Nitrate is a water pollutant of concern and its concentration in New Zealand’s water bodies has shown an increasing trend during the past three decades due in part to its insufficient removal from wastewater before discharge. Nitrate is commonly removed from wastewater by biological means, in a process known as denitrification. Many environmental factors, such as pH and toxic substances, can affect the biological denitrification process. Cell- encapsulation technology has been claimed to provide protection to microorganisms under hash environmental conditions. However, there is still a need for investigating nitrate removal using encapsulated cells under suboptimal pH conditions and in the presence of potentially toxic organic and inorganic substances. Therefore, the aim of this research was to examine denitrification rates by free suspended cells and cells encapsulated in alginate under different pH levels and different concentrations of nZVI and clopyralid.

The research took place in four phases: Phase I (Preliminary tests), Phase II (Batch tests under different pH conditions), Phase III (Batch tests under different nZVI concentrations), and Phase IV (Batch tests under different clopyralid concentrations). The aim of Phase I was to stablish and operate an anoxic sequencing batch reactor (SBR) as a sludge (i.e. denitrifying biomass) generator for smaller batch tests. However, the SBR failed to attain adequate anoxic, denitrifying conditions as assessed from oxidation-reduction potential (ORP) values in the SBR being regularly around -300 mV and observed spikes of nitrate-nitrogen at the beginning of denitrification tests. Therefore, a decision was made to collect sludge from a nutrient removal WWTP which had a reasonable biomass-specific denitrification rate of 0.114 mg N/(g VSS∙min) and to use this sludge as inoculum in subsequent batch tests (Phases II, III, and IV).

In Phase II, the denitrification rate of freely suspended cells was observed to be negatively affected by too low (5.0) or too high (9.3) pH values. In both cases, the biomass specific denitrification rate was similar and around 0.058 ± 0.005 mg N/(g VSS∙min). Contrary to the expectation, the denitrification activity of the encapsulated cells appeared to be even more affected than the free cells under all pH conditions.

In Phase III, the biomass-specific denitrification rate of the freely suspended cells was found to be severely affected by the addition of nZVI nanoparticles at concentrations of 0.5 and 3 mg/L. A similar denitrification rate of about 0.023 mg N/(g VSS∙min) was observed in both cases. However, the denitrification rate (0.007 mg N/(g VSS∙min)) of the encapsulated cells exposed to 0.5 g/L of nZVI was the lowest.

In Phase IV, the denitrification rates were 0.027 and 0.010 mg N/(g VSS∙min) for biomass exposed to 50 and 300 mg/L of the herbicide clopyralid. The denitrification rate was more severely impacted by a high clopyralid concentration as compared to a low concentration. The denitrification rate by the encapsulated cells was also affected by the exposure to 50 mg/L of clopyralid. However, in this case, the denitrification rate was twice the rate observed with the free-cells.

Overall, this study confirms the negative effect on the biological denitrification process by suboptimal environmental conditions (i.e. high and low pH, nZVI and clopyralid). However, the expected enhancement caused by cell encapsulation was only observed in the 50 mg/L clopyralid case. In all other cases, cell encapsulation further affected the biological denitrification rates.