Access to potable water is a basic human right; however, for one third of the world population this right is not met. Membrane ultrafiltration is increasingly used to remove particulate matter, bacteria and large viruses from greywater, rainwater, and wastewater. A simple, low cost, portable variant of membrane ultrafiltration is gravity-driven membrane (GDM) filtration, which uses elevation head to impose hydrostatic pressure and induce flow, eliminating the need for expensive external pumping systems. GDM filtration inherently operates within a low hydrostatic pressure range (10–200 mbar), meaning the permeate flux is determined by the membrane hydraulic resistance, a parameter that increases over time due to membrane biofouling. GDM systems exploit the long-term stabilisation of biofouling layers to yield a stable permeate flux, a phenomenon known as flux stabilisation, enabling low-maintenance long-term system operation (5–8 years). However, GDM flux stabilisation values are typically an order of magnitude lower than conventional ultrafiltration systems, a major obstacle inhibiting community-scale adoption of GDM systems. Therefore, developing approaches to increase flux stabilisation values are critical to the furtherance of this technology.
The biofouling layer in GDM filters is a mixture of passive particulate and living biological matter, the major constituent being microbial biofilms. Ubiquitous in nature, biofilms are microbial communities bound together by a matrix of self-secreted extracellular polymeric substances (EPS). The EPS matrix is primarily composed of proteins, polysaccharides, and extracellular DNA, which together form a hydrated viscoelastic gel. This polymer network provides mechanical stability and acts as a small molecule diffusion barrier, contributing over 80% of the total GDM system hydraulic resistance. Biofilm hydraulic resistance is primarily a function of EPS pore size; however, EPS composition and spatial distribution is heterogeneous, with regulation of EPS secretion determined by local microbial responses to biological and chemical cues. Significantly, the application of external stresses invokes structural rearrangement through stress relaxation, and there is emerging evidence suggesting that mechanical cues can trigger EPS secretion. Biofilm hydraulic resistance has been shown to correlate with changes in mesoscale morphology, EPS volume and viscoelastic behaviour, factors seemingly shaped by hydrostatic pressure. However, despite these mesoscale observations, the microscale biophysical mechanisms and microstructural rearrangements that shape biofilm hydraulic resistance remain underexplored.
Currently, membrane-bound biofilm studies are performed at the meso- and macro-scale, and so require extended growth periods (days to months) and must rely upon ex-situ sacrificial analysis techniques with limited temporal and spatial resolution. Despite the important contribution of biofilm microstructure and EPS composition to the determination of hydraulic resistance, a systematic understanding of how hydrostatic pressure shapes biofilm microstructural development, EPS composition and mechanical properties is still lacking. This knowledge gap stems primarily from the difficulty of directly probing microscale rearrangements of biofilm structure and EPS composition under controlled hydrodynamic conditions. This project will fill this gap.
The aim of this project was to understand how hydrostatic pressure shapes the biophysical development of biofilm microstructure and EPS composition to affect biofilm hydraulic resistance. Insights gained at the microscale enabled the synthesis and rapid testing of pressure control strategies to engineer biofilm hydraulic resistance. To achieve these aims the project was split into two main objectives (O).
O1. To determine how hydrostatic pressure shapes microscale EPS distribution and composition of membrane-bound biofilms and how these properties interact to determine hydraulic resistance.
O2. To establish a scalable relation between hydrostatic pressure and biofilm hydraulic resistance based on the microscale investigations of O1 and demonstrate its validity at the macroscale using membrane biofouling simulators.
