Release behavior of chloride from MSW landfill simulation reactors with different operation modes

The chloride ion (Cl−), a very common monatomic anion, has high ecological toxicity at high concentrations because of its non-biodegradability, and can easily migrate from landfill site into the surrounding environment. Four lab-scale landfill simulation reactors were established to investigate Cl− release behavior: the anaerobic landfill mode (R1), the semi-aerobic landfill mode (R2), the anaerobic landfill with leachate re-circulation mode (R3), and the semi-aerobic landfill with leachate re-circulation mode (R4). The landfill operation modes had a great influence on the release of Cl−. In 256 days, the cumulative release amounts of Cl− in the four reactors were 64.52, 132.07, 56.10, and 33.1 g for R1-R4, respectively. Once air enters anaerobic landfill, the leachate Cl− concentration may sharply increase. The highest leachate Cl− concentrations were 6.6 g L−1 in anaerobic reactor and 18 g L−1 in semi-aerobic reactor. However, the leachate re-circulation can maintain the release of Cl− at dynamic equilibrium state. Theoretically, the Cl− release behavior from anaerobic landfill with leachate re-circulation (R3) will be continuous. In contrast, under the other conditions, it can be anticipated to occur once the leachate recirculation stops (R1) or when the landfill encounters air incursion (R2 and R4). The semi-aerobic operation modes had significantly lower COD/Cl and NH4-N/Cl ratios than the anaerobic modes. This indicates that the Cl− pollution risk from semi-aerobic modes is lower than that from anaerobic modes.