The photo-electrochemical disinfection of water using a novel bubble column reactor P. A. Christensen

“You will have a blade upon your weapon; when you “ease yourself” outside, you will dig with the blade, and cover what comes from you” Deuteronomy Chapter 12 Verse 12 and 13

Housesteads Roman Fort, Hadrain’s Wall, Northumberland, GOC Netties N (Latrines)

The latrine at Housesteads Roman fort

Structure of Presentation Aim Introduction-Photocatalysis by TiO 2 Reactor design Reactor characterisation Light sources Electrodes Disinfection data: E. coli Disinfection data: Cryptosporidium Disinfection data: E. coli -the effect of Fe A warning for the academic Conclusions Further work

Aim To develop a photoelectrochemical detoxification system to facilitate the recycling of wastewater from industrial processes. To explore the potential of such a system for disinfection. (Does EFE take place?)

Introduction: Photocatalysis by TiO 2 an example of an AOP Advanced Oxidation Process

The generation of hydroxyl radicals at irradiated TiO 2 ( < 410 nm) h - OH ads h+h+ e-e-. OH ads e-h+e-h+ e-e- TiO 2 particle

Semiconductor Photocatalysis Slurry Minority carrier length 0.1  m Small particle size - Long settlement times - Membrane filtration Low throughput Immobilised film Severe MT limitations AND - EFE BUT - reactor design

V e-e- O2O2 H2OH2O e - h + h e-e- e-e- ‘Fatal Attraction’ The Electric Field Enhancement Effect (How?) TiO 2 photoanodeCounter electrode

Reactor Design (Not designed specifically for disinfection)

To reservoir Air Sparge From reservoir SIDE VIEW UVLAMPUVLAMP Ni Counter electrode TiO 2 Working electrode PLAN VIEW UV lamp Glass walls Gauze 125 mm x 115 mm The Bubble Column Reactor Electrode cassettes

The Bubble Column Reactor (Single electrode cassette)

Reactor Characterisation

Dependence of Reynolds number on bubble rise velocity. Volumetric flow rate = 2 x m 3 s -1 Reynolds number sufficiently high to maintain turbuluent flow

The light source(s)

Emission profiles from the 8W, 32W and 400W lamps used in our experiments. Actinometry shows that the emission of the 400W lamp is 100 x that of the 2 x 32W lamps at 435 nm AnataseRutile

The Electrodes (Photoanodes) (As coatings on a mixture of substrates: 1 cm 2 and 25 cm 2 Ti plates, and 11.5 cm x 12.5 cm Ti mesh rolled into a cylinder)

Thermal TiO 2 films were prepared by first cleaning the Ti metal substrates (ie. mesh) with ethanol and then placing them in a furnace preheated at the required temperature for 10 min.in air, after which time they were allowed to cool slowly. The TiO 2 sol-gel electrodes were prepared from the product of the acid catalysed reduction of titanium di-isopropyl acetoacetonate with water [L. Kavan and M. Gratzel, Electrochim. Acta, 40 (1995) ]. Dip-coated films were applied by immersing the titanium substrate into the gel product and withdrawing slowly to ensure good coverage of the catalyst. The resulting amorphous layer was heated at the required temperature for 10 mins. The procedure was repeated five times. Electrode Preparation Geometric surface area High surface area.

Tapwater + Methanol

‘Kill’ mechanism?

No change on addition of methanol Note - max c. 750 C The effect of heating temperature on the photocurrent density of the resultant thermal TiO2 film. 5 cm x 5 cm Ti plates, 2 x 32W ‘sunbed’ lamps.

Disinfection data (Data for batch operation, reactor volume 500 cm 3, unless otherwise stated)

5 x 5 cm 2 plate photo- anode in 250 cm 3 cell

The inactivation of Cryptosporidium oocsysts in tap water in a 135 cm 3 batch reactor, 2 x 8W UVA lamps, 25 cm 2 sol-gel TiO 2 electrode, 1.2V vs Ag/AgCl applied potential. The error bars represent multiple comparisons at the 95% level

But- catalytic activity too low

The catalyst activities need improving… ….a little thing(!?), but... “…well, that certainly screws up our chance of conquering the universe!”

However- - Iron

1 cm 2 Sol gel TiO 2 /Ti photoanode, fabricated at 500 C. Photocurrents measured in tap water with and without added methanol as a function of the atom % of added Fe. 2 x 32W sunbed lamps. Steady decline in photocurrent as Fe content increases- reflected in ‘activity’? Added methanol No methanol

% Kill of E. Coli (10 7 cm -3 ) in the BCR at 1.3V after 15 minutes, using various TiO 2 photoanodes Kill mechanism?

But….

Conclusions Although not designed for disinfection, the BCR showed good mass transport and light utilisation characteristics, with supply of oxidant to the counter electrode, and reduction thereat, not being rate limiting. The photoelectrochemical disinfection of water inoculated with a range of pathogens including Cp, E. coli and Clostridium Prefringens spores has been observed using simple TiO 2 photoanodes and relatively low intensity UVA lamps and low applied voltages (< 3V). The kill rate is increased substantially over that observed at zero applied potential.

The mechanism of disinfection remains unclear. However, it is clear that disinfection activity depends very much upon the nature of the TiO 2 photoanode employed, and may not simply involve OH radicals as the active agent. In our hands, the photoelectrochemical disinfection of water proved more effective than photochemical disinfection using a TiO 2 slurry. The catalytic activity of the photoanodes remains too low.

The most recent reactor design: the Tower reactor

Ti Rod Ti-mesh Ni-mesh PTFE wheel 60 mm 40 mm 60 mm Stainless Steel supporting rod UV lamp housing Sintered glass gas distributor Ni Rod

Preliminary disinfection data from the tower reactor. 4.3 x 10 6 CFU ml -1, 4.6 dm 3 aqueous 1.4mM Na 2 SO 4, 5 x thermal TiO 2 /Ti mesh + Ni mesh cassettes. 2 x 25W UVA lamps, 50% intensity. Reactor volume 7 dm 3. Electrochem (1.6V) Photocatalytic Photoelectrocat (1.6V)