The accumulation of nitrogen compounds represents a pivotal problem in the management of fish culture. Several methods were investigated in the last decades for treatment of waste waters, and the use of photocatalytic materials has received increasing attention. The photocatalytic degradation (PCD) process with titanium dioxide (TiO2) represents the most promising single-step method to promote the removal of nitrogen compounds from water. The present study compares for the first time the effects on fish culture of a classical mechanical, biological, and ultraviolet purification system to a TiO2-PCD one, with particular emphasis on water chemistry and on physiological responses in zebrafish. Fish were exposed for 14 days to the two different purification systems and samplings were performed 7 and 14 days after the experiment beginning. The photocatalytic system showed excellent efficiency in removing nitrogen compounds from water with no significant adverse effects on fish. Physiological analysis on fish samples included histological analysis of gills and gut, TUNEL assay of the gills, and real-time polymerase chain reaction (liver) of genes involved in stress response and growth. No significant biological alterations were detectable on the cultured fish.
