Feasibility assessment of Photoreactors for Water Treatment

Photocatalytic processes are acquiring increasing attention as a mean to exploit solar energy in promoting chemical transformations. Numerous applications can be found in the literature on the use of semiconductors for water treatment or to produce renewable fuels. Very challenging applications combine also the two concepts. These topics are usually addressed from the point of view of material science, in the search of very specific, often na\uefve, materials for each reaction. On the contrary, process design is very poorly addressed, with few literature outcomes if compared with catalyst development. The result is that the question often remaining unanswered is: is the process feasible? Is the reactor realistic for the given application, based on activity data? Therefore, attention is here paid on the possible scale up of the technology, trying to determine the reactor size, in terms of catalyst mass needed to achieve the desired conversion. The application example we are dealing with is the photooxidation of ammonia for wastewater treatment. We have set up and compared different types of photoreactors (ca. 300 ml and ca. 1.5L), operated in semibatch mode. Based on a preliminary catalyst screening we have calculated the maximum reaction rate, through which we calculated the catalyst mass needed to fulfil pollutant abatement below law limits in some representative cases. The process demonstrated feasible to manage the water treatment needs of a small community with the highest reaction rate, even in the case of a high concentration of pollutants. Some feasibility options are also found in case of a medium community need. Tests have been carried out also on samples of real waste water, monitoring the conversion of COD and N-containing pollutants in a 10 L pilot photoreactor, in collaboration with ISWA (Stuttgart, Germany, which is gratefully acknowledged). These tests evidenced that there is a competition between the oxidation of organics present in water, with respect to ammonia. This point has to be carefully taken into account when designing the whole water treatment process