A treatment system is a combination of physical, chemical and biological processes applied to wastewater to affect its decontamination. The objective of the present study was the evaluation of the residual water treatment system installed in the central sector of EARTH University. The system consists of three components: a septic tank for separation of solids and non-biodegradable materials, two biodigestors in tandem, and two sets of four ascending-flow gravel filters. The quality of the residual water in the effluent of the system was compared to the effluent of the septic tank as it entered the biodigestors, by analyzing residual water samples in the field and in the laboratory. The biodigestor component presented the highest percentage of decontamination of the residual waters; in addition, the biogas produced is being used at the university’s cafeteria for food preparation. During the 128 days of monitoring, the levels of turbidity, DBO₅, and DQO in the effluent of the system exceeded the maximum permissible limits as stipulated by the law in Costa Rica. However, the system has been in operation for only six months and in the final weeks of this study, the system showed an improvement for decreasing the levels of contamination in the residual waters. It is possible that with time, the efficiency of the system will improve due to an increase in the microbial populations responsible for the decomposition of the organic material in the residual waters. As well, an increase in the retention time of the residual water in the biodigestors is recommended to allow for maximum degradation of the organic material present. Due to their inefficiency in reducing the contamination of the residual waters, the gravel filters should be removed and replaced with sedimentation channels.