3. Results and discussion
3.1. Effect of pressure on SS and COD removal
The effects of air pressure on SS and COD removal from domestic
wastewater were studied for when the influent concentrations
of COD and SS for the bioreactor were 180–200 mg/L and 159–
205 mg/L, respectively. The results are presented in Fig. 2.
As shown in Fig. 2, the effluent SS concentrations were lower
than 25 mg/L. The effluent SS concentration decreased from 25 to
12 mg/L when the air pressure was increased in incrementally
from 0.10 to 0.50 MPa. This suggests that pressure played an
important role in SS removal. Air bubbles were generated in the
contact zone where the pressure was instantaneously reduced,
and attached to the flocs in water after contact [8,13]. The generation
of bubbles depended on the pressure in the air dissolving vessel.
The amount of bubbles increased while the size of bubbles
decreased as pressure was increased. These conditions were generally
more favorable for collisions and adherence among the bubbles
and suspended particles. Therefore, the treatment efficiency
of SS might be improved by the use of coagulant. The experimental
result showed that SS removal could reach a satisfactory level
which met the requirement at a high pressure through PA–DAF,
thus higher pressure was not necessary. Gravity sedimentation is
the dominant mechanism for gas/liquor separation in the conventional
activated sludge processes including oxidation ditch technique,
anaerobic–anoxic–oxic processed and sequencing batch
reactor process, and lasts more than 1 h. So, PA–DAF is more efficient
than sedimentation for gas/liquor separation, and overcome
the existing shortcomings of PA mentioned above.