Nitrifying bacteria characterisation to identify and implement factors leading to nitrogen removal via nitrite in activated sludge processes

Nitrogen removal in biological wastewater treatment plants by nitrification and denitrification can be short-cut via nitrite instead of the traditional nitrate and has the potential for up to a 25% reduction in aeration and 40% reduction in COD requirements. While the potential monetary savings for nitrogen removal via nitrite are significant, the difficulty lies in achieving specific inhibition or removal of the nitrite oxidising bacteria (NOB; those that oxidise nitrite to nitrate) whilst retaining ammonia oxidising bacteria (AOB; those that oxidise ammonia to nitrite). In approaching this general problem, characterisation of NOBs (both Nitrobacter and Nitrospira) were used to determine, in conjunction with AOB data, reactor operating factors which may lead to NOB removal. From literature, the AOBs have been hypothesised to have a higher oxygen affinity (lower oxygen half saturation constant, K[subscript]o) than NOBs. The K[subscript]o values of both Nitrospira and Nitrobacter enrichments (NOBs) and a Nitrosomonas enrichment (AOB) were determined with floc size distributions that indicated oxygen mass transfer was negligible as 0.54 ± 0.14 mg.L¯¹, 0.43 ± 0.08 mg.L¯¹ and 0.033 ± 0.003mg.L¯¹, respectively. The relative AOB and NOB K[subscript]o values confirm the hypothesised difference in oxygen affinity. The growth rate values of the same Nitrosomonas and Nitrobacter enrichments were determined using a novel nitrifier growth rate method (developed in this thesis) as 0.47 ± 0.09 day¯¹ and 0.60 ± 0.03 day¯¹, respectively. Therefore, whilst the NOB maximum growth rate was determined as greater than that of the AOB maximum growth rate at saturated dissolved oxygen (DO) concentration, at low DO concentration the maximum growth rate of AOBs was determined as greater than the maximum growth rate of the NOBs. Hence, washout of NOBs was achieved by operating a continuous reactor at a DO concentration (0.4 mg.L¯¹) and a sludge retention time of 2.4 days which allowed for AOB retention but not NOBs. The results from the continuous reactor experiments suggested that the growth rate of NOBs may not be exclusively greater than the AOB growth rate, contrary to previous thought. Therefore, if aeration is terminated at the end of ammonium oxidation in a batch process (such as a sequencing batch reactor; SBR) with accumulated nitrite remaining, then the relative population of AOBs would eventually dominate. Indeed this concept was investigated using an SBR treating domestic wastewater (TKN concentration of about 43 mgN.L¯¹) with a pre-denitrification configuration and only the termination of aeration at the end of ammonium oxidation (aerobic duration control) as the selection factor for AOBs. The process proved effective in achieving a steady state whereby 80% nitrification to nitrite (20% to nitrate) was observed. Investigation of the cause of nitrification to nitrite by a calibrated ammonium and nitrite oxidation model showed the aerobic duration control as the key selection factor for AOBs. The results of numerous simulations with the same computer model also verified the importance of a greater AOB growth rate than a NOB growth rate in achieving sustained nitrite as the product of nitrification. The processes developed in this thesis to remove NOBs, i.e. low DO concentration and aerobic duration control could theoretically be implemented in current operational wastewater treatment plants. The low DO concentration selection factor against NOBs would probably be more effective in combination with other selection factors against NOBs (e.g. high temperature) as this would enhance the effectiveness of such processes. However, the aerobic duration control could theoretically be implemented immediately in similar SBR processes treating similar domestic wastewater.