Limiting factors of heavy metals removal during anaerobic biological pretreatment of municipal solid waste landfill leachate

An anaerobic biofilm reactor was used to pretreat a typical municipal solid waste landfill leachate. It was challenging to remove Fe, Pb, and Ni to meet the discharge-to-sewer standards at a hydraulic retention time (HRT) typically used in previous studies. This work further systematically studied the factors that limited the metal removal. The HRT limited metal removal because the required metal sulfides precipitation time was more than 3.5 times of the HRT. Sulfide availability only slightly limited the metal removal since adding sulfate above the stoichiometric requirement improved the metal removal by only 5–11%. Via experiments combined with modeling, it was found that metal bisulfide was the dominant complex that limited Fe removal, but humic acids-metal complex was the dominant complex that limited the removal of Pb and Ni. When the total dissolved sulfide concentration is <18 mg/L, humic substances are more limiting the removal of the three metals than bisulfide. On the other hand, when the total dissolved sulfide concentration is >250 mg/L, bisulfide is more limiting than humic substances.