Caihua Wang

Performance of high-loaded ANAMMOX UASB reactors containing granular sludge

The performance of high-loaded anaerobic ammonium oxidizing (ANAMMOX) upflow anaerobic sludge bed (UASB) reactors was investigated. Two ANAMMOX reactors (R1 with and R2 without effluent recycling, respectively) were fed with relatively low nitrite concentration of 240 mg-N L(-1) with subsequent progressive increase in the nitrogen loading rate (NLR) by shortening the hydraulic retention time (HRT) till the end of the experiment. A super high-rate performance with nitrogen removal rate (NRR) of 74.3-76.7 kg-N m(-3) day(-1) was accomplished in the lab-scale ANAMMOX UASB reactors, which was 3 times of the highest reported value. The biomass concentrations in the reactors were as high as 42.0-57.7 g-VSS L(-1) with the specific ANAMMOX activity (SAA) approaching to 5.6 kg-N kg-VSS(-1) day(-1). The high SAA and high biomass concentration were regarded as the key factors for the super high-rate performance. ANAMMOX granules were observed in the reactors with settling velocities of 73-88 m h(-1). The ANAMMOX granules were found to contain a plenty of extracellular polymers (ECPs) such as 71.8-112.1 mg g-VSS(-1) of polysaccharides (PS) and 164.4-298.2 mg g-VSS(-1) of proteins (PN). High content of hemachrome (6.8-10.3 μmol g-VSS(-1)) was detected in the ANAMMOX granules, which is supposed to be attributed to their unique carmine color.

Suppression of anaerobic ammonium oxidizers under high organic content in high-rate Anammox UASB reactor

The effect of organic matter on the nitrogen removal performance of anaerobic ammonium oxidation (Anammox) process was investigated in an upflow anaerobic sludge blanket (UASB) reactor fed with nitrogen loading rate of 13.92 kg Nm(-3)day(-1) at an HRT of 0.83 h. Mass balance showed that the heterotrophic denitrification prevailed in the UASB reactor, and became the dominant reactions when high influent COD/NO(2)(-)-N ratios of 2.92 were applied. The Anammox bacterial growth was significantly suppressed by denitrifying communities under high organic matter content due to the weaker competition for nitrite (electron acceptor) and living space. Long-term operation of the Anammox UASB reactor under relatively high organic content resulted in weak recovery performance.

Influence of substrates on nitrogen removal performance and microbiology of anaerobic ammonium oxidation by operating two UASB reactors fed with different substrate levels

Both ammonium and nitrite act as substrates as well as potential inhibitors of anoxic ammonium-oxidizing (Anammox) bacteria. To satisfy demand of substrates for Anammox bacteria and to prevent substrate inhibition simultaneously; two strategies, namely high or low substrate concentration, were carefully compared in the operation of two Anammox upflow anaerobic sludge blanket (UASB) reactors fed with different substrate concentrations. The reactor working at relatively low influent substrate concentration (NO(2)(-)-N, 240 mg-NL(-1)) was shown to avoid the inhibition caused by nitrite and free ammonia. Using the strategy of low substrate concentration, a record super high volumetric nitrogen removal rate of 45.24 kg-Nm(-3) day(-1) was noted after the operation of 230 days. To our knowledge, such a high value has not been reported previously. The evidence from transmission electron microscopy (TEM) showed that the morphology and ultrastructure of the Anammox cells in both the reactor enrichments was different.

Floatation and control of granular sludge in a high-rate anammox reactor.

The granule floatation is a serious issue of the anammox (anaerobic ammonium oxidation) process when high loading rates are applied that results in instability or even system collapse. The present study reports the granule floatation in an anammox reactor when high loading rates were applied. The comparison of enlarged photos taken for the settling and floating granules showed that the two kinds of granules both contained macroscopic gas pockets accounting for 11 +/- 14% of total volume. The settling granules had gas tunnels that could release the gas bubbles, while the floating granules did not. The presence of gas bubbles enclosed in the gas pockets led to the small density of 979.2 +/- 15.8 mg L(-1) and flotation of anammox granules. Consequently, the flotation caused washout of anammox granules and the deterioration of anammox process (volumetric removal rate decreased from 4.00 to 2.46 kg N m(-3) d(-1)). The collection of floating granules, breaking them into small pieces and then returning to the anammox reactor proved an effective control strategy. The volumetric removal rate was finally up to 16.5 kg N m(-3) d(-1) after the control strategy was put into use.