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.
During dead-end ultrafiltration all particles and pathogenic microorganisms are retained on a porous membrane, while water and dissolved salts pass through
In gravity-driven membrane (GDM) technology, no backflushing or cleaning is used. Pressure necessary to press water through the membranes is generated by gravity generated by difference of water levels between two storage tanks. As a feed, natural water (river, spring, well or rainwater) can be used without pre- or post-treatment. Although also turbid waters can be used, a pretreatment is required if the water becomes extremely turbid (> 500 NTU).
According to common membrane filtration theory, operation of such a system on a long term should result in the decline of water flux and clogging of the membrane. However, this does not occur due to the phenomenon of flux stabilization.
A broad range of water qualities can be used to feed the GDM system. No backflushing, cleaning or addition of chemicals is required
Flux stabilization occurs after 2-10 days of filtration of untreated water through UF membranes. Flux levels of around 4-10 Liter per hour and square meter of membrane is observed. This flux value is low for conventional membrane systems. However, it allows the production of at least 48 L of safe drinking water per day only with 0.5 m2 of membrane, which is sufficient to cover drinking water needs of a family. A standard membrane module of 25 m2 can provide at least 2.4 m3 of water per day which is sufficient for a small village. Thus, GDM technology has a high potential for implementation for drinking water treatment in households or for communities.
Representative flux values for different raw waters during GDM filtration