Ultrafiltration

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 output 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.

Aqualose

Microbial cellulose membranes are considerably cheaper than competitors. Whilst Polyvinylidene fluoride (PVDF) ultrafiltration membranes typically sell to the wastewater industry at upwards of $100 /m² (Shenzhen Youber Technology Co., Ltd. no date). Current bacterial cellulose production for nata de coco costs around $1.6 /gram (Lee et al. 2014) and our cellulose with G. xylinus igem cost only £0.09 ($0.14) /g. Our thin cellulose membranes for filtration weighed around 20 g/m² so would cost just $2.80 /m2. Whilst Our studies and existing literature (Fifield 2012) suggest bacterial cellulose is a strong and durable material however we would expect a shorter membrane life than the highly durable PVDF. Being considerably cheaper and easier to dispose of however, cellulose membranes could be more regularly replaced and still provide a more cost effective solution. This would also save on chemical washes regularly required to sustain the extended life of PVDF membranes.

Future Functionalisation

Our system for functionalising cellulose membranes is inherently modular. Proteins to bind or degrade a limitless range of contaminants can be bound to cellulose membranes. Filtration can then be adapted to the particular contaminants, altering seasonally, geographically and adapting over time to emerging threats. Metaldehyde contamination in the English home counties (Mathiesen 2013) could be addressed with specific degradation enzymes and microcystin could be removed from supplies in Ohio contaminated from algal blooms (Queally 2014).

Process Engineering

“To meet new environmental regulations, a step change is required in how we process water. Functionalised cellulose filtration could be a viable part of the solution.” - Dr. Michael Chipps, Principal Research Scientist at Thames Water We envisage functionalised bacterial cellulose membranes utilised in a plate and frame, dead end filtration setup for easy backflush cleaning and replacement when fouled. https://static.igem.org/mediawiki/2014/3/3f/IC-14_Membrane_backwash.gif (from www.meco.com) This process is particularly applicable for water recycling. Ultrafiltration combines well with activated sludge for purification when organic solids content is high and the additional membrane functionalisation can target removal of recalcitrant small molecules that can accumulate in recycled water. Current wastewater treatment plants could be easily retrofitted with the technology with dedicated facilities constructed in the longer term.

Disposal of Contaminants

Heavy metal contaminants pose a serious problem for all water processing solutions as they ultimately cannot be destroyed. To prevent escape into the environment they must be recovered and disposed of into controlled landfill (European Commission 2002). Trapping contaminants on filtration membranes concentrates them for easy recovery. To prolong filter life, contaminants could be regularly eluted and collected. When membranes are spent they could either be disposed of directly into controlled landfill or to reduce volume - incinerated and the flue scrubbed. Incineration is the most effective solution to destroy recalcitrant organic pollutants and unlike for other materials, incineration of cellulose membranes produces no harmful by products.

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