Introduction

Ultrafiltration (UF) membranes have a pore size of 0.1 to 0.01um (10 to 100nm) and are capable of removing particulates, bacteria and viruses. Microbial cellulose sheets naturally have pore sizes in this range (Gatenholm, P., & Klemm, D. (2010), Mautner et al 2014). Current ultrafiltration cannot remove small molecule contaminants such as pesticides and heavy metals however. Whilst nanofiltration and reverse osmosis membranes can exclude these small molecules they are expensive and energy intensive to use. Flow rates are low, they require very high pressures and the input water must be already purified by primary and secondary processes to avoid damaging the membranes.

https://static.igem.org/mediawiki/2014/4/47/IC-14_Water_Purification_Spectrum1.JPG Size exclusion for different grades of filter (from http://www.edstrom.com/) Depending on input water quality UF systems may replace or complement existing secondary (coagulation, flocculation, sedimentation) and tertiary filtration (sand filtration and chlorination) systems in water treatment plants. Pretreatment of feed water is usually required to prevent reduce damage to the membrane units though ultrafiltration may be used in standalone systems for isolated regions. UF processes have the following advantages over traditional treatment methods:

• Chemical free (aside from cleaning)
• Constant product quality regardless of feed quality (excluding small molecule contaminants, changes in input water quality affect only the life of the membrane, not the quality of the flow through)
• Compact plant size, efficient for small scale, decentralised purification
• High quality of output water particularly with regards to pathogen removal

UF processes are currently limited by the high cost of membranes, inevitable membrane fouling means they must be regularly replaced. There use is also restricted by limitations in removal of small molecule contaminants. They can only be employed where feed water is free of these contaminants or in tandem with other (often slow or energy intensive) treatment methods for removing them.

Ultrafiltration

Phytochelatin-dCBD metal binding assay

Nickel filtration assay

The nickel ions are an example of heavy metals, poisonous even in relatively small concentrations in water and notoriously difficult to filter with current filtration methods. Therefore our filtration concept was tested against a concentration of nickel in water that far exceeds the safe limits. We have attempted to filter high amount of nickel (250 μM) through the cellulose filters grown by the G. Xylinus ATCC53582 strain (K1321305). The phytochelatin-dCBD fusion (K1321110) was coated on the surface of the cellulose to make a nickel specific functionalised ultra filtration membrane. To test the two membranes we have used coffee press. As a control measure we have also attempted to the filter the nickel solution through the cellulose that was not further functionalised.

Results