Disinfection of municipal wastewater by TiO2 photocatalysis with UV-A, visible and solar irradiation and BDD electrolysis

The efficiency of TiO2 photocatalysis induced by ultraviolet and visible irradiation and salt-free
electrolysis over boron-doped diamond electrodes to inactivate total (TC) and fecal (FC)
coliforms in secondary treated municipal wastewater was evaluated. Photocatalytic
experiments were conducted with two types of titania (Degussa P25 and sulfur-doped
catalyst) at loadings in the range 0.1-0.5 g l-1 and three types of irradiation, i.e. artificial UV-A,
artificial visible and solar. Electrolysis was conducted in a flow-through cell at anodic current
densities of 14.3 and 28.6 mA cm-2 without the addition of auxiliary chemicals. Inactivation
followed a first-order kinetic expression with regard to bacteria population and the rate was
dependent of the experimental conditions in question. In general, electrochemical disinfection
was up to about two orders of magnitude faster than photocatalysis; for instance, TC
population decreased to less than 5% of the initial population after 9 and 4 min at 14.3 mA
cm-2 and 28.6 mA cm-2 respectively. Similar efficiencies with TiO2 photocatalysis would
require at least 30 min of contact time. Interestingly, energy consumption for the
electrochemical process was computed to about 0.5 kWh per m3 of treated effluent, well
below the respective value for photocatalysis.