Effect Of Zn²⁺ On Biological Nitrogen And Phosphorus Removal In Single-stage Oxic SBR

Eutrophication has been increasingly serious which results in enormous harm to ecological environment and human health, and biological nitrogen and phosphorus removal can be achieved efficiently in single-stage oxic sequencing batch reactor?SBR? without anaerobic phase. Trace amounts of heavy metals are essential for microbes to grow, however, over height of concentration of heavy metals will poison and even kill the microbes, moreover, zinc is one of the most abundant heavy metals in sewage and sludge. For these reasons, the effect of Zn²⁺?0, 1, 5, 10 and 20mg/L? on biologtcal nitrogen and phosphorus removal in single-stage oxic SBR was investigated, using synthetic wastewater with sodium propionate as the sole carbon source.The experimental results showed that 96.54% and 97.48% of the influent phosphorus could be removed in the reactors fed with 0 and 1mg/L Zn²⁺. However, the phosphorus removal efficiency decreased to 89.20%, 76.43% and 57.29% respectively in the reactors fed with 5, 10 and 20mg/L Zn²⁺, suggesting that a lower concentration of Zn²⁺ promoted the phosphorus removal while a higher concentration of Zn²⁺ inhibited the phosphorus removal induced by the single-stage oxic regime. It was demonstrated that the effect of a higher concentration of Zn2 + on phosphorus removal was due to the inhibition to the degradation of COD, synthesis and hydrolysis of polyhydroxyalkanoates and the activity of polyphosphate kinase. Moreover, the metabolic shift from polyphosphate accumulating organisms to glycogen accumulating organisms at high concentrations of Zn²⁺ could also decrease the phosphorus removal ability of the system.Meanwhile, the average efficiencies of total nitrogen removal of the 5 SBRs were 89.6%, 90.7%, 81.3%, 60.3% and 42.4% respectively, and the average efficiencies of ammonia nitrogen removal were 99.8%, 99.8%, 96.0%, 82.9% and 67.0%, showing that 1 mg/L of Zn²⁺ had no obvious influence on ammonia nitrogen removal, but was advantageous to remove the total nitrogen of the system, on the contrary, a higher concentration of Zn²⁺ could inhibit the biological nitrogen removal induced by the single-stage oxic process. Mechanism studies revealed that denitrification was promoted when 1 mg/L of Zn²⁺ was existed, and the denitrification ability was weakened in the reactor fed with 5 mg/L of Zn²⁺, which caused the inhibition of 5 mg/L of Zn²⁺ on nitrogen removal. Furthermore, the deterioration of biological nitrogen removal caused by 10 mg/L and 20mg/L of Zn²⁺was due to the inhibition to both the nitrification and the denitrification. Moreover, the effect of Zn²⁺ on the activities of ammonia monooxygenase, nitrite oxidoreductase, nitrate reductase and nitrite reductase was another reason why the 5 reactors showed different nitrogen removal abilities when fed with different concentrations of Zn²⁺.