An important advantage of GDM is that these systems decrease the concentration of biodegradable compounds in the permeate, in comparison to conventional ultrafiltration (UF). These characteristics are not only of major interest for community scale systems, but also for centralized systems. For instance, enhancing the removal of biodegradable compounds is one of the first objective of reverse osmosis pre-treatment step, given that they are major membrane foulants. Yet, a principal limitation for GDM applicability in centralized systems is the higher footprint and associated capital costs caused by higher membrane surface requirements, due to lower fluxes than achieved with conventional UF.

Since 2019, we actively work on approaches that could increase GDM fluxes and reduce space requirements by replacing flat-sheet with hollow fiber membranes. Using our new GDM platform (picture), we successfully proved that GDM filtration is feasible with hollow fiber (HF) membranes operated in inside-out mode. The implementation of a backwash allows maintaining high fluxes despite the low space availability for biofilm growth in the lumen of the fibers, as discussed in Stoffel et al., (2021). With this project, we showed that the GDM step footprint could be decreased by up to 20 times, when implementing HF membrane operated in inside-out mode with low cost maintenance instead of GDM systems equipped with non-maintained flat sheet membranes.

Our current activities focus on assessing procedures to lowering the capital costs and optimizing the operation.

Application examples

Several treatment systems based on “biofilm+membrane” composite were developed at Eawag: The Safir water filter and the Autarky Toilet. Both the Safir water filter and the Autarky Toilet are decentralized filtration systems (Table 1). The Safir water filter is a point-of-use system applied to the decentralized production of drinking water in developing countries (Peter-Varbanets et al., 2014). The Autarky Toilet is an updated version of the Blue Diversion Toilet and has been developed as part of the “reinvent the toilet challenge” funded by the Bill&Melinda Gates Foundation (Larsen et al., In press). The Autarky Toilet operates “off the grid” (without connections to piped water, sewer system, etc). It separates fresh urine, feces and used wash-water. The used wash-water is treated on-site for further reuse.

Blue Diversion Toilet (Reinvent The Toilet Challenge)

In 2011 the Bill & Melinda Gates Foundation initiated and funded the competition "Reinvent-the-Toilet-Challenge" (RTTC) in which eight universities with promising entries had to proof their concepts. The target is to develop a mass-produced sanitation system operating grid-free (not connected to electricity grid, piped water, or sewer) with total costs not exceeding 5 US cents per person and day. High user comfort and total resource recovery are also key requirements.

Almost 2.6 billion people worldwide use unsafe toilets or defecate in the open. Poor sanitation causes severe diarrhea, which kills 1.8 million people each year. This problem is especially daunting in dense urban settlements especially affecting the urban poor. Therefore people living in under such conditions are the target group for RTTC.

A key feature of the toilet system developed in collaboration with the Austrian design office EOOS is an on-site water recovery system which is based on gravity-driven ultrafiltration and is integrated in the back wall of the toilet. GDM filtration is applied in the RTTC project to the recovery of wash and flush water. The special feature of the gravity-driven ultrafiltration is its passive biological activity maintaining the membranes permeable, thus making chemical cleaning or maintenance unnecessary as long as moderate aeration is provided.

More information about this project

Publications associated

  • Larsen, T.A., Gebauer, H., Gründl, H., Künzle, R., Lüthi, C., Messmer, U., Morgenroth, E., Niwagaba, C.B., Ranner, B. (2015). Blue Diversion: a new approach to sanitation in informal settlements. J. Water Sanit. Hyg. Develop., 2015, 5(1) 64-71.
  • Ravndal, K., Künzle, R., Derlon, N., Morgenroth, E. (In Press). On-site treatment of used wash-water using biologically activated membrane bioreactors operated at different solids retention times. J. Water Sanit. Hyg. Develop.
  • Künzle, R., Pronk, W., Morgenroth, E., Larsen, T.A. (In Press). An energy-efficient membrane bioreactor for on-site treatment and recovery of wastewater.  J. Water Sanit. Hyg. Develop.

Kontakt

Other related projects

GDM for drinking water treatment in low-income countries

"SELF" living module

About us

Department of Process Engineering (Eng)

Prof. Dr. Eberhard Morgenroth Tel. +41 58 765 5539 Send Mail
Dr. Nicolas Derlon Tel. +41 58 765 5378 Send Mail

Department of Water and Sanitation in Developing Countries (Sandec)

Dr. Sara Marks Water Supply and Treatment Group Tel. +41 58 765 5631 Send Mail
Regula Meierhofer Group Leader Water Safety Management Tel. +41 58 765 5073 Send Mail

Department of Environmental Microbiology

Dr. Frederik Hammes Group Leader Tel. +41 58 765 5372 Send Mail
Dr. Tim Julian Group Leader of Pathogens and Human Health Tel. +41 58 765 5632 Send Mail

Collaborators

  • Thomas Neu, Head of group "Microbiology of interfaces", Department of river ecology, Helmholtz Centre for Environmental Research - UFZ.
  • Judith Blom, Limnology Station of Zürich, University of Zürich.
  • Jakob Pernthaler, Prof., Limnology Station of Zürich, University of Zürich.

Former members

  • Richard Johnston, former Head of the water supply group (SANDEC).
  • Maryna Peter-varbanets, former post-doc in the water supply group (SANDEC)
  • Selina Derksen-Müller, former project officer in the water supply group (SANDEC).
  • Anna Chomiak, former PhD student in the process engineering department
  • Joao Nuno Maximino Mimoso, former PhD student in the process engineering department.

Downloads

PhD theses

Anna Chomiak (2015). Influence of inorganic and organic matter in gravity-driven membrane ultrafiltration. Diss. ETH no. 22669.

Maryna Peter-Varbanets (2010). Ultra-low pressure ultrafiltration for decentralized drinking water treatment. Diss. ETH no. 19134. 

Presentations

Other publications