Sponge iron reliefing nitrate accumulation in wastewater anaerobic ammonium oxide reactor

Anaerobic ammonia oxidation (ANAMMOX) is a process in which ammonia, as the electron donor, is oxidized by nitrite. Nitrate is accumulated as the main production in addition to N2 in the solution, causing incomplete removal of total nitrogen in water. Some processes, such as oxic denitrification, anaerobic denitrification or dozing of chemicals, can be used to achieve partial denitrification (reduction of nitrate to nitrite). The reduction of nitrate to nitrite can couple with ANAMMOX process, achieving complete removal of nitrogen. In this paper, sponge iron was added into an ANAMMOX sequencing batch reactor (SBR) to decrease nitrate accumulation. Sponge iron is a kind of special zero-valence iron (ZVI) which has a larger specific surface than that of the normal scrap iron, and is cheaper than nano-ZVI. The sponge iron can react with oxidants, such as oxygen and nitrate, immediately in solution and produce Fe2+ or Fe3+. The reactions between sponge iron with nitrite and nitrate were studied in a static state, in which the nitrite and nitrate were reduced into ammonia by sponge iron in solutions. Several factors affecting the reactions between sponge iron with nitrite and nitrate were investigated, such as the dose and particle size of sponge iron, solution pH value and dissolved oxygen (DO). The results showed that small sponge iron particle size and high DO concentration were favorable for nitrite and nitrate removal. Low solution pH value had slight enhancement in nitrate and nitrite removal. In the dynamic experiments, two ANAMMOX SBRs (marked as A and B, both with an effective volume of 2 L) acclimatized with the municipal wastewater plant anaerobic sludge were operated about 20 days, then the reactor B was added with 300 g sponge iron with diameter of 5-8 mm, and the reactor A was used as control. The nitrate concentration in the outlet of reactor B dosed with sponge iron could be kept around 25-30 mg/L, while in reactor A without sponge iron, it could reach 55 mg/L, solving the problem of large quantity of nitrate accumulation. In the reactor A without sponge iron, the nitrate concentration was increasing, showing obvious nitrate accumulation. The mitigation of nitrate may come from the reaction of iron with nitrate, providing the reducing product nitrite to couple with ANAMMOX. Obvious red granule sludge could be seen in the reactor with sponge iron, which suggested that the ferric and ferrous ions may be beneficial for the production of granule sludge. High-throughput sequencing showed that the proportion of ANAMMOX bacteria in the SBR with sponge iron (22.55%) was three times of that (8.85%) in the SBR without sponge iron, proving that the sponge iron was favorable for the start-up of the ANAMMOX SBR. Also the ferric and ferrous ions produced from oxidation of sponge iron by nitrate may also provide necessary element demand of the microorganisms, for example, the ANAMMOX bacteria.