During an analysis of water from the River Arlanza, where the Castrovido dam is located, 200 meters downstream from the discharge point pH levels of 10.59 were detected. These limits are higher than the established pH limits between 6 and 9. The presence of these high pH levels presents a risk to the riverbed flora and fauna, and therefore the discharge must be pre-treated. These pH levels in the water discharge are mainly due to the use of concrete for construction, especially for stripping and cleaning works.

To neutralise the effluent discharge, a concrete tank was set up with two compartments separated by a wall. In the first compartment, the pH of the water from the decanting tanks was neutralised by means of CO2 diffusion through a gridiron (CO2 is fed from a cryogenic tank of liquid carbon dioxide adjacent to the tank of concrete). Once the pH of the water is neutralised, this then overflows to the second compartment where the neutralised discharge is pumped to the river. These water quality parameters are within the limits set by law. The decision to opt for the use of CO2 as a neutralising agent has advantages over the use of other reagents such as hydrochloric acid, sulphuric acid or acetic acid (used more frequently), since the use of these acids involves greater storage and handling difficulty due to their high corrosive nature.

The implementation of this process meant that the effluent discharge maintained a constant pH within the limits set by law. After periodic inspections conducted both by an accredited independent laboratory and quality control personnel from the works, it was determined that the discharge did not affect the fish fauna, with trout, crayfish and other species typical of this type of environment present downstream from the discharge area of treated water.