Effect of nickel on nutrient removal by selected indigenous protozoan species in wastewater systems

Nutrient and heavy metal pollutions are major concern worldwide. This study aimed at comparing the effect of Ni2+ on nutrient removal efficiency of four indigenous wastewater protozoan species (Aspidisca sp., Paramecium sp., Peranema sp., Trachelophyllum sp.). Specific physicochemical parameters and microbial growth/die-off were measured using standard methods. The results revealed that protozoan species were able to simultaneously remove phosphate, nitrate and Ni2+ at concentrations ranging between 66.4–99.36%, 56.19–99.88% and 45.98–85.69%, respectively. Peranema sp. appeared to be the isolates with the highest removal of nutrients (Phosphate-99.36% and Nitrate-99.88%) while Paramecium sp. showed higher removal of Ni2+ at 85.69% and low removal of nutrients. Aspidisca sp. was the most sensitive isolate to Ni2+ but with significant nutrient removal (Phosphate-66.4% and Nitrate-56.19%) at 10 mg-N2+/L followed by an inhibition of nutrient removal at Ni2+ concentration greater than 10 mg/L. Significant correlation between the growth rate and nutrient removal (r = 0.806/0.799, p < 0.05 for phosphate and nitrate, respectively) was noted. Except for Peranema sp. which revealed better nutrient removal ability at 10 mg-Ni2+/L, an increase in Ni2+ concentration had a significant effect on nutrient removal efficiency of these indigenous protozoan species. This study suggests that although Ni2+ appeared to be toxic to microbial isolates, its effect at a low concentration (10 mg-Ni2+/L) towards these isolates can be used to enhance the wastewater treatment process for the removal of nutrients. Peranema sp., which was able to remove both Ni2+ and nutrients from wastewater mixed-liquor, can also be used for bioremediation of wastewater systems.