Effects Of Heavy Metal Ions On Anammox Granules And Strategies For Remediation

Certain heavy metals are necessary micronutrients of microorganism growth, whereas excessive heavy metals have inhibitory or even toxic effects on them in biological treatment processes. Given the prospective application of anaerobic ammonium oxidation (anammox) in the treatment of high-strength ammonium wastewaters from landfill leachate, livestock farm, fertilizer production and semiconductor manufacture, considerable amounts of heavy metals could be introduced to anammox reactors. This study firstly evaluated the interactive effect of Cu(Ⅱ) and Zn(Ⅱ) on anammox granules and possible mechanism. And then, the behavior, distribution and form dynamics of overloaded Cu(Ⅱ) (selected as an example) in the granule-based anammox reactor were tracked. Finally, two novel remediation strategies respectively from biochemical and physico-chemical views were proposed and their feasibilities were also investigated. The main results are as follows:(1) The interactive effect of Cu(Ⅱ) and Zn(Ⅱ) on anammox activity was evaluated using response surface methodology with a central composite design. The joint inhibitory effect tended to be synergistic, additive, independent and antagonistic as the concentrations increased. The most severe inhibition, resulting in an inhibition ratio of 80%, occurred at Cu(Ⅱ) and Zn(Ⅱ) concentrations of 16.3 and 20.0 mg L-1, respectively. Notably, the cumulative toxicity was mitigated with the aid of intermittent exposure acclimatization. Additionally, pre-exposure to Cu(Ⅱ) in the absence of substrates strongly inhibited anammox activity. However, the presence of nitrite (without ammonium) significantly enhanced Cu(Ⅱ) inhibition.(2) The behavior, distribution and form dynamics of overloaded Cu(Ⅱ) in a granule-based anammox reactor were investigated. Results showed that the Cu distribution migrated from the extracellular polymeric substances-bound to the cell-associated Cu and the Cu forms shifted from the weakly bound to strongly bound fractions over time. Cu loading as high as 0.24 g L-1 d-1 exceeded sludge bearing capacity, and precipitation dominated the removal pathway. Pearson correlation and fluorescence spectra analyses showed that the increase in protein contents in the EPS was a clear self-defense response to Cu(Ⅱ) stress. Two remediation strategies, i.e.,ethylenediamine tetraacetic acid (EDTA) washing and ultrasound-enhanced EDTA washing, weakened the equilibrium metal partition coefficient from 5.8 to 3.41 and 0.45 L mg-1 SS, respectively, and resulted in a redistribution of the Cu in the anammox granules.(3) The feasibility of using EDTA washing followed by Ca2+ regulation for the remediation of anammox granules suppressed by copper was investigated. After washing with 2 mM EDTA (at Vliquid/Vsludge=10) for 210 min, a critical SAA recovery (74.7%) was achieved with a relatively optimal Cu removal efficiency (80.5%).The two-step desorption (or nonhomogeneous diffusion) process revealed that the Cu in anammox granules was primarily introduced via adsorption (approximately 80.5%). Afterwards, the Cu internalized in the cells (approximately,14.7%) could diffuse out of the cells and was gradually washed out of the reactor over the next 20 days. The following Ca2+ addition (82 mg L-1) accelerated biomass growth rate from 0.018 d-1 to 0.054 d-1.The nitrogen removal rate (NRR) and granule characteristics returned to normal levels within 90 days.(4) The feasibility of using ultrasound-enhanced EDTA washing followed by intermittent ultrasound enhancement for the remediation of anammox granules suppressed by copper was also investigated. The low-intensity ultrasound (Freq:28 kHz, UI:0.7 w cm-2, ET:1.9 min) improved the remediation effects:SAA, dehydrogenaseactivity and Cu removal efficiency were increased by 107%,47.7% and to 84.9%, respectively. After washing, the Cu internalized in the cells (approximately 12.7%) could diffuse out of the cells and was gradually washed out of the reactor whithin 20 days. Thereafter, ultrasound irradiation at intervals of 8 days contributed to an accelerated NRR increasing rate, increasing from 0.0773 to 0.1971 kgN m-3 d-2. Accordingly, the NRR and granule characteristics returned to normal levels within 64 days.