Nitrogen removal in a modified anaerobic baffled reactor (ABR): 2, nitrification

The oxidation of ammonia to nitrite/nitrate using chemolithoautotrophic nitrifying bacteria immobilised in polyvinyl alcohol (PVA) cubes and placed within an aerobic stage of a modified anaerobic baffled reactor (ABR) was investigated. Various system parameters known to affect nitrification were tested to determine the viability of the process. Nitrification efficiency was improved by increased mixing, increased oxygen levels and the initial addition of copper. In contrast, nitrification was reduced by; a reduction in bulk ammonia levels, a reduction in pH, continued copper addition and COD levels above 1000 mg/l. Whilst immobilisation appeared to protect against un-ionised ammonia, it simultaneously increased the saturation constant Ks for both oxygen and ammonia by orders of magnitude. Consequentially, the bacteria could survive in abnormally high bulk ammonia concentrations with no signs of inhibition, but were limited for oxygen below a partial pressure of 0.42 atm, and for ammonia below 160 mg NH3/l. A series of non-typical trends became evident during this work: the continued dependence of nitrite oxidation on the performance of ammonia oxidation; persistently low bulk nitrite concentrations; the ability of the pellets to produce high levels of nitrate even when bulk un-ionised ammonia concentrations were orders of magnitude higher than the inhibition coefficient for Nitrobacter; and, the high sensitivity of the bacteria to low concentrations of ammonia and oxygen. These results were due to significant increases in apparent Ks values for both groups, and were in accordance with reduced diffusion coefficients for the pellets. These findings were then used to propose a layered structure for the pellets, and in conjunction with other work on denitrification in an ABR, lay the basis for an integrated nitrogen removal technique in a single reactor.