Emission of H2S and mass balance of sulfur in anaerobic ponds.

Anaerobic ponds are highly efficient at removing BOD from wastewater with a reduced land area requirement. However, their use is often limited because of the problem of odor release, primarily due to the emission of hydrogen sulfide (H2S). The experiments were conducted on full scale anaerobic ponds used for the primary treatment of urban wastewater under Mediterranean climatic conditions (south of France). A model was developed to estimate the emission of H2S from water characteristics (temperature, pH and sulfides concentration). The emission rate from anaerobic ponds varied between 20 and 576 mg-S/m2.d, leading to concentrations of H2S in the surrounding atmosphere between 0.2 and 5.2 ppm. These concentrations risked creating odor nuisances for nearby inhabitants. The whole of the results and the analysis of sulfur species in sludge permitted the production of a complete mass balance for sulfur. Methods of reducing the emission of odorous compounds were tested on a large scale. The recirculation of secondary effluent and the use of impermeable or permeable covers appeared to be the most interesting solutions.