Removal of hydrogen sulphide BOD from brackish water using vertical flow wetlands in a Caribbean environment

Wastewater from a 550-inhabitant community had been treated and discarded using an anaerobic filter. Due to seawater intrusion in the aquifer that supplies the water, high concentrations of hydrogen sulphide were detected in the effluent. A vertical flow wetland was designed in 1998 for treating this effluent. Four parallel reed beds with a total area of 556 m² were constructed. During the first months of operation, a mean BOD₅ removal efficiency of 91% was obtained, with loads to the wetland system up to 4g/m²/d of grease and oils (G&O). In 1999, problems of soil clogging were found due to high G&O content in the wastewater, with loads up to 15g/m²/d of G&O, which highly influenced the hydraulic conductivity of the beds, generating the clogging problems. The low hydraulic conductivity and the high effluent G&O content, caused low BOD5 and COD removal efficiencies. As G&O accumulated in the soil, the removal efficiencies decreased. Despite the clogging problems, there has been a high sulphide removal throughout the system operation. The wetlands removed sulphides successfully, under loads up to 20 gS⁼/m²/d. Four native species of macrophytes were planted: Paspalum penisetum, Typha sp, Conocarpres erectus and Scirpus lacustris. All of them but Typha sp. were established in the system.