A pH-dependent enhancement effect of co-cultured Bacillus licheniformis on nutrient removal by Chlorella vulgaris

A pH-dependent enhancement effect of a co-cultured bacterium Bacillus licheniformis on nutrient removal by a green alga Chlorella vulgaris was evaluated in a 6-day experiment. In the single algal system, steady removal efficiencies of ammonia nitrogen (NH4-N) and total phosphorus (TP) were achieved at 63 and 91% with 10 × 105 algal cells mL−1, meanwhile the optimal removal rates of NH4-N and TP were achieved at 10.2 and 7.3 mg 10−10 cells day−1 with initial concentrations of 10 mg L−1 NH4-N and 2 mg L−1 TP, respectively. In the bacteria-co-cultured algal system, the removal efficiency of NH4-N was significantly increased to 78% with an algae/bacteria cell density ratio of 1:1, compared with 63% in the single algal system under the same conditions. However, a decrease in pH was detected in the algal-co-cultured system and the removal efficiency of NH4-N could be raised to 86% by regulating pH from acidic (pH 3.5) to neutral. Furthermore, algal cells in the pH-unregulated algal-co-cultured system (acid medium) were observed as close-grained spheres by scanning electron microscopy (SEM), while cells were wrinkled in the pH-regulated algal-co-cultured system (neutral medium) at the end of tests. These results suggest a potential application of bacteria-co-cultured algal system for effective nutrient removal from wastewater by adjusting pH (base addition) during the process.