Desempenho de tanques vegetados em um ecossistema engenheirado para tratamento descentralizado de esgotos domiciliares na Ilha Grande, RJ

Abstract

The lack of appropriate infrastructure for sewage collection and treatment affects several communities living under unhealthy conditions in coastal areas and in insular systems in Rio de Janeiro State. The objective of this study was to assess the performance of an engineering ecosystem for the descentralized treatment of domestic wastewater located at the Centre of Environmental Studies and Sustainable Development (CEADS), UERJ, Vila Dois Rios, Ilha Grande, Rio de Janeiro State. The emphasis was put on the contribution of macrophytes and algae in vegetated tanks as a terciary treatment for the general improvement of the quality of the effluent, particularly regarding nutrients removal. The system includes a flow controlling box, grease box, septic tank, submersed aerated filter, secondary decanter and a set of vegetated tanks planted with aquatic Eichhornia crassipes, Schoenoplectus sp., Panicum cf. racemosum plus Cyperus ligularis besides an algae tank between the first two macrophytes tanks. The main parameters monitored in samples taken every 15 days during the period from September 2009 to July 2010 in 10 sampling points equivalent to the outflow of each treatment unit were: OD, pH, electric conductivity, temperature, DBO5, DQO, SST, SDT, Ammonia Nitrogen, Nitrite, Nitrate and total Phosphorus. The average flow from the system was 52 litres per hour. The primary biomass production by macrophytes and the metabolism (primary production and respiration) from the algae tank were also estimated. The average efficiency of the engineering ecosystem was 93% for SST, 93% for DBO, 90% for DQO, 30% for Ammonia Nitrogen and, 38% for total Phosphorus. The set of vegetated tanks promoted an average increase for dissolved oxygen (DO) of 182%, compared to the average DO found in the decanter outflow. It was concluded that the system of vegetated tanks contributed with a general improvement of the quality of the final effluent in relation to practically all parameters, even if in some cases only as an incremental level. There was more production of biomass (dry weight.day-1) and water retation by E. crassipes (water hyacinth) in comparison to the species Schoenoplectus sp, P. cf. racemosum and C. ligularis. The low primary production (measured as O2 production) in the algae tank in relation to the respiration (measured as O2 consumption), at least during the monitoring period, indicated unfavourable conditions to the nutrients removal (N and P) in these tanks. A more detailed investigation about the project parameters which includes an eventual decanter and algae tank re-dimensioning is suggested.