Abstract:
Lack of access to clean and safe potable water, especially for people living in rural areas
of developing economies, is a matter of great concern in different parts of the world.
Measures taken to address the challenges arising from this problem include the
improvement of existing water purification methods and development of new
appropriate technologies such as point of use (POU) water treatment technologies.
One such appropriate POU technology is the Remote Rural Water Treatment System
(RRWTS) developed at Durban University of Technology (DUT) in South Africa. The
RRWTS is based on polyester woven fabric microfiltration (WFMF) membranes and
other locally sourced materials. The filtration unit consists of flat sheet modules
assembled into a pack and permeate outlets connected to a manifold and then to a tap.
The system is gravity driven and therefore eliminates the use of pumps and electricity.
This system has shown potential for use in water treatment as it produces permeate with
turbidity below 1 NTU, has a high permeate flow rate, and is easy to use and maintain.
However, the major challenge facing its use is that permeate does not meet the set
microbiological standards for drinking water (zero E. coli in 100 mL treated water). The
RRWTS can ideally remove 95 to 99 per cent of the influent E. coli. This necessitates
the use of a separate disinfection step, often using chlorine for complete removal of
microbial contaminants.
The objectives of this study were: to investigate the incorporation of silver nanoparticles
(AgNPs) into the WFMF membrane; to evaluate the disinfection efficacy of the AgNPs
impregnated filter (coated filter); and to determine the long term performance of the
coated filter in terms of disinfection and silver elution (90 days).
The study was conducted in four stages. Firstly, AgNPs were incorporated on the
membrane using in situ chemical reduction of silver nitrate using sodium borohydride.
Secondly, the filters were characterized using scanning electron microscopy (SEM) to
determine the morphology, and the Sessile drop method for contact angle measurement
was employed to determine the membrane hydrophilicity. In addition, X-ray diffraction
(XRD), Fourier Transform Infrared (FT-IR) spectroscopy and UV- Visible Spectroscopywere used to investigate the presence of AgNPs on the coated filter. Thereafter, the
antibacterial efficacy of the filters was evaluated using a laboratory scale experimental
rig and different microbial assays. Raw river water and deionized water spiked with
E.coli (synthetic feed) were used as feed. Lastly, the effects of concentration of E.coli in
the feed and silver elution on the disinfection performance of the coated filter over time
were investigated. The performance criteria were based on permeate quality and they
included: turbidity, concentration of E.coli, and silver concentration.
The characterization results depicted that AgNPs were successfully immobilized on the
WFMF membranes by in situ chemical reduction. The incorporation of AgNPs was
ascertained using UV-Vis Spectrophotometry, FT-IR and XRD. The Sessile drop test
indicated that the membrane became more hydrophilic (77 per cent decrease in water
contact angle) and the permeability increased significantly as a result of the coating (p
<0.05). The coated filters demonstrated excellent filtration performance producing
permeate with turbidity less than 1 NTU for feed turbidities between 40 and 700 NTU.
The disinfection efficacy was found to be excellent, producing permeate with zero E.coli
concentration for feed concentrations between 10,000 CFU/ 100 mL and 85,000
CFU/100 mL. The E.coli removal efficiency was 100 per cent for a period of 63 days of
continuous filtration. The ICP Atomic Emission Spectrometer (ICP-AES) results
showed that the leaching of silver from the coated filters over time (90 days) was always
below 0.1 mg/L which is the widely accepted guideline for potable water.
From the literature surveyed, this is the first study which investigated the use of AgNPs
in WFMF membranes for potable water disinfection. The coated filters treated water to
the set international standards for potable water in terms of physical and microbiological
quality.However, the study did not comprise investigation into the effect of different silver
loadings on the filter performance. The study also employed E.coli as the indicator
organism for faecal contamination. The results obtained can be used as a model for
future work using other microorganisms and different silver loadings in order to
compare the performance.